Modification of the Surface Layer of Grey Cast Iron by Laser Heat Treatment

Abstract

This paper presents possible modifications to the properties of grey cast iron by laser heat treatment. These modifications are analyzed especially with regard to wear properties as a result of graphite content, which is a well-known solid lubricant. Examples of applications of grey cast iron in cases where good wear resistance is required are presented. Laser hardening from the solid state, laser remelting, and laser alloying are characterized. In this study, changes in the surface layer caused by these treatments were analyzed (especially the influence on the microstructure—including graphite content—and wear properties). It was shown that all of these treatments enable the wear resistance of the surface layer to be enhanced, mostly due to the increase in the hardness and microstructure homogeneity. It was also proven that it is possible to retain the graphite phase (at least partially) in the modified surface layer, which is crucial in the case of friction wear resistance. In particular, laser hardening from the solid state does not eliminate graphite. Laser remelting and alloying cause the dilution of carbon from the graphite phase to the melted metal matrix, but, in the case of nodular cast iron, it is possible that not all of the valuable graphite in the surface layer is lost.

1. Introduction—Grey Cast Iron Characterization

1.1. Properties of Graphite and Grey Cast Iron

1.1.1. Graphite Characterization

Graphite, as an allotropic type of carbon—among others such as diamond, fullerene, and nanotubes—takes a crystalline form. It is composed of regular structures whose vertices are occupied by carbon atoms (graphene). Each of these atoms in graphene has a strong covalent bond with three adjacent ones arranged in the same location. The fourth electron forms van der Waals bonds between layers of atoms [1]. Graphite exhibits good chemical stability at high temperatures and in a non-oxidizing atmosphere, as well as a low thermal expansion coefficient, high gas adsorption, good workability with high electrical and thermal conductivity (in directions parallel to the plane with hexagonal arrangements of carbon atoms), and good resistance to thermal shocks. Therefore, graphite is used on the heating elements of electric furnaces; on arc welding electrodes; in meltable pieces for casting molds for metal alloys and for sintering ceramic materials; as a fire-proof material and insulator; for chemical reactor tanks, rocket nozzles, electrical contacts, collector brushes and resistors, and battery electrodes; and in air purifier devices [1].

In addition, what is particularly important is that graphite is characterized by very good lubricating properties. Taking into account the general direction of research on reducing the use of lubricants for ecological reasons, graphite has attracted particular interest especially with regard to machine part materials operating in friction pairs. The special properties of graphite are used, for example, in the creation of graphite–metal pairs, in which graphite is transferred to the metal surface, which enables operation in the graphite–graphite system after some time [2]. The sliding layer created in this way delays the destruction process.

Graphite plays an important role in cermet materials, which are applied in power supplies and used on brushes, electrical contacts, lubricating components, and abrasion-resistant elements. Sintered solid bearings with an appropriate proportion of graphite or soft low-melting metals are self-lubricating. They are most often made of ferrographite or copper–graphite [1].

1.1.2. Grey Cast Iron Characterization

As graphite is usually categorized as a ceramic engineering material, grey cast iron can also be classified as a composite material because it consists of a metal matrix and a ceramic part.

Grey cast iron is a well-known cast iron alloy due to its relatively simple production and relatively low price. It is characterized by casting properties better than cast steel, such as lower linear shrinkage (lower tendency to create casting stresses), lower volume shrinkage (lower tendency to create shrinkage cavities and usually removes the need to use risers), and better castability (allows complex or thin-walled castings to be made) [3].

However, there are also other beneficial properties of this material, leading to its wide application. Although graphite in cast iron is characterized by low strength, especially impact strength, it also has some valuable characteristics; it dampens vibrations, decreases the sensitivity of the material to the notch effect, and makes the density of cast iron lower than that of steel. Depending on the type of grey cast iron, its density ranges from 6.8 (flake cast iron with ferrite matrix) to 7.4 g/cm³ (flake cast iron with pearlite matrix). The density of spheroidal cast iron seems to not depend on matrix composition (7.1 g/cm³) [4].

