State of Knowledge in the Field of Regenerative Hardfacing Methods in the Context of the Circular Economy

Abstract

Regenerative hardfacing of steel substrates is an important technology for restoring the surface layer of components operating under wear conditions, supporting the goals of the circular economy (CE) by extending the service life of components, reducing material and energy consumption throughout their life cycle, and shortening downtime during machine repairs. The article provides a synthetic analysis of the literature on the production of functional layers exclusively on steels and systematizes process → structure → properties (PSP) relationships in the context of technological quality and the prediction of the functional properties of welds. The review covers methods used and developed in steel hardfacing (including arc processes and variants with increased energy concentration), analyzed on the basis of measurable process indicators: energy parameters (arc energy/heat input/volume energy), dilution, bead geometry, heat-affected zone characteristics, and the risk of welding defects. It has been shown that these factors determine the structural effects in the weld and the area at the fusion boundary (including phase composition and morphology, hardness gradient, and susceptibility to cracking), which translates into functional properties (hardness, wear resistance, adhesion, and fatigue life) and durability after regeneration. The main result of the work is the development of a PSP table dedicated to hardfacing on steel substrates, mapping the key “levers” of the process to structural consequences and trends in functional properties. This facilitates the identification of optimization directions (minimization of energy input and dilution while ensuring fusion continuity), which translates into longer durability after regeneration and a lower risk of defects—key, measurable effects of CE. Research gaps have also been identified regarding the comparability of results (standardization of energy metrics) and the need to determine and verify “technology windows” within the WPS/WPQR (welding procedure specification/welding procedure qualification record) for layers deposited on steels.

1. Introduction

Hardfacing is one of the key technologies for shaping and restoring the surface layer of machine parts [1,2,3]. Today, it is one of the main techniques to reduce the life cycle costs of machines and shorten downtime. In industrial practice, it is used in two ways [4]:

• For the regeneration of machine parts—wear in the form of geometric defects within the surface layer is more intense than contact fatigue wear, which is the basis for the possibility of multiple regeneration by surfacing (shafts, journals, rollers, sockets) [5,6,7,8,9];
• For the manufacture of functional layers with predetermined properties (resistance to wear, corrosion, erosion) [1,10,11,12,13].

These solutions fit well into the circular economy (CE) action plan and are supported by the development of high-efficiency arc welding processes and their robotization [7,14,15]. CE is based on maximizing the “useful life” of a product and minimizing primary waste streams. Hardfacing fits in with the CE principles because it allows products to be kept “in circulation for as long as possible” through repair and regeneration, which is one of the pillars of the EU’s circular economy strategy and EU policies aimed at designing durable, repairable products and extending their life cycle. This is explicitly emphasized by the European Commission and the European Parliament, which point to “repair, renew, and regenerate” activities as key measures for extending the life of products and reducing waste streams and consumption of primary materials. Regenerative hardfacing allows the surface layer to be selectively restored and/or functionality (e.g., tribological resistance, corrosion resistance) to be added without replacing the entire component. For maintenance organizations, this means shorter restoration times and lower logistical risks, and for designers, it means the need to define “technological windows” that stabilize technological and functional quality with acceptable process variability [16,17,18].

From a product life cycle assessment perspective, studies show that regeneration/repair—including the restoration of the functional layer by hardfacing methods—can significantly reduce the environmental footprint compared to the manufacture of a new component; significant reductions in energy and material consumption and a decrease in environmental impacts have been demonstrated in comparative analyses of various product groups (e.g., turbochargers, mining machinery, electrical equipment). The literature emphasizes that the selection of process parameters determines the geometry and fusion of the weld, dilution, arc stability, and, consequently, the functional properties of the welds and their adhesion to the substrate. The type and flow rate of the shielding gas are also important [2,19,20]. It is therefore necessary to determine “technological windows”, i.e., recommendations for the welding procedure specification (WPS) and welding procedure qualification record (WPQR) for specific material–process combinations.

According to a study by the American Welding Society [21], surfacing processes may be grouped as surface cladding, buildup, buttering, and hardfacing. In a study entitled The Practical Reference Guide for Hardfacing, these processes are defined as follows:

• Cladding—a surfacing variation that deposits or applies surfacing material, usually to improve corrosion or heat resistance;
• Buildup—a surfacing variation in which surfacing material is deposited to achieve the required dimensions;
• Buttering—a surfacing variation that deposits the surfacing metal on one or more surfaces to provide metallurgically compatible weld metal for the subsequent completion of the weld;
• Hardfacing—a surfacing variation in which surfacing material is deposited to reduce wear.

A strong reason for undertaking research on regenerative hardfacing is the growing demand from the steel, energy, mining, metallurgical, and automotive industries for stable and repeatable hardfacing technologies using gas metal arc welding (GMAW)—a widely used, easily robotized, and highly efficient process. Reviews and case studies show that conscious shaping of thermal history (e.g., by controlling linear energy, interpass temperature, interpass cooling) translates into the quality and predictability of the properties of the deposited layers and the economy of the process [7,22,23].

In recent years, there has also been an evolution from classic hardfacing towards wire arc additive manufacturing (WAAM) technology, which belongs to the additive manufacturing processes of directed energy deposition (DED). This applies to the reduction in dilution and control of microstructure, stresses, and surface quality through interpass strategies (e.g., forced cooling between passes/layers, interpass rolling, local shielding) [24,25,26,27]. The same concepts are successfully transferred to functional layers made using the GMAW method, which reinforces the need for research on process maps and dependency models: parameters → technological quality → functional properties.

From the operational perspective of industrial companies, correctly validated “technological windows” for GMAW hardfacing enable the following:

• Reduction in downtime and scrapping costs due to effective regeneration of high-value parts.
• Increased reliability due to predictable properties of the hardfaced layer (hardness, adhesion, wear resistance).
• Increased productivity through conscious management of bead geometry, blending, and dilution without excessive increase in heat input.
• The possibility of robotization and standardization (WPS/WPQR), based on methodical experiment planning and statistical analysis.

The application and scientific context of regenerative hardfacing on steel substrates, defined above, indicates its importance both for surface engineering (recreation and modification of the surface layer, increased wear resistance) and for the circular economy (extending the service life of components, reducing material and energy consumption during the life cycle). At the same time, it has been shown that despite the wide range of research and the maturity of the technology’s implementation, the literature in the field of hardfacing on steels is characterized by limited comparability of results, due to inconsistent definitions of thermal parameters, differences in reporting technological quality, and fragmentary descriptions of the relationships between process parameters, microstructure, and performance properties. This allowed us to establish a problem framework and clearly formulate the objective of the work, which is to organize the process → structure → properties (PSP) relationship for hardfaced layers on steels and prepare methodological foundations for developing a PSP table as the main result of the article.

2. The Amount of Heat Introduced and Its Significance for the Hardfacing Process

Hardfacing is a welding process that involves the controlled deposition of additional metal onto a substrate to restore the dimensions of worn surfaces or give them specific operational functions (tribological, corrosion, and erosion resistance). Hardfacing is used in repairs to restore the serviceability of machine or equipment parts (regenerative hardfacing), or it can be used in the production process of new products (technological hardfacing). A feature that distinguishes the surfacing process from other methods of applying metallic coatings is the melting of the substrate material onto which the surfacing material is applied. Meeting this condition is essential to achieve the appropriate depth of fusion, which guarantees the adhesion of the surfaced layer to the substrate material [1,2,3,4].

The properties of the fusion zone, which is a transition layer between the weld and the base material, have a significant impact on the welding result. The technological quality of welds is determined by the bead geometry (b—width, h—height, p—fusion), dilution D (proportion of base metal in the weld), the presence of dissimilarities, and the corresponding functional properties (microstructure, (micro)hardness, adhesion, and wear resistance). It is crucial to manage the amount of heat input and the thermal history (pre- and interlayer temperature and cooling rate).

Heat input in welding/hardfacing is a commonly used technological parameter for selecting welding conditions and a quantity that serves as the basis for other indicators that are calculated and used in welding [2,10].

The heat input of welding/hardfacing is a measure of the amount of heat per unit length of the weld [28]. It cannot be measured directly—it is calculated based on the measured values of the arc voltage, welding current, and welding speed, using the following Equation (1):

$$Q=k\times {\displaystyle \frac{U·I}{v}}\times {10}^{-3},\mathrm{kJ}/\mathrm{mm},$$

(1)

where Q—heat input of welding/hardfacing, kJ/mm; k—thermal efficiency coefficient of the welding/hardfacing process; U—arc voltage, V; I—welding/hardfacing current, A; and v—welding/hardfacing speed, mm/s.

The heat transfer coefficient k values for different welding/hardfacing methods are shown in Table 1.

Table 1. Thermal efficiency coefficient k for different welding/hardfacing methods [29].

Method No.	Welding/Hardfacing Method	Coefficient k
121	Submerged arc welding with wire electrode (SAW)	1.0
111	Manual-arc welding with a covered electrode (MMA)	0.8
131	Metal inert gas (MIG) welding	0.8
135	Metal active gas (MAG) welding	0.8
114	Flux-cored wire metal-arc welding without gas shield	0.8
136	Flux-cored wire metal-arc welding with active gas shield	0.8
137	Flux-cored wire metal-arc welding with inert gas shield	0.8
138	Metal-cored wire metal-arc welding with active gas shield	0.8
139	Metal-cored wire metal-arc welding with inert gas shield	0.8
141	Tungsten inert gas (TIG) welding	0.6
15	Plasma arc welding (PAW)	0.6

During welding/hardfacing, significant fluctuations in electrical parameters (in particular, current intensity and arc voltage) are observed, resulting from the high dynamics of the process and the complexity of the phenomena occurring in the glowing welding arc. In practice, however, this variability is usually ignored when calculating linear energy, as the calculations are based on time-averaged values, rather than instantaneous parameter values [30].

