Study of Surface Integrity Evolution During Laser Hardening of 42CrMo4 Steel Using a 4 kW Diode Laser

Abstract

Laser surface hardening (LSH) is an efficient and flexible technique for improving the surface integrity of steel components used in high-load automotive applications. In this study, the surface changes occurring during laser hardening of 42CrMo4 steel were systematically investigated using a 4 kW high-power diode laser. The influence of laser power and scanning speed on surface roughness, hardness distribution, hardened layer depth, tribological behavior, and phase composition was analyzed. Surface topography was evaluated using three-dimensional laser scanning microscopy, while mechanical performance was assessed through microhardness and scratch testing. Phase transformations and residual structural changes were examined by X-ray diffraction (XRD) at different depths beneath the treated surface. The results demonstrate that laser processing parameters strongly affect surface integrity through competing mechanisms of surface melting, oxidation, and self-quenching. High laser power combined with low scanning speed produced deep hardened layers but promoted surface melting and retained austenite formation, whereas lower power and higher scanning speed yielded a stable martensitic surface with reduced roughness and a steep hardness gradient. XRD analysis confirmed that oxide formation was limited to the near-surface region, while the subsurface hardened zone consisted predominantly of martensitic/bainitic phases. An optimal processing window was identified that balances surface hardness, roughness, and microstructural stability without compromising surface integrity. These findings provide practical guidelines for optimizing diode laser hardening of 42CrMo4 steel gears in industrial automotive applications.

1. Introduction

Laser surface hardening (LSH) has become an advanced and energy-efficient surface engineering technique for improving the surface integrity of steel components subjected to high contact loads, cyclic stresses, and severe tribological conditions in automotive applications, particularly gears, shafts, and transmission elements. By delivering precisely controlled laser energy to a localized surface region, LSH enables rapid austenitization followed by laser hardening through heat conduction into the bulk material without liquid cooling, resulting in the formation of a hardened martensitic surface layer while preserving core toughness and dimensional stability. Compared with conventional heat-treatment methods, laser hardening offers superior process flexibility, reduced thermal distortion, and the ability to tailor surface hardness, microstructural gradients, and roughness through controlled adjustment of laser parameters, such as power density and scanning speed [1,2,3].

Among laser-based surface treatments, LSH is an industrially mature process that has demonstrated significant improvements in wear resistance, fatigue performance, and service life of steel components [4,5,6]. Since only a shallow subsurface layer—typically several hundred micrometers to approximately 2 mm—is thermally affected, the bulk mechanical properties of the component remain largely unchanged, which is critical for precision parts such as gears and camshafts [7,8,9]. The effectiveness of LSH is governed by the transient thermal cycle imposed by the laser–matter interaction, which controls austenitization depth, cooling rate, and the resulting martensitic transformation [10,11].

The development of high-power diode lasers has significantly accelerated the industrial adoption of laser surface hardening. Compared with CO₂ and Nd:YAG lasers, diode lasers offer higher electrical-to-optical efficiency, improved beam homogeneity, and good absorption capacity on steel surfaces. Studies demonstrated the suitability of diode lasers for large-area hardening, while subsequent experimental and numerical studies showed that laser power and scanning speed are the dominant parameters controlling surface temperature evolution, hardened layer depth, and final microstructure [9,12,13,14,15]. Recent investigations on 42CrMo-based steels have reported substantial hardness increases and refined martensitic structures when optimized diode laser parameters are employed, confirming the material’s suitability for laser hardening applications [16].

A critical review of recent studies on 42CrMo4 laser hardening reveals well-established relationships between key process parameters and primary outcomes, such as hardened depth and surface hardness. For instance, ref. [17] demonstrated the significant influence of power and speed on surface roughness, while [18] correlated high energy input with deep case hardening but also noted the risk of retained austenite formation. Ref. [19] further highlighted how laser parameters control the martensitic transformation and wear resistance. However, these studies predominantly focus on isolated aspects of performance—either topography, hardness, or wear. What remains underexplored is an integrated analysis that systematically links the full spectrum of laser parameters (power, speed) to the complete surface integrity of 42CrMo4, encompassing not only hardness and depth but also the evolution of 3D surface roughness, depth-resolved phase gradients (via XRD), and direct tribological response under a high-power (≈4 kW) diode laser. This gap is particularly relevant for industrial gear applications, where the final performance depends on a delicate balance between these competing properties.