Moreover, graphite, which is a solid lubricant, is an important component in the material with regard to friction and wear properties (and improves machinability) [1]. Therefore, grey cast iron is characterized by the unique ability of self-lubrication. Even if part of the surface layer wears out during operation, a new part of the material with graphite will be exposed. Taking into account a possible lack of lubrication in the kinematic node or a general tendency to reduce the use of lubricants for ecological reasons, graphite is a valuable ingredient in cast iron.

Other properties of grey cast iron, such as its thermophysical properties, differ from those of steel. For example, the heat capacity of different types of grey cast iron is generally higher than that of steel and ranges from 450 to 545 J/kg·K [4]; additionally, that of spheroidal cast iron is lower than that of flake cast iron. It can also be noted that cast iron with a ferrite matrix is characterized by a lower heat capacity than that with pearlite [5]. Thermal conductivity is higher in most types of grey cast iron compared to steel (45 W/m∙K), and it ranges from 30 to 60 W/m·K. Additionally, that of spheroidal cast iron is lower than that of flake cast iron. More pearlite in the cast iron matrix decreases the thermal conductivity, with the difference in thermal conductivity between them being almost double [5].

The mechanical property values of grey cast iron are generally lower, as mentioned above, than those of steel or cast steel due to the presence of graphite. Nevertheless, for example, nodular cast iron with a spheroidal graphite morphology has similar values of some mechanical properties to those of cast steel [6], and the values of some properties of this cast iron are not much lower than those of steel. Cast iron with graphite (especially nodular cast iron) is characterized by better properties (except the impact toughness and modulus of elasticity) than cast steel [7]. It has been shown that the presence of spheroidal graphite significantly reduces the depth of fatigue cracks. The authors of [8] showed that the precipitates of graphite stop fatigue–thermal cracks, thus limiting their spread into the material.

Since the development of the technology for obtaining nodular cast iron and ausferritic and malleable cast iron, there has been an increase in the application of a group of these materials in various industries. In mechanical engineering, cast iron is one of the most widely used foundry materials because its mechanical properties are usually completely sufficient for many applications.

1.2. Application of Grey Cast Iron

Grey cast iron has found wide application in many fields—machinery, transport, and agricultural industries. It is not only used in many machine housings but is also (particularly nodular cast iron) used in the motor industry for engine parts such as crankshafts, camshafts, cylinder liners, piston rings, cylinder blocks, gears, valve lifters, valve levers, valve guides, and other automobile components like brake discs and drums and brake pumps and cylinders. A number of cast iron types are also used for machine parts in agriculture (e.g., harvester shafts, harrow teeth, disc harrows, coulters), in metallurgical applications (e.g., shafts), or in hydraulic components such as pumps [9,10,11,12,13].

1.3. The Requirements of the Surface Layer Properties

In most applications, good wear resistance is required. During their operation, many machine parts are exposed to intensive wear, mainly by friction, but also corrosion and heating. In most cases, intense tribological wear does not affect the entire component but only part of it. There are many examples of automobile components. The following surfaces are affected:

• Journals of crankshafts (and camshafts)—the main and connecting rod operating with bearings—are exposed to frictional wear;
• Cams of camshafts cooperating with the caps of the valve tappets are exposed to wear;
• Parts of edges of the piston rings are exposed to frictional wear; the seat of valves is also exposed to wear by friction and high temperatures;
• Brake components are exposed to abrasive and corrosive wear and temperature shocks;
• Edges of the culture flap in seeders, harrow teeth, or disc harrows in agriculture machines are exposed to abrasive wear in soil, so corrosion can also appear (as an example of agriculture component).

An additional difficulty in many cases of friction wear and corrosive wear is the fact that corrosion is accelerated by wear and vice versa. Therefore, in order to increase the durability of these parts, their surface layer should exhibit resistance to the following:

• Wear by friction (abrasive or adhesive in most cases);
• Corrosion (in some cases);
• High temperatures (in some cases).

