High-cycle Fatigue Behavior And Fatigue Threshold Of 25Cr2Ni2MoV Steel Overlaying Welded Joints

During the service of steam turbine rotor,local damage such as scratches,cracks and wear are prone to occur,which affects the normal operation and safety of the equipment.Replacing the entire rotor is not only technically complex,but also causes huge economic losses.Therefore,overlaying repair technology is often used to repair the turbine rotor locally.25Cr2Ni2MoV steel is a commonly used rotor steel material for low-pressure,medium-low pressure and high-low pressure composite welding rotor.Since the welded joint is the worst part of the fatigue resistance of the entire welded rotor,studying the fatigue behavior of the overlaying welded joint is crucial to the fatigue design and life prediction of the steam turbine rotor.In present thesis,the high-cycle fatigue behavior and fatigue threshold of 25Cr2Ni2MoV steel overlaying welded joints were studied,the main work and conclusions are as follows:(1)The effect of heat treatment and stress ratio on the high-cycle fatigue behavior of 25Cr2Ni2MoV steel overlaying welded joints were studied.The results showed that with the increasing of R,the fatigue limit decreased significantly,but the heat treatment under the same R had little effect;With the increasing of the cycle number,the location of crack initiation gradually changed from the surface of the specimen to the interior of the specimen,and the location of fracture changed from the base material to the weld;Most of the short-life specimen cracking sources are small size forging defects in base metal,while long-life specimen cracking sources are mostly large size weld defects.Heat treatment and R affected the fatigue life by the form and location of crack initiation.(2)The effect of heat treatment and stress ratio on fatigue crack growth behavior of 25Cr2Ni2MoV steel overlaying welded joints were studied.By conducting fatigue crack growth tests under different heat treatments and R conditions,the da/dN-?K curve with different heat treatment and R was obtained,and the microstructure morphology and crack growth mechanism of the fracture were analyzed by scanning electron microscopy.The results showed that with the increasing of R,da/dN increased and the fatigue threshold ?Kth decreased for the base metal,heat-affected zone and weld metal;The base metal was not significantly affected by heat treatment when R is 0.1 and 0.7,and when R is 0.3 and 0.5,the fatigue resistance under 580?×20h heat treatment was better than 550?×20h heat treatment;The weld metal had little influence by the heat treatment under each R;The heat affected zone was affected by the heat treatment in a complicated situation.At low R,the base metal had a face fracture morphology in the near-threshold area with the crack closure.At this case,?Kth was not the real ?Kth;When R?0.7,the facet fracture morphology disappears and the crack closure was small,?Kth could be regarded as the inherent property of the material,and the estimation formula was established.(3)Based on the obtained crack growth rate results of different areas of overlaying welded joints,the applicability and accuracy of the two crack closure-crack and propagation driving force joint models for predicting fatigue crack growth rate were compared and studied.The basic contents of Zhu model and Kwofie-Zhu model were briefly introduced,and the fatigue crack growth test data of 25Cr2Ni2MoV steel was used to verify the model.The results showed that the prediction result of the Kwofie-Zhu model was more accurate,but the application process involving parameter solving was more complicated;The application process of the Zhu model was simple,but the prediction result was not as accurate as the Kwofie-Zhu model.Both models showed good prediction results in the near-threshold region and the Paris region of fatigue crack growth of 25Cr2Ni2MoV steel welded joints,which verified that it was reasonable and feasible to unify the crack closure mechanism and the propagation driving force mechanism to study the stress ratio correlation of fatigue crack growth behavior.