Structure and Mechanical Properties of Tubular Steel Products Processed by Cold Rotary Swaging

Abstract

Rotary swaging (RS) is applied for the manufacturing of bars, stepped shafts, tubes with complex internal profiles, bimetallic composites, and similar products. This process falls under the category of severe plastic deformation (SPD) methods, which produce ultrafine-grained materials that provide superior properties in service. Our study investigated the effect of cold RS on the structure, grain size, and microhardness of AISI 304 stainless steel and CK45 carbon steel. Tubular specimens were processed by RS with the purpose of obtaining conical parts with a closed end, achieving a maximum reduction of nearly 44%. Samples were taken by longitudinal sectioning along the diameter from three zones with different degrees of deformation and subjected to structural analysis using scanning electron microscopy (SEM). The investigations were complemented by microhardness measurements in the axial direction for samples of both steels. The resulting structures revealed material texturing and a continuous decrease in grain size with increasing swaging ratio. The average grain size was reduced by approximately 46% in AISI 304 steel and by around 50% in CK45 steel. The microhardness of the materials increased by about 179% for AISI 304 steel and by approximately 95% for CK45 steel. The obtained results are discussed, highlighting the effect of cold RS processing on the two steels studied.

1. Introduction

RS is a net-shape method of plastic deformation processing of metals and alloys, in which a workpiece reduces its cross-sectional dimensions and increases its length. The method consists of cyclically applying blows in a radial direction, in a dynamic regime/mode, to a workpiece that is axially displaced and rotated during processing. The reduction of the cross-section occurs progressively, as the workpiece advances axially, through high-speed impacts applied by means of dies (2, 3 or 4) mounted in a rotating tool. The applied deformation forces are decomposed into two components acting in the axial and radial directions. The axial component causes the material to flow in the longitudinal direction, while the radial component produces severe shear strains, which result in crystal lattice rotations, defect accumulation, and grain refinement [1,2]. Unlike traditional forging, where the deformation force is applied over the entire surface of the workpiece, in RS the force is applied locally, which leads to a fine and uniform crystal structure [3,4].

RS is also known as an SPD process, widely used for producing tubular components and bars with enhanced mechanical properties and refined microstructures. Danno et al. [5] conducted studies on a titanium alloy subjected to an SPD process using the RS method and showed that this processing mode can improve the elastic modulus of the alloy. Mertová et al. [6] also investigated titanium used in the medical field as a biocompatible material, analyzing the influence of the SPD process on both mechanical performance and biocompatibility. Tokar et al. [7] studied the structural transformation and mechanical properties of a low carbon steel (0.2% C) and a 0.09% C-Mn-V steel produced by RS. They also conducted comparative studies between the results obtained by RS and those obtained by an SPD (equal channel angular pressing—ECAP) method. Chen et al. [8] studied the behavior of an Mg-Mn-Al-Zn-Ca alloy subjected to an RS process carried out through multiple successive passes, showing that this technique leads to significant microstructural refinement and improved mechanical properties. Based on these studies, it can be concluded that RS is an SPD method by which products with ultrafine structures can be obtained, which lead to increased strength, fatigue and wear properties. The listed studies were carried out on other materials, other types of semi-finished products, other plastic deformation conditions, which is why the results presented cannot be directly correlated with those that could be obtained on steels. Even though the existing results are convincing, they do not provide sufficient answers regarding the effects of cold RS processing of tubular steel products, which justifies the need for the present study.

The fine and uniform crystal structure of RS-processed parts enhances fatigue strength and durability, making it ideal for austenitic stainless steel AISI 316L components subjected to intense loading [9]. It has also been observed that microstructural development depends on processing parameters, with subgrain structures evolving more rapidly when the workpiece was rotated 90° after each pass. Moreover, Tian et al. [10] evaluated the effects of RS on 42CrMo steel rods and reported an overall improvement in torsional and fatigue performance. In another study, Alkhazraji et al. [11] observed increased fatigue strength of commercially pure titanium (grade 1) processed by RS. The increase in fatigue strength with the refinement of polycrystalline grain size is attributed to the higher number of microstructural barriers in the path of cracks and the reduced slip length before the crack tip, which slowed crack propagation.

