Very High Cycle Fatigue Behavior Of25Cr2Ni2MoV Steel Welded Joints

Fatigue fracture is still a major security threat of large scale equipment. Recently, aerospace, high-speed rail and nuclear are developed quickly. Much important engineering structure works from108to1010cycles. Very high cycle fatigue has been concerned by more and more people, In this work, very high cycle fatigue behavior of nuclear turbine welded rotor steels (25Cr2Ni2MoV steel welded joint) were systematically investigated in terms of experiment, theoretical calculation and numerical simulations. The main research contents and conclusions are listed as follows:(1) The material’s S-N curve was obtained by high frequency fatigue test, SEM and FEA. The fatigue behavior difference of welded joint was investigated, In high cycle fatigue regime, the fatigue performance of welded metal is strong, however in the very high cycle fatigue regime, the fatigue performance of base metal is strong. Small surface defect and large interior defect had similar potential for crack initiation. Stress concentration around pore and inclusion in the weld metal were larger than inclusion in base metal.(2) S-N curve, crack initiation mode and defect distribution were obtained by ultrasonic fatigue experiment and SEM. Compared with high frequency fatigue test, ultrasonic fatigue test can obviously enhance the fatigue strength of specimen. The influence of loading frequency of bcc metal is much larger than fcc metal. The S-N curve of ultrasonic fatigue test was corrected by frequency modifying coefficient. The corrected data was coincided with the data of high frequency fatigue test.(3) The influence of specimen size and welding process to the welded joint were investigated in terms of ultrasonic fatigue test. It is found that the fatigue strength of small size specimen were larger than larger size specimen, In the very high cycle fatigue regime, the weaken influence of welding is increasing.