Thermal cycling induced stress–assisted sigma phase formation in super duplex stainless steel

Abstract

Thermal cycling induced the phase transformation originated from the physical simulation of power plant on/off operation. The effect of thermal cycling on duplex stainless steel was studied to understand its phase transformation comprehensively. Cyclic thermal processing is a nonisothermal heat treatment method known to accelerate the kinetics of phase transformation. In this study, when the number of thermal cycles increased, the ferrite grains gradually contracted from the grain boundaries and finally disappeared due to the eutectoid reaction. However, the austenite phase demonstrated a parabolic increase and the intermetallic sigma (σ) phase, formed at the grain boundaries, exhibited a similar trend. The KAM maps reveal the highest degree of misorientation (up to 2°) may have either been at the γ′ or σ phase at 40 and 80 cycles. In DSS, the misorientations (>15°) among grains also affect σ formation. A high crystallographic misorientation between the austenite and ferrite phases favored σ precipitation. The calculated thermal stress of 422 MPa assisted the σ formation by reducing the activation energy and accelerating the alloying element diffusion. The σ phase formation caused by thermal cycling increased the yield strength and ultimate strength but degraded ductility. The formation of tensile cracks was related to the σ phase at the interface or the σ matrix. Keywords: Super duplex stainless steel, EBSD, Thermal cycling, σ phase, Thermal stress