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Fatigue crack growth (FCG) behaviors of weld metal (WM) in turbine part and WM in compressor part of a gas turbine rotor after 14 years of service were investigated. Phosphorus (P) segregation at prior austenite grain (PAG) boundaries occurred in the turbine part WM but not in the compressor part WM, which resulted in local intergranular fracture and high da/dN in Paris regime of the turbine part WM. However, P segregation at PAG boundaries had little effect on near-threshold FCG and Δ K th because the cyclic plastic zone sizes were much smaller than PAG sizes and cracks mainly grew inside PAGs.

The two driving forces (TDF) model was modified based on the dislocation dipoles mechanism of fatigue crack growth (FCG) and corrected by K max effect for a better prediction of the R dependent FCG thresholds Δ K th and K max , t h . This model can well describe transformation of the crack growth mechanism from fatigue fracture to quasi-static fracture when R tends to 1, and agrees well with experimental data for a broad round of alloy systems. An empirical relationship between the fatigue threshold and the fracture toughness was proposed, and then an approximately material-independent fatigue threshold prediction model was established by normalization based on this relationship.

The near-threshold fatigue crack growth (FCG) behavior under different stress ratios (R) of a 9%–12% Cr steel welded joint was studied in 538 °C air. The most significant oxide-induced crack closure (OICC) occurred in weld metal (WM) due to the lowest Cr content, resulting in the highest threshold value (Δ K th ) of WM. The base metal (BM) with coarser grains and denser lath structures had more obvious roughness-induced crack closure (RICC) and more severe crack deflection, which led to a higher Δ K th of BM than heat affected zone (HAZ). The Δ K th of WM, HAZ and BM decreased as R increased.