Beyond phase transformation, surface integrity after laser hardening is strongly influenced by changes in surface topography, hardness distribution, residual stresses, and tribological behavior. Previous studies have shown that increasing scanning speed or reducing energy density decreases the interaction time, leading to shallower hardened layers and reduced surface roughness, whereas excessive heat input may induce surface melting, oxidation, and roughness heterogeneity [19,20,21]. In addition, excessive thermal input may promote retained austenite formation near the surface, resulting in localized surface softening despite increased hardened depth [18]. These competing mechanisms highlight the complex relationship between laser parameters, microstructural evolution, and functional surface performance.

In industrial practice, large functional surfaces are typically processed using multiple adjacent laser tracks, making overlap effects an important technological consideration. Variations in overlap ratio can modify local heat accumulation and reheating conditions, potentially leading to hardness gradients or softened regions between tracks [22]. Although overlap strategies are widely used, systematic experimental investigations under high-power diode laser irradiation remain limited, particularly for gear-grade 42CrMo4 steel.

Laser surface hardening also directly affects tribological performance through the combined influence of surface roughness, microstructure, and residual stresses. Martensitic surface layers produced by laser processing have been shown to reduce friction and improve wear resistance; however, these improvements are highly sensitive to processing conditions and surface quality [23,24]. Recent studies on laser-processed 42CrMo-based steels indicate that surface oxidation and melting phenomena can significantly alter friction behavior, underscoring the need for integrated analysis of mechanical, topographical, and phase-related effects [24]. Based on the results obtained in the studies, it is possible to model or predict laser hardening parameters [25]. This is essential in the manufacturing industry when, for example, the depth of hardening needs to be changed.

Despite the extensive body of literature on laser surface hardening, detailed experimental correlations between laser power, scanning speed, surface roughness evolution, hardness depth profiles, tribological response, and phase composition for 42CrMo4 steel under high-power diode laser irradiation (≈4 kW) remain insufficiently documented. In particular, the identification of processing windows that maximize hardened depth while maintaining favorable surface integrity and avoiding detrimental surface melting or oxidation is still an open challenge for industrial gear applications.

The present study directly addresses this gap by providing a comprehensive, integrated analysis. We systematically investigate the surface integrity evolution during laser hardening of 42CrMo4 steel using a 4 kW diode laser, explicitly linking process parameters to the competing mechanisms of solid-state transformation, surface oxidation, and localized melting. The effects of laser power and scanning speed on (i) surface roughness evolution, (ii) surface and subsurface hardness distribution, (iii) hardened layer depth, (iv) friction coefficient behavior, and (v) depth-dependent phase composition and microstructural stability were analyzed. Surface topography was characterized using three-dimensional laser scanning microscopy, mechanical performance was evaluated by microhardness, and phase transformations were examined by X-ray diffraction at different depths beneath the treated surface. The results provide practical guidelines for optimizing diode laser hardening parameters to achieve a balanced combination of hardness, surface integrity, and tribological performance in automotive gear applications.

2. Materials and Methods

2.1. Materials

In this study, 42CrMo4 steel, a low-alloy chromium–molybdenum steel, was selected as the material for laser hardening experiments. The specimens were machined from an industrially supplied hot-rolled and normalized 42CrMo4 steel bar, providing a ferrite–pearlite microstructure. This material is widely used in the automotive industry for high-performance components, such as gears, shafts, and transmission elements, due to its excellent combination of strength, toughness, and hardenability. Its ability to undergo a well-defined martensitic transformation under rapid thermal cycles makes it particularly suitable for laser surface hardening applications.

