Recrystallized grains characteristics after cold rolled and the effect of σ phase precipitation by thermal cycling

• Main Authors: Li, Jian-Sin, 李建欣
• Other Authors: Wang, Shing-Hoa
• Format: Others
• Language: en_US
• Published: 2019
• Online Access: http://ndltd.ncl.edu.tw/handle/ta7bdt

碩士 === 國立臺灣海洋大學 === 機械與機電工程學系 === 107 === After cold-rolled (CR) deformation, super duplex stainless steel (DSS) was annealed at 1100 °C for a two-phase solid–solution region. Preferred orientations in δ and γ phases changed to (111)δ, (100)δ, and (101)γ from the rough random orientation of as-received super DSS. The kernel average misorientation (KAM) angle was less than 5° implying the grain boundary strain shifted to a higher value after cold rolling, which was consistent with the X-ray diffraction (XRD) microstrain results. After annealing, the misoriented angle was restored to a low angle of approximately 0.5°, which implied the elimination of grain boundary strain when the annealing time increased from 15 to 30 min. The calculated microstrain based on XRD peaks demonstrated that the γ phase deformed before the δ phase at 50% CR. The recrystallized δ grains with slow growth and high staking fault energy (SFE) mainly showed recovery and a wide distribution of high angle grain boundary (HAGB) in the range between 15°–58°, whereas the recrystallized γ grains with low SFE exhibited rapid recrystallization; the majority of HAGB angles were greater than 50°, because of depletion of the stored strain energy. This recrystallized austenite adhered to the Kurdjumov–Sachs relationship with ferrite. 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 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.