Characterization of reverse martensitic transformation in cold-rolled austenitic 316 stainless steel

In this work the reversion of the strain-induced alpha'-martensite to gamma'-austenite was studied in a series of AISI 316 stainless steels (SS) specimens. The samples were submitted to 63% reduction in thickness by rolling at a temperature of -70 degrees C, to achieve high martensite content. The reversion of martensite to austenite was made by means of isochronic, isothermal heat treatments at temperatures between 200 and 900 degrees C, so as to induce partial martensite-austenite phase transformation. The samples were studied by optical and electronic microscopy, X-ray diffraction, magnetization measurements, microhardness, and electromagnetic non destructive methods: feritscope, conductivity measurements using Van der Pauw's technique, and magnetic permeability assessments by an eddy current inverse method. The metalography study showed an alpha' lath martensite type structure and a similar one for the reverted austenite (gamma'). Magnetization measurements and magnetic permeability showed that the reversion is active between 400 and 800 degrees C. From the magnetic saturation measurements, a new calibration curve for the assessment of %alpha' through the feritscope readings was obtained. The results of the different techniques were compared, in order to assess the scope of the studied techniques in the characterization of rolled SS products, and in the evaluation of progress of reversion reaction.