When using a pulsed current in the welding/hardfacing process, the average current value is calculated using Equation (2) [31]:

$$I_{av}={\displaystyle \frac{I_i\times t_i+I_b\times t_b}{t_i+t_b}}, \mathrm{A},$$

(2)

where I_av—average current intensity, A; I_i—pulse current intensity, A; I_b—base current intensity, A; t_i—pulse current duration, ms; and t_b—base current duration, ms.

Heat input in welding and hardfacing processes is a quantitative technological parameter describing the amount of heat introduced into the joint/layer, thus allowing the process conditions to be numerically related to changes in the properties of the base material and weld metal. At the same time, this parameter is commonly used as a comparative measure for comparing the effects of different welding processes, including pulsed variants, on the weldability of the base material [32].

To enable a proper comparison of heat input in various welding/hardfacing processes on different types/generations of welding equipment (including devices that allow current waveforms to be shaped), the concept of heat input of the welding arc—arc energy—was introduced in the Technical Report [33]. The dimensionless thermal efficiency coefficient was omitted from the calculation of arc energy. Depending on the measuring systems with which welding power sources are equipped, various relationships giving equivalent results, (3), (4) and (5), are used to calculate the arc energy:

• Formula (3) determines arc energy based on the welding/hardfacing current and arc voltage values:

$$E={\displaystyle \frac{U·I}{v}}\times {10}^{-3} ,\mathrm{kJ}/\mathrm{mm},$$

(3)

where E—welding/hardfacing arc energy, kJ/mm; U—arc voltage, V; I—welding/hardfacing current, A; and v—welding/hardfacing speed, mm/s;

• Equation (4) defines arc energy using instantaneous energy:

$$E={\displaystyle \frac{IE}{L}}\times {10}^{-3} ,\mathrm{kJ}/\mathrm{mm},$$

(4)

where IE—instantaneous energy, J and L—weld length, mm;

• Equation (5) defines arc energy using instantaneous power:

$$E={\displaystyle \frac{IP}{v}}\times {10}^{-3} ,\mathrm{kJ}/\mathrm{mm},$$

(5)

where IP—instantaneous power, J/s and v—welding/hardfacing speed, mm/s.

Any of the above relationships can be used to calculate the heat input of the welding arc during standard welding/hardfacing. During welding/hardfacing using controlled pulse waveforms, it is permissible to use relationship (4) or (5). Work [33] recommends the use of a data acquisition system that is capable of capturing arc voltage and welding current samples at a sampling frequency of at least 10 times the pulse frequency.

Instantaneous energy (IE), used in Equation (4), is a measure of the total energy of the welding/hardfacing process, which is determined based on the recording of instantaneous current and voltage waveforms. In practice, it is calculated by the discrete sum of the products I(t), U(t), and the time step Δt between successive measurement samples, recorded at high frequency, which allows us to capture rapid changes that are characteristic of the dynamic course of the process. Consequently, IE represents the energy supplied to the system for a given weld/joint throughout its execution time.

Instantaneous power (IP), used in Equation (5), is a measure of the welding/hardfacing process power, determined based on arc current and voltage values sampled at short intervals. In practice, it is calculated as the time-averaged value of the product U_i × I_i recorded at high frequency, which allows for rapid fluctuations in process parameters to be considered. Thus, IP corresponds to the average instantaneous power (i.e., average power in the sense of time) over the time interval under consideration.

The amendment to EN ISO 15614-1 in 2017 [34] allows the heat input of welding/hardfacing to be replaced in welding/hardfacing technology testing, with the heat input of the welding arc calculated according to the ISO/TR 18491:2015 standard [33]. When calculating heat input, the k factor according to [29] must be considered. The WPQR welding/hardfacing technology qualification protocol must specify the method used to calculate heat input or arc energy.

Wojsyk et al. [35,36] demonstrated that a clear measure of heat input in welding/hardfacing can be the volume of material melted by it, or its representation, assuming a constant process speed—the cross-sectional area of the weld within the joined or hardfaced elements. The method of assessing the amount of heat input by measuring the cross-sectional areas of welds allows for a comparison of the actual thermal efficiency of different welding/hardfacing methods and is particularly useful in the assessment of pulse and hybrid methods.

Winczek and Wojsyk [37] proposed an alternative method for determining the amount of heat supplied to a welded/hardfaced joint. Instead of the classic measure referring to a unit of stitch length, they introduced an indicator normalized to a unit of volume, i.e., the volumetric energy of welding/hardfacing. The presented concept and the general expression of the relationship (6) are based on the basic technological parameters of the process (in particular, the energy generated by the electric arc and the welding/surfacing speed) and on the cross-sectional area of the weld/surfacing joint, which is the reference value for the volume of the deposited material:

$$E_{vw}={\displaystyle \frac{U\times I}{v\times A}}, \mathrm{J}/{\mathrm{mm}}^3,$$

(6)

where E_vw—welding/hardfacing volumetric energy, J/mm³; U—arc voltage, V; I—welding/hardfacing current, A; A—cross-sectional area of the weld, mm²; and v—welding/hardfacing speed, mm/s.

Lowering the Q value (while maintaining process stability) helps to reduce D and the width of the heat-affected zone, which is of paramount importance in material systems with increased sensitivity to the heat cycle. In practice, the relationships between the current, voltage, deposition speed, wire diameter and feed rate, and electrode stick-out determine both arc stability (and metal transfer mechanism) and bead shape and penetration depth.

In the weld, especially in its upper part, there is almost pure molten material of the filler metal used, while in the lower part, the molten material partially mixes with the base material, forming a fusion zone, as in welding. The transition line is a conventional boundary between the base material and the weld metal of the filler material [2,3,38].

The change in the concentration of the base material components in the weld metal decreases from 100% at the transition line to 0% outside the fusion zone, as the distance from the transition line and the base material (BM) increases. The course of this change depends on the dimensions (thickness) of the deposited layer. The BM area adjacent to the weld, heated by the heat supplied during the deposition process, is the heat-affected zone, where structures depend on the temperature reached by individual weld layers and the cooling rate. These structures change as they move away from the transition line, until they reach the intact BM structure [38].

The degree of mixing is determined by the coefficient of substrate metal content in the weld. The substrate metal content in the weld U_p (7) is the ratio of the cross-sectional area of the melted substrate metal F_w to the sum of the cross-sectional areas of the weld F_n and the substrate metal F_w [3,39]:

$$U_p={\displaystyle \frac{F_w}{F_n+F_w}}\times 100\%$$

(7)

where U_p—substrate metal content in weld, %; F_w—cross-sectional area of melted substrate metal, mm²; and F_n—surface area of the weld overlap, mm².

Other indicators characterizing the hardfacing process are as follows [39]:

• Hardfacing efficiency—This is the mass of the hardfacing layer per unit of time, most often given in kg/h;
• Surface hardfacing efficiency—This is the surface area hardfaced per unit of time, m²/h;
• Width of the deposited layer in a single pass, mm;
• Thickness of the deposited layer in a single pass, mm.

The quantitative description of energy input into the process as the primary factor determining the thermal cycle was supplemented with characteristics of dilution, stitch geometry, heat-affected zone features, and an analysis of the risk of hardfacing nonconformities in steels. It has been shown that heat input alone, even when corrected by a thermal efficiency factor, does not automatically ensure the comparability of results between different methods and technological conditions if it is not supplemented by unambiguous definitions, consistent units, and a minimum set of data enabling the reconstruction of the heat balance and deposition efficiency. Therefore, the method of defining and applying energy input measures in further analysis has been standardized, and the conditions for their correct application and reporting have been indicated. Thus, this Section creates a methodological bridge between the description of the process and the analysis of structural and property effects, enabling the consistent use of energy metrics as an organizing axis in the PSP table.

3. Hardfacing Methods in Regeneration

Hardfacing can be performed using various methods. The most-used methods of regenerative hardfacing are described below.

3.1. Gas Hardfacing (Acetylene–Oxygen)

Gas hardfacing (acetylene–oxygen) involves applying a layer of additional metal—melted by the heat of a gas flame—to the surface of an object, onto a previously melted substrate, to form a durable weld.

The quality of the resulting layer is determined by flame stability and composition, burner operation, additional material supply kinetics, and heat balance control in the hardfacing zone [1,3,10,39,40,41,42]. Solid wires, powder wires, cast rods (sticks), or metal powders are used as binders.

A variation in this process is gas–powder hardfacing, in which the filler material is in the form of a metallic or cermet powder with a grain size of approx. 0.03–0.10 mm. The powder is sucked from the container by a stream of combustible gas, transported through the burner nozzle, melted in the flame, and then transferred by the kinetic energy of the gases, settling on the molten substrate to form a weld [1,3,10,39,40,41,42].

In industrial practice, acetylene (flame temperature ≈ 3100 °C) is almost exclusively used as a combustible gas, due to its clearly defined flame zones and the possibility of adjusting the nature of the flame from oxidizing to carburizing. Other gases are used sporadically, due to their lower flame temperature, higher oxygen consumption, and limited adjustment possibilities compared to acetylene [1,3].

The process requires precise flame control, as its composition and enthalpy determine the chemical interaction with the metal pool and the overmelt zone. The quality of the welds is largely a function of the accuracy of the torch guidance and the thermal stability of the substrate. Depending on the material combination (substrate/binder), the following are often necessary: preheating, temperature control during surfacing, slow cooling after the process, and, in some cases, heat treatment to stabilize the microstructure and reduce residual stresses [1,39].

Gas hardfacing allows coatings of virtually any chemical composition to be applied to steel, cast steel, cast iron, and other metal and alloy substrates. It can be performed manually (most often), as well as semi-automatically or automatically, which facilitates the treatment of geometrically complex surfaces—both small and large and flat and rotary, including in hard-to-reach places [1,3,42].