The chemical composition of the 42CrMo4 steel used in this study was determined via optical emission spectroscopy (OES). The measured values are presented in

Table 1. Chemical composition (wt.%) of the investigated 42CrMo4 steel.

Element	C	Si	Mn	Cr	Mo	Pmax	Smax	Fe
EN 10083 Nominal	0.38–0.45	0.15–0.35	0.60–0.90	0.90–1.20	0.15–0.30	0.025	0.035	Balance
Measured (OES)	0.36	0.23	0.71	1.06	0.21	<0.025	<0.025	~97.3

alongside the nominal composition range specified in the EN 10083 standard [26], confirming the material’s conformity and suitability for reproducible experiments.

Before laser processing, the samples were ground to a uniform roughness (S_a ~ 1.7 µm) to minimize variability in laser energy absorption. For the experiments, cylindrical blanks with a diameter of Ø30 mm and a height of 20 mm were used (see Figure 1).

2.2. Laser Hardening

System laser hardening experiments were conducted using a high-power diode laser system. The laser technological setup included a Fanuc M20-iD industrial robot, a single-axis positioner, and a laser power supply (LDM4000-100, 4 kW) (Figure 2a,b). The laser head (OTZ-5 VR) was equipped with vari-optics, allowing for the adjustment of the laser spot size from 5 × 8 mm to 22 × 44 mm. For precise temperature control, the system incorporated an integrated two-color pyrometer (LDC01 Mergenthaler) to measure and regulate the temperature of the hardened surface during the process. The experiments were planned according to the matrix shown in

Table 2. Planned Experimental Matrix—numbers of the studied samples from 1 to 18 (speed varies from 12 to 20mm/s, and power is 3.0, 3.5, and 3.8 kW).

Power	Speed (mm/s)
	12	13	14	15	16	20
3.0 kW	10	11	12	13	14	16
3.5 kW	6	7	8	9	15	17
3.8 kW	1	2	3	4	5	18

.

Laser hardening was performed using the following parameters: laser power of 3.0—3.8 kW, processing speed of 12 ÷ 20 mm/s, spot size of 15 × 8 mm, and 0% overlap. Three samples were used for additional measurements: S1—Reference, the untreated sample, and two laser-hardened samples. Sample S2—Air was cooled in air after laser hardening, while Sample S3—Oil was cooled in oil after hardening.

2.3. Surface Roughness Measurements

Surface topography and 3D roughness parameters were characterized using a laser scanning confocal microscope (OLYMPUS LEXT OLS500, Olympus Corporation, Tokyo, Japan) according to ISO 25178 [27]. Microstructural images were captured with a 20× objective lens at 451× magnification, with a measurement field of 650 × 650 µm. The surface roughness parameters, including S_a, S_z, and S_q, were measured perpendicular to the hardened stripes across the entire investigated area of 650 × 650 µm. The measurement accuracy is within ±2.0 µm.

2.4. Hardness Measurements

The sample surface hardness was measured using a Micro-Vickers indenter HM220D (Mitutoyo, Japan). The hardness tester was equipped with a CNC XYZ (Mitutoyo, Japan). sample stage. Together with AVPACK 3.0 software, it provided semi-automatic hardness measurements. Hardness measurements were conducted using a line pattern with 1 mm spacing between indentations. A test force of 1 kgf (9.81 N) and a 50× magnification lens were used.

Indentations were performed using a standardized loading sequence: 4 s to reach the test force, 10 s dwell at the peak load, and 4 s unloading. The approach speed was set to 60 µm/s; the same settings were used for all measurements.

2.5. Microstructure Analysis

The samples for microstructure analysis were prepared using a Metkon Forcipol 1V (Metkon, Bursa, Turkey) [28] grinder–polisher. Various SiC abrasive grinding wheels and diamond suspensions, as well as lubricants, were used for surface preparation of the samples. The microstructure of the material was examined on polished and etched (by Nital–Ethanol nitric acid) sections of the samples using light optical microscopy, Meihji Techno IM 7200 (Meiji Techno Co., Ltd., Saitama, Japan), equipped with a Brunel Microscopes LP605100A (Brunel Microscopes Ltd., Chippenham, United Kingdom) digital camera. The structure was investigated under different magnifications: 200×, 500×, and 1000×. The microstructure picture was taken and examined through ToupView 3.7 and a LP605100A (Brunel Microscopes Ltd., Chippenham, United Kingdom) digital camera.