The localized application of the deformation force allows the use of smaller and more economical forging machines, reducing energy consumption [12]. Studies have shown that this technique reduces defects, such as cracks or porosities, and improves the mechanical strength of swaged parts [13]. This method allows the obtaining of metal components with a high degree of homogeneity, increased hardness and a fine crystalline structure, being used in various applications in the automotive, aerospace, military and energy fields. This method minimizes material losses due to high-precision deformation, thus reducing the costs associated with scrap [14]. The microstructure of metals can be significantly modified by subjecting the material to SPD processes, such as that obtained by RS, ECAP [15] or high-pressure torsion—HPT [16]. These processes are part of the “top-down” approach and are capable of leading to a substantial refinement of the crystal grains, so that their size can be reduced to the submicron or even nanometer level. The microstructure of materials processed by RS undergoes a profound transformation under the effect of the accumulation of plastic strains and the increase in the density of dislocations. According to the study carried out by Abdulstaar et al. [17] on pure aluminum (Al 1050), the RS process causes a significant reduction in the crystal grain size, which leads to an improvement in mechanical strength and hardness by up to 120%. These results are also valid for other metals, including carbon or stainless steels, which react by strain hardening, dynamic recrystallization or mechanically induced phase transformations. In the case of metastable austenitic stainless 321-type steel, the RS process can induce martensitic transformation even at moderate temperatures, leading to a significant increase in hardness and yield strength [18]. Recent work has shown that through cold-applied RS, AISI 304 develops a microstructure composed of submicron austenitic grains and lamellar martensite, which leads to hardness values of about 450 HV in highly deformed areas [19]. Also, medium carbon steels, such as CK45, respond to RS by strain hardening, due to an increase in dislocation density and fragmentation of crystal grains. In the absence of a phase transformation, hardness and mechanical strength are improved mainly by controlling the crystal size and the homogeneity of the ferrite-pearlitic network. Domblesky and Shivpuri [20] developed a numerical model for a multi-stage radial forging process using the finite element simulation method to observe the material flow through the three die zones. Ameli and Movahhedy [21] conducted a study in which they analyzed the residual stresses and the forging force required during radial deformation and they also emphasized the importance of deformation zones in the die for maintaining process stability. Liu and his team [22] investigated the material flow in the axial and radial directions during RS of 1.0308 steel tubes, and compared the experimental data with finite element simulations. Their results highlight the importance of specific zones in the die for achieving the desired dimensions of the final product. In the last three studies it has been demonstrated the existence of three distinct zones in the deformation focus in RS: the reduction zone, in which both diameters (internal and external) are reduced; the calibration zone, where the final dimensional tolerances are sought to be obtained and the material exit zone between the dies. Each of these zones influences the microstructure and local mechanical properties differently.

Although there are studies that present the influence of technological parameters, such as axial feed, stroke frequency or tool geometry on the swaging load, properties and structure obtained or residual stresses, the specialized literature does not provide sufficient experimental data on the evolution of the microstructure and mechanical characteristics in tubular products obtained by RS from certain steels.

Relatively recent, cold RS has been applied as a new method for net-shape plating of tubular products to obtain bimetallic components from aluminum and copper [23]. Bimetallic components are advanced materials with diverse applications in fields such as electrical engineering, automotive, naval and aerospace industries [24]. The combination of two different metals allows obtaining properties superior to those of the metals taken separately, such as high electrical conductivity, lightweight, superior anti-friction properties, and increased mechanical strength [25,26]. Aluminum–copper composites can also be produced using conventional methods, such as rolling [27], extrusion [28], or deep drawing [29], as well as unconventional methods, such as combined cast-roll and hot-roll laminations [30], accumulative roll bonding—ARB [31,32], or twist channel angular pressing—TCAP [33]. Some of these processes are conducted at high temperatures, which can cause the formation of intermetallic phases and inhomogeneous structures, which is why cold SPD methods are preferred in many applications.

