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.

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.

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