Comparison of Microstructure and Hardening Ability of DCI with Different Pearlite Contents by Laser Surface Treatment

Abstract

Laser surface treatment (LST) has been employed on ductile cast iron (DCI) parts to obtain a good performance and a long service life. There is a need to understand the laser surface-treated microstructure and hardening ability of DCIs with different matrix structures to facilitate the scientific selection of DCI for specific applications. In this study, a Laserline-LDF3000 fiber-coupled semiconductor laser with a rectangular spot was used to harden the surface of ductile cast irons (DCIs) with different pearlite contents. The hardened surface layer having been solid state transformed (SST) and with or without being melted–solidified (MS) was obtained under various process parameters. The microstructure, hardened layer depth, hardness and hardening ability were analyzed and compared as functions of pearlite contents and laser processing parameters. The results show that the MS layers on the DCIs with varied pearlite contents have similar microstructures consisting of fine transformed ledeburite, martensite and residual austenite. The microstructure of the SST layer includes martensite, residual austenite and ferrite, whose contents vary with the pearlite content of DCI. In the pearlite DCI, martensite and residual austenite are found, while in ferrite DCI, there is only a small amount of martensite around the graphite nodule, with a large amount of unaltered ferrite remaining. There exists no significant difference in the hardness of MS layers among DCIs with different pearlite contents. Within the SST layer, the variation in the hardness value in the pearlite DCI is relatively small, but it gradually decreases along the depth in the ferrite DCI. In the transition region between the SST layer and the base metal (BM), there is a steep decrease in hardness in the pearlite DCI, but it decreases gently in the ferrite DCI. The depth of the hardened layer increases slightly with the increase in the pearlite content in the DCI; however, the effective hardened depth and the hardening ability increase significantly. When the pearlite content of DCI increases from 10% to 95%, its hardening ability increases by 1.1 times.

1. Introduction

DCI is a ferrous alloy which has a higher strength and ductility, good castability and machinability, and a lower production cost compared to other alloys [1,2]. The matrix structure of most commonly used DCIs varies from a fully ferritic structure to a mixed ferritic and pearlitic or a fully pearlitic structure [3,4], which gives DCI a wide range of mechanical properties [5,6]. A ferritic matrix provides a DCI with good ductility and impact resistance and with a good tensile strength. A pearlitic matrix results in a DCI with high strength, good wear resistance, and moderate ductility and impact resistance. These characteristics enable DCI to be suitable for casting parts where a high resistance to wearing or a high level of ductility is needed, such as camshafts, crankshafts, and suspension parts in automobiles and general industrial machinery [2,7,8].

In some applications where the mechanical parts are subjected to high alternating loads and friction, the DCI exhibits insufficient performance. For the purpose of obtaining good performance and a long service life, surface treatment technologies can be applied to these DCI parts [9,10,11,12].

Laser surface treatment (LST) is a technique to modify the microstructure at the surface region without affecting the bulk. This technique is a fast thermal process, in which the heat input from a laser beam quickly raises the temperature at the part’s surface and then rapidly transfers to the other part of the components, and meanwhile, a rapid transformation occurs in the microstructure near the surface [12,13,14]. LST has received particular attention because it has many beneficial properties such as minimal distortion of the treated workpiece, highly localized application, the absence of quenching media and easy automatic control of the procedure. Among the LST techniques, laser surface transformation hardening (LSTH) [14,15,16,17] and laser surface melting (LSM) [10,17,18] are the most commonly used surface hardening techniques, which involve rapid heating of the surface with and without melting, respectively, followed by quenching to modify the near-surface microstructure without changing the chemistry composition.

