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

## TL;DR

This study examines how laser hardening affects the surface properties of 42CrMo4 steel, identifying optimal laser parameters for industrial automotive applications.

## Contribution

The paper identifies an optimal laser processing window for balancing surface hardness, roughness, and microstructural stability in 42CrMo4 steel.

## Key findings

- High laser power and low scanning speed produce deep hardened layers but increase surface melting and retained austenite.
- Lower power and higher scanning speed yield stable martensitic surfaces with reduced roughness and a steep hardness gradient.
- XRD analysis shows oxide formation is limited to the surface, while the subsurface is dominated by martensitic/bainitic phases.

## 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.