Strengthening of heat resistant martensitic steel by Cu addition

Microstructure and creep deformation behavior were investigated in martensitic 9%Cr-(0-4)%Cu steels, and the strengthening mechanism by Cu was discussed in terms of microstructural change during creep deformation. As-quenched specimens have typical lath-martensitic structure. Cu addition gives no influence on the morphology of martensite-laths, but austenite grains tend to be refined with increasing the Cu content. As a result of creep testing for the quenched specimens with a same grain size, it was found that minimum creep rate is greatly lowered and rupture time are prolonged with increasing the Cu content. TEM observations of a creep-deformed 9%Cr-4%Cu steel revealed that dislocations attractively interact with ε-Cu particles within laths, and also lath boundaries are pinned by the ε-Cu particles. Through these interactions between dislocations or lath boundaries and ε-Cu particles, recovery in martensitic structure is greatly retarded, and dislocation density is kept higher during creep deformation. But once the pinning force by ε-Cu particles falls below the applied stress, the recovery rapidly proceeds and the strain rate is accelerated.