Lots of studies have been conducted on LST of DCI since the 1980s [19,20,21,22]. The studies cover LST process variables [20,21,23], microstructure [22,23,24,25], hardness, wear/corrosion resistance [26,27,28,29,30], the thermo-metallurgical model [31,32], etc. The LST techniques for DCI have now been applied in manufacturing of automotive parts, molds and dies, metallurgy tools and other fields [33,34,35,36]. This advanced technology demonstrates significant potential for industrial applications, as it effectively enhances components’ performance and extends their service life. However, there is little comparative analysis of the microstructures and hardening abilities of DCIs with different matrix structures performed by LST. To enable the scientific selection of DCIs for particular applications, it is imperative to comprehend the laser surface-treated microstructure as well as the hardening ability of DCIs with different matrix structures. Thus, in this study, the microstructure hardness and hardened depth of DCI samples with different pearlite contents after LST were evaluated in detail, and their hardening ability by LST was analyzed and compared.

2. Materials and Methods

The metallographic microstructures of five types of DCIs with different pearlite contents were used in this study. Their chemical composition is shown in

Table 1. Chemical composition of DCI with different pearlite contents (%).

C	Si	Mn	Cu	Fe
3.45~3.65	2.10~3.12	0.18~0.28	as required	balance

, and the pearlite contents of them are approximately categorized as 10%, 35%, 55%, 75% and 95%, according to the Chinese standard GB/T 9441-2021 [37]. Shown in Figure 1 are the microstructures of the five kinds of DCIs.

The specimens with the size of 200 mm × 100 mm × 20 mm were prepared for LST by a cutting process, and the surfaces for laser treatment were polished by using 300-grit sandpaper (Fuste, Shanghai, China).

The laser beam output to the surface to be treated was generated by a Laserline-LDF3000 semiconductor fiber-coupled laser, and the laser spot was rectangular, with a size of 20 mm × 2 mm, and its schematic diagram is shown in the Figure 2. Based on preliminary experiments, 9 sets of process parameters were selected with laser power (P) ranging from 1200 to 1800 W, scanning speed (V) from 2 to 6 mm/s, and energy density (E) between 15 and 30 J/mm², which ensured that the hardened surface layer having been solid state transformed (SST) and with or without being melted–solidified (MS) could be obtained. The specific P and V values are shown as a matrix in Figure 3.

The energy density (E), representing the laser energy applied to the specimen surface per unit area, was calculated by using the following formula in this study:

$$\mathrm{E}={\displaystyle \frac{P}{S}}\times {\displaystyle \frac{W}{V}}={\displaystyle \frac{2P}{SV}}$$

(1)

where S represents the laser spot area; W is the laser spot width. The E values are provided in

Table 2. Process parameters of LST applied to DCI.

	P (W)	V (mm/s)	E (J/mm2)
1	1800	6	15.0
2	1650	5	16.5
3	1650	4	20.6
4	1500	5	15.0
5	1500	4	18.8
6	1500	3	25.0
7	1350	4	16.9
8	1350	3	22.5
9	1200	2	30.0

.

The microstructures of the cross-sections of the treated specimens were analyzed by using optical metallographic microscopy (OM) and scanning electron microscopy (SEM) after grinding, polishing and etching with a 4% nitric acid–alcohol solution. The hardness at different depths was measured by using a Vickers hardness tester (Zhongte, Dongguan, China) with a test load of 1000 gf and loading time of 15 s. The hardened depths were measured according to GB/T 18683-2002 standard [38].