2. Laser Heat Treatment

Therefore, to increase the durability of these parts, the surface layer of the parts most exposed to wear should be modified. It is possible to obtain such properties by forming a suitable microstructure of the surface layer characterized by homogeneity, fine grains, and hard phases (particularly to achieve wear resistance). The chemical composition can also form new phases to achieve corrosion and heat resistance. High hardness is usually achieved through the formation of a martensitic microstructure or other hard phases such as fine carbides, which favor wear resistance [14]. It would also be valuable to not lose the lubricating properties offered by graphite as a solid lubricant in grey cast iron. It is important to analyze such surface treatments, especially with regard to the possibility of retaining graphite in the surface layer. The enhancement in wear properties of grey cast iron can be achieved through laser heat treatment.

Laser heat treatment is mostly applied in the case of steel. Therefore, the greatest amount of information available in the literature on the subject concerns steel. This possibility is also increasingly being described with respect to grey cast iron. Good effects of laser treatment have been noted with aluminum alloys (including Al-Si alloys) [15,16] and titanium alloys like Ti15Mo and Ti6Al7Nb [17], copper alloys [18], nickel-based superalloys [19], and molybdenum alloys [20].

With a laser beam, it is possible to achieve different effects in the surface layer of the treated object. According to [21], the applications of lasers in surface treatment currently mainly include surface heating (transformation hardening and annealing); surface melting (homogenization, microstructure refinement, generation of rapid solidification structures); surface alloying; surface cladding; laser casting; surface texturing; surface roughening: plating by laser chemical or physical vapor deposition; bending; laser marking; micro-machining; shock hardening; and magnetic domain control.

The purpose of laser heat treatment is usually to modify the surface layer (cast iron or other alloys) by means of heating with a laser beam, consisting of hardening from a solid state, remelting (i.e., hardening from a liquid state), or alloying (i.e., remelting with simultaneous introduction of an alloying substance). Tempering of the surface layer of the hardened element can also be performed. Consequently, the principle of laser surface layer modification (mainly for metals) can be divided into three general areas [22,23,24]:

• Heating:

  -

  Transformation hardening.

  -

  Annealing.

• Melting:

  -

  With chemical modification (alloying, cladding).

  -

  Without chemical modification (glazing, grain refining).

• Shocking (shock hardening).

Laser shocking is an effective method with regard to improving the wear resistance and fatigue life of nodular cast iron without austenite transformation [25]. Laser shock peening allows the compressive stresses in the surface layer to be increased. Considering the research presented in [26], as well as the influence of the laser cavitation peening of grey cast iron on surface morphology and roughness, residual stress, and hardness, it has been shown (among other aspects) that higher laser beam energy causes higher hardness and residual stresses. The aim of such treatment is to increase the resistance to cavitation erosion.

There is also the possibility to repair damage to grey iron parts through laser treatment, for example, by laser cladding [27,28]. The refurbishment of wear-damaged cast iron die radii was presented in [29].

In this paper, an analysis of three types of laser treatment related to austenite transformation, laser hardening from the solid state, laser remelting, and laser alloying, with regard to microstructural changes and wear resistance is presented (taking into account their influence on graphite). Laser treatment could be used to modify different types of grey iron like flake and nodular [30,31], which are the most popular, but also vermicular [32] or austempered ductile cast iron [33,34].

It needs to be underlined that laser heat treatment is recommended and economically justified in the case of components that require the surface treatment of only relatively small areas, as mentioned above. If the whole surface of the component needs to be modified, then, for example, diffusion methods are recommended (e.g., nitriding). In the case of small areas that need to be modified, laser treatment seems to be a good solution. As shown by economical calculations, taking as an example one of the agricultural components, the coulter flap, which is strongly exposed to friction wear on only the part that operates in the soil, has a 25% lower cost after 10 seasons of its application after laser hardening from the solid state than that for untreated couture, and that of the coulter flap after laser alloying is 63% lower. It is worth emphasizing that the replacement time with regard to the laser-hardened coulter flap is 2 times longer than that of the untreated one and that in the case of an alloyed one it is 5 times longer [35].

