Magnetic properties and microstructural characterization of cold-rolled and annealed 317L austenitic stainless steel

The annealing behavior of cold-rolled 317L austenitic stainless steel was investigated. The material was rolled to a true strain (epsilon) of 2.04 and subjected to both stepwise and continuous annealing up to 1000 degrees C, the latter conducted in the presence of an external magnetic field. Electron backscatter diffraction, dilatometry, thermo-dynamic calculations, Vickers microhardness testing, and electron channeling contrast imaging were used to follow the microstructure evolution upon annealing. The microstructure of the cold-rolled steel has about 2.3% of strain-induced alpha'-martensite, 4% of delta ferrite, and austenite as the predominant phase. Eye-shaped defor-mation heterogeneities are also noticeable and contrast with the predominant lamellar structure typical of cold-rolled materials. Microstructure changes were followed by means of magnetic measurements, with emphasis on both Ms (saturation magnetization) and Hc (coercive field) parameters. Our findings confirm the occurrence of austenite reversion, decrease of delta ferrite, and massive sigma phase precipitation for the annealing temper-ature and time intervals herein investigated. At 800 degrees C the steel is almost fully recrystallized, except for the eye-shaped structures, in whose interior precipitation is much less intense. Precipitation of sigma phase occurs preferentially at the delta ferrite lamellae. Magnetization was able to capture the fragmentation of the ferro-magnetic delta ferrite lamellae due to sigma phase precipitation and the changes associated with the decrease of delta ferrite and austenite reversion upon annealing.