3. Results and Discussion

3.1. Microstructure of Laser Surface-Treated DCI with Different Pearlite

3.1.1. Microstructure Characteristics

The LSM and LSTH have been achieved on DCIs with different pearlite contents by using the process parameters in Table 2, and Figure 4 shows the typical low-magnification OM images of the cross-section of the DCIs treated with 1500 W laser power at scanning speeds of 3, 4 and 5 mm/s, respectively. As can be seen from the figure, the microstructure of the treated layer is influenced by the process parameters. At a scanning speed of 3 mm/s, under which the energy density is higher, two distinct layers can be observed, which indicates that LSM is achieved, and a melted–solidified (MS) layer and a solid state transformed (SST) layer are formed. Both of these layers possess high hardness and can be collectively referred to as the hardened layer. In contrast, at a scanning speed of 5 mm/s, only one layer can be seen. This suggests that under a low energy density condition, LSTH is obtained and only an SST layer is formed. It can also be seen from Figure 4 that the microstructure of the treated layer is related to the pearlite content of the tested DCI, and the boundaries between the MS layer, the SST layer and the BM are clear in the case of high pearlite content, and the boundaries between the SST layer and the BM are not clear in the case of low pearlite content, which indicates that the pearlite content has an effect on the microstructure of the treated layer.

High-magnification OM and SEM examinations of the laser-treated layer show that there are both distinctive and common microstructural features across specimens with varying pearlite contents and processing parameters. Firstly, the specimens treated with a laser power of 1500 W and scanning speed of 3 mm/s were selected for detailed examination of their microstructure and comparison of the influence of the pearlite content of DCIs on the microstructure.

Shown in Figure 5 are the high-magnification OM images of four distinct regions of the laser-treated layer of DCI with 95% pearlite content, as marked in Figure 4 and denoted as A, B, C and D. These regions correspond to the MS layer, the transition region between the MS layer and the SST layer, the SST layer, and the transition region between the SST layer and the BM, respectively.

The microstructure of region A, as shown in Figure 5a, consists mainly of transformed ledeburite (Ld), accompanied by the partially dissolved graphite. Additionally visible are the martensite (M) from transformation of the fine austenitic dendrites formed during solidification of the melted iron, and coarse high-carbon martensite and residual austenite formed from the transformation of the unmelted iron. The unmelted iron has an increased carbon content due to the diffusion of carbon dissolved from graphite into it.

Region B, a relatively narrow transition region between the MS and SST layer, exhibits the microstructure shown in Figure 5b. As shown in the figure, partial melting occurs around the graphite (G) in this region, which subsequently transforms into a double-layered shell-type microstructure morphology during cooling, i.e., there is a layer of martensite shell around the graphite nodules, with an outer shell of ledeburite. As shown in Figure 6, the SEM image clearly reveals that the graphite nodule is encapsulated by an inner layer of martensite shell and an outer layer of ledeburite shell. Additionally, Figure 5b shows the presence of coarse martensite and residual austenite, resulting from the transformation of the unmelted iron during the cooling process.

Figure 5c presents the microstructure of region C, which is predominantly composed of martensite, with some residual austenite.

Shown in Figure 5d is the microstructure of region D. It can be seen that there are microstructures both undergoing phase transformation and with remaining pearlite (P), which are mixed with each other, and it is difficult to distinguish the phase-transformed microstructure under OM. Figure 7 is its SEM image. It can be seen from the image that the two microstructures are mixed with each other more clearly, and Figure 7b shows that the microstructure undergoing phase transformation has the characteristics of lamellar interlacing, and most of its laminae are connected with the pearlitic laminae. According to the relevant literature [39], this microstructure can be identified as martensite with residual austenite.

Figure 8 shows the high-magnification OM images of different regions of the laser-treated layer of DCI with 10% pearlite content. It can be seen that the microstructure of region A is similar to that of the above DCI with 95% pearlite content as shown in Figure 8a, but ferrite(F) exists in regions B, C and D as shown in Figure 8b–d. A three-layer shell microstructure of martensite, ledeburite and then martensite is present in region B as shown in the SEM photo in Figure 9. As shown in Figure 8c, region C consists of martensite and ferrite. The martensite is transformed from initial pearlite and ferrite. The ferrite-transformed martensite is around the graphite nodule, where a martensite shell is formed, and the farther away from the surface, the thinner this shell becomes. In region D, the initial ferrite is basically unchanged, and the initial pearlite is partially transformed into martensite, forming a microstructure with pearlite and martensite mixed with each other similar to that in Figure 7.

