In Situ Thermo-magnetic Investigation of the Austenitic Phase During Tempering of a 13Cr6Ni2Mo Supermartensitic Stainles
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INTRODUCTION
SUPERMARTENSITIC stainless steels (SMSS) are increasingly being applied in the offshore oil and gas industry, since they offer an outstanding combination of high strength, good toughness, good corrosion resistance, and reasonable weldability.[1–4] Due to their lower cost, they are, in some applications, a suitable alternative to the more expensive duplex stainless steels.[1,2] The microstructure, consisting of tempered martensite, finely dispersed retained austenite and a small fraction of carbides, nitrides, or carbo-nitrides, is obtained by an austenitization and single or double tempering treatment. The mechanical properties of SMSS, particularly the yield stress, are strongly dependent on the fraction of retained
A. BOJACK, PhD Researcher, is with the Materials Innovation Institute (M2i), Mekelweg 2, 2628 CD, Delft, The Netherlands, and also with the Department of Materials Science and Engineering, Delft University of Technology, Mekelweg 2, 2628 CD, Delft, The Netherlands. L. ZHAO, Research Fellow formerly with the Materials Innovation Institute (M2i), also 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, formerly Research Fellow with the Tata Steel Europe, 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 13, 2014. Article published online September 16, 2014 5956—VOLUME 45A, DECEMBER 2014
austenite,[5] which can be controlled by the heat treatment. The austenitization is carried out above Ac3, after which air cooling might be sufficient for SMSS to obtain an almost fully martensitic microstructure[2] of highly dislocated laths.[6] Due to the low C-content the morphology of martensite is lath- or plate-like with a bcc crystal structure.[7] The subsequent first tempering step is carried out above Ac1 to form austenite that should be stable during cooling to room temperature. If Ac1 is exceeded by too much, fresh martensite will form during cooling, having a detrimental effect on the ductility.[5,8] The reason for the fresh martensite formation lies in the increase of the volume fraction of austenite, having lower concentrations of austenite-stabilizing elements, which makes these areas less stable against transformation during cooling. During a second tempering step fresh martensite will be tempered and partly transformed to austenite that is stable during cooling to room temperature,[5,9–11] provided the tempering temperature is again above the Ac1 of the fresh martensite. The stabilization of austenite at room temperature is obtained by austenite-stabilizing elements like Ni, Mn, and C, that enrich austenite.[5,10–12] Since the C-content in SMSS is very small ( 753 K (480 C) it increases until the final tempering temperature. The minimum in austenite fraction
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