Creep-fatigue Behavior Research Of P92 Steel And Its Weld Metal Under Strain Control

Serve environment of P92 steel used in ultra supercritical thermal power unit is severe.Because P92 steel has to suffer not only high temperature but also fatigue load,it undergoes low cycle fatigue(LCF)and creep fatigue interaction(CFI)damage.Therefore,with the method of researching the failure behavior of LCF and CFI and predicting the fatigue life of P92 steel and its weld metal,we can make the reasonable arrangement of maintenance and replacement of thermal power unit components.It is of great significance to ensure the safety and reliability for thermal power unit and avoid casualties and property losses.This paper is based on the tests of LCF and CFI of P92 steel base metal and its weld metal at 630?,and it researches the effect of strain amplitude on LCF and the effect of hold time on CFI.The research content is as follows.For LCF tests,the increase of strain amplitude can reduce the fatigue life of P92 steel base metal and its weld metal.And in the LCF tests,both P92 steel base metal and its weld metal show the cyclic softening behavior,but the degree of softening will be saturated as the strain amplitude increases.The LCF life and strain amplitude satisfy the Coffin-Manson relationship and Basquin relationship.The LCF life of P92 steel base metal and its weld metal at 630? can be precisely predicted by the method of plastic strain energy density.For the CFI tests,the introduction of hold time can reduce the CFI life of P92 steel base metal and its weld metal,meanwhile,as the increase of hold time,the CFI life of them tends to be stable.For P92 steel base metal and its weld metal,the CFI life prediction can be made by the phenomenon of stress relaxation during tensile hold time.For P92 steel,modified Coffin-Manson relationship and Basquin relationship can describe the relation between CFI life and strain amplitude precisely.When life fraction method,ductility exhaustion method and strain energy density exhaustion model are used to predict the CFI life,the energy density exhaustion model has the maximum accuracy,then ductility exhaustion method and last is life fraction method.By considering the effect of creep limit stress,the modified energy density exhaustion model can predict the CFI life of P92 steel at 630? with a factor of 1.5.Scanning electron microscope is used to analyze the fracture morphology of LCF and CFI of P92 steel base metal and its weld metal.It can be obtained that the LCF and CFI fracture includes three different regions: fatigue crack source,fatigue crack growth and fracture zone.The crack initiates on the surface of sample and propagate in a transgranular mode.And with the increase of strain amplitude and hold time,the secondary crack and fatigue stripe become more obvious,the increase of strain amplitude and hold time can accelerate the propagate of crack.