Large delta T thermal cycling induced stress accelerates equilibrium and transformation in super DSS

Abstract

Based on the predicted phase diagram of super duplex stainless steel (DSS) calculated by Thermo-Calc, the maximum peak temperature 1100 °C was selected to ensure no σ phase existence. This target temperature fell into the two-phase solid solution (SS) region. A series of different thermal cycling tests were carried out with the notations of 2SS, 2SS + 3 cycles, 2SS + 7 cycles, 2SS + 13 cycles, and 2SS + 20 cycles. It was found that the trend of two-phase volume ratio variation by thermal cycling followed the predicted thermodynamic equilibrium trend. After 2SS + 7 cycles, the ratio of two-phase δ/γ tended toward the ideal 1:1. According to the electron backscatter diffraction (EBSD) analysis, the δ phase crystal orientation changed from the most frequent directions of <001> and <111> of the as-received sample to the most frequent orientation of <113> after two SS treatments. While the γ phase grain always remained at <101> orientation. The grain boundary misorientation angles of the γ grains were relatively stable, ranging from 53° to 63°, but those of the δ grains were widely distributed actively presuming the lattice rotation. The Kernel Average Misorientation (KAM) value of the local strain in face center cubic (fcc) γ grains was varied and greater than that of the body center cubic (bcc) δ phase, indicating that the former, with a large grain boundary misorientation had larger local deformation than the latter, which possesses wide random misorientation angle distribution.