The process enables the production of welds with a smooth, even surface, with minimal melting of the substrate and a low proportion of substrate material in the coating (typically 2–10%). In a single pass layer, thicknesses of 0.02–3.5 mm are achieved, which often allows the required chemical composition and target performance properties to be achieved after the first layer [1,3,10,39,41,42].

The main advantages include low equipment costs, the possibility of hardfacing elements with complex shapes and very small surfaces, and conducting the process in various welding positions. The limitations include the need for very careful surface preparation, the frequent requirement for preheating and heat treatment after hardfacing, an increased risk of deformation and welding stresses, and possible adverse structural changes in the substrate material. The relatively low deposition rate (approx. 0.5–5 kg/h), depending on the technique and equipment used, should also be considered [1,2,3,10,39,40,41,42].

Gas hardfacing is a technology with high application flexibility and low implementation barriers. It ensures high-quality welds with a properly selected thermal regime and torch parameters but requires technological discipline due to its sensitivity to surface preparation conditions and limited efficiency compared to methods with higher energy density [1,3].

3.2. Manual Arc Hardfacing with Coated Electrode (Shielded Metal Arc Welding)

Manual arc hardfacing with a coated electrode, also known as shielded metal arc welding (SMAW), involves melting the end of a coated electrode and the molten layer of the substrate with the heat of an electric arc, creating a metallic coating with specified performance characteristics. Due to the manual nature of the process, heat input is difficult to determine unambiguously and varies significantly, depending on the operator’s technique (hardfacing speed, arc length, downtime), which limits the repeatability of the thermal parameters and makes it difficult to strictly control the heat-affected zone [1,2,3].

Compared to methods with higher energy density, the SMAW process usually causes greater dilution of the weld metal with the base metal and a wider heat-affected zone. This results in a modification of the composition and microstructure of the hardfaced layer (including a reduction in the effective content of alloying elements in Fe-Cr-C welds, reduction/dispersion of hard phases, and grain growth in the heat-affected zone) as well as a different cooling curve, which promotes the formation of heterogeneous phase structures. As a consequence, internal stresses and deformations increase (greater thermal shrinkage), which can lead to a deterioration of selected operational properties: a decrease in hardness and wear resistance (due to dilution and change in phase proportions), a reduction in impact strength (due to grain growth and microstructural gradient), and (in some alloy systems) a deterioration in corrosion resistance (e.g., due to local Cr depletion). For these reasons, SMAW is sometimes used as a reference or complementary option in local repairs and smaller-scale applications, while for tasks requiring low dilution, a narrow heat-affected zone, and high repeatability, it is more advantageous to use processes with a higher energy density [1,3].

Hardfacing electrodes are most often made as medium- or thick-coated (rutile, basic) with a solid core (Ø 2.5–6 mm) or powder core (Ø 4–11 mm). The use of a powder core expands the possibilities for shaping the chemical composition of welds and, due to the higher electrical resistance of the core, allows for more than 100% increase in efficiency compared to solid core electrodes, reducing melting time [1,2,3]. The process can be carried out with an alternating current (AC) or direct current (DC) with positive or negative polarity, depending on the application requirements and the type of electrode [1,2,3].

The high temperature of the arc (up to approx. 6000 °C) promotes significant melting of the substrate. The proportion of substrate material in the weld typically reaches 10–40%, depending on the technique and process settings. As a result, only the third layer achieves a chemical composition that is consistent with that of the electrode. A layer thickness of 1–5 mm is achieved in a single pass, and the deposition rate is in the range of 1–5 kg/h [1,3,42]. The method allows for the application of materials with a composition identical to that of the substrate or with different properties (e.g., hardened, wear-resistant layers) [42].

The quality of welds manufactured using the SMAW method is highly dependent on the welder’s manual skills, the stability of the arc length, the speed of movement, and consistency in cleaning the slag from the weld pool [3,39]. The advantages include low equipment costs, the ability to work in various positions (including forced ones) and the possibility of hardfacing elements with complex geometry and limited access. The disadvantages include, above all, low productivity resulting from the manual nature of the process (breaks for electrode replacement, slag cleaning), increased risk of non-conformities (pores, blisters), and greater susceptibility to deformation and adverse structural changes associated with a wide heat-affected zone [2,39,41].

SMAW remains a valuable hardfacing method for repair and field tasks, but when high repeatability, heat input control, and dilution size are required, mechanized processes with greater thermal control are more advantageous. When selecting a technology, a compromise must be made between mobility and cost on the one hand, and the quality and metallurgical requirements of the material system on the other.

3.3. Arc Hardfacing with a Non-Consumable Electrode in an Inert Gas Shield (Gas Tungsten Arc Welding/Tungsten Inert Gas)

Arc hardfacing with a non-consumable electrode in an inert gas shield, using the gas tungsten arc welding (GTAW) or tungsten inert gas (TIG) method, involves depositing filler material in the arc zone between a non-consumable tungsten electrode and the hardfacing substrate in an inert gas shield. The filler material (wire, solid or powder rod, tape, or possibly powder melted directly onto the substrate) melts in the metal pool and forms a weld on the previously melted surface [1,3,10,39]. The non-consumable electrode is made of tungsten. To increase durability and glow stability, oxide additives (ThO₂, La₂O₃, CeO₂, ZrO₂, Y₂O₃) are used to improve the ignition and current-carrying capacity. The filler material is usually supplied from the outside (wire/rod), and in variants with tape or powder, wider beads with precisely controlled geometry are formed [1,3,10]. Argon and helium are standard gases. Helium increases the enthalpy of the arc and the depth of penetration, while argon promotes stability and easy ignition. To increase the temperature of the arc, hydrogen or nitrogen additives (approx. 5–10% in the mixture) are sometimes used. However, the addition of hydrogen is not acceptable when welding aluminum, copper, and their alloys, and it is not recommended for steels that are susceptible to hydrogen cracking (the need to assess the risk of hydrogen-induced cracking—HIC, and dewatering conditions) [2,3,10].

The process is usually carried out using DC with negative polarity on the electrode, which ensures high arc stability, precise heat input control, and low dilution. In the case of aluminum and magnesium (or their alloys), positive polarity on the electrode or AC is used, which is beneficial due to the cleaning effect of the oxide layer [3,10]. Heat input can be kept low without losing weld pool stability, which limits the proportion of base metal in the weld and the width of the heat-affected zone [19,26].

For manual hardfacing, typical productivity is 0.5–2 kg/h and the proportion of substrate material in the weld is around 5–15%. The use of the hot wire technique (resistive heating of the wire from a separate source) allows the productivity to be increased to 5–8 kg/h, while reducing the melting of the substrate (lower dilution). A layer with a thickness of 1.5–5.0 mm is usually deposited in a single pass [2,3,10,39].

GTAW hardfacing is generally performed in a downward position or at an angle of less than 30°. The process can be manual, semi-automatic, or automatic, including robotic systems. The method is used on steels, cast irons, cast steels, and copper and aluminum alloys. High-alloy steels, nickel, cobalt, copper, and aluminum alloys, as well as selected cermets, are applied as welds [1,2,3,10,39].

GTAW is characterized by the highest precision and purity of the weld pool. The absence of spatter and slag reduces post-hardfacing processing. In practice, a narrow heat-affected zone and low dilution are achieved, which helps us to reach the target chemical composition that is already in the initial layers. The texture and microstructure are strongly dependent on the heat history, which can be consciously programmed (e.g., by controlling the cooling rate, the maximum allowable temperature of the previous weld layer before starting the next layer, and cooling breaks), due to the precise regulation of the arc current/voltage and hardfacing speed [19,26].

In functional applications and hybrids (e.g., GTAW + hot wire, GTAW + interlayer rolling), as well as WAAM/DED, it is possible to additionally modulate the heat input and control the height and width of the deposit, which improves the homogeneity of the layer and reduces properties’ gradients. Such solutions increase repeatability and reduce residual stresses, as well as structural anisotropy [19,26].

The key advantages of GTAW hardfacing include metallurgical inertness (Ar/He shielding), narrow heat-affected zone, low mixing ratio, and high surface quality. Limitations include lower productivity (especially in manual mode), the requirement for stable gas-shielding conditions (usually in enclosed spaces), and greater sensitivity to gas flow disturbances. When selecting gas application, limitations must be considered, e.g., no H₂ addition for Al and Cu and caution with steels that are susceptible to hydrogen embrittlement [2,39].

When hardfacing on steel substrates with difficult weldability, the priority is to maintain a low heat input while maintaining arc stability, controlling preheat and interpass temperatures, and avoiding factors that increase the risk of cold cracks (including hydrogen in the shielding gas). GTAW, due to its precise thermal control and low dilution, promotes the safe formation of functional layers on such substrates, albeit at the expense of a lower deposition rate compared to GMAW/SAW [3,10,19,26].

3.4. Arc Hardfacing with a Consumable Electrode in a Gas Shield (Gas Metal Arc Welding)

Arc hardfacing with a consumable electrode in a gas shield using the gas metal arc welding (GMAW) method involves depositing additional metal from a consumable electrode in the arc area between the end of the wire and the hardfaced substrate in a gas shield. The heat of the arc melts the end of the electrode and overheats the substrate. After mixing and crystallization, a weld with the specified properties is formed [19,43,44,45,46]. Depending on the type of shielding gas used, the GMAW hardfacing process is classified into two basic technological variants (MIG and MAG) [2,3]:

• MIG (metal inert gas)—Hardfacing in a chemically inert gas shield, i.e., argon (Ar), helium (He), or mixtures thereof.
• MAG (metal active gas)—Hardfacing in a shield of chemically active gases, such as CO₂, H₂, O₂, N₂, and NO, used alone or as additives to Ar or He.