The axial feed and the rotation speed of the workpiece during RS have a direct influence on the mechanism of crystal sliding, the deformation texture and the dynamic recrystallization degree of the material [34]. Studies carried out on aluminum have shown that due to the high energy of the packing defects, dynamic restoration occurs more easily, reaching a balance between dislocation accumulation and dynamic softening at a certain reduction ratio [35]. In copper, dynamic recrystallization requires a higher energy [36], which leads to different microstructural evolutions in the products processed by RS, which are reflected in their mechanical and electrical properties [37].

Current research in the field of RS highlights specialists’ interest in processing an increasingly diverse range of metals and alloys, including magnesium alloys. The study presented in [38] provides information on the fabrication of 1 mm-diameter MgB₂ wire using a strategy alternating RS with drawing, while [39] reports the production of a high-strength magnesium–lithium alloy (Mg–4Li–3Al–3Zn) in which a nanocrystalline structure was obtained via RS. Also, the magnesium alloy Mg–4.4Al–0.9Zn–0.4Mn was studied by Estrin et al. [40], and the magnesium alloy WE43 by Martynenko et al. [41], with regard to the effects of RS on microstructure, texture, and mechanical properties.

The aim of this study is to experimentally investigate the microstructure, grain size and hardness of AISI 304 and CK45 steel tubes processed by cold RS and closed at one end. The evolution of the microstructure and mechanical properties of the two steels was systematically investigated to identify the effects that this incremental deformation method exerts on the material. The research aimed to provide reference data for RS cold stainless and carbon steel tubular products by establishing the correlations between the chemical compositions of the steels, the swaging ratio and the results obtained regarding the structure and mechanical properties.

2. Materials and Methods

The experimental program was carried out on two steel grades in tubular form: AISI 304 stainless steel and CK45 carbon steel.

The chemical composition of the austenitic steel is presented in

Table 1. Chemical composition (wt%) of the AISI 304 stainless steel.

C	Mn	Si	P	S	Cr	Ni	N	Fe
0.07	1.90	0.93	0.04	0.02	19.00	10.50	0.11	Balance

.

As can be seen from Table 1, austenitic stainless steel AISI 304 has a low C content (0.07%), which prevents the appearance of hard carbides and ensures its high plasticity. The main alloying elements of this steel are Cr and Ni, present in significant amounts, namely 18–20% Cr and 8–11% Ni. The element Cr, in a proportion of 18–20%, confers the corrosion resistance of steel by forming a passive protective oxide layer. Chromium forms inclusions in the form of oxides that influence the germination capacity, favoring fine structures. The element Ni, in a proportion of 8–11%, contributes to the further increase in the corrosion resistance of stainless steels. Its role is to stabilize the austenitic structure at ambient temperature and to favor the formation of a face-centered cubic—FCC crystal lattice. This type of structure determines the high plasticity and toughness specific to AISI 304 steel, properties that are maintained even at low temperatures. Other elements, such as Mn (up to 2%) and Si (maximum 1%) contribute to the stabilization of austenite and to the increase in oxidation resistance.

The chemical composition of the ferritic steel is presented in

Table 2. Chemical composition (wt%) of the CK45 steel.

C	Mn	Si	P	S	Fe
0.46	0.62	0.35	0.03	0.03	Balance

.