The technology of laser hardening consists of the following steps. Prior to laser heat treatment, the surface of the samples is coated. To achieve only hardening from the solid or liquid state, the surface layer of the grey cast iron sample is covered only with a coat that absorbs the laser beam radiation. In the case of laser alloying, to realize such an effect, the sample is covered with a special coat consisting of the alloying substance [30]. The next step is the selection of laser heat treatment conditions, especially the values of laser beam parameters such as laser beam power (P), its diameter (d), and its velocity relative to the treated object (V). The appropriate selection of those parameters allows for proper laser beam power densities (E) and its interaction times (t) to be obtained. The density and interaction time influence the temperature in the surface layer. Finding appropriate laser treatment parameters was also the subject of [5,31]. By generating a specified temperature, different thermal effects in the surface layer can be reached; for example, the remelting at the temperature caused melting and hardening from the solid state if there was only austenite in the surface layer. In the case of the laser heat treatment of grey cast iron, it is possible to use lower laser beam densities to remelt the surface layer in comparison to other alloys. The thermal as well as thermophysical properties of grey cast iron are different from those of steel (and there are even quite important variances among different cast iron types) as mentioned above. Such values of parameters as density, melting temperature, specific heat capacity, thermal conductivity, or coefficient of surface absorption could vary significantly within metal alloys and even cast-iron types. In the surface layer in nodular cast irons, higher temperatures during the treatment can be expected compared to the case of flake cast iron. Therefore, the range of changes from the surface layer to the core of nodular cast iron caused by laser heat treatment can be expected to be larger than in the surface layer of flake cast iron archived under the same laser treatment conditions [5]. Depending on the interaction time, even a laser beam power density of 10 ÷ 20 W·mm⁻² (in the case of a continuous-wave molecular CO₂ laser) would be enough to achieve remelting of the surface layer of grey cast iron. Lower laser interaction times require higher laser beam power densities. To achieve only hardening from the solid state, a lower laser beam power density is not only generally needed but also a longer interaction time [30]. Another important aspect of laser heat treatment is the time of treatment and especially the cooling rate of the treated surface layer from the treatment temperature. Through appropriate selection of the cooling rate, it is also possible to control the microstructure and effects of properties in the surface.

An additional significant aspect of laser heat treatment is that the laser wavelength (that is related to laser type) is crucial in its effect on the treated surface. Generally, the shorter the wavelength, the better the absorption that can be expected. For example, the absorption coefficient in the case of a CO₂ laser (with λ = 10.69 μm) is approximately 4%, and that in the case of an excimer KrF laser (with λ = 0.248 μm) is 60% [15]. Therefore, methods for increasing absorption (e.g., coatings) are necessary during laser heat treatment in the case of lasers producing long waves. A crucial aspect is to develop appropriate values of laser treatment parameters, such as the laser beam density, which needs to be adjusted when using different lasers.

The general purpose of the following revision is to show the effects of three types of laser heat treatments with austenite transformation on the example of one very specific material, which is grey cast iron due to its double phase: metallic and non-metallic.

This work aims to present the similarities and differences between the presented treatments in terms of their operation in the surface layer of grey cast iron, taking into account the presence of graphite.

This comparison demonstrates the potential of these laser heat treatments, in terms of changes in the microstructure (taking into account their impact on graphite and, in particular, the potential of its presence in the surface layer after processing and contributing to friction reduction) and hardness and wear resistance.

2.1. Laser Hardening from the Solid State

Laser hardening from the solid state is generally a similar method to traditional induction surface hardening but is much faster and does not use a coolant. After laser hardening, the microstructure of the surface layer consists of martensite, some ferrite grains, and graphite [30]. An example of such a microstructure is presented in Figure 1. It can be seen that some carbon diffused from the graphite phase to the austenite matrix during the heating. Thus, the vicinity around the graphite was enriched in carbon. Furthermore, as a consequence, some so-called shells of martensite are visible around the graphite phase, which strengthens graphite areas. As not all the carbon from the graphite phase was diluted into the metal matrix during heating, graphite was visible in the section of the modified surface layer (and also on the surface). As laser hardening from a solid state allows the graphite to remain in the surface layer, it does not limit the self-lubricating abilities of this material. Thus, graphite effectively helps in friction operation, for example, in journals in crankshafts. If graphite is removed during the operation, empty spaces can even constitute oil reservoirs in such a pair.