The microstructural analysis of the LST layer in DCI with pearlite contents of 75%, 55% and 35% indicates that the microstructure in region A is similar to that of the previously described DCIs, predominantly comprising ledeburite, martensite and residual austenite. The microstructure in region B, however, varies depending on the pearlite content. For samples with 75% and 55% pearlite, this region is primarily composed of martensite and ledeburite, displaying a double-layer shell-like structure of martensite and ledeburite, akin to the microstructure observed in DCI with 95% pearlite, as shown in Figure 10a,c. In contrast, when the pearlite content is reduced to 35%, ferrite is observed in this region, accompanied by a distinctive three-layer shell-like structure consisting of martensite, ledeburite and martensite, as shown in Figure 10e. The microstructures in regions C and D resemble those of the DCI with 10% pearlite mentioned earlier, with the key distinction being the ferrite content. Notably, the ferrite content decreases proportionally with the increase in pearlite content in the original microstructure. Figure 10b,d,f provide OM images of the microstructures in region C for these three DCIs, highlighting the differences in pearlite content.

As for the microstructure of the LSTH, its characteristics are similar to the SST layer microstructure in the LSM-treated specimens described above. In some cases, minor melting at the rims of the nodules occurs, forming a microstructure analogous to region B in the LSM-treated specimens, which will not be described in detail here.

The above results indicate that the pearlite content has a limited influence on the MS layer microstructure of DCI under LSM. This microstructure primarily consists of transformed ledeburite, martensite and residual austenite. However, the pearlite content significantly affects the SST layer microstructure in both LSTH and LSM treatments. In DCIs with fully pearlitic structures, the SST layer microstructure is predominantly composed of martensite. In contrast, for DCIs with mixed microstructures of pearlite–ferrite, the SST layer microstructure comprises both martensite and ferrite, with the martensite content being directly correlated to the initial pearlite content and the distance away from the surface. The higher the initial pearlite content is, the more martensite will eventually appear, and the farther away from the surface, the less martensite content will appear. In the transition region between the MS layer and the SST layer in the LSM-treated specimens, two distinct microstructures appear with different pearlite contents. When the pearlite content exceeds 55%, a bilayer shell-like microstructure of ledeburite and martensite forms, and when the pearlite content is below 35%, a triple shell-like microstructure of martensite–ledeburite–martensite is observed. In the region between the SST layer and the BM, some of the initial pearlite is transformed into martensite, which results in the formation of a mixture of pearlite and martensite, while ferrite remains essentially unchanged.

3.1.2. Mechanism of Microstructure Formation

LST is characterized by rapid heating and cooling rates, and the temperature and carbon concentration in the treated region cannot reach a uniform distribution. As a result, different locations in the treated region experience different temperature and carbon concentration variations, and evolve into distinct microstructures. Figure 11 illustrates the microstructure evolution process of the DCIs with a mixed matrix of pearlite–ferrite pearlite.

When the laser irritates the surface of DCI (as illustrated in Figure 11a), the temperature of the surface rises. When the temperature exceeds the eutectoid temperature, the pearlite and ferrite transform into austenite (Figure 11b). Subsequently, the graphite nodules dissolve, releasing carbon atoms that diffuse into the austenite. This diffusion creates a carbon concentration gradient centered around the graphite nodules, with the concentration decreasing gradually with increasing distance from the nodules, as illustrated in Figure 11c,f,g. If rapid cooling occurs at this time, the austenite with high carbon concentration (A_HC) that is around the graphite nodules and is from pearlite transformation will transform into martensite, while the austenite with low carbon concentration (A_LC) that originates from ferrite transformation and locates far from the graphite will transform back into ferrite. This process produces a martensitic ring around the graphite nodule (Figure 11j,k), as shown in the OM image (Figure 8c and Figure 10f).

