Experimental Study On Very High Cycle Fatigue Of Martensitic Steel Of2Cr13under Different Environment

Research in the area of fatigue of engineering materials has always been a key concern due to fatigue failure occurred frequently in engineering materials and structures, causing economic losses or even loss of lives. Traditionally,107loading cycles is used as a testing standard in engineering design. However, an increasing number of cycling loads existing in components, such as steam turbine blades, railway wheels and medical devices, are often subjected to109-1010loading cycles due to the high frequency and low load amplitude. The convenient design below10’cycles is no longer suitable for very high cycle fatigue over10cycles. Therefore, it is essential to investigate the very high cycle fatigue properties in order to make it clear about failure mechanism and characters.Very high cycle fatigue test was carried out on martensitic steel of2Crl3which is used as steam turbine blade material, by using a four-axis rotating bending fatigue rig under the environment of air and3.5%NaCl solution. Stress life curve (S-N) curves were plotted for two environments. Detailed inspections of fracture surfaces were conducted at various stress levels. The analyzing results were carefully compared with existing available data from other’s work.In both air or3.5%NaCl corrosive environment, the S-N curves for martensite stainless steel2Crl3both showed a step-wise tendency over the range of106-108cycles. In air fatigue,2Cr13steel has the fatigue limit of530MPa within the range of10-10cycles. Failure continued to happen as cycles goes above10cycles. Existence of a fatigue limit in a corrosive medium is strongly dependent on material microstructure and corrosive environment. Over the range of10-108cycles,2Cr13steel tested presents a fatigue limit of250MPa in3.5%NaC1, which is47%of the air fatigue limit of530MPa.In air fatigue, cracks initiated from the sample surface and inclusions at subsurface and no typical fish eye feature in very high cycle fatigue was observed for all samples tested up to6X108cycles. In3.5%NaCl solution, multiple cracks initiated from the surface and the number of crack origins increased with increasing stress level and surface proportion of fatigue propagation increased as number of cycles increased. The fatigue crack propagation proportion exceeded50%of the entire fracture surface at10cycles and only20%at10cycles. By comparison with other materials in corrosive environment, it can be concluded that the number of crack origins is highly dependent on materials, electrochemical conditions and stress levels. For the materials of2Cr13tested in the present study, the stress level is the main factor. The most effective method to prevent corrosion fatigue failure for steam turbine blade is the reduction of the design stress.The number of fatigue crack origins increases with increasing stress levels. Also, the area proportion of fatigue crack propagation on fracture surface is getting large as the number of cycles increases in3.5%NaCl, which accounts for50%or more at108cycles and only20%at104cycles.