Low Cycle Fatigue Behavior And Microstructure Evolution Of G115 Martensitic Steel At High Temperature

G115 steel,a new 9Cr-3W-3Co martensitic heat-resistant steel developed by China Iron and Steel Research Institute（CISRI）,can be used in USC power plants at650°C.Steam generators and their components are often subjected to repeated thermal stress as a result of the temperature gradients that occur on heating and cooling during start-ups and shut-downs or during temperature,then the risk of low cycle fatigue（LCF）failure increases at high temperature.Thus,it is significance to explore the LCF behavior to ensure the safety and reliability for thermal power unit.A series of uniaxial strain-controlled fatigue tests were performed at 650°C to explore the LCF behavior and corresponding microstructure evolution of G115 steel.The fatigue test results showed that the steel exhibited a cyclic softening behavior,which was related to the decrease of dislocation density,disappearance of subgrain boundary,M₂₃C₆ coarsening and subgrain development.In addition,the variation of degree of softening（35）S was used to make comparison between G115 steel and P92steels to evaluate the effects of the strain amplitudes on the respective S.The G115steel experienced the minimum relative increment compared with P92 steel,which resulted from the strengthening by fresh and fine Cu-rich precipitates.These particles try to offset the impact of strain amplitude but not enough to completely prevent the cyclic softening behavior.A series of strain-controlled,interrupted LCF tests under a total strain amplitude of±0.5%and a control test were performed on G115 steel to explore the precipitation and coarsening behavior of Cu-rich phase.Formation of this precipitates was rapid at around 750K before the start of the tests.Then,both the high temperature and cyclic loading promoted the coarsening of Cu-rich particles and accelerated their structural transformation.The coarsening of this phase during the LCF tests agreed well with the LSW theory.The sequence of transformation was as follows:BCC→9R→3R→FCC.In addition,TEM observation manifested the Cu-rich precipitates attracted the dislocation lines.The cut-through mechanism is proposed based Friedel-Brown-Ham theory to show that the weak Cu-rich particles are easy to cut by to-and-fro motion of dislocations and then the dissolution occurred.SEM was employed to investigate the fracture morphology.Representative images showing three regions:fatigue crack initiation sites,stable crack growth region and rupture region.Fatigue cracks initiate at the specimen surface defects and then propagate into the matrix until failure.