CK45 steel is an unalloyed carbon steel, with a less complex composition, but much richer in C—around 0.45%. This high C value determines a significant increase in mechanical strength and hardness, but reduces the plasticity and toughness of the material. The Mn element, in a proportion of 0.50–0.80%, strengthens the matrix and favors the formation of pearlite, respectively, martensite after heat treatments, but can induce coarse grain growth and low thermal conductivity, which makes the steel susceptible to cracking. The Si element is alphagenic, dissolves in Fe without forming stable carbides and acts as a graphitizing element. In the proportions in which it is found in CK45 (0.15–0.40%), Si contributes to stabilizing the structure, increases the yield strength, hardness, resistance to wear, fatigue and oxidation at high temperatures. Residual elements, such as P and S, even in small quantities, can reduce toughness and must be kept below the limits allowed by standards.

By comparing the chemical compositions of the two steels, we can anticipate that they will behave differently when processed by cold RS. AISI 304 steel, rich in alloying elements such as Cr and Ni, has a stable austenitic structure and the low percentage of C ensures high ductility and a relatively low yield strength. In contrast, unalloyed carbon steel CK45, with its higher C and Mn content, develops a ferritic, ferrite–pearlite, or martensitic microstructure (depending on the applied heat treatments), providing superior mechanical strength and hardness, but with a significant compromise in plasticity and corrosion resistance.

The chemical composition differences between the two steels directly explain the observed variations in their mechanical properties.

The elastic and plastic properties for the two steels are indicated in

Table 3. Mechanical properties for the AISI 304 and CK45, respectively.

Material	E [GPa]	Rp0.2 [MPa]	Rm [MPa]	HB	A, %	Z, %
AISI 304	193	210	605	152	55	70
CK45	210	340	670	201	19	48

.

The notations in Table 3 represent: E is the elasticity modulus, R_p0.2 is the conventional yield strength, R_m is the ultimate tensile strength, HB is the Brinell hardness, A is the percentage elongation to fracture and Z is the percentage necking.

As can be seen from Table 3, the mechanical properties of the two steels show significant differences, which are explained by both their chemical composition and structural constituents.

The elasticity modulus (E) is a property that maintains its almost constant value for most steels, being about 200 GPa. The two steels investigated have E = 193 GPa in the case of AISI 304 stainless steel and E = 210 GPa in the case of CK45 carbon steel, a difference explained by the different structural constituents and the high Cr and Ni content of the stainless steel.

Regarding the conventional yield strength (R_p0.2), the following aspects can be observed. AISI 304 stainless steel exhibits lower resistance to initial plastic deformation, with typical values of 190–230 MPa, making it more easily deformable. In contrast, CK45 steel, especially in normalized or heat-treated (quenched and tempered) condition, reaches significantly higher values, on the order of 300–350 MPa, providing greater stiffness and the ability to withstand higher mechanical loads.

The ultimate tensile strength (R_m) has a wider range of values (500–750 MPa) for AISI 304 steel, compared to a narrower range (600–750 MPa) for CK45 steel. These values explain the sensitivity of stainless steel to different processing conditions and the degree of strain hardening and the better strength of carbon steel.

Regarding the Brinell hardness (HB) of the two steels, the following elements are noteworthy. CK45 steel, due to its high carbon content (~0.45%), is naturally harder than AISI 304, especially in the normalized condition, with typical values of 170–220 HB compared to 140–190 HB for the stainless steel in the same state. The higher hardness values of CK45 steel recommend that it is suitable for applications where wear resistance is an important criterion.

The plasticity of the two steels can be evaluated by the values of elongation at fracture (A) and necking (Z). AISI 304 stainless steel is characterized by high elongation at fracture (A) of 40–60% and a reduction in cross-sectional area (Z) of 65–75%, demonstrating excellent capacity to accommodate large plastic deformations before fracture. In contrast, CK45 steel exhibits lower plasticity, with typical elongation values of 16–25% and cross-sectional area reduction of 40–55%, making it more brittle compared to the austenitic steel.

By comparing the mechanical properties of the two steels, we can see that AISI 304 steel is a more ductile material and can accept a wide range of plastic deformations, being suitable for applications where plasticity and toughness are essential. The other steel, CK45, has superior mechanical strength and hardness, simultaneously with lower elongation and necking, which is why it is recommended for parts where loads are high and wear resistance is a priority.

