Thermally induced pores (TIPs) are generally the source of fatigue crack initiation in the powder metallurgy (PM) Ni-based FGH96 superalloy. The effect of TIPs on fatigue crack initiation on the surface of the FGH96 superalloy was detected using scanning electron microscopy (SEM). The cause of fatigue crack deflection was studied using electron backscatter diffraction (EBSD) analysis. The results indicated that there are two states of TIPs including isolated TIPs and clustered TIPs located at the grain boundary. The investigation of crack initiation and propagation around TIPs was conducted in detail through the comprehensive integration of experimental findings and computational results. For cracks initiated by isolated TIPs, the maximum equivalent size and the ratio of the vertical–parallel axis to the loading direction of the TIPs reveal a linear relationship, and both of them determine crack initiation. Regarding clustered TIPs, the constituent pores of the clustered TIPs will compete to initiate cracks based on the experimental results, and the largest pore will be more likely to initiate cracking. Moreover, the results showed that fatigue crack propagation can be hindered by hard-orientation grains and twins with a low Schmid factor (SF). Large-angle crack deflection due to twins with a low SF can significantly increase crack length and resistance to crack propagation.

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.

P91 steel and P91 steel joints experience performance degradation after serving for 30,000 h in working conditions. To clarify the damage and failure behavior and remaining life of the joints during subsequent service, further creep testing was conducted on the welded joints of P91 steel that had been in service for 30,000 h at three temperatures: 550 °C, 575 °C, and 600 °C. The fracture surface and the cross-section damage behavior were characterized by SEM and EBSD methods. The results show that there are two types of fracture modes in the joints at different temperatures: ductile cracking occurring at the BM, and type IV cracking occurring in the FGHAZ. The threshold stress for fracture mode transition decreases with an increase in working temperature. Type IV cracking near the HAZ is the main reason for the premature failure of joints during service. And based on the fracture mode, the dual-constant L-M method was proposed to predict the strength of in-service joint materials. The testing data are discussed and classified based on the fracture mode in this method, which has high accuracy and can prevent the premature failure of joints.