The effectiveness and quality of the GMAW hardfacing process are determined by the following parameters: hardfacing current, arc voltage, hardfacing speed, free wire length, wire diameter, grade and shielding gas composition, and flow rate, while heat input is used to measure the heat load. The appropriate selection of these parameters allows for control of penetration, dilution, weld geometry, arc stability, and spatter level [2,3,19,45].

The form of liquid metal transfer from the end of the wire to the weld pool depends on the current and voltage parameters, wire diameter, shielding atmosphere, and length of the electrode’s free outlet. The transfer of liquid metal in the arc during GMAW hardfacing can take place in various ways. Each mode affects arc stability, spatter, penetration depth and hardfacing process efficiency differently [10,45,46]:

• Short-circuit metal transfer mechanism—Low heat input, low penetration and dilution; beneficial for thin layers and constrained positions;
• Globular metal transfer mechanism—Larger droplets, moderate stability, increased splatter;
• Spray metal transfer mechanism—High stability and efficiency, high fusion;
• Pulsed-spray mechanism—Controlled droplet detachment in an electrical pulse; compromise: stable spraying with reduced heat input;
• Rotational metal transfer mechanism—Droplet separation due to the rotational momentum of the liquid column; used in special configurations.

In the GMAW hardfacing process, solid wires (metal) with a diameter of approx. 0.5–2.4 mm and flux-cored wires with a diameter of approx. 1.2–4.8 mm are used. Hardfacing is performed semi-automatically or automatically (including robotically), typically with a direct current, with positive polarity on the wire (DC+) [3,10].

With DC+ and spray transfer, a yield of 4–10 kg/h is achieved, but the proportion of substrate metal in the weld is high (20–40%) due to high penetration. For short-circuit arcs and thin wires of 0.5–1.2 mm, dilution can be limited to less than 5%, and stresses and deformations can be significantly reduced. Typical productivity is 2–3.5 kg/h [1,3,42]. The thickness of the layers in a single pass is usually in the range of 0.5–6 mm [1] and in some applications, it is up to 10 mm [3].

GMAW hardfacing can be performed with numerous process modifications and strategies to limit the amount of heat input:

• Pulsed GMAW—Precise heat input and droplet control; less base metal melting, easier work in forced positions [3,42];
• Electro-vibration (vibrostic) GMAW—Wire vibration along the axis (50–100 Hz, amplitude~wire diameter), limits substrate heating; useful for thin coatings of 0.5–3 mm on rotating parts; currently used sporadically [1,39,42];
• Hot wire—an additional, resistance-heated wire introduced into the arc zone; the efficiency increases up to 20–30 kg/h [1,2,3,39];
• Varieties with reduced heat, e.g., cold metal transfer (CMT)—an advanced welding/hardfacing method, a development of the MIG/MAG process, which uses the forward and backward movement of the welding wire to precisely control the metal transfer process)—combine stable spraying with a low heat input, which helps to reduce dilution and improve geometry [47,48,49];
• WAAM/DED—Conscious control of the heat history (interpass rolling) reduces dilution (the proportion of base metal in the weld), improves layer homogeneity and operational properties [24,25,26,27].

The GMAW method enables hardfacing of elements of almost any shape and size: carbon and alloy steels, cast steel, alloy cast iron and nickel, titanium, copper, and aluminum alloys. Low- and high-alloy steels (Cr, Cr-Ni), nickel, cobalt, copper and aluminum alloys are used as welds [1,3,42].

The advantages include high efficiency, easy robotization, operation in all positions, no slag, and a wide range of parameter adjustment (I-U-v, free wire outlet length, shielding gas). Limitations include susceptibility to pores/gas bubbles and spatter at suboptimal settings and the need to protect the work area from drafts due to the stability of the gas shield [39].

GMAW (MIG/MAG) is the process of choice for hardfacing, due to its combination of high productivity and high controllability. Low dilution and limited heat input requirements are met by selecting the transfer mode (short circuit/pulse), thin wires, conscious heat input control, and interpass strategies. When maximum productivity is a priority, spray transfer is preferred, accompanied by measures to limit the effects of increased penetration (e.g., interpass rolling).

3.5. Submerged Arc Hardfacing Under Flux (Submerged Arc Welding)

Submerged arc welding (SAW) involves creating an electric arc between a consumable electrode and a hardfaced substrate, with the arc zone and molten metal pool completely covered by a layer of flux. The heat of the arc melts the end of the electrode and the surface layer of the substrate. After crystallization, a weld layer with specific geometry and properties is formed. Thermal and metallurgical phenomena occur in an environment shielded from atmospheric air, which promotes high metallurgical purity of the weld metal [1,3,10,42].

The process is carried out, mechanized or automatically, mostly in a downward position (exceptionally in a wall-mounted position, with safeguards against flux spillage). The electrode is fed continuously at a constant speed and can be in the form of solid or powder wire, as well as tape (solid or powder). The use of tape increases the width of the seam and improves the uniformity of the face while maintaining high deposition efficiency [1,3,10,42].

The selection of the type and granulation of the flux is an important tool for controlling the quality and geometry of welds. The flux performs the following functions [1,3,10,42]:

• Shielding—This completely isolates the arc and weld pool from the atmosphere.
• Metallurgical—This refines liquid metal, affecting oxygen/nitrogen content and modifying the chemical composition of welds (e.g., by transferring Si, Mn).
• Technological—This stabilizes the arc, shapes the face, and reduces the rate of heat dissipation, which affects bead formation and temperature distribution.

SAW is characterized by very high efficiency and uniformity of beads, which makes the method ideal for large surfaces and thick layers (rollers, skids, shafts). Due to the possibility of using high current intensities, efficiencies of up to >40 kg/h can be achieved. However, the high thermal load usually results in increased heat input, and with it, greater penetration depth, a higher proportion of base metal in the weld (often 30–40%), and a wider heat-affected zone. In practice, this means that to ensure the target chemical composition of the weld, it may be necessary to deposit at least two or three layers [1,3,10,42].

Significant heat input values can cause deformation of components with insufficient cross-sectional rigidity. For this reason, SAW is mainly used for the regeneration of large components (according to [42]—thicknesses > 30 mm or diameters of rotating components >100 mm; according to [1,3]—also plates with a thickness of 10–30 mm and rotating parts with a diameter of 50–60 mm). In each case, thermal history management is required: preheating, control of the maximum interlayer temperature, and cooling rate [1,3,10,42].

To improve efficiency and/or reduce melting, the following are used, among others:

• Oscillation (pendulum motion) of the head, transversely to the direction of hardfacing, allows us to reduce the depth of penetration, reduce flux consumption, and at the same time increase productivity;
• SAW with filler (additional powder/chips fed into the weld pool) increases the deposition rate;
• Additional wire (cold or hot-wire) increases productivity without a proportional increase in arc energy;
• Multi-electrode systems (tandem, twin, multi-wire) increase efficiency and bead width with profile and fusion control.

These treatments allow a compromise to be achieved between efficiency and thermal impact to meet the requirements of specific hardfaced layers [1,3,10,42].

The typical layer thickness after one pass is 2–8 mm. The literature also states that it is possible to obtain layers > 100 mm (in multi-track/multi-layer configurations, with tape and high-performance systems) [1,3,42].

SAW is used for applying layers of low-carbon and low-alloy steels, high-alloy and special steels, nickel, cobalt and chromium alloys, high-chromium cast irons, and selected copper and aluminum alloys. The method is suitable for the regeneration and production of functional layers on large machine and equipment components [1,3,39].

Advantages and limitations of the method [39,41]:

• Advantages: Very high efficiency and homogeneity of beads, high metallurgical purity of welds, safe working conditions (invisible arc), and extensive possibilities for mechanization/automation;
• Limitations: The need to work in a downward position (or wall-mounted position with flux protection), increased substrate metal content in the weld and a wide heat-affected zone, lack of direct observation of the weld pool during the process, and higher costs of specialized equipment and accessories.

SAW is the method of choice for hardfacing large surfaces and thick layers when efficiency and repeatability are priorities. In material systems that are sensitive to the thermal cycle, it is necessary to precisely control the heat input and thermal history, as well as to consciously select process strategies to limit melting, dilution, and adverse metallurgical effects [1,3,42].

3.6. Plasma Hardfacing

Plasma hardfacing involves melting filler material in a very high temperature plasma arc (around 15,000–20,000 K) and then depositing it on a slightly molten substrate to produce a weld layer with specified properties [3,10]. In industrial practice, a plasma transferred arc (PTA) is the most common method, in which granulated metallic or cermet powder with a grain size of 60–300 µm is introduced into the arc zone. The grains melt in the plasma stream and are kinetically deposited on the molten substrate. After solidification, they form a weld with minimal dilution—typically <5%. The powder melting efficiency is usually 90–95%, depending on the composition and fraction of the powder. As the current intensity increases, so does the deposition efficiency, but also the melting of the substrate (and the proportion of native material in the weld), which requires conscious management of heat input (I-U-v) and torch spacing. The plasma-forming gases are argon, helium, or mixtures thereof. The outer shield is usually argon or mixtures of Ar/H₂ or He/H₂ (the addition of H₂ increases the enthalpy of the arc and the fluidity of the weld pool). In the powder configuration, there is also a carrier gas for the powder. Alternatively, wire feeding (PTA with wire) is used when higher deposition rates or dust reduction are desired [1,3,10,42].

Balancing the plasma, shielding and carrier gas flows is critical for arc column stability, penetration depth, and temperature distribution in the heat-affected zone. The process can be performed manually, semi-automatically, or automatically. Due to the mobility of the weld pool and powder feed, powder PTA is performed in practice in a downward position. Wire variants allow for a wider range of positions, including forced ones, with appropriate parameter restrictions [3,10].

