The aim of this study was to investigate cracking behavior of AISI 304 stainless steel that had been exposed to a high temperature MgCl₂ solution for several years. The microstructure of the cracked area of the reactor was studied by in-depth microstructural characterization. Transgranular stress corrosion cracking only occurred at the cold-formed part of the reactor. It was observed that approximately 10–20% of the austenite matrix was transformed into alpha prime martensite due to cold forming at the lower head of the reactor. The preferential path for crack propagation was found to be strain-induced alpha prime martensite. The present study reveals that strain-induced martensitic transformation in austenitic stainless steel has a negative effect on stress corrosion cracking.

Die-casting molds undergo wear, erosion, and repeated thermal loading, which progressively degrade their surfaces and reduce their service life. Compared with conventional gas nitriding, sulfonitriding can improve wear resistance; however, its effects on other reliability-related properties, particularly erosion and thermal-shock fatigue, and the relationships between these properties and the compound-layer characteristics remain insufficiently understood. In this study, duplex surface treatments combining sulfonitriding with three microshot peening conditions, resulting in surface-roughness values of Rz 14, 18, and 22 μm, were applied to SKD61-based (FDAC) die-casting mold steel. The duplex-treated layers were evaluated based on their microstructure, hardness, wear resistance, erosion resistance, and thermal-shock-fatigue resistance, followed by a nondestructive assessment using Barkhausen noise. Sulfonitriding produced FeS-containing compound layers and reduced the wear of the counterbody and the tribo-pair compared with the results for gas nitriding, which is consistent with the solid-lubrication effect of FeS. The erosion resistance increased with the compound-layer thickness. The thermal-shock-fatigue tests indicated a preliminary trade-off between crack initiation and crack propagation as a function of the compound-layer thickness. These results demonstrate that sulfonitriding modifies the surface performance of SKD61 and that the compound-layer thickness is a key parameter governing mold reliability. Moreover, Barkhausen-noise analysis offers a nondestructive assessment of the compound-layer thickness after treatment.