A laser-hardened layer can be characterized by a hardness over 600 HV0.1 [5]. Such a modified surface layer allows the wear resistance to be increased. A lower mass loss of treated samples was noted in the case of nodular cast iron [36] and, for example, austempered cast iron [37]. The results shown in [36] indicate that, after the friction wear test (lasting only 15 min), nodular cast iron samples previously subjected to laser hardening from the solid state were characterized by a nearly ninefold lower mass loss compared to samples of untreated cast iron. Tribological tests carried out on the engine part (cylinder liner) also proved reduced wear (almost three times) after laser hardening from the solid state compared to the wear of the treated part [38]. In addition, research on laser hardening of an engine component—namely, the crankshaft—showed that this treatment caused the improvement in its wear resistance [39].

2.2. Laser Remelting

Another type of laser treatment with austenite transformation is laser remelting (hardening from the liquid state) [31,40,41,42]. After laser remelting of the surface layer of cast iron, three zones (in order of the treated surface) appear: remelted, transition, and hardened from the solid state [5,30,43]. The microstructure of the surface layer of nodular cast iron as an example is shown in Figure 2.

The result in the surface layer of cast iron is a fine-grained (especially compared to the base material), dendritic microstructure with some similarities to hardened ledeburite. Such a microstructure consists of, among others, martensite, cementite, retained austenite, and graphite [30,43]. In the case of nodular cast iron, not all graphite is dissolved during the laser treatment in the molten metal matrix (Figure 3). As opposed to nodular cast iron (with spheroidal graphite) in the case of flake cast iron, almost all graphite, because of its morphology, is dissolved. Therefore, nodular cast iron is interesting especially with regard to the possibility of additional lubrication arising from the presence of graphite. In Figure 4, the surface of treated nodular iron is visible. Some of the graphite nodules are visible in the melted area. On the cross-section of the treated cast iron, it can be noted that graphite nodules are ‘pushed’ to the surface during the laser melting by dendrites that, nucleating in the unmelted area, immediately crystallize and grow towards the surface. Spheroidal graphite was also observed during the laser remelting of nodular iron in [44]. A graphite nodule on the surface is visible in Figure 5.

Such a microstructure can be characterized by a hardness of over 1000 HV [36]. Other than a hardness increase, a better wear resistance of grey cast iron is noted [36,45,46]. A lower mass loss of laser-treated samples (than untreated) after the tribological test was noted, for example, in [36,47,48]. As [36] showed, after the wear test (lasting only 15 min), nodular cast iron samples previously subjected to laser remelting were characterized by a nearly 14-fold lower weight loss compared to untreated samples. Laser-treated samples with remelting were noted to have a better fatigue wear resistance [25,48] and a lower coefficient of friction in comparison to untreated samples [49]. The wear intensity after the treatment can be reduced more than 100 times, and the linear wear can be reduced by more than 50 times, as was noted in [50]. In addition, some increase in resistance to thermal fatigue [47] and to erosion wear [51] was noted in the case of laser-melted samples. It is also possible to increase the corrosion resistance due to the high saturation of carbon in martensite [52].

Laser remelting causes a more uniform and finer microstructure with higher hardness and, in some cases (particularly nodular cast iron), allows graphite to be especially left in the surface, which favors the increase in wear resistance.

