Abstract
The purpose of the study is to understand the effect of welding process on the fatigue crack growth rate of 309L austenitic stainless steel weldments. Two welding processes, the GTAW (Gas Tungsten Arc Welding) and SMAW (Shield Metal Arc Welding) were adopted to weld 309L austenite stainless steels and test them in 25°C and 288°C. The results show that the solidification mode of 309L austenitic stainless steel weldments is ferrite-austenitic mode and the primary microstructure is dendritic structure. The ferrite content decreases with increasing welding times. The hardness increases with the increasing ferrite content and the latter welding layer shows the lower hardness, resulting from the tempering effect. The residual stress is related to the heat input by the different welding procedures. In order to realize the effect of the residual stress on fatigue crack growth rate, this study measured the residual stress by XRD and neutron diffraction. The measurement by XRD was conducted to detect the surface residual stress. By GTAW, the compressive residual stress is small and average stress. By SMAW, the compressive residual stress is relatively large with more fluctuation. The heat input due to different process may result in different cooling rates, which play an important role on the distribution of surface residual stress. The inner residual stress is determined by neutron diffraction. The residual stress of the weldments by GTAW is higher than that by SMAW. The weldments of fatigue crack growth was tested at 25°C and 288°C. All the specimens were transgranular fracture. The results reveal that the base metal and weld display different trends at 25°C. The fatigue crack growth of both weldment are lower than base metal due to microstructure. The inhomogeneous structure has rough fracture surface which retards the fatigue crack growth rate. At 288°C, all the specimens shows similar crack growth rate. The weldments by GTAW and SMAW have the dynamic recovery and planar slip phenomenon, respectively. The weldments by GTAW have high stacking fault energy which results in dynamic recovery and increases the fatigue crack growth rate. In contrast, the weldments by SMAW have low stacking fault energy meaning that the planar slip could occur difficultly. As a result, the weldments by SMAW have longer the fatigue life. The experimental results indicate that the residual stress has no significant effect on the fatigue crack growth rate. The microstructure and hardness have a more prominent effect on fatigue crack growth rate and the residual stress has no significant effect on fatigue crack growth rate.