The tubular specimens prepared for cold RS had an outer diameter of 9.4 mm, a length of 310 mm, and a wall thickness of 0.7 mm. They were subjected to reduction along a 170 mm length with a constant axial feed (8 mm/s) applied throughout the swaging process. The round tubes were plastically deformed at room temperature from a diameter of 9.4 mm down to 5.6 mm in a single stage, corresponding to a true strain of 0.57 and a swaging ratio of 43.7%. The true strain resulting from the reduction of the cross-sectional area was calculated using the formula [42]:

$$\phi =\mathrm{l}\mathrm{n}(S_0/S_1)$$

(1)

where $S_0$ is the cross-sectional area of the tube before swaging, and $S_1$ is the cross-sectional area of the tube at the closed end through swaging.

The swaging ratio was calculated by the equation:

$$\epsilon =\left[\right(S_0-S_1)/S_0]\times 100\hspace{0.33em}\hspace{0.33em}\%$$

(2)

where $S_0$ and $S_1$ have the same meanings as in Equation (1).

Another objective of our research was to achieve closure of the tube ends at the final stage of the cold RS process. A practical use of these tubular products is as plungers and coolers for the manufacture of glass containers.

The plastic deformation of the tubes made from the two steels was carried out using a swaging machine (Rungoal Machinery Equipment, Foshan, China) equipped with a tool operating on a principle similar to that shown in Figure 1.

Plastic deformation was carried out through an incremental forming process based on cyclic loading. The tubes were reduced between four identical swaging dies, arranged concentrically around the workpiece, which perform high-frequency radial movements (800 blows/min) with short strokes (2.2 mm). To prevent the formation of longitudinal burrs in the gaps between the dies, there is a relative rotational movement between the dies and the workpiece [23]. The RS process has several variants; in this application, cold RS was performed with tubular workpieces fed without a mandrel.

The evaluation of the results for the swaged-reduced tubes was performed in three zones for each specimen: zone 1 at the beginning of deformation, zone 2 at the middle of the reduced length, and zone 3 at the end, where tube closure occurs (Figure 2).

From the three zones, 20 mm-long specimens were extracted by wire cutting for the purpose of analyzing structural changes and determining the mechanical properties resulting from the cold RS process.

SEM was used to analyze the structural changes. To prepare the samples for microscopic analysis and microhardness testing, the swaged-reduced tubes were cut longitudinally so that the longitudinal section planes became visible, as shown in Figure 3, where the direction along which the micrographs were taken and the microhardness measurements were performed is also indicated.

The samples were embedded in heat-resistant resin and subjected to metallographic preparation. Their longitudinal section planes were manually sanded and subsequently mechanically polished using diamond suspensions with particles of 3 μm and 1 μm, respectively.

The swaged samples were first analyzed by SEM using a Thermo Scientific Quattro C microscope (Thermo Fisher Scientific Brno s.r.o., Brno, Czech Republic) with the following technical specifications: magnification over 1,000,000×; maximum accelerating voltage of 30 kV; high vacuum; software for microscope control and image capturing.

In the second part of the investigation, Vickers microhardness (HV 0.2) was measured along the longitudinal sections of the RS specimens, with indentations made every 2 mm, using a Micro-Vickers Hardness Tester (CV Instruments Ltd., Wolverhampton, England, UK). The applied load was 200 g, and the dwell time for each indentation was 15 s.

3. Results and Discussion

This section is divided into three subsections to provide a concise and precise description of the experimental results regarding the structural analysis and the determination of the mechanical properties of the parts after cold RS.

3.1. Microstructure Analysis

Microstructural analysis was performed for both materials that underwent RS, including both the stainless steel and the carbon steel. The results of the analyses regarding grain orientations in the three examined zones revealed the details of the crystalline structures, as shown in Figure 4 for AISI 304 steel.

