Study on the microstructure and creep fracture behavior of 22Cr 15Ni steel welded joint after aging at 650 °C

It has been carefully investigated that microstructure and creep fracture behavior of Fe–22Cr–15Ni (SP2215) steel welded joints on aging at 650 °C. The extrapolated stresses for welded joint after aging for 0 h (Aging-0h WJ) and 2012 h (Aging-2012h WJ) at 105 h are 105.2 and 91.7 MPa, respectively. The fracture location for Aging-0h WJ and Aging-2012h WJ was respectively heat affected zone (HAZ) and weld metal (WM). HAZ grain size of Aging-0h WJ was 53–88 μm because of thermal cycling. After 2012h aging, HAZ grain size did not change, but grain boundaries were roughened and M23C6 has been found on grain boundaries. Welding and aging treatment did not cause element migration near the fusion line. However, the decrease in Nb elements on the weld side resulted in a decrease in the amount of NbC, which delayed creep damage. The 2012h aging had no significant effect on the hardness difference between HAZ and WM near fusion line, indicating that the transfer of creep fracture position was caused by microstructure changes. The cluster-like NbC formed in WM of Aging-2012h WJ, which not only reduced Nb solid strengthening effect, but also caused local strain concentration. Moreover, in WM of Aging-2012h WJ, the orientation and hardness differences between large-sized (Nb, Ti) C and matrix, and M23C6 on the grain boundary jointly reduced the WM creep performance.