Creep-ratcheting-fatigue Behavior Of 2(1/4)Cr-1Mo-V Steel At Elevated Temperature

2(1/4)Cr-1Mo-V steel is widely used in the fabrication of hydrogenation reactor due to its high mechanical strength as well as good resistance against creep and hydrogen corrosion at elevated temperature.Hydrogenation component faces more than fatigue,ratcheting or creep damage under the co-existence of high temperature and pressure.It also suffers the damage created by the synergetic effects among fatigue,ratcheting and creep.Therefore,it is necessary to investigate the cyclic mechanical property of 2(1/4)Cr-1Mo-V steel near its working temperature.In this study,a series of low-cycle-fatigue tests,ratcheting-fatigue tests and creep-ratcheting-fatigue tests of 2(1/4)Cr-1Mo-V steel were conducted at 455 oC in air,and the influence of test parameters,such as strain amplitude,stress level,loading rate,stress holding periods and holding direction,on deformation behavior and fatigue life were investigated.It was found that 2(1/4)Cr-1Mo-V steel featured cyclic softening.The increase of stress level stimulated the accumulation of ratcheting strain,which was also greatly influenced by stress rates.A creep strain recovery phenomenon was found in creep-ratcheting-fatigue tests when the peak holding periods were short.The creep recovery rate reduced when the peak holding period was extended or the cyclic number was increased.The increase of strain amplitude,stress level as well as peak holding periods reduced the fatigue life of test specimen.The relationship between strain amplitude and fatigue life could be well described by the Basquin-Coffin-Manson equation,while the fatigue life in creep-ratcheting-fatigue tests could be well predicted by the linear damage summation method.By introducing stress ratio into time fraction rule,prediction results for double holding tests were improved.Taking the minimum strain rate as a parameter,the predicted fatigue life in ratcheting-fatigue and creep-ratcheting-fatigue tests fell within a scatter band of 1.5.The fracture surface,free surface and longitudinal cross section of failed specimens were observed by optical microscope and scanning electronic microscope.More than one fatigue crack initiation sites were found when strain amplitude was increased,and the propagation of fatigue crack led to the final brittle fracture of test specimen.For ratcheting-fatigue and creep-ratcheting-fatigue tests,the formation of plastic dimples and creep cavities resulted in loss of carrying capacities and led to the final ductile fracture of test specimen.ASME Code Case 2605,ASME-NH and R5 methods were used to evaluate the creep-fatigue damage accumulated during the design life of a catalyst drain tank,and evaluation results from three methods satisfied design requirements.The fatigue damage evaluated by ASME Code Case 2605 was the largest,in contrast with the lowest estimated from ASME-NH.For creep damage evaluation,time fraction rule provided by ASME-NH gave the most conservative result,followed by the ductility exhaustion theory in R5,and the Omega method adopted in ASME Code Case 2605 gave the lowest creep damage estimation.For total damage evaluation,the evaluation result of ASME-NH was the largest while ASME Code Case 2605 gave the lowest damage value.