Strengthening mechanism of Cu bearing heat resistant martensitic steels

Relation between microstructure and creep deformation behavior was investigated in 9%Cr base Cu bearing martensitic steels, and the strengthening mechanism by Cu particles was discussed in connection with the microstructure of lath martensite. All of as-quenched 9%Cr-(0∼4)%Cu steels have typical lath-martensite single structure characterized by martensite-block and -packet structures, with high density of dislocation. As a result of creep testing at 873K-140MPa, it is found that minimum creep rate of these martensitic specimens is lowered and the rupture time is prolonged with increasing Cu content. The 9%Cr-4%Cu steel has about seven times as long creep-rupture time as the 9%Cr base steel. TEM observations in the crept 9%Cr-4%Cu steel revealed that dislocations tangled with fine Cu particles which precipitated within laths. This dislocation pinning effect by Cu particles contributes to retarding the recovery of martensite. Hence the dislocation density is kept higher in the 9%Cr-4%Cu steel than the 9%Cr base steel during creep deformation. The relation between the recovery and the creep deformation behaviors was successfully explained by the change in pinning force by Cu particles in all of 9%Cr-Cu steels. Discussed in detail, once the pinning force falls below the applied stress, the recovery rapidly proceeds and the creep strain rate is accelerated corresponding to the recovery behavior.