Typical outputs range from 0.5 to 15 kg/h (up to 22 kg/h for high-power burners) and the layer thickness after a single pass range from 0.25 to 7 mm [1,3,10,42]. Due to low heat input and a concentrated plasma column, a narrow heat-affected zone, low penetration depth, and low substrate material content are achieved, which facilitates the formation of the weld composition in the first layer. A smooth, even surface (typically Sa < 0.5 mm) often allows for a reduction in machining allowances [39,41]. At the same time, the relatively low heat input allows for the hardfacing of small elements (thickness of 3–5 mm, diameter of 20–50 mm) with limited deformation [1].

PTA allows for welding of a wide range of alloys: based on Co, Ni, Fe, Cr, Mo, Zr, Cu, and Sn, with the addition of refractory metals, ceramic phases, and cermet composite layers. Steel elements (carbon, alloy, stainless), cast steel, selected cast irons, copper, and aluminum alloys are hardfaced, both on flat and rotating surfaces [1,3,10,42].

The key advantages of the PTA hardfacing process include high metallurgical purity of welds, very low dilution, narrow heat-affected zone, and a wide range of thicknesses that are achievable in a single pass and smooth surface finish, which together translate into low thermal interference and high repeatability. The limitations are the high costs of PTA sources and equipment, expensive powder materials, the requirement for precise surface preparation, and a stable power supply. In addition, powder PTA is usually performed in a downward position. For some applications, the efficiency is lower than in GMAW/SAW, which requires balancing quality and economic criteria [39,41].

The high energy density plasma arc and powder/tape feed enable the formation of narrow, repeatable beads with low dilution and a narrow heat-affected zone (parameters that are difficult to achieve in processes with a higher heat input). Compared to GMAW/SAW, the plasma transferred arc offers better controllability of heat input at the expense of lower productivity. As a result, it is the process of choice for layers with high quality requirements (wear/erosion resistance, composition stability) and on substrates that are sensitive to thermal cycling [1,3,42].

3.7. Laser Beam Hardfacing (Laser Cladding/Laser Metal Deposition)

Laser beam hardfacing, also known as laser cladding (LC) or laser metal deposition (LMD), involves the localized melting of a thin layer of substrate and filler material (powder fed coaxially/laterally or wire), using laser beam energy to produce a controlled weld layer with specified functional properties.

Compared to arc processes, LC/LMD is characterized by a high power density, a narrow heat-affected zone, low dilution, and high cooling rates, which promotes the formation of a fine-grained microstructure and precise bead geometry [50].

The key controllable parameters and conditions for LC/LMD hardfacing are beam power, travel speed, spot diameter, filler material, feed method, flow rate, focusing distance, and gas shield parameters. These settings determine the heat balance of the weld pool, bead height/width, penetration depth, and dilution. Process stability is ensured by coaxial powder feeding and atmosphere control (Ar/He, local chambers), supplemented by vision/pyrometric sensors for monitoring and feedback controllers [50]. Coaxial powder feeding refers to a head arrangement in which the powder stream is axially coaxial with the laser beam axis. The particles, carried by the carrier gas, are focused into a “powder cone,” in which the powder focus converges near the beam focus on the substrate surface. As a result, the powder penetrates directly into the metal weld pool, regardless of the direction of the head movement, forming a weld layer with controlled geometry and low dilution.

LC/LMD is being intensively developed in three interrelated areas [51,52]:

• Material and metallurgical—A wide range of welds, including Ni/Co/Fe-based alloys (including stellites and Inconel), stainless and tool steels, and metal matrix composite (MMC), i.e., composite weld metals (Ni/Co/Fe matrix) containing dispersed particles of tungsten carbide (WC) or WC cermet with cobalt binder (WC-Co), which are designed for use in conditions of severe abrasion/erosion. The appropriate selection of the hardfacing process and parameters is aimed at minimizing particle dilution and degradation (dissolution/decarburization), ensuring high tribological resistance with stable adhesion to the substrate;
• Process and quality—Process maps and “technological windows” limiting the following typical non-conformities: porosity (powder quality/atmosphere, deposition speed), cracks (residual stresses, hot brittleness), spatter/powder loss (jet aerodynamics), and excessive melting (power/focus selection). This indicates the effectiveness of the following strategies: substrate heating, interpass temperature control, remelting, trajectory and overlap shaping, and modification involving the feeding of two powders;
• Modeling and control—Hybrid models predicting geometry and temperature fields, i.e., analytical models using the finite element method (FEM) or computational fluid dynamics (CFD), machine learning correlations (data-driven) for parameter selection, and surveillance systems (coaxial camera, color pyrometry) coupled with quality control algorithms during the hardfacing process.

Due to low dilution, a narrow heat-affected zone and the ability to precisely manage the energy supplied, LC/LMD is a technology that is particularly suited to forming functional layers on difficult-to-weld steels. With the right choice of parameters and thermal strategy, the method allows the target composition and geometry to be achieved in the initial layers, limiting the risk of adverse metallurgical phenomena in the heat-affected zone and at the weld–substrate interface [50,51,52].

3.8. Other Methods of Regenerative Hardfacing

Complementing classic arc methods and processes with increased energy density, e.g., PTA (plasma transferred arc) and LC (laser cladding), are technologies in which the energy flow or deposition mechanism leads to a significantly different heat balance and degree of fusion and dilution, and thus to different characteristics of the process → structure → properties (PSP) relationship. This group includes, among others, hardfacing/deposition using an electron beam in vacuum conditions, as well as spraying processes, including cold spray (solid deposition) and detonation gun spraying (DGS).

Hardfacing/electron beam cladding/electron beam surfacing is a process in which a small area of the substrate surface and additional material (usually wire, less often powder) are locally melted, and the resulting pool of liquid metal solidifies as a hardfaced layer. The process is usually carried out in a vacuum, which limits the influence of the atmosphere, reduces the risk of oxidation and contamination, and promotes stable bead/layer formation [53,54]. Compared to arc methods, the electron beam allows for very high energy concentration and precise control of heat input and weld geometry, which is advantageous for local hardfacing, precision repairs, and when heat-affected zones need to be limited. Vacuum conditions and high energy density promote the production of layers with increased metallurgical quality, but the process requires specialized infrastructure (vacuum chamber) and is limited by the size of the components [54]. From the PSP perspective, the key factors are dilution at the fusion boundary, the nature of the heat-affected zone, and the microstructure of the hardfaced zone. Due to the local, intense energy input and rapid solidification, it is possible to obtain fine-grained microstructures, but the triad that is typical of remelting processes still occurs—weld metal–fusion line–heat-affected zone—with consequences for gradient hardness, susceptibility to cracking, and internal stresses. The literature also describes variants of electron beam hardfacing with powder (multi-pass), indicating the possibility of forming tool layers on steel substrates [55]. EB-cladding can serve both as hardfacing (wear layers) and cladding (protective layers), depending on the choice of filler material and working environment. In the context of steel components, an important advantage is the possibility of precise “dosing” of energy and material in critical areas without excessive heating of the entire part [53,54].

Cold spray (CS) is a deposition technique in which powder particles are accelerated to very high speeds in a gas stream (de Laval nozzle), and their adhesion to the substrate results from intense plastic deformation during impact, at temperatures remaining below the melting point of the particles. For this reason, CS is classified as a solid deposition process with minimal thermal impact on the substrate [56,57]. Unlike remelting processes, CS does not involve a classic heat-affected zone and dilution in the metallurgical sense, and thermal deformation is severely limited. This means that metrics such as heat input have less organizing power here than in arc/laser processes; in practice, the role of “process leverage” is taken over by the kinetic parameters of the jet (gas type and pressure, gas temperature, nozzle design, spray distance, powder granulation, and morphology), which determine the critical adhesion velocity [58,59]. The microstructure of CS coatings is usually characterized by a low degree of oxidation (compared to classic thermal spraying) and strong deformation strengthening in the particle–particle and particle–substrate contact zones. The literature emphasizes that solid deposition allows the characteristics of the powder material to be largely preserved, while at the same time obtaining favorable mechanical and tribological properties that are suitable for regenerative applications. The limitation is the difficulty of depositing brittle and very hard materials without the use of a matrix/binding metal (composite systems are often used) [56,57,59].

Detonation gun spraying (DGS) is a thermal spraying process in which cyclic detonations of a gas mixture in the barrel of a detonation device simultaneously heat and accelerate powder particles, directing them onto a prepared substrate surface at very high speed. As a result, it is possible to produce coatings with high density, high hardness, and good adhesion, which makes this technology useful in anti-wear applications [60,61]. The structure of DGS coatings is typical for spray coatings: lamellar (splat-based), with the possible presence of oxides, inter-lamellar discontinuities, and porosity, with detonation parameters and surface preparation significantly determining the degree of compaction and adhesion. In practice, DGS is sometimes used to produce carbide-based layers and metal–ceramic systems, where the key factors are resistance to abrasion/erosion, stability at elevated temperatures, and resistance to contact forms of degradation [60,61,62]. The limitations of the DSG process include safety and noise emission requirements, difficulties with complex geometry (line-of-sight), the need for rigorous substrate preparation (roughness, degreasing), and the sensitivity of properties to detonation cycle stability [60,61].

3.9. Critical Assessment of the State of Research: Trends, Contradictions, and Limitations of Data Comparability

An analysis of the literature on hardfacing functional layers on steel substrates indicates a clear trend away from qualitative descriptions toward attempts to quantitatively link the “leverage” of the process with structural effects and functional properties, in terms of process → structure → properties (PSP). At the same time, the comparability of results between publications remains limited due to the heterogeneous definitions of the metrics used, variability in process boundary conditions, and fragmentary reporting of input data. As a result, some of the interpretative discrepancies in the literature may not be due to actual contradictions in the phenomena, but to methodological incompatibility and a lack of consistent reporting standards.

