Microstructural Evolution and Its Influence on Low-Temperature Toughness of Simulated Heat-Affected Zone in Super Duplex Stainless Steel Welded Joint

The microstructures of heat-affected zone (HAZ) during multipass welding procedure of super duplex stainless steel (DSS) are simulated by a thermomechanical simulator. The influences of ferritization, 1st-reheating, and 2nd-reheating process on the microstructural evolution and toughness of the HAZ are studied. The results show that the alternating bands of ferrite and austenite are transformed into coarse equiaxed ferrite and different types of primary austenite (gamma 1). The austenite content significantly decreases and a great number of Cr2N precipitate in the ferritized HAZ comparing with as-received super DSS. In addition, the 1st-reheating temperature has significant influence on microstructure of the ferritized HAZ. The austenite content increases but the precipitation tendency of Cr2N reduces with the raising of 1st-reheating temperature. Secondary austenite (gamma 2) is prone to precipitate after the 1st-reheating at 1000 degrees C. A combined precipitation behavior of gamma 2 and Cr2N is clarified during the reheating process. Furthermore, the 2nd-reheating at 900 degrees C promotes Cr2N precipitation again as well as austenite formation. The toughness is mainly related with ductile austenite content, hard-brittle Cr2N amount, and N supersaturation degree in the ferrite. In order to raise toughness of the ferritized HAZ, it is recommended to reheat at above 1200 degrees C. This article shows that welding thermal cycle has obvious effect on microstructure and toughness of super duplex stainless steel heat-affected zone (HAZ). The precipitation behavior of gamma 2 and Cr2N at different reheating temperatures and the combined precipitation mechanism are also investigated. It is finally concluded that reheating at above 1200 degrees C is recommended to improve the toughness of HAZ.image (c) 2023 WILEY-VCH GmbH