Near-threshold Fatigue Crack Growth Behavior In Welded Joint Of NiCrMoV Rotor Steel

NiCrMoV steels are widely used in manufacturing critical components of the third generation nuclear power plants, such as the low and intermediate pressure rotors in ultra-supercritical steam turbines. Recently, equipment often works under extreme conditions such as high temperature and high pressure. Manufacturing of rotors by welding has become an advanced technology. The research on the fatigue performance of the welded joint of NiCrMoV rotor steel is considered as the basis for new rotor development, design and manufacturing. In this paper, welded joints of NiCrMoV rotor steel are employed for the experimental study. Fatigue crack growth mechanism and the influence of the microstructure in BM, WM and HAZ are discussed. The combination of experiment and theoretical modeling is used to investigate systematically the effect of stress ratio, microstructure of welded joint, specimen geometry and long-term aging treatment on fatigue behavior and mechanism in the near-threshold regime. A new model in term of the stress ratio and microstructure is put forward to predict fatigue crack growth behavior in the near-threshold regime. The optimization of fatigue design curve for welded structure is analyzed. The main research contents and conclusions are listed as follows.(1) The selection of specimen shape for fatigue crack growth in the near-threshold regime and fatigue threshold measurement is discussed. Fatigue crack propagation experiment of NiCrMoV rotor steel is performed on CT, SENB3 and SENT specimens. The degree of crack closure and its influence factor have been analyzed in detail. It is found that specimen geometry has unneglectable influences on FCG in near-threshold regime, but slight effect on FCG in Paris regime. Fatigue crack growth rates by CT and SENB3 with higher constraint are faster than that by SENT with low constraint. In the near-threshold regime, the crack closure mechanism of CT and SENB3 is dominated by roughness-induced crack closure (RICC), while that of SENT is affected by both plastic induced crack closure (PICC) and RICC due to low constraint induced change of stress state, which makes the plastic zone at crack tip larger. By comparison, CT specimen with high constraint is suitable for research on FCG in near-threshold regime and fatigue threshold measurement.(2) Research on transitional behavior and its influence factors of FCG in near-threshold regime has been studied. The material investigated was a 25Cr2Ni2MoV steel welded joint. The initial cracks of FCG test are notched in BM, WM and HAZ. Mechanism of transitional behavior is investigated by Scanning Electron Microscope and Surface Plasmon Resonance Analyzer. Transitional behavior of FCG is observed for initial notches located at BM, WM and HAZ at various load ratios. Both microstructures and load ratio is involved in this behavior. The transition point in BM is corresponding to the faceted fracture morphology whereas it is related to the changes of ridges in size and distribution in WM and HAZ. The roughness of crack surface presents a valley at the transition point of FCG curve, especially at lower R. Under lower R, the transition point of FCG curve is dominated by ΔK while it is driven mainly by Kmax under higher R. Consequently, an estimation model for the stress intensity factor range at the transition point, ΔKt, is proposed by taking both microstructures and R into account. The predicted results of AATt agree fairly well with experimental data at various R.(3) The experiments on the influence of long-term aging treatment for FCG in near-threshold regime are performed. It is indicated that the effect of long term aging treatment at 350 ℃ for 3000 h on FCG behavior is mainly observed for low da/dN below 10"6 mm/cyc where da/dN values are varied with R. After the aging process, ΔKth for notch in the BM decrease due to carbides precipitation and reduced resistance from lathy martensites; whereas FCG resistance for notch in the WM increases at high stress ratio R because of microstructure homogenization and carbide dissolution in WM after aging treatment.(4) A new modified model is put forward to predict fatigue crack growth in near-threshold, which could consider the influence of microstructure. The different parts of welded joint have different microstructures, which are subjected to change after long-term aging treatment. The difference of microstructure has significant influence on FCG in the near-threshold regime. Firstly, the application of Kujawski, Huang and Zhang model is discussed by using the data of FCG of the 25Cr2Ni2MoV welded joint. It is found that the applicability of two-parameter model is related to stress ratio and transitional behavior of FCG. The parameter λ that is related to stress ratio and microstructure quantifies the effect of long-term aging treatment on the fatigue threshold. A new modified model is put forward to predict fatigue threshold in WM after long-term aging treatment. On the other hand, a new modified crack closure model is developed, which predicts FCG in near-threshold regime at various R well and can estimate crack closure under different FCG.(5) The effect of near-threshold FCG on fatigue design of welded joint is modified. In the first place, FCG data in welded joint and fatigue design curves from existing codes and standards are compared. It indicates that design of welded structure based only on base metal may cause risk in other positions of the welded joint, especially the heat-affected zone. For example, FCG design curve in WES2805 can guarantee the safety of BM at high R, but it is dangerous for other parts of welded joint. The new fatigue design curves, which are proposed based on transitional behavior and new crack closure model, describe FCG well. The results provide a useful pathway to optimize fatigue design curve of NiCrMoV welded joint achieving both safety and economy.