When the temperature exceeds the eutectic temperature and is maintained for a period of time, the carbon content in the austenite surrounding the graphite nodules increases, thereby lowering its melting point and causing the austenite near the graphite nodules to melt, as illustrated in Figure 11d. If cooling occurs at this stage and the temperature falls below the eutectic temperature, the molten iron will solidify following the stable Fe–graphite system. This is because the graphite nucleation is not required in this solidification, and carbon can precipitate on the existing graphite. Meanwhile, austenite nucleates on the graphite and grows, forming an austenite shell around the graphite nodules.

After graphite nodules become encapsulated by austenite shells, carbon atoms in the molten iron can still diffuse through the austenite to precipitate on the graphite. However, atomic diffusion in solid phase becomes increasingly difficult, and the decreasing temperature hinders solidification of the remaining molten iron following a stable Fe–graphite system. Consequently, the remaining molten iron transforms into ledeburite following the metastable Fe-Fe₃C system, so that a multilayer structure is formed around the graphite nodules (Figure 11h).

As the temperature continues to decrease, the austenite transformed from ferrite farther away from the graphite transforms into ferrite again due to its low carbon concentration; the rest of the austenite transforms into martensite and residual austenite. Theoretically, a four-layer shell-type microstructure with martensite, ledeburite, martensite and ferrite in sequence can eventually be formed (Figure 11l), but in practice the fourth layer is often incomplete, as shown in Figure 9 and Figure 10e.

When staying above the eutectic temperature for an extended period, more graphite dissolves, and the iron will completely melt (Figure 11e). During the cooling process, the molten iron first transforms into high-temperature ledeburite (Figure 11i) and then low-temperature ledeburite (Figure 11m).

The above discussion indicates that the main factors influencing the formation of microstructures during LST of DCIs are the initial matrix microstructure, and heating and cooling rates. These factors affect the carbon concentration and its distribution in the iron matrix, and the carbon concentration determines the resulting microstructure. Based on the above discussion, it can be inferred that the temperature–carbon concentration history at five distinct positions in Figure 11l or Figure 10e are as illustrated in Figure 12. The varying temperature–carbon concentration histories at different positions lead to the formation of different microstructures.

3.2. Hardness of Laser Surface-Treated DCI with Different Pearlite

Among laser-treated DCIs, both LSTH and LSM are capable of producing hardened layers. The hardness of these hardened layers is a critical factor influencing their service performance. Figure 13 shows the variation in the surface hardness of the laser-hardened layer of DCI with respect to energy density for different pearlite contents. As can be seen from the figure, at low energy densities, the surface hardness increases slightly with higher pearlite content. However, at high energy densities, the pearlite content has negligible influence on the surface hardness. This result is consistent with the observed microstructure. At relatively low energy density, ferrite appears on the surface of the hardened layer of DCI with low pearlite content, resulting in slightly lower hardness. Conversely, when the energy density is relatively high, the surface microstructure of the hardened layer of DCI, regardless of pearlite content, is all ledeburite, so the surface hardness is independent of the initial pearlite content.

Figure 14 shows the average microhardness distribution along the depth direction of laser-treated layers of DCIs with different pearlite contents under three typical process parameters: a constant laser power of 1500 W with scanning speeds of 3, 4 and 5 mm/s. The results demonstrate that both the laser process parameters and the pearlite content significantly affect the hardness distribution. At a scanning speed of 3 mm/s, the surface hardness of DCIs with different pearlite contents is relatively high, but there is a low-hardness zone among the hardened layer. When the scanning speed is 4 mm/s, except for the DCIs with 10% pearlite content, the hardness distributions of other DCIs are similar to those at the scanning speed of 3 mm/s. Conversely, at a scanning speed of 5 mm/s, the surface hardness of DCIs is relatively low, with the exception of the DCI containing 10% pearlite content.

