Microstructure and Mechanical Characteristics of Dissimilar TIG Welded 9% Cr Heat-Resistant Steels Joints

Abstract

Dissimilar 9% Cr heat-resistant steels (G115 and CB2) with good creep properties for ultra-supercritical steam turbines were butt-joined by tungsten inert gas welding. The microstructure of welded metal (WM) was quenched martensite without carbide precipitates and lath packets existed inside prior austenite grains (PAGs), which leaded to higher hardness of WM. Partially melted zone at G115 side was composed of untempered martensite within equiaxed PAGs. The lowest hardness occurred in both G115 and CB2 steels which was attributed to tempered martensite with many M23C6 precipitates. The heat-affected zone consisted of three sub-grains and their microstructure was detailly analyzed in current work. As current increased from 130 to 150 A, both the tensile strength at room temperature and 650 ℃ increased while strength had no obvious change with further increasing current. The values of 673 MPa and 309 MPa corresponded to the tensile stress with 150 A at room temperature and 650 ℃, respectively. The fracture mode of joints at room temperature was cleavage and ductile failure at 130 and 150 A, respectively. The high-temperature fracture surface at 150 A was composed of deep and fine dimples.