정밀금형용 SKD61 강의 복합표면처리에 의한 표면신뢰성 향상

Purpose: The lifespan of a die-casting steel mold depends on its surface hardness and wear resistance. Gas nitriding treatments for forming surface hardening layers have been widely applied to fabricate reliable die-casting molds. In the present study, we propose a complex surface treatment consisting of a micro-shot peening process and a sulfo-nitriding treatment to improve the surface hardness of SKD61 mold steel.
Methods: The cross-sectional microstructure of the complex surface-treated SKD61 mold steel was observed using an optical microscope and a scanning electron microscope. The phase of the surface hardening layer was analyzed using an X-ray diffractometer. The micro-hardness and residual stress of the complex surface-treated SKD61 mold steel were evaluated as well.
Results: The surface hardening layer consisted of the top-most compound layer and a diffusion layer. The micro-shot peening pretreatment increased the thickness of the compound layer. The general nitriding (GN) treatment led to the formation of the FexN compound layer, while the sulfo-nitriding (SN) treatment led to the formation of the FexN and FeS composite compound layer, which was thicker than the compound layer of the GN specimen. The hardness of GN and SN hardening layers improved by 125% and 135%, respectively, relative to that of the as-received SKD61 mold steel with a hardness of 450 HV.
Conclusion: The SN treatment formed a reliable hardening layer with superior hardness than the GN treatment. Furthermore, the thicker sulfo-nitride compound layer produced by the SN treatment can further improve the lifespan of SKD61 steel molds.