In recent years, three dominant directions in the development of hardfacing process research have been observed:

• Energetization of process description—The growing role of energy input metrics (linear, mass, and volumetric) as organizing parameters and as a basis for transferring results between processes and workstations.
• Reduction in dilution and control of the transition zone—Increased interest in strategies that minimize the proportion of substrate metal in the weld and stabilize the geometry/fusion, as these parameters strongly determine the microstructure and functional properties (especially the wear resistance and corrosion behavior of protective layers).
• Shift in emphasis from set parameters to achieved (actual) parameters—increased importance of resultant and intermediate parameters (e.g., dilution, weld cross-sectional area, process stability indicators, non-conformities), which are a more direct link to PSP than current and voltage settings alone.

The literature reports conflicting conclusions regarding the influence of thermal metrics on microstructure and properties. For example, for seemingly similar heat input conditions, there are reports of both an increase in hardness/wear resistance (associated with a finer microstructure and limited dilution) and a decrease (associated with a higher proportion of Fe from the substrate, longer exposure to high temperatures, or an increase in the proportion of defects). Such contradictions most often result from the overlap of at least four effects:

• Incompatibility of energy metrics (different definitions and methods of calculating heat input, different assumptions regarding source efficiency);
• Lack of control or reporting of boundary conditions (preheating, inter-stitch temperature, thickness/thermal mass of the element, heat dissipation);
• Variability of dilution and stitch geometry (the same settings do not necessarily lead to the same melt volume and the same D);
• Differences in filler material and its form (solid wire vs. powder vs. powder) and in the multi-layer strategy.

Consequently, without parallel reporting of the parameters achieved (D, geometry, heat-affected zone, defects), interpreting PSP trends based solely on U, I, v, and heat input carries a significant risk.

The literature review identified recurring reporting deficiencies that directly limit the possibility of replication and inter-center comparisons:

• Energy metrics and their definitions—Part of the publication does not specify whether heat input was calculated from average values or instantaneous values, does not provide sampling frequencies, and does not indicate assumptions regarding thermal efficiency/effectiveness. This results in a situation where the same numerical value in kJ/mm may refer to different metric designs, and thus to incomparable energy states of the process.
• The boundary conditions of the heat cycle are often overlooked—Interpass temperature, preheating, cooling method, workpiece clamping, and the geometric characteristics of the sample (thickness/thermal mass). The lack of this data makes it impossible to interpret differences in the heat-affected zone, microstructure, and deformations as a function of the energy supplied alone.
• Dilution and geometry of the weld—In many studies, dilution is reported without indicating the method of determination (field definition, number of cross-sections, location of cross-sections, uncertainty), and weld geometry parameters are presented selectively. Meanwhile, D and geometry are critical PSP links for steel substrates.
• Microstructure description and quantitative structural measures—The description is often limited to qualitative images without quantitative characteristics (phase fraction, particle/column size distribution, hardness as a function of distance from the fusion line). This makes it difficult to establish reliable structure–property relationships.
• Methodology for testing functional properties—Comparisons of wear or corrosion resistance are often hampered by inconsistent test conditions (tribological configuration, load, friction path, counter sample, environment and corrosion exposure time, surface preparation). There is also a lack of consistent reporting of result dispersion (number of repetitions, deviations, confidence intervals).

To increase data comparability and enable critical synthesis in terms of PSP, it is recommended that publications on hardfacing on steels report the following, at least:

• Energy metrics with definitions (calculation variant, instantaneous vs. average values, efficiency/assumptions);
• Full thermal context (preheat, interpass, component geometry, cooling/clamping conditions);
• Parameters achieved: dilution (with determination method), weld geometry, heat-affected zone, defects;
• Quantitative characteristics of the structure (hardness profile, phase/carbide content, transition zone features);
• Explicit functional test conditions and statistical evaluation of the repeatability of results.

4. From Process Parameters to Technological Quality and Performance Characteristics

Based on the literature review, the technological quality of the hardfaced layer is characterized by a set of features that are directly resulting from the hardfacing process and assessable without operational testing. These include bead geometry (weld width, weld height, penetration depth), dilution (proportion of substrate metal in the weld), weld continuity and lack of continuity of the metallurgical bond between the weld and the substrate or between adjacent beads/layers (LOF, lack of fusion), porosity, wave face, heat-affected zone, and residual stresses. In turn, the service quality of the hardfaced layer refers to the fulfillment of functional service criteria: hardness/micro-hardness, wear resistance (abrasive/erosive/adhesive), corrosion resistance (passivation, pitting corrosion), fatigue resistance, and adhesion of the layer to the substrate. In the literature, both concepts are combined in the Process → Structure → Properties (PSP) relationship. The distribution of heat fields and metal flow (resulting from I-U-v, the type of shielding gas, the length of the free electrode outlet, and the mode of metal transfer in the welding arc) shapes the geometry, inconsistencies, and microstructure that determine the performance characteristics [63,64,65].

4.1. Benefits and Drawbacks of the Feedstock Materials for Hardfacing

Feedstock is one of the key factors determining the outcome of the hardfacing process, as it influences the following:

• The chemical composition and dilution of the hardfacing.
• Solidification mechanisms and phase transformations (and thus, microstructure).
• Susceptibility to defects (e.g., cracks, porosity, lack of fusion).
• Functional properties (wear resistance, corrosion resistance, fatigue life, adhesion).

From the perspective of the process → structure → properties (PSP) relationship, the selection of the type of filler material is equivalent to the selection of the energy source and thermal parameters, as it determines both what is deposited (chemistry and phases) and how it is deposited (process stability, productivity, deposition efficiency).

In practical terms, hardfacing consumables should be evaluated simultaneously in four dimensions:

• Design possibilities (including composites and phase additives).
• Process compatibility (arc/jet stability, susceptibility to automation).
• Metallurgical and technological quality (dilution, heat-affected zone, defects, repeatability).
• Economics and logistics (cost, availability, storage, health and safety requirements).

Table 2, Table 3, Table 4, Table 5 and Table 6 summarize the main groups of feedstock materials used in regeneration/hardfacing: solid electrodes/wires/rods (Table 2), flux-cored/metal-cored and powder-cored electrodes (Table 3), powders (Table 4), strip electrodes and strips (Table 5), and pre-applied powders and mixtures (Table 6) [1,2,3,10,19,26,39,40,41,42,45].

Table 2. Characteristics of solid electrodes/wires/rods used as the feedstock materials for hardfacing.

General description	Solid electrodes/wires/rods—this group of feedstock includes shielded metal arc welding (SMAW) electrodes, solid wires for GMAW/MAG and GTAW (mechanically fed), and rods for gas metal arc welding/GTAW. Their chemical composition is achieved by using a homogeneous metal without a “powder charge” in the core.
Advantages	High repeatability of composition and homogeneity of feedstock (lower risk of segregation of components in feedstock).
	Good feeding stability (in the case of solid wires) and high reliability in robotic applications.
	Relatively simple logistics (storage, batch traceability, lower risk of material degradation due to improper storage than with some powders).
	Predictable behavior in the process (especially in GMAW/MAG), which promotes WPS/WPQR qualification and standardization.
Limitations	Limited freedom in designing microstructure compared to powder feedstock (it is more difficult to introduce high proportions of hard phases/composites, e.g., carbides, in a technologically stable manner).
	Less flexibility in shaping the chemical composition (especially for high-alloy brazing), where non-standard alloy systems or mixtures of components with different melting points are required.
	Depending on the method, greater sensitivity to dilution and thermal effects (wider heat-affected zone, greater deformation) is possible, which results not so much from the form of the additional material as from the typical energy of arc processes used with solid wires.

Table 3. Characteristics of flux-cored/metal-cored and powder-cored electrodes used as the feedstock materials for hardfacing.

General description	Flux-cored/metal-cored and powder-cored electrodes consist of a metal casing (mantle) filled with powder(s). In practice, there are powder wires for GMAW/MAG/FCAW (including rutile/basic/metallic varieties) and coated electrodes of a “powder” nature (similar functions: process stabilization, composition shaping, and weld pool protection).
Advantages	High flexibility in composition design: possibility of introducing alloying additives, hard phases, and metallurgical modifiers in a manner that is difficult to achieve with solid wire.
	High deposition efficiency (often higher than for solid wires under comparable conditions), which facilitates the regeneration of large-size components.
	Possibility of improving performance properties (e.g., abrasion/erosion resistance) by controlling the proportion and morphology of reinforcing particles and the composition of the matrix.
	Additional process stabilization mechanisms (depending on the type of core and coating), which can reduce spatter and facilitate face shaping.
Limitations	Risk of variability related to filling quality (homogeneity, moisture, powder bulk density, mantle tightness), which may affect the repeatability of composition and susceptibility to porosity.
	Sensitivity to operating parameters (electrode extension, feed stability, synergistic settings)—critical for repeatability of dilution and weld geometry.
	In slag-containing variants, the need to remove slag and the risk of interpass inconsistencies in multi-layer welds if the procedure is not strictly followed.

Table 4. Characteristics of powders used as the feedstock materials for hardfacing.

General description	Powders are used in processes such as PTA, LC/LMD, electron beam deposition, thermal spraying, and cold spray. The powder can be a single-component alloy, mixed, or composite (e.g., metallic matrix + hard phase).
Advantages	Maximum freedom in material design: possibility to compose multi-component systems (e.g., matrix + carbides, borides), control grain size and particle morphology, and thus the microstructure and properties of the coating/weld.
	Possibility of supplying components with different properties and particle sizes, which is particularly important for composite layers and for coatings with an exceptionally high wear resistance.
	In many powder processes, it is possible to limit dilution (especially in LC) and precisely control the heat-affected zone.
Limitations	Powder quality requirements—particle size distribution, sphericity, flowability, bulk density, purity, and degree of oxidation—directly affect feeding stability and layer quality.
	Sensitivity to storage conditions (moisture, oxidation) and risk of segregation in mixtures of powders with different densities and granulations.
	Greater complexity of equipment (dispensers, powder flow calibration, process gas system), which increases qualification costs and repeatability requirements.
	For spray and cold spray processes: no classic metallurgical fusion (depending on the method), which shifts the quality criteria from heat-affected zone/dilution to adhesion, porosity, oxidation, and lamellar cohesion.