Comparative analysis of the hardness distributions across DCIs with different pearlite contents reveals distinct patterns. The DCI with 95% pearlite content exhibits higher hardness over a specific depth range with minimal variation, and the transition to the BM hardness occurs over a narrower interval, resulting in a steeper hardness gradient. As the pearlite content decreases, the depth range of higher hardness diminishes, and the hardness gradually decreases to the BM hardness over a wider interval. Notably, for the DCI with 10% pearlite content, the hardness decreases rapidly from the surface, except for the scanning speed of 3 mm/s.

The hardness distribution within the hardened layer is fundamentally determined by its microstructure distribution. Previous analysis results show that the hardened layer of laser-treated DCI has a variety of microstructures, including ledeburite, high-carbon martensite, low-carbon martensite, austenite and ferrite. These microstructures possess different hardnesses, and their relative volume fractions and distributions significantly influence the overall hardness profile. The formation of these distinct microstructures is primarily attributed to the non-uniform distribution of carbon atoms during the LST process.

When the scanning speed is low, the high laser energy density causes the surface of the specimen to melt, transforming the microstructure into ledeburite, which results in high surface hardness. However, a low-hardness region forms in the middle of the hardened layer due to the high carbon content in the transition region between the martensitic layer and the SST layer, leading to the formation of a significant amount of residual austenite. In contrast, at high scanning speeds, no melting or only slight melting occurs in the surface layer. In this case, the carbon content in the iron surrounding the graphite is high, and a substantial amount of residual austenite is present, leading to lower surface hardness.

The SST layer of pearlite DCI is mainly a martensite microstructure, resulting in consistent hardness distribution. However, a sharp hardness drop is exhibited in the transition zone between the SST layer and the BM due to their significant hardness differences. In contrast, the SST layer of ferritic DCI contains ferrite, with ferrite content increasing with distance from the surface. Consequently, the hardness gradually decreases within the SST layer, and the transition zone between the SST layer and the BM exhibits a gradual hardness reduction.

Based on these observations, a model can be established to correlate the concentration of carbon in the hardened layer, the resulting microstructure and the hardness distribution, which effectively explains the microhardness distribution patterns described earlier. This model for pearlite–ferrite DCI is shown in Figure 15.

3.3. Hardened Depth of Laser Surface-Treated DCIs with Different Pearlite Contents

The hardened depth is the maximum vertical distance from the surface to the point where the microstructure has changed, including the MS and SST layers. Figure 16 shows the variation in hardened depth with energy density for DCIs with different pearlite contents. As can be seen from Figure 16, within the range of experimental process parameters, the hardened depth exhibits a positive correlation with the laser energy density. Furthermore, at a constant energy density, the hardened depth increases with higher pearlite content in the DCI.

The different hardened depths of DCIs with different pearlite contents under the same energy density indicates that the hardened depths of DCIs are influenced not only by process parameters but also by the initial pearlite content of DCIs. This phenomenon can be explained by analyzing the temperature distribution within the laser-treated region. During the LST process, heat is introduced to the workpiece surface by the laser while simultaneously being conducted from the surface to deeper regions of the workpiece. The difference between the heat input and output will determine the depth of the region where the temperature exceeds the austenitizing temperature, that is, the hardened depth. The heat input Q_in is proportional to the energy density, as expressed by Equation (2):

$$Q_{\mathrm{in}}=AE$$

(2)

where A is the laser absorption rate.

According to the principles of heat transfer, the heat output Q_out from the surface region can be expressed as:

$$Q_{\mathrm{out}}=\lambda gradT{\displaystyle \frac{2}{V}}$$

(3)

where λ is the thermal conductivity of the material, and gradT is the temperature gradient.

If the hardened depth D is proportional to the net heat accumulation (Q_in − Q_out), it can be expressed as:

$$D=\propto (Q_{\mathrm{in}}-Q_{\mathrm{out}})=\propto (AE-\lambda gradT{\displaystyle \frac{2}{V}})$$

(4)

Equation (4) indicates that the hardened depth is influenced by both the laser surface treatment (LST) process parameters and the thermal conductivity of the material, with higher thermal conductivity corresponding to a lower hardened depth. Based on this relationship, it can be inferred that under identical energy density conditions, the variations in case depth among DCIs with different pearlite contents result from their distinct thermal conductivities. Since the thermal conductivity of pearlite is lower than that of ferrite [40], DCIs with higher pearlite content exhibit a greater case depth.