AISI 304 stainless steel has a face-centered cubic—FCC crystal lattice, characteristic of austenitic steels, resulting from its high Cr and Ni content. This structure provides the material with good plasticity and high corrosion resistance. Microstructural analyses of AISI 304 stainless steel revealed characteristic metallographic structures, including polyhedral grains with deformation twins in zone 1. In the swaged zones with medium and high degrees of plastic deformation (zones 2 and 3, respectively), deformation twins in polyhedral grains are also evident, with a progressively decreasing grain size. Material texturing is additionally observed, resulting from the preferential orientation of the crystal lattices due to rotations caused by the radial and axial components of the force.

Two of the microscopic images obtained by SEM from the studied zones of CK45 steel are shown in Figure 5.

CK45 carbon steel has a body-centered cubic—BCC crystal lattice, characteristic to ferritic steels. The microstructural images taken from the samples show crystal structures with fibers oriented in the plastic deformation directions, determined by displacements and rotations of the crystal lattice through slip and twinning mechanisms. Once an average degree of plastic deformation is reached (about 20%), regions with polyhedral structures with deformation twins appear. In zone 3 of swaging with a high reduction ratio, deformation twins are more clearly highlighted, specific to processes that take place at low temperatures and with a high strain rate.

3.2. Grain Size

The next evaluation focused on grain size and the evolution of grain refinement with increasing reduction ratio through cold RS. For this purpose, 15 measurements were performed in a region of 600 µm × 300 µm in each of the three studied zones for both steels, as illustrated in Figure 6.

The measurements performed in the three zones of each swaged sample made it possible to generate the graphical representations shown in Figure 7 for AISI 304 steel.

From Figure 7a we observe that in zone 1 the average size of the crystalline grains is 67.1 µm, which indicates a relative coarseness of the microstructure. The distribution of measured values shows a fairly wide variation, with grains even exceeding 120 µm. This predominance of large grains is due to the fact that the material has undergone plastic deformation with a small degree of reduction. The material retains important reserves of plasticity, but its mechanical strength remains relatively low.

Figure 7b shows that in zone 2 the grain size becomes finer, with an average crystal size of 47.4 µm. The grain size distribution is more uniform, without very pronounced extremes, which suggests a more balanced and homogeneous microstructure. This shows that the forging was more intense than in zone 1, but not enough to lead to complete fragmentation of the grains. The mechanical properties of AISI steel in this zone reflect an increase in strength and hardness compared to zone 1 and it can be estimated that the material also retains good plasticity.

Figure 7c, corresponding to zone 3, shows a significant reduction in average grain size, reaching 37.8 µm, highlighting a fine and compact microstructure. The distribution of values is tight and concentrated around the mean, with minimal variation (maximum grain size 67.5 µm), indicating pronounced grain refinement. This result is explained by the high swaging ratio, which had the effect of grain fragmentation and the emergence of new grains through dynamic recrystallization mechanisms. This fine microstructure that emerged gives AISI 304 steel increased mechanical strength and lower plasticity values. The standard deviations for the data were 9.4 (zone 1), 8.2 (zone 2), and 6.9 (zone 3).

The overview of the three zones of the austenitic AISI 304 steel highlights the evolution of the microstructure from large and inhomogeneous grains (zone 1) to fine and uniform grains (zone 3). The evolution is explained by the differences in the intensity of the cold RS to which the material was subjected. The zones with a low swaging ratio present a coarse grain size, favorable to plasticity, while the zones with a high reduction ratio have developed a refined microstructure, which offers superior mechanical strength.

The grain size evolution for CK45 steel samples is shown in Figure 8.

For ferritic CK45 steel, Figure 8a shows that in zone 1 the average crystal size is 53.1 µm, indicating a relatively balanced microstructure. The distribution of values also exhibits wide variations, with grains exceeding 80 µm. This microstructure is due to the fact that the material has undergone low-intensity plastic deformation. The mechanical properties are characterized by a mechanical strength comparable to that of the original material and by a sufficiently good plasticity.

