Martensite formation during heat treatments of AISI 304 steel with biphasic structure

X-ray diffraction (XRD) and Mössbauer spectroscopy (MS) measurements were used to evaluate the content of α′ ferromagnetic phase in AISI 304 steel cold rolled and then heat treated for increasing times (up to 7.2×104 s) at 673 K. Both analytical techniques show the same trend of α′ content versus annealing time, characterized by two maxima. The first maximum has been attributed to the growth of preexisting martensitic zones due a different microstress relaxation in the two phases, which induces a stress gradient through the α′/γ interface forcing it to move. The second maximum was explained assuming that local composition variations were induced by carbide precipitation and that a new martensitic phase may form during cooling to room temperature. MS data, i.e. the increase of average hyperfine field of magnetic fraction and the constancy of austenite isomer shift, and the analysis of precision XRD peak profiles support this assumption. Furthermore, MS measurements evidenced in cold rolled samples the presence of layers with decreasing martensite content towards the inside.