M23C6 precipitation and Si segregation promoted by deep cryogenic treatment aggravating pitting corrosion of supermartensitic stainless steel

The microstructure evolution and the pitting corrosion resistance of a supermartensitic stainless steel after deep cryogenic treatment process were clarified through X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy (TEM) and electrochemical methods. The results showed that the microstructure of supermartensitic stainless steel mainly consisted of reversed austenite, tempered martensite, and M23C6 carbides after tempering. The deep cryogenic treatment promoted the refinement of the martensite laths and the precipitation of the carbides in comparison with the traditional process. TEM analysis indicated that the segregation of Si atoms at the boundary was found at the interface between carbide and martensite. The pitting corrosion potential of the specimens subjected to deep cryogenic treatment decreased with the elevated tempering temperature, and the lowest pitting corrosion potential was found at the tempering temperature of 650 degrees C. The sensitivity of the pitting corrosion potential was attributed to the precipitation of M23C6 carbides and Si atoms segregation. Si atoms segregation engendered the formation of Cr-depleted zone near M23C6 and impeded the recovery of Cr-depleted zone.

Automated summary

The microstructure evolution and pitting corrosion resistance of a supermartensitic stainless steel after deep cryogenic treatment were studied. The deep cryogenic treatment refined the martensite laths, promoted carbide precipitation, and caused Si atom segregation at the carbide-martensite interface. With increasing tempering temperature, the pitting corrosion potential decreased, with the lowest value observed at 650 degrees C. This sensitivity was attributed to the precipitation of M23C6 carbides and Si atom segregation.