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INTRODUCTION

DUPLEX stainless steels deliver an excellent combination of mechanical and corrosion properties, making them an attractive alternative to austenitic and super austenitic stainless steels.[1] However, phase separations occur relatively rapidly below 500 C, resulting in what is referred to as 475 C-embrittlement, which restricts their application at temperatures between 250 C and 500 C.[2,3] Spinodal decomposition of d-ferrite to Fe-rich (a) and Cr-rich (a¢) ferrite and the precipitation of a¢ (by nucleation and growth), G-phase, R-phase, v-phase, nitrides, and Cu-clusters have been reported in this temperature range, which contribute to the embrittlement.[3,4] Super duplex stainless steels (SDSSs) have high contents of Cr, Mo, and Ni and are prone to this type of embrittlement which is why kinetics of phase

VAHID A. HOSSEINI is with the Department of Engineering Science, University West, 461 86 Trollha¨ttan, Sweden and also with Innovatum AB., Trollha¨ttan, 461 29 Trollha¨ttan, Sweden. Contact e-mail: [email protected] MATTIAS THUVANDER is with the Department of Physics, Chalmers University of Technology, 412 96 Gothenburg, Sweden. STEN WESSMAN and LEIF KARLSSON are with the Department of Engineering Science, University West. Manuscript submitted November 14, 2017.

METALLURGICAL AND MATERIALS TRANSACTIONS A

transformations play a crucial role in processing and applications of these steels.[5] A considerable variation of data reported in the literature and industrial datasheets points to a knowledge gap about the kinetics of 475 C-embrittlement in SDSSs. Hilders et al. found that 9 hours heat treatment of 2507 SDSS at 475 C did not decrease the impact toughness while it was significantly reduced after 72 hours.[6] Gutie´rrez-Vargas et al. observed only minor changes in microhardness after 1-h heat treatment of 2507 SDSS at 475 C.[7] An industry datasheet for 2507 SDSS,[5] in contrast, introduced a time-temperature transformation (TTT) diagram indicating that only 3 minutes aging at 475 C will result in a 50 pct toughness drop in 2507 SDSS. Several indirect and direct methods can be used to detect 475 C-embrittlement. Indirect methods monitor the changes in properties caused by the phase separations, which include hardness testing,[8] impact toughness testing,[9] magnetic ferrite measurement,[10] electrochemical tests,[10] alternating current potential drop,[7] electrical resistance measurement,[11] scanning Kelvin probe force microscopy and magnetic force small-angle neutron scattering microscopy,[12] (SANS),[13] etc. Direct methods, on the other hand, detect the phase separations by analyzing the microstructure using scanning and transmission electron microscopy[14] and atom probe tomography (ATP).[15]

Especially, APT builds a 3D reconstruction of the atomic configuration and provides high spatial resolution allowing to measure nanoscale compositional fluctuations.[16] However, the direct methods are generally more time-consuming, less easily accessible and expensive compared to the indir