Table 5. Characteristics of strip electrodes and strips used as the feedstock materials for hardfacing.

General description	Strip electrodes and strips are mainly used in high-performance hardfacing processes (e.g., SAW strip cladding) to produce wide layers, often in protective applications (cladding) and surface restoration of large components.
Advantages	Very high deposition rate and surface coverage efficiency with large weld widths, which significantly reduces regeneration time.
	Good repeatability of geometry under stable operating and automation conditions.
	Possibility of achieving layers of uniform thickness on large surfaces.
Limitations	Limited geometric flexibility (less suitable for local repairs with complex geometry).
	Typically, higher “thermal inertia” of the process (depending on parameters) and the need to control deformation on slender elements.
	Less freedom in shaping complex composite systems than in the case of powders and powder wire parts.

Table 6. Characteristics of pre-applied powders and mixtures used as the feedstock materials for hardfacing.

General description	Pre-applied powders and mixtures—powder or mixture is pre-applied to the substrate (e.g., as a powder layer, paste with binder, powder tape) and then melted or consolidated by an energy source (flame, arc in special variants, laser, electron beam). This solution is sometimes used when it is important to control the location of the material and limit losses.
Advantages	Very high composition flexibility (like powders) and the ability to create gradient or multi-component layers.
	Good material utilization in local applications (less waste than in some powder feed solutions).
	Easier implementation in selected spot repairs and hard-to-reach areas.
Limitations	Risk of heterogeneity in the thickness and composition of the initial layer (segregation, uneven distribution, variability in binder content), which may translate into porosity and variability in properties.
	Sensitivity to the remelting process (overheating, excessive dilution, loss of volatile components/oxidation) requiring a careful procedure.
	An additional technological stage (preparation and consolidation of the layer), which increases time and operating costs.

From the point of view of steel component regeneration, there is no universally best form of feedstock material. The choice of feedstock is a compromise between functional requirements (wear/corrosion/temperature), substrate thermal limitations, required performance, and the ability to ensure technological quality. Solid wires and electrodes ensure high repeatability and simplicity of implementation and powder wires/electrodes significantly expand the scope for designing composition and microstructure, while powders (including mixed and pre-placed systems) offer the greatest material flexibility at the expense of higher equipment and quality requirements. Strips remain a highly efficient solution for large areas. This classification of additional materials allows for a clear link between the material (composition, form) and possible “process levers” and predicted structural and property consequences.

4.2. Key Indicators of Technological Quality

Dilution (D) is an indicator of the metallurgical purity of the weld, increasing with heat input and the susceptibility of the process to deeper penetration into the hardfaced material. Dilution directly modifies the chemical composition of the weld and the nature of the passive layer and co-determines the microstructure (proportion of strengthening/eutectic phases), microhardness, and modes of degradation in tribocorrosion [66]. For nickel welds (e.g., Inconel 625), Xue et al. [67] demonstrated that reducing the Fe content in the weld (extremely low D < 0.5% obtained during hardfacing with the CMT variant of the GMAW method) reduces susceptibility to intergranular corrosion by changing the composition and stability of the passive film. This relationship is threshold-like and is strongly evident in the near-surface zone. Hanif et al. [68] showed that in high-temperature environments (KCl, 600 °C), the effect of D on the composition of oxides and the rate of metal loss on FeCrAl layers is unambiguous: an increase in D worsens high-temperature corrosion resistance.

Bead geometry and imperfections—The response surface methodology (RSM) for the GMAW method shows that I-U-v, wire diameter, and electrode free exit length determine the width and height of the weld deposit and thus the risk of overlaps, discontinuities in the metallurgical bond between the weld and the substrate, or between adjacent beads/layers and local stress concentrators. Optimization of the “parameter windows” allows us to both reduce D and stabilize the face profile (which reduces technological notches and improves fatigue/tribological properties) [69]. In welds with Inconel 625, due to D of 30–40% in the first layer, ≥2 layers are often required to achieve the target composition and properties, which additionally links technological quality (geometry and overlap control) with functional quality (corrosiveness, hardness) [70].

Another group of technological quality indicators are residual stresses (RS) and heat-affected zone width. RS during hardfacing arise because of the following [22,45]:

• Uneven heating and cooling (hot weld + cool substrate → during cooling, the weld shrinks but is “held” by the substrate ⇒ tensile stresses in the layer and/or heat-affected zone, compressive stresses in other areas, the system balances itself);
• Phase transformations (e.g., austenite → martensite/bainite) accompanied by a change in volume;
• Sequence of bead/layer deposition and interpass cooling cycles.

In hardfacing on steels, the RS value and its sign result mainly from Q, thermal history, and path sequence. High heat input widens the heat-affected zone and promotes tensile stresses, which can be critical for difficult-to-weld steels, degrading fatigue life. In arc-based additive manufacturing (WAAM) processes, it is recommended to control the heat history by reducing the temperature gradients, controlling the preheating and interpass temperatures, and using interpass/interlayer rolling to reduce RS as well as improve grain uniformity [63,71]. Studies on AM/DED have shown that controlling the thermal history and mechanical treatments shifts RS towards compressive, which directly increases resistance to fatigue crack initiation [72].

4.3. Performance Indicators and Their Dependence on Technological Quality

This section describes the corrosion resistance of protective coatings (corrosion-resistant overlays/cladding) applied to steel substrates, as in the practice of steel component regeneration, resistance to the environment (general and local corrosion) alone or in conditions of corrosion-wear synergy is a frequent requirement. Unlike hardfacing in the strict sense, where the primary goal is to reduce wear, in protective coatings, it is critical to maintain the composition and ability to form a stable passive layer. For this reason, the key technological parameter is dilution D, which controls the proportion of Fe from the substrate in the weld volume and the local chemical composition of the top layer, and consequently, the composition and stability of the passive layer.

For Ni-based welds on carbon steels, dilution D directly affects the Fe content in the coating and the ratio of passivating elements in the surface layer. The literature indicates that local fluctuations in D (and thus, local fluctuations in composition) can be as important as the average value, which justifies controlling the geometry of the weld, the stability of the arc glow, and the repeatability of the process. Wang et al. [73] showed that dilution heterogeneity is a factor initiating local corrosion heterogeneity. In nickel welds on carbon steels, the interaction between D, microstructure, and passive layer determines the course of pitting and intergranular corrosion. At very low D (<1%) and in welds of varying thickness [67], more stable passivation and lower susceptibility to damage penetration are observed. Additionally, the technological parameters shaping the application of welds (pitch, degree of coverage) affect local corrosion by modulating the microstructure and roughness. It has been shown that the optimum application of ~80% welds in multi-welds hardfacing can reduce surface porosity and electrochemical activity [74].

The mechanism D → chemical composition of the layer → passive/discontinuous state → local corrosion is not specific to Ni alloys but is a general rule for protective layers on steels for the following groups of materials [75,76,77,78,79,80].

• Austenitic stainless steels (e.g., 304L/316L) and duplex steels: An increase in D reduces the effective Cr/Mo/N content in the weld (especially in the first layer), which impairs local corrosion resistance. In this group, PREN (pitting resistance equivalent number) indices are often cited, which approximately rank the influence of passivating elements on susceptibility to pitting corrosion. A decrease in the Cr/Mo/N content due to dilution translates into a decrease in the passivation potential of the layer. At the same time, thermal cycling and dilution can disturb the phase equilibrium (important for duplex) and generate zones of increased corrosion susceptibility near the fusion line.
• Co-based alloys (e.g., stellite): In many applications, they combine wear resistance with good stability in corrosive environments and/or at elevated temperatures, but their protective effectiveness also depends on maintaining the composition in the layer and limiting local discontinuities (porosity, micro-shrinkage), and material costs often force the optimization of thickness and number of layers.
• Fe-Cr alloys and Fe-Cr-(Mo, Ni) systems: Corrosion resistance is strongly related to the minimum Cr content in the layer. Dilution can locally reduce the Cr content below the effective passivation threshold, while uneven stitch geometry and surface roughness can create sites for crevice/pitting corrosion initiation.
• Composite systems (metallic matrix + hard phase): The presence of hard phases increases wear resistance but may introduce micro-pores/discontinuities and local galvanic cells. In such layers, corrosion behavior is determined by matrix continuity, tightness, and chemical homogeneity on a local scale, which remain sensitive to D and the stitching strategy.

From the perspective of the PSP relationship, the corrosion resistance of hardfacing layers on steels should be interpreted because of the chain: process parameters and application strategy → dilution (average and local) and metallurgical quality of the clad zone → microstructure and chemical homogeneity of the surface zone → stability of the passive layer and susceptibility to local corrosion. Consequently, in addition to alloy selection, it is critical to control D, limit discontinuities, and stabilize the geometry and roughness of the face, as these factors determine the initiation and development of pitting/crevice corrosion on a local scale.

Tribological wear—Fine-grained microstructure and low substrate content (e.g., in LC/PTA) increase hardness and abrasion resistance. Additionally, MMCs (e.g., Ni/Fe-based with WC/WC-Co) increase layer durability if particle dissolution/decarburization is limited (energy and powder flow control). Studies by Chen et al. [81] and Xu et al. [82] confirm the effectiveness of WC gradient architectures in achieving high wear resistance with acceptable ductility. In Ni- and Fe-based welds, a reduction in D and a fine-grained microstructure (LC/PTA) increase the hardness and wear resistance. Further improvement is provided by MMC (e.g., Ni/WC, Ni/WC-Co) if particle dissolution/decarburization is limited by controlling energy and powder flow. Studies by Xiang et al. [83] and Gain et al. [84] for LC combine high technological quality (low D, fine grain, few defects) with low wear rates under abrasive/erosive conditions [83,84].