Figure 8b shows that in zone 2 the average grain size decreases to 44.6 µm. The grain size distribution becomes more balanced, with maximum values of about 74 µm and a clearer concentration around the mean. The beginning of structural refinement indicates that the material was subjected to swaging with a higher reduction ratio, exceeding 20%, which led to a more homogeneous microstructure. The mechanical strength in zone 2 shows a significant increase, while the plasticity properties are decreasing.

In zone 3 (Figure 8c), the grain size is the finest, with an average crystal size of 29.6 µm. The distribution of values is concentrated around the average value, without large variations (a single value is 58.3 µm), which shows a pronounced refinement of the structure (the smallest dimension is 9.8 µm). This result is due to the high swaging ratio (over 40%), which led to grain fragmentation and the formation of new grains by recrystallization. The mechanical properties of the steel in this zone are characterized by higher strength and lower plasticity compared to zone 2. The standard deviations for the grain sizes from CK45 were 13.6 (zone 1), 12.3 (zone 2), and 10.9 (zone 3).

The grain size evolution of the CK45 samples shows the transition from a large and inhomogeneous grain structure to a fine and uniform structure. The gradual decrease in grain size is closely related to the progressive increase in the swaging ratio.

In the case of AISI 304 stainless steel, the average grain size decreases from about 67 µm in zone 1, to about 47 µm in zone 2, and to below 38 µm in zone 3. This gradual reduction reflects the specific way in which the austenitic structure (stabilized by Ni and Cr) responds to deformation: the grains are elongated and fragmented, while static or dynamic recrystallization leads to the formation of new grains, which are still relatively large compared to those in carbon steel.

For CK45 carbon steel, the tendency for structural refinement is more pronounced: the average grain size decreases from about 53 µm in zone 1, to over 44 µm in zone 2, and reaches nearly 30 µm in zone 3. The gradual reduction in grain size reflects the ferritic nature of CK45 steel and the higher C content, which favors both grain fragmentation by cold RS and subsequent recrystallization processes. CK45 steel developed a finer microstructure compared to austenitic AISI 304 steel for the same swaging ratios.

The interpretation of these results confirms the expected trend: the decrease in the average grain size leads to an increase in mechanical strength and hardness, accompanied by a reduction in plasticity. Thus:

• In zone 1, both materials exhibit a relatively coarse-grained structure and good plasticity; however, AISI 304 starts from significantly larger grain sizes, which explains its lower mechanical strength.
• Zone 2 represents a state of equilibrium, in which structural refinement is significant and the grain size distribution becomes more uniform. CK45 steel exhibits the advantage of higher strength due to the smaller crystal size, while AISI 304 remains superior in terms of plasticity at the same forging ratio as CK45.
• In zone 3, the end zone, SPD is the most pronounced for both materials, which determines a much finer structure in terms of grain size. In CK45 steel, strength and hardness increase significantly, but a substantial reduction in plasticity occurs. AISI 304 steel, with a less refined structure, maintains an optimal combination of strength and toughness at the same swaging ratio as CK45, and additionally benefits from corrosion resistance.

Recent results confirm the interpretation that reducing the average grain size leads to an improvement in the mechanical properties of the swaged steels [43]. The authors processed 304 SS bars by RS, with an equivalent strain of 1.5, and obtained a decrease in the average grain size up to 80 nm, which caused an increase in strength from 727 MPa, as received, to 2220 MPa after swaging. The same correlation between grain size and mechanical properties was also confirmed by Yang et al. [44], who radially cold forged 30SiMn2MoVA alloy steel tubes.

