# Graded Brittle–Ductile Transition via Laser-Induced Thermal Gradient for Broaching of Z10C13 Steel

## TL;DR

A laser technique is used to modify the surface of Z10C13 steel, enabling efficient and precise broaching with reduced cutting force and improved surface quality.

## Contribution

The novel use of laser defocus gradient modification to induce surface embrittlement in martensitic stainless steel for low-speed precision machining.

## Key findings

- A 50 μm gradient remelting layer with ultrafine grains and high GND density was created.
- Surface hardness increased by 12.95%, cutting force decreased by 34.07%, and surface roughness improved by 63.74%.
- A crack-deflection mechanism prevents subsurface damage propagation, creating a crack-free zone.

## Abstract

This paper presents a breakthrough in activating the skin effect at conventional broaching speeds (1–8 m/min) by using laser defocus gradient modification to induce surface embrittlement in martensitic stainless steel Z10C13. Through controlled defocusing, a 50 μm gradient remelting layer was created, which features ultrafine grains (0.8 μm) and a high-density geometrically necessary dislocation (GND) zone (ρGND = 2.27 μm−3). The quasi-cleavage fracture was triggered via dislocation pinning by non-oriented low-angle grain boundaries (28.4% LAGBs). Multiscale characterization confirms that this microstructural transformation enhances surface hardness by 12.95% (reaching 31.4 HRC), reduces cutting force by 34.07%, and improves surface roughness by 63.74% (Sz = 28.80 μm). Simultaneously, a parallel crack-deflection mechanism restricts subsurface damage propagation, resulting in a crack-free subsurface zone. These results demonstrate the effectiveness of the embrittlement–toughening dichotomy for precision machining of difficult-to-cut materials under low-speed constraints.