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

X-ray diffraction (XRD) and Mossbauer spectroscopy (MS) measurements were u sed to evaluate the content of alpha' ferromagnetic phase in AISI 304 steel cold rolled and then heat treated for increasing times (up to 7.2 x 10(4) s) at 673 K. Both analytical techniques show the same trend of alpha' conte nt versus annealing time, characterized by two maxima. The first maximum ha s been attributed to the growth of preexisting martensitic zones due a diff erent microstress relaxation in the two phases, which induces a stress grad ient through the alpha'/gamma interface forcing it to move. The second maxi mum was explained assuming that local composition variations were induced b y carbide precipitation and that a new martensitic phase may form during co oling 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. Furth ermore, MS measurements evidenced in cold rolled samples the presence of la yers with decreasing martensite content towards the inside. (C) 1999 Elsevi er Science S.A. All rights reserved.