3.3. Microhardness Evaluation

Vickers microhardness measurements for the processed samples were carried out along a characteristic line on the longitudinal sections of the swaged specimens. For this purpose, 10 measurements were performed at every 2 mm, starting from 1 mm away from the specimen edge, for a total specimen length of 20 mm. The same procedure was applied to each of the three studied zones for both steels. The average microhardness values obtained for the materials in their initial state were 154 HV 0.2 for the AISI 304 stainless steel and 204 HV 0.2 for the CK45 carbon steel. The microhardness values increased considerably for the swaged AISI 304 samples (Figure 9) and significantly for the CK45 samples (Figure 10). The average microhardness values for zones 1, 2, and 3 of the AISI 304 specimens were 170.1 HV 0.2, 268.3 HV 0.2, and 372.7 HV 0.2, respectively. The standard deviations for these values were 4.3, 3.4, and 2.7, respectively.

Comparable hardness values, of approximately 420–430 HV, were obtained for metastable austenitic stainless 321-type steel specimens processed through cold RS by Panov et al. [18]. Type 321 steel is an austenitic stainless steel alloyed with Cr and Ni, but stabilized with Ti, which provides corrosion resistance similar to that of AISI 304 steel.

For the CK45 specimens, the corresponding average microhardness values were 227.1 HV 0.2 (zone 1), 287.5 HV 0.2 (zone 2), and 353.9 HV 0.2 (zone 3). The standard deviations for these values were 6.5, 4.9, and 3.4, respectively.

Overall, the microhardness measurements confirmed that the most significant structural changes occurred in AISI 304 steel, as the HV 0.2 values increased by 178.6% compared to the initial state; however, in CK45 steel, the increase in HV 0.2 by 95.1% is also considerable.

Figure 11a,b illustrates the influence of the swaging ratio on the microhardness of the two investigated steels.

The same trend in the evolution of microhardness with increasing swaging ratio can be observed in both AISI 304 and CK45 steels. The established regression equations are third-degree polynomials, which provide a good approximation of the experimental data and the functional dependence between the studied parameters, with a modeling error of less than 5% in both cases.

Comparable results were obtained by Pachla et al. [45] when deforming 316L steel bars by hydrostatic extrusion followed by cold RS. Thus, HV 0.2 increased from 205 in the initial state to 465 in the final state. Other characteristics of 316 steel had spectacular evolutions: yield strength 283 → 1335 MPa; ultimate tensile strength 612 → 1405 MPa; elongation to fracture 56 → 12.4%. All these changes in mechanical characteristics confirm the results and discussions presented in this study for cold RS processing of steels, but we must also note the lack of data or studies on the processing of carbon steels by this method.

4. Conclusions

The cold RS process of tubular products made of AISI 304 stainless steel and CK45 carbon steel was investigated by applying plastic deformation, followed by structural analysis and measurement of the mechanical properties obtained after processing.

• A first conclusion shows that CK45 steel exhibits the same tendency as AISI 304 steel, namely the homogenization of the crystalline structure and the formation of deformation texture as an effect of cold RS, but with greater intensity in CK45 compared to AISI 304. This is explained by its ferritic nature and higher carbon content, which provide higher mechanical strength but lower plasticity than in stainless steel.
• Regarding the evolution of grain size in the two steels, the same tendency can be observed: a transition from a coarse structure in the initial zone (zone 1) toward visibly more refined structures in the zones subjected to higher deformation intensity (zones 2 and 3). However, the magnitude of this evolution differs between the two steels. The decrease in the average grain size was nearly 46% in AISI 304 steel and about 50% in CK45 steel.
• Regarding the microhardness of the investigated steels, a clear trend of increasing microhardness with rising swaging ratio is evident, with a greater increase observed in austenitic stainless steel AISI 304. This can be explained by the material’s higher plasticity reserve, which allowed strain hardening to occur more intensely than in ferritic CK45 carbon steel. The hardness of the materials increased by approximately 179% in AISI 304 and by about 95% in CK45.
• Overall, it is observed that although both materials follow the same structural trajectory (intense deformation → smaller grains → higher strength), CK45 steel reaches lower values of average grain size, which gives it added strength and hardness. In contrast, AISI 304 steel has the advantage of greater plasticity, which makes it more suitable for applications where plastic behavior and corrosion resistance are a priority.