According to the definition of effective hardened depth in GB/T 18683-2002 and assuming the minimum required hardness of the surface layer is 510 HV, the effective hardened depths of DCIs with different pearlite contents are shown in Figure 17 as a function of laser energy density. It can be seen that the effective hardened depth increases with higher laser energy density for all pearlite contents. Furthermore, DCIs with higher pearlite content exhibit a greater effective hardened depth. A comparison of Figure 16 and Figure 17 reveals that the effective hardened depth of DCI with 95% pearlite content is nearly equivalent to its total hardened depth. In contrast, the effective hardened depth of DCIs with lower pearlite content is relatively smaller. When no MS layer is present, the effective hardened depth of DCI containing 10% pearlite is less than 0.3 mm.

3.4. Hardening Ability of DCI with Different Pearlite Contents

The hardened depth and hardness are the primary indices representing the hardening effect. The above experimental results show that the hardened depth and hardness of DCIs with different pearlite contents are different under the same laser process parameters. This indicates that DCIs with different pearlite contents have different abilities to harden their surface layers with laser energy. To comprehensively compare this ability of DCIs with different pearlite contents, the laser hardening ability (Ha) is defined by referencing the concept of laser surface hardening efficiency [16,41,42] as the magnitude of the hardness change per unit volume per unit laser energy, which can be expressed as follows:

$$H_a={\displaystyle \frac{{\displaystyle {\int }_0^{\mathrm{D}}}\Delta H_V\times \mathrm{d}x}{E}}$$

(5)

where ΔH_V is the change in hardness before and after hardening at a certain depth from the surface, D is the total hardened depth and x is the distance from the surface.

Shown in Figure 18 is the calculated variation in hardening ability with energy density for DCIs of different pearlite contents. As can be seen from the figure, under the same energy density, the hardening ability increases with higher pearlite content. When the pearlite content of DCI increases from 10% to 95%, its hardening ability increases by 1.1 times.

The DCI containing 95% pearlite exhibiting the best hardening ability can be attributed to the fact that the pearlite in the LST process easily forms austenite with a higher carbon content, which subsequently transforms into martensite. In contrast, austenite transformed from ferrite requires sufficient carbon diffusion from graphite to facilitate martensitic transformation. However, austenite located far from graphite has difficulty acquiring carbon during rapid heating, thereby failing to transform into martensite. From an engineering perspective, to achieve the best laser hardening effects, DCIs with higher pearlite content should be prioritized.

4. Conclusions

• The laser MS layer of the DCIs with different pearlite contents has a similar microstructure consisting of fine transformed ledeburite, martensite and residual austenite.
• The microstructure of the SST layer of the DCIs varies with pearlite content. In the pearlite DCI, it is a martensite and residual austenite; in the ferrite–pearlite DCIs it is composed of martensite, ferrite and residual austenite. In the ferrite DCI, there is only a small amount of martensite around the graphite nodule.
• The hardness of the laser MS layer of the DCIs with different pearlite contents has no obvious difference. Within the SST layer, the hardness in the pearlite DCI varies relatively little, but in the ferrite DCI it gradually decreases along the depth. In the transition region between the SST layer and the base metal, there is a steep decrease in hardness in the pearlite DCI, and it decreases gently in the ferrite DCI.
• The increase in pearlite content in DCI slightly raises the hardened depth, but significantly increases the effective hardened depth and the hardening ability. When the pearlite content of DCI increases from 10% to 95%, its hardening ability increases by 1.1 times. From an engineering perspective, to achieve the best laser hardening effects, DCIs with higher pearlite content should be prioritized.