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SUPERMARTENSITIC stainless steels (SMSS) have been developed from soft-martensitic stainless steels, containing a much lower level of the interstitials C and N[1] and possessing an outstanding combination of properties, i.e., high strength, good toughness, good corrosion resistance, and reasonable weldability.[1–4] Due to the increasing need for a more cost-effective use of materials, they have been further developed in the past 20 years and are increasingly being applied in the offshore oil and gas industry to replace highly alloyed alternatives.[2,3] The properties of SMSS, particularly the yield stress, are strongly dependent on the fraction of retained austenite.[5] This can be controlled by the heat treatment, which consists of an austenitization treatment followed by tempering, usually double tempering, just above the Ac1 temperature. The resulting microstructure after double tempering consists of A. BOJACK, Ph.D Researcher, is with the Department of Materials Science and Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands, and also with the Materials innovation institute (M2i), Mekelweg 2, 2628 CD Delft, The Netherlands. L. ZHAO, Research Fellow, formerly with the Materials innovation institute (M2i), and with the Department of Materials Science and Engineering, Delft University of Technology, is now with VDL Weweler B.V., Ecofactorij 10, 7325 WC Apeldoorn, The Netherlands. P.F. MORRIS, was formerly Research Fellow at Tata Steel, Swinden Technology Centre, Moorgate, Rotherham, South Yorkshire S60 3AR, U.K. J. SIETSMA, Professor, is with the Department of Materials Science and Engineering, Delft University of Technology. Contact e-mail: [email protected] Manuscript submitted January 19, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A

low-carbon martensite and finely dispersed austenite that is retained at room temperature.[5,6] It is well known that the stability of austenite depends on the concentrations of austenite-stabilizing elements in the phase,[5–8] which are mainly Ni[9–12] and Mn[13] in SMSS. This is critically dependent on the microstructural evolution during the annealing steps. It was found from austenitization experiments with a 13Cr6Ni2Mo SMSS, presented in a previous study,[14] that the formation of austenite from martensite takes place in two stages during heating. However, these experiments have been carried out with as-received, double-tempered material, containing about 21 vol pct retained austenite. It was argued that the reason for the two-stage transformation could be the partitioning of Ni to austenite, where the martensite with a lower Ni concentration has a higher Ac1 temperature and therefore transforms to austenite at a higher temperature. These two stages were found by dilatometry to be distinct, even though most of the austenite-stabilizing elements that would have caused this effect are already dissolved in austenite. As a consequence, the differences in Ni concentration were considered to be the most likely cause of the two-stage austen