Hyper Duplex Stainless Steel의 상변태 및 기계적 물성에 미치는 합금원소의 영향

Abstract

Duplex stainless steels consisting of ferrite and austenite represent excellent mechanical properties and superior corrosion resistance, and accordingly they are widely used as the structural components for chemical and nuclear power industries. Recently, to meet the need for higher performance (enhanced life in more severe corrosive environment) of materials, hyper duplex stainless steels with PREN value of over 50 is being developed. However, hyper duplex stainless steels often reveal undesired microstructure after heat treatment at certain temperature range and holding time, resulting in the degradation of excellent properties. One of the main reasons is known to be due to the precipitation of sigma phase. Therefore, it is important to control the amount of sigma phase during heating, holding and cooling.In this work, by the addition of various alloy elements, W, Ce, Cu, microstructure and mechanical properties of hyper duplex stainless steel were investigated with the special emphasis on the mechanical properties variation of in relation to precipitation of sigma phase and chi phase. Based on TTT diagram of secondary phases and the results of mechanical tests, optimum alloy compositions were suggested. Important results obtained in this study is summarized as follows.Toughness was drastically reduced by the precipitation of secondary phases, sigma phase and chi phase. Increasing the amount of the precipitation of the precipitation of sigma phase was significantly suppressed, slowing down the toughness reduction of the alloy. However with the increase of the chi phase, the toughness was not noticeably changed.The tensile properties were also changed by the precipitation of secondary phases. The yield strength was increased and the total elongation was reduced with the increase in the amounts of secondary phases. It was observed that the sigma phase played more important role in influencing the tensile properties because cracks were formed to follow along the sigma phase rather than the chi phase. The ultimate tensile strength was similar between the before and after heat treatment due to the change of hardening rate.This study clarified the role of W, Ce and Cu in the precipitation of secondary phases. The lowering of Mo and addition of W promoted the precipitation of the chi phase and effectively retarded the precipitation of sigma phase. The addition of Ce also retarded the precipitation of sigma phase, but not the precipitation of chi phase. The addition of both Ce and Cu effectively retarded the precipitation of the sigma phase, including the chi phase.