2.3. Laser Alloying

During laser remelting, alloying elements could be added to the surface layer. Grey cast iron can also be effectively modified by such treatment [11,24,36,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69]. A single element or combination of elements can be applied during laser alloying. Commonly popular elements in other, more traditional methods (as diffusive processes) such as chromium, boron, nitrogen, and titanium are used. In addition, a combination of elements can be used like boron and silicon or boron, silicon, and cobalt. In this case, ultra-fine grains [61] or even an amorphous structure can be achieved in the grey iron surface layer [62]. Such elements like boron and silicon are typically used to create an amorphous phase during the laser treatment, improving the wear resistance, which can be, for example, approximately 1.71 times greater than for the substrate, as shown in [70]. Sometimes, compounds such as silicon carbide and tungsten carbide [54] are added. As laser remelting favors expanding the solubility of solid solution, it is possible to introduce elements characterized by small solubility such as yttrium. Other than elements or compounds, laser alloying may also be performed with used metal alloys like steel [63].

After laser alloying, a comparable microstructure to that achieved via laser remelting is formed. An example of the microstructure of the surface layer of nodular cast iron after laser alloying is presented in Figure 6. It can be noticed that, in the case of nodular cast iron, similar to the laser remelting effect, graphite nodules have been ‘pushed’ during the solidification of crystals in the melted area in the surface direction.

Usually, additional phases made of alloying elements also appear. In Figure 7, iron borides in the alloyed layer after laser alloying with boron are visible [36]. Their morphology is different from the morphology of iron borides formed during diffusion boronizing. In the case of research presented in [60], chromium carbides were noted after laser alloying with chromium.

The microstructure after laser alloying can be characterized by higher hardness compared to the microstructure after laser remelting. A hardness of over 1200 can be achieved depending on the kind of alloying substance and laser treatment parameters [36]. If a high-velocity laser treatment and, as a consequence, high cooling velocity are applied (by the appropriate combination of laser beam parameters), a hardness of 1800 HV0.1 is possible [61].

Other than increasing the hardness and wear resistance of the surface layer, other mechanical properties are changed as a result of laser heat treatment. For example, after laser alloying, a 2-fold increase in the modulus of elasticity, 15% decrease in the creep parameter, and 2.5-fold reduction in the plastic work were noted during the nanoindentation test in comparison to the core material [71].

High-velocity treatment causes ultrafast remelting of the metal alloy in the surface layer. If such conditions of the treatment are applied, it is possible to leave the graphite not completely dissolved even in the case of flake cast iron [61].

Cast iron after laser alloying can increase wear resistance (also in the case of white cast iron [72]). After the laser alloying of grey cast iron with boron, a mass loss of the sample over 18-fold lower than that of untreated samples was achieved [36]. A lower mass loss of samples was noted in the case of laser alloying with the combination of carbon, boron, tungsten, and chromium (with achieved a hardness of 1200 HV0.05) in comparison to untreated samples [11]. It was noted that applying compounds like silicon carbide increased the wear resistance by 18 times, increasing by 24 times if tungsten carbide was applied (in comparison with the initial material) [54]. A better resistance to fatigue wear was also noticed after laser alloying than after laser remelting [64]. In addition, thermal wear can be increased as was noted after laser alloying with chromium [60]. After laser alloying with chromium and nickel, it was observed that the achieved surface layer stopped appearing and blocked the propagation of thermal cracks [60]. After laser alloying with those elements, better corrosion resistance was also noted [65]. In addition, better corrosion resistance was observed after laser alloying with copper [66] and yttrium [67].

Thermal effects are different in the case of different cooling rates, as has been shown in [73]. Different cooling rates of the surface layer are possible when using an appropriate combination of laser beam density and its interaction time. Such a combination allows for different maximum temperatures of the surface layer to be reached during the treatment and its different cooling rates. For example, in [73], a nominal cooling rate of the treated surface was achieved in the range from 0.9 to 14.3 · 10³ °C/s. The lowest cooling rate was reached with the combination of the lowest laser beam density, the longest interaction time, the fastest cooling rate of the highest laser beam density, and the shortest interaction time. The layers cooled at a higher cooling rate are characterized by a better homogeneity as well as by a fine-grained microstructure in comparison to the layers cooled with a lower cooling rate. The more crystal nuclei occurring under conditions of faster cooling rates caused fine dendrites in the layer. A higher amount of retained austenite and cementite was also noticed in the layer cooling with a higher cooling rate. The hardness is also dependent on laser treatment conditions. In the case of lower cooling rate, a lower hardness was measured. It was noted that the hardness increased with increasing cooling [73]. In addition, a higher amount of undiluted graphite was noted in the case of layers formed with higher cooling rates, as it was achieved with higher process velocities than in the case of a layer formed with lower rates, which is important information with respect to the presence of a valuable graphite phase in the surface.