Fatigue—Residual stresses and surface condition (smoothness, technological notches) influence crack initiation. Energy reduction, smoothing (material-free passage of a heat source over a previously laid weld to remelt only the upper part of the layer, followed by recrystallization, known as remelting) and post-process treatments (e.g., interpass rolling, hammering) reduce tensile RS and improve fatigue life [27].

Based on the analyzed literature, a PSP relationship for the hardfacing process on a steel substrate was developed. It organizes the relationship between the controllable process variables (P), the resulting weld structure and heat-affected zone (S), and the final functional properties (P), Table 7.

Table 7. PSP characteristics for the hardfacing process on a steel substrate: P—controllable process parameters (source, I-U-v, filler material, gas, temperature, trajectory); S—macro-/microstructure of the weld and heat-affected zone (geometry, D, defects, RS, phases); and P—functional properties (HV, wear, corrosion, adhesion, fatigue). Optimization direction: min. Q and D while maintaining arc/weld pool stability and joint continuity.

Process Parameters and Conditions (P)	→	Structural Effect (S)	→	Consequence in Functional Properties (P)
↓ Q (pulse/short circuit, ↑ v, thinner wire, proper gas)		↓ p, ↓ D, narrower heat-affected zone, less segregation		↑ corrosion resistance (less Fe in the weld), ↑ hardness (finer grain), lower risk of cracking in the heat-affected zone
Stable transfer (pulse/CMT), correct U		Smooth surface, no pores/possible lack of fusion		↓ notches, ↑ fatigue, ↓ adhesive wear
Interpass temperature control, cooling breaks		Uniformity of the layer microstructure and heat-affected zone		Repeatable HV, smaller property gradients, ↓ deformations
Interpass rolling/remelting (material-less passage of the heat source over the previously laid weld, aimed at re-melting only the upper part of the layer and its recrystallization and then re-solidification)		Grain refining, profile alignment (height and waviness)		↓ tensile RS, ↑ fatigue; more stable geometry of subsequent paths
Coaxial powder feed (LC)/PTA		Low D, narrow heat-affected zone, few pores		High adhesion and tribological resistance with small allowances
Increasing U (at constant I)		↑ b, ↓ p, risk of spatter/pores if U is too high		Possible reduced adhesion and corrosivity (surface defects)
Excessive Q/spray metal transfer mechanism in welding arc (SAW/GMAW)		↑ p, ↑ D, wide heat-affected zone		≥2–3 layers required for final composition; ↑ risk of cracks/deformation

↑—high, ↓—low.

5. Conclusions

The analysis of the subject literature confirms that regenerative hardfacing is a technology that is of significant importance for the implementation of circular economy (CE) principles, as it enables repeated extension of the service life of machine components, reduction in primary material and energy consumption, and reduction in waste streams throughout the entire product life cycle. In the case of functional layers on steel substrates, it is crucial to control the process → structure → properties (PSP) relationship, which allows layers to be designed for operational requirements (wear, corrosion, fatigue) while maintaining high reliability of the cladding–substrate bond.

The literature review covers both traditional hardfacing methods (gas, SMAW, GTAW/TIG, GMAW/MAG, SAW) and processes with increased energy density and/or limited dilution (PTA, LC/LMD), as well as the developing WAAM/DED technologies that utilize the phenomena characteristic of additive manufacturing. An important result of the work is the PSP table, which synthesizes the literature data between controllable process variables (P), the resulting weld and heat-affected zone structure (S), and the final functional properties (P) in the form of directional relationships and is a tool that enables a comparable interpretation of results and identification of critical “technological levers”.

It has been demonstrated that a prerequisite for the comparability of process results and technology transfer between research centers and industry is the consistent use of energy metrics (energy input/arc energy, linear energy, and, where justified, volumetric energy) calculated based on actual process parameters, depositing speed, and bead geometry. At the same time, it has been confirmed that energy metrics are only useful for classification when analyzed in parallel with “achieved” parameters, i.e., dilution D, weld geometry (fusion, height, width), heat-affected zone width, and the occurrence of non-conformities (especially lack of fusion—LOF). Consequently, the PSP table deliberately combines energy metrics with technological quality parameters to reduce the risk of misleading comparisons resulting from heterogeneous definitions of heat input and incomplete reporting of input data.

Significant research gaps have been identified, including the lack of verified “technology windows” for GMAW cladding on steel substrates with difficult weldability, limited quantitative data on the influence of shielding gas composition and metal transfer mode on dilution and face topography, and a lack of models that coherently combine process parameters, technological quality indicators, and service properties (including tribocorrosion and fatigue life). Comparisons of hardfacing technology variants, i.e., the influence of electrode wire type, path-laying strategy, and different thermal conditions while maintaining the same heat input value, also need to be supplemented, which is necessary for a reliable assessment of quality–economic trade-offs under CE conditions.

Based on the analysis of source materials, several detailed conclusions were formulated, which are presented below.

• Technological windows for the process of hardfacing on steel substrates with difficult weldability using the GMAW method. The current state of research does not provide complete, experimentally verified ranges of settings I, U, v, contact tip to work distance (CTWD), and shielding gas composition/flow for surfacing steel with difficult weldability. This results in large fluctuations in D, heat-affected zone width, and surface smoothness. In PSP logic, this means that there is no stable transition from process descriptors → process effects, which makes it impossible to predict the performance properties. The priority is to develop process maps and practical setting cards based on WPS/WPQR, with clear acceptance criteria: target D range, no LOF, and repeatable geometry.
• Composition of shielding mixtures—no quantitative thresholds. The effect of Ar-CO₂ mixtures with added O₂ on edge wetting, surface smoothness, and dilution is described mainly in qualitative terms in the literature. To generalize the PSP, it is necessary to determine the threshold content of components (CO₂, O₂), ensuring a simultaneous reduction in LOF and the required face topography when surfacing steel with increased hardenability.
• Limits of metal transfer modes for difficult-to-weld steels. The location of the boundaries between short-circuit, globular metal, and spray transfer as a function of U, v, wire diameter, and CTWD is not clearly defined for difficult-to-weld steels. The lack of this data limits the possibility of reducing the energy input without increasing the risk of LOF and porosity, thus hindering the simultaneous control of two key PSP nodes: Q-D-LOF.
• CTWD and current intensity as variables controlling short-circuit dynamics and dilution. The contact tip to work distance and the current intensity determine the dynamics of short circuits, droplet size, splashing, penetration depth, and, indirectly, dilution. A methodology for selecting CTWD for a specific wire diameter and source characteristics is required, formulated as a technological compromise: minimization of LOF ↔ minimization of D.
• Deficit of multi-response models of technological quality. Statistical models simultaneously covering D, weld geometry, porosity, and surface roughness are rare. In terms of CE, local process models (material–wire–gas–source) are desirable, which will enable the selection of settings that ensure the reduction in energy input while meeting quality criteria (no LOF, porosity control, acceptable face topography).
• Preheat and interpass—compromise between heat-affected zone ↔ D ↔ deformation. Increasing the preheat and interpass temperatures reduces the tendency for hardening structures and cracks in the heat-affected zone but may increase D and deformation. It is necessary to determine the functional relationships linking these temperatures with the base metal content and deformations as a function of the geometry of the component and the cooling conditions, which has a direct impact on the reliability of the regeneration of high-value components.
• Welds layout strategy, stresses, and fatigue life. Coverage, oscillation, welds layout sequence, and cooling intervals determine the distribution of residual stresses and deformation and consequently, fatigue life. For steels with difficult weldability, there is a lack of quantitative data linking the welds strategy to stresses and durability, which limits the informed design of regeneration in critical nodes.
• Feedstock material form—no comparisons at the same heat input. Solid wires, metal powders, and flux-cored wires differ in heat balance, face cleanliness, slag presence, melting efficiency, and dilution, which affects the finishing costs. There is a lack of systematic comparisons conducted at the same linear energy value and controlled conditions that would allow for an unambiguous assessment of quality–economic trade-offs under CE conditions.
• No consistent correlations: D and geometry → hardness/wear/corrosion resistance. In the literature, dilution and stitch geometry are not consistently linked to hardness, abrasion resistance, and pitting corrosion resistance within a single model. Quantitative correlations are needed, including tribocorrosion conditions, to enable the design of coatings based on operational criteria, rather than solely on geometric correctness.
• LOF as a critical criterion for joint reliability. The reliability of welded joints and connections between welds requires the elimination of lack of fusion (LOF), while maintaining a minimum penetration depth and the lowest possible D. This regime promotes welds’ composition stability and reduces heat-affected zone hardening and cracking, which is reflected in the PSP table by the dominant coupling between the metallurgical quality of the joint and its functional durability.
• Preferred technologies for high-hardenability substrates. For high-hardenability steels, technologies that reduce the heat input and D (e.g., pulsed GMAW, PTA, LC/LMD) with strict interpass temperature control and, if necessary, interpass rolling, are advantageous. These approaches reduce the risk of unfavorable structures in the heat-affected zone, promote homogeneity of welds, and can improve the corrosion and fatigue resistance of refurbished components.
• Definition and function of “technological windows” in CE. The limits of acceptance of process parameters (“technological windows”) should be defined using multiple criteria: ensuring that minimum performance requirements are met (tribological resistance, corrosion resistance, adhesion, fatigue life) while maintaining high technological quality (low D, repeatable geometry, no LOF, and limited porosity) and acceptable deformations. In practice, this means that the technological window is determined not only by the heat input range, but also by a set of criteria indicated in the PSP table, which is a prerequisite for the implementation of regeneration as a CE tool on an industrial scale.