2.6. X-Ray Diffraction Analysis

Phase and structural analyses were carried out using PANalytical Empyrean (Malvern Panalytical Ltd., Malvern, United Kingdom) and X’Pert PRO MPD (Brunel Microscopes Ltd., Chippenham, United Kingdom). diffractometers. Three samples were prepared for XRD analysis: one as-received (unprocessed) and two laser-hardened samples processed with an average power of 3.5 kW and a scanning speed of 20 mm/s. One laser-hardened sample was polished in air, and the other in oil. X-ray diffraction was performed using a copper tube operating at 30 kV and 20 mA, with a dwell time of 10 s per angular step. The irradiated volume and X-ray penetration depth were defined based on the Bragg–Brentano method, with an effective penetration depth of approximately 10 µm. XRD is performed in the direction of the laser beam movement on the surface of the hardened zone. This methodology provided insight into phase transitions, including martensitic, bainitic, and ferritic phases, which were characterized based on peak separation and crystallographic features.

2.7. Tribological Tests

The tribological properties of laser-hardened steel samples were measured using a ball-on-flat type tribometer TRB3 (CSM Instruments, Peuseux, Switzerland) under dry friction conditions according to ASTM G133–05 (ASTM International: West Conshohocken, PA, USA, 2016) [29], where the ball was a stationary counter body and hardened steel samples were flat. The test balls with a diameter of 6 mm were made of ceramic Al₂O₃ (ISO 3290 2:2014 Rolling bearings—Balls—Part 2: Ceramic balls. International Organization for Standardization: Geneva, Switzerland, 2014.) [30], with a ball hardness of 75 ± 1 HRC. The total tribotest track length was 50 m, and the length of one cycle was 10 mm, which resulted in 5000 cycles when applying the ball indenter load of 3 N, and there was a linear sliding speed of 0.05 m/s. Following the tribology test, wear track profiles of the ceramic samples were measured at three evenly spaced locations using a Surftest SJ-500 profilometer (Mitutoyo, Kawasaki, Japan). The wear rate of the ceramic samples was calculated by taking the mean cross-sectional area of the three measured wear profiles. The wear scar on the ball was assessed using a KH-7700 digital microscope (Hirox, Tokyo, Japan). All tribological tests were performed at 20 ± 1 °C temperature and 39 ± 2% relative humidity.

3. Results

The experimental investigation into the laser hardening of 42CrMo4 steel using a 4 kW diode laser revealed distinct correlations between processing parameters (laser power P and scanning speed v) and the resulting surface integrity. The results are presented in the following sequence: surface topography, microstructure, mechanical properties (hardness), and phase composition.

3.1. Surface Topography Analysis

The influence of processing speed on surface roughness was evaluated using three key parameters: Arithmetic Mean Height S_a, Maximum Height S_z, and Root Mean Square Height S_q. The experiments were conducted at power levels of 3.0 kW, 3.5 kW, and 3.8 kW across a speed range of 12 ÷ 20 mm/s.

3.1.1. Arithmetic Mean Roughness S_a

The evolution of S_a in Figure 3 highlights two competing mechanisms, depending on the linear energy density P/v.

High Power Regime (3.5 kW and 3.8 kW): A distinct U-shaped trend was observed. At low scanning speeds (<15 mm/s), the S_a values for the 3.8 kW samples dropped significantly below the baseline of the untreated material. This reduction is due to the effect of reducing roughness (glazing). The highpower density leads to the formation of a thin molten film on the surface.

Conversely, by increasing the scanning speed above 15 mm/s, S_a increased sharply, exceeding the roughness before processing. This increase correlates with the formation of “solidification ripples”. At higher speeds, the interaction time decreases, causing the melt pool to solidify before surface tension can level the waves created by fluid flow within the unstable melt pool and rapid solidification.