2.4. Comparison of Laser Heat Treatment Types with Austenite Transformation

Other than the differences in the microstructure (and/or chemical composition) of the metal matrix and hardness that are achieved after laser hardening from the solid state, laser remelting, and laser alloying, there are also some differences with regard to graphite phase morphology. Therefore, laser heat treatment clearly affects the shape of graphite particles.

In all analyzed types of processing, a reduction in graphite size was observed. The conditions and velocities of treatment are particularly important. The size of graphite decreases due to the diffusion of carbon from the graphite phase into the austenite matrix in the case of solid-state laser hardening or into the liquid phase in the case of laser remelting or alloying. Considering spheroidal graphite, smaller spheroids were visible after laser processing, and its quantity also decreased. In most types of laser processing involving melting or alloying, flake graphite is completely dissolved in the matrix. Only in the case of high laser processing velocities is it possible to leave small graphite flakes.

A general comparison of laser heat treatments, laser hardening from the solid state, laser remelting, and laser alloying is presented in

Table 1. A comparison of the three types of laser treatment effects.

The Surface Layer Factor	State of the Surface Layer
	Untreated	Laser-Hardened	Laser-Remelted	Laser-Alloyed
Grain size	Coarse-grained	Less coarse-grained	Fine, ultrafine	Fine, ultrafine, amorphous
Homogeneity	Poor	Poor	High	High
Compounds of metal matrix	Typically ferrite and pearlite	Martensite, ferrite	Martensite, austenite	Martensite, austenite, new formed phases containing alloyed element
Graphite presence	Present	Present (almost in the same amount)	Present but in a smaller amount in the case of nodular cast iron and visible especially in the vicinity of the surface; in the case of flake cast iron, usually all carbon from the graphite phase is diluted in the metal matrix during the treatment
Hardness of the metal matrix	~200 H0.1	~600 HV0.1	>1000 H0.1	1200–1800 HV0.1
Wear resistance	-	Increased	More increased

. The following could be seen:

• All types of treatment presented increase the hardness and microstructure homogeneity;
• All types of treatment presented enable better wear resistance to be achieved;
• Laser hardening from the solid state does not eliminate graphite;
• Laser remelting and laser alloying cause the dilution of carbon from the graphite to the melted metal matrix, but, in the case of nodular cast iron, it is possible that not all of the valuable graphite in the surface layer is lost.

3. Conclusions

This study characterized grey cast iron with regard to its properties, especially the wear properties resulting from the presence of graphite. Examples of applications of this material were given, particularly in the machinery, transport, and agricultural industries, where good wear resistance is required. Laser heat treatment is a surface treatment that enables these requirements to be met. Three types of laser treatments related to austenite transformation—namely, laser hardening from the solid state, laser hardening followed by remelting, and laser alloying—were characterized. The microstructure changes (taking into account their influence on graphite) and the wear resistance were analyzed. Generally, these three types of treatment enable better wear resistance of the surface layer to be achieved, mostly due to the increase in the hardness and microstructure homogeneity. It is also possible to retain the graphite phase (at least partially) in the modified surface layer, which is crucial in the case of friction wear resistance; in particular, laser heat treatment (especially laser hardening from the solid state) does not eliminate the presence of graphite (which is crucial for lubrication processes) in the surface layer. Laser remelting and laser alloying cause a dilution of carbon from the graphite to the melted metal matrix, but, in the case of nodular cast iron, it is possible that not all of the valuable graphite in the surface layer is lost.