Creep Strength and Microstructure in Welded Joints of ASME Gr.91 Type 1 and Type 2 Steels

High-chromium (Cr) ferritic heat-resistant steels, such as ASME Gr.91 steel are used for steam pipes in thermal power plants. ASME (2019) divided Gr.91 steel into two standards: conventional Type 1 and newly-established Type 2. In this study, creep strength and microstructure in welded joints of conventional Gr. 91 Type 1 steel and new standard Type 2 steel were compared. Type 2 steel is a heat-resistant steel with reduced tramp elements to improve creep ductility. Two steels were prepared for creep test: one was the tramp element free steel satisfying Type 2 and the other was tramp element added steel doing Type 1. The effects of these tramp elements on creep strength and microstructure were investigated for welded joints. Creep tests conducted at a temperature of 650°C and stresses of 40~80 MPa (maximum rupture time of 18,620 h) showed that Type 2 steel exhibited superior creep strength compared to Type 1 steel. The detailed microstructural observations in the creep interruption tests showed that the creep deformation, microstructural changes, and creep damage evolution in each steel were similar, differing only in their time-dependence. In other words, Type 2 steel showed better thermal stability of microstructure than Type 1 steel, and was less susceptible to creep strength deterioration. The thermal stability of the microstructure was related to the number density of carbides on the grain boundaries. Detailed microstructural analysis revealed that the number density of the carbides in HAZ was affected by the tramp element concentration in the steel.