Low Power Regime (3.0 kW): No melting was observed in this regime. The roughness is driven solely by oxidation scaling. At slower speeds (12 mm/s), the prolonged thermal exposure allows for the formation of a thick, flaky oxide layer, resulting in higher roughness. As speed increases (approaching 20 mm/s), the oxidation time reduces, leading to a thinner oxide scale and a consequent decrease in S_a.

3.1.2. Maximum Height S_z and Root Mean Square Roughness S_q

The S_z, and S_q measurements (Figure 4 and Figure 5) corroborated the S_a findings while providing insight into the severity of the effect.

S_z Analysis: The 3.5 kW and 3.8 kW samples exhibited high S_z values at elevated speeds. This confirms that the “solidification ripples” identified in the S_a analysis are high-amplitude defects (deep valleys and sharp peaks) that can act as stress concentrators. In contrast, the 3.0 kW samples showed reduced S_z values, confirming that the roughness was caused by surface oxides rather than deep topographical valleys.

S_q Analysis: As a parameter sensitive to outliers, S_q validated the process stability. The low error bars and stable S_q values for the 3.0 kW settings indicated a thermally stable process dominated by solid-state transformation. The higher variance in S_q for the high-power settings reflected the turbulent nature of the melt pool and the instability of the fluid flow dynamics.

3.2. Results of Microstructure Analysis

The microstructural change across the hardened zone was analyzed to link the topographical features with subsurface transformations. Figure 6 presents an overview of the microstructure before and after laser hardening for both quenching conditions.

The initial, as-received material exhibits a ferrite–pearlite microstructure (Figure 6a). After laser hardening under high-energy conditions (e.g., 3.8 kW, 12 mm/s), the near-surface region transforms significantly. A thin, bright-etching remelted zone (RZ) is visible at the very surface for both air- and oil-quenched samples (Figure 6b,c), resulting from localized melting and rapid solidification. Directly beneath the RZ lies a deep hardened zone (HZ) characterized by a fine, acicular martensitic structure. This zone results from solid-state austenitization and subsequent rapid cooling (self-quenching). The transition from the HZ to the unaffected core is marked by a heat-affected zone (HAZ), where the temperature was sufficient to temper the original microstructure but not to form austenite, leading to a gradient in hardness and etching response. For conditions without melting (e.g., 3.0 kW, 20 mm/s, Figure 7), the RZ is absent, and the hardened martensitic zone transitions directly to the base material with a very narrow HAZ.

3.3. Hardness Analysis

3.3.1. Surface Hardness (HV1)

The surface hardness measurements (Figure 8) were compared against the roughness trends to assess functional quality.

3.8 kW: Hardness decreased as speed increased. While low speeds yielded high hardness, they coincided with the “melting/glazing” regime. High speeds resulted in lower hardness and the “rippling” defect regime.

3.0 kW: Surface hardness remained stable and high across the entire speed range. This indicates that the energy input was sufficient to exceed the austenitizing temperature Ac3 for 42CrMo4 without the risks associated with overheating or melting.

Figure 9 shows that the untreated reference sample S1 exhibits a uniform hardness profile with an average value of approximately 316 HV. In contrast, laser-hardened samples S2 and S3 display a pronounced increase in hardness within the laser-treated region, reaching average values of about 733 HV and 718 HV for air- and oil-cooled conditions, respectively. Outside the treated zone, hardness rapidly decreases to levels comparable to the reference material, confirming the localized nature of the hardening effect. Overall, laser hardening significantly enhances the surface hardness of 42CrMo4 steel, with slightly higher values obtained under air cooling.

3.3.2. Hardness Depth Profiles

Microhardness profiles (Figure 10) revealed the trade-off between case depth and surface integrity.

Depth vs. Energy: As expected, the 3.8 kW/lowspeed parameters produced the greatest hardening depth (1.88 μm maximum depth), as shown in

Table 3. Summary of selected parameter combinations.

