Microstructure of tempered AISI 403 stainless steel

Abstract

The microstructure of AISI 403 martensitic stainless steel (AISI 403 MSS) tempered at various temperatures for various times was investigated by transmission electron microscopy (TEM). The microstructure consisted of tempered martensite laths together with carbides and a small amount of σ-ferrite. At lower tempering temperatures of 480 and 540 °C, two types of carbides, Cr23C6 and Cr6C3, were present. The majority of the Cr23C6 carbides were in the martensitic matrix and a chain of Cr23C6 carbide was seen at and in the vicinity of grain boundaries. Upon tempering at 600 or 660 °C, Cr23C6 became the predominant carbide and was mainly distributed at the boundaries of the martensitic laths. The results showed that secondary hardening in AISI 403 MSS, produced by tempering at lower temperatures, especially 480 °C, resulted from Cr23C6 carbides heterogeneously precipitated in the martensicic matrix. For specimens tempered at 600 °C and above, severe sensitization and loss of corrosion resistance of the steel occurred owing to large Cr23C6 precipitates which were concentrated at the grain boundaries, forming a zone of reduced chromium content adjacent to the carbides in the grain boundaries. © 1993.