MICROSTRUCTURE OF TEMPERED AISI-403 STAINLESS-STEEL

The microstructure of AISI 403 martensitic stainless steel (AISI 403 M SS) tempered at various temperatures for various times was investigate d by transmission electron microscopy (TEM). The microstructure consis ted of tempered martensite laths together with carbides and a small am ount of delta-ferrite. At lower tempering temperatures of 480 and 540- degrees-C, two types of carbides, Cr23C6 and Cr7C3, were present. The majority of the Cr23C6 carbides were in the martensitic matrix and a c hain of Cr23C6, carbide was seen at and in the vicinity of grain bound aries. Upon tempering at 600 or 660-degrees-C, Cr23C6 became the predo minant carbide and was mainly distributed at the boundaries of the mar tensitic laths. The results showed that secondary hardening in AISI 40 3 MSS. produced by tempering at lower temperatures, especially 480-deg rees-C. resulted from Cr23C6 carbides heterogeneously precipitated in the martensitic matrix. For specimens tempered at 600-degrees-C and ab ove, severe sensitization and loss of corrosion resistance of the stee l occurred owing to large Cr23C6 precipitates which were concentrated at the grain boundaries, forming a zone of reduced chromium content ad jacent to the carbides in the grain boundaries.