Laser Power, kW	Scanning Speed, mm/s	Linear Energy, J/mm	Case Depth, mm	Surface Hardness (HV1)	Observed Surface Condition
3.8	12	317	~1.5	~720	Melting marks/ glazed surface/ surface oxidation.
3.8	20	190	~0.9	~650	Semi-melted/ traces of initial processing/ surface oxidation.
3.5	16	219	~1.2	~710	Partially melted/ traces of initial processing/ surface oxidation.
3.0	16	188	~0.8	~730	Slightly melted/ traces of initial processing/surface oxidation.
3.0	20	150	~0.6	~735	Slightly melted/ traces of initial processing/thin oxidation.

. However, the profiles suggested a potential “softening of the immediate surface skin” of 0.0–0.1 mm compared to the subsurface. This is likely caused by “retained austenite” resulting from the excessive heat input and slow cooling rates at the very surface of the melt pool.

The 3.0 kW profiles exhibited the steepest hardness gradients. The absence of excess heat resulted in a narrow heat affected zone (HAZ) and a sharp transition from the hard martensitic case to the soft base metal, indicative of a clean phase transformation.

3.4. X-Ray Diffraction Phase Analysis

To correlate the surface topography and mechanical properties with the microstructural change, X-ray diffraction (XRD) phase analysis was performed on the hardened samples. The investigation was conducted on two levels: comparing quenching environments and analyzing the phase gradient as a function of depth.

3.4.1. Influence of Quenching Environment

The initial analysis confirmed that there are no significant changes in phase composition between samples hardened in air versus those quenched in oil. This indicates that the dominant cooling mechanism is self-quenching by the cold substrate, and the surrounding medium (air/oil) primarily affects surface oxidation rather than the bulk phase formation.

All analyzed samples were fundamentally composed of a matrix of ferrite (and/or bainite and/or martensite), retained austenite, and iron oxides.

3.4.2. Phase Composition Gradient (Surface vs. Depth)

A detailed depth profile analysis was conducted at three distinct levels: a surface of 0 μm depth, 100 μm depth, and 200 μm depth (Figure 11, Figure 12 and Figure 13). This analysis provided critical insights into the nature of the surface layer and the consistency of the hardened zone.

Surface Layer (0\μm): The diffractograms exhibited the most complex phase composition. Strong peaks corresponding to iron oxides (Fe₃O₄, FeO) were detected.

Link to Roughness: This confirms that the increase in roughness S_a observed in the 3.0 kW regime is strictly a surface phenomenon caused by oxide scale formation.

Link to Hardness: The presence of retained austenite was also most pronounced at the immediate surface. This provides direct crystallographic evidence for the “soft skin” effect hypothesized in the hardness depth profiles (Section 3.2), particularly for the high-power samples where surface cooling is slowest.

Bulk Hardened Zone (100 μm): At a depth of 100 μm, the diffraction patterns showed a “stabilized metallic phase”. The intense peaks corresponding to iron oxides were completely absent, and the pattern was dominated by the “martensitic/bainitic matrix” with significantly reduced retained austenite compared to the surface.

This confirms that the oxide layer is superficial (likely < 20–30 µm). The structure at 100 µm represents the core of the heat-treated zone. The dominance of the hard martensitic phase here explains the high hardness values (HV1) measured in the previous section, confirming that the surface roughness or oxidation has no negative impact on the internal mechanical integrity at this depth.

Deep Hardened Zone (200 μm): At a depth of 200 μm, the phase composition remained consistent, consisting primarily of “martensite/bainite”.

The absence of oxides and the stability of the matrix at this depth confirm that the laser hardening process has produced a uniform microstructure throughout the case depth. The consistency between 100 µm and 200 µm in the experiment indicates a high-quality hardening zone with a consistent crystalline structure, free from the instabilities seen on the surface.

3.5. Comparative Summary

The correlation of roughness, hardness, and phase gradient data identifies four distinct process regimes:

➢

3800 W/Low Speed: Deep case depth and low average roughness S_a that are compromised by surface melting. The XRD confirmation of retained austenite at the surface (Figure 7) suggests potential subsurface weaknesses, despite the oxide-free glazed appearance. Recommended only if post-processing is used.

➢

3800 W/High Speed: Unacceptable due to high S_z (ripples) and decreasing hardness.

➢

3000 W/Low Speed: Moderate case depth with roughness driven by iron oxides. The depth profile analysis (Figure 10) confirms that this roughness is a superficial oxide layer. The underlying bulk (100–200 µm) is high-quality martensite. Requires post-machining to remove oxide scale.

➢

3000 W/High Speed: Optimal compromise. It provides high surface hardness (martensitic matrix), low roughness (minimal oxidation), and a steep hardness gradient. While the case depth is shallower, it offers the best surface integrity and fatigue resistance for applications where extreme depth is not the primary constraint.

3.6. Tribological Test Results

Three samples were selected for ball-on-flat tribological tests (described in Section 2.7). The coefficient of friction and wear resistance were determined for the selected samples. A summary of these results is provided in

Table 4. Selected sample tribological properties, coefficient of friction, sample wear rate, and counter body wear rate.

Sample	Coefficient of Friction	Sample Wear Rate, mm3/N/m	Counter Body Wear Rate, mm3/N/m
S-1—Reference	0.83	1.41 × 10−4	1.37 × 10−5
S-2—Air	0.81	8.90 × 10−5	6.10 × 10−6
S-3—Oil	0.78	9.24 × 10−5	3.18 × 10−6

, where it can be seen that the values of the coefficient of friction for all three samples do not differ significantly, but the wear resistance of the treated samples is significantly higher.

4. Discussion

The present study demonstrates that surface integrity during laser hardening of 42CrMo4 steel using a high-power diode laser is governed by a delicate balance between energy input, interaction time, and thermal gradients. The experimental results reveal that laser power and scanning speed not only control the depth of the hardened layer but also strongly influence surface roughness, hardness distribution, phase stability, and tribological response. These findings emphasize that optimization of laser hardening parameters cannot be based solely on achieving maximum hardness or case depth but must consider surface integrity as an integrated performance criterion.

4.1. Influence of Energy Density on Surface Topography

The observed evolution of surface roughness parameters S_a, S_z, and S_q reflects the transition between three distinct thermal regimes: solid-state transformation, surface oxidation, and localized surface melting. At lower laser power 3.0 kW, no evidence of surface melting was detected, and roughness evolution was primarily governed by oxidation kinetics. Prolonged thermal exposure at low scanning speeds promoted the formation of a thick oxide scale, resulting in increased roughness, whereas higher scanning speeds limited oxidation time and led to smoother surfaces.

In contrast, higher laser powers 3.5–3.8 kW introduced a melting-dominated regime at low scanning speeds. The reduction in average roughness S_a under these conditions is attributed to surface tension-driven flow within the molten layer, which partially smooths pre-existing machining marks. However, this apparent surface smoothing is accompanied by the formation of solidification-induced ripples at higher scanning speeds, as reflected by elevated S_z and S_q values. These high-amplitude surface features are associated with unstable melt pool dynamics driven by Marangoni convection and rapid solidification, and they may act as stress concentrators during service.

4.2. Relationship Between Surface Hardness and Hardened Layer Depth

Surface hardness measurements revealed that high laser power combined with low scanning speed produced the deepest hardened layers, but not necessarily the highest functional surface hardness. The slight reduction in hardness observed at the immediate surface for high-power conditions can be attributed to the formation of retained austenite resulting from excessive heat input and reduced cooling rates near the surface. This interpretation is supported by the XRD results, which showed an increased retained austenite fraction at the surface compared with subsurface regions.

Conversely, laser hardening at 3.0 kW resulted in a stable and consistently high surface hardness across the investigated speed range, despite producing a shallower hardened layer. The absence of melting and the steep hardness gradient observed in these samples indicate a rapid thermal cycle dominated by solid-state austenitization and self-quenching.

4.3. Phase Stability and Depth-Dependent

X-ray diffraction analysis confirmed that surface oxidation and phase heterogeneity were confined to the near-surface region, while the subsurface hardened zone was dominated by a stable martensitic/bainitic matrix. The presence of iron oxides and retained austenite at the immediate surface explains the roughness and localized softening observed under certain processing conditions. Importantly, the disappearance of oxide phases and the stabilization of the martensitic structure at depths beyond approximately 100 µm demonstrate that surface degradation phenomena do not compromise the integrity of the load-bearing hardened zone.

The consistency of phase composition between 100 µm and 200 µm depths further indicates that the laser hardening process produced a uniform hardened layer under optimized conditions. This depth-dependent stabilization is critical for gear applications, where subsurface stresses are often higher than surface stresses during rolling contact fatigue.

4.4. Tribological Implications and Industrial Relevance

The scratch testing results highlight the strong coupling between surface roughness, microstructure, and friction behavior. Samples exhibiting excessive roughness due to oxidation or solidification ripples showed increased friction coefficients and unstable scratch responses. In contrast, surfaces hardened under moderate energy input displayed stable friction behavior, reflecting the dominance of a homogeneous martensitic matrix and reduced surface defects.

From an industrial perspective, these findings indicate that maximizing laser power to achieve deeper hardened layers may be counterproductive if surface integrity is compromised. The optimal processing window identified in this study—characterized by moderate laser power and higher scanning speed—offers a favorable balance between surface hardness, roughness, and microstructural stability. Such conditions are particularly suitable for automotive gear applications, where surface fatigue resistance and tribological performance are as critical as hardened depth.

4.5. Implications for Process Optimization

The combined analysis of surface topography, hardness profiles, and phase composition underscores the necessity of integrated process optimization in laser surface hardening. While high-power diode lasers provide the capability for deep and rapid hardening, careful control of scanning speed and energy density is essential to avoid undesirable surface phenomena, such as melting-induced roughness and retained austenite formation. The results of this study provide quantitative guidance for selecting laser hardening parameters that enhance surface integrity without sacrificing mechanical performance.

5. Conclusions

This study systematically investigated the surface integrity evolution of 42CrMo4 steel subjected to laser surface hardening using a 4 kW high-power diode laser. Based on the experimental results and their analysis, the following conclusions can be drawn:

• Laser power and scanning speed strongly govern the thermal regime during laser hardening, determining whether the process is dominated by solid-state transformation, surface oxidation, or localized surface melting.
• High laser power combined with low scanning speed produces deeper hardened layers but promotes surface melting and increased retained austenite at the immediate surface, which can lead to localized surface softening despite increased case depth.
• Lower laser power with higher scanning speed enables stable solid-state laser hardening, resulting in a homogeneous martensitic surface layer, reduced surface roughness, and a steep hardness gradient without detrimental surface melting.
• X-ray diffraction analysis confirmed that oxidation and phase heterogeneity are confined to the near-surface region, while the subsurface hardened zone (≥100 µm) exhibits a stable martensitic/bainitic structure, ensuring mechanical integrity under load.
• Tribological performance is closely linked to surface integrity; surfaces exhibiting minimal oxidation and controlled roughness demonstrate more stable friction behavior and improved resistance to surface damage.

Based on the integrated analysis of topography, microstructure, hardness, and phase composition, an optimal processing window for balancing surface integrity and mechanical performance is identified for the used beam geometry: laser powers of 3.0–3.5 kW combined with scanning speeds of 16–20 mm/s (linear energy density ~150–220 J/mm). This window produces a hardened case depth of 0.8–1.2 mm, high surface hardness (>700 HV1) with minimal retained austenite, low surface roughness (Sa < 2 µm) dominated by a thin oxide layer rather than melting defects, and a stable martensitic microstructure. These findings provide quantitative, practical guidelines for the industrial application of diode laser hardening of 42CrMo4 steel gears, where surface quality and fatigue performance are as critical as hardened depth.