The relation between microstructure and yield strength in tempered low-carbon lath martensite with 5% nickel

The tensile yield strength of quenched and tempered low-carbon (< 0.10%) lath martensite with 5% nickel is analysed with respect to grain size and substructural parameters and at the same time effects from an addition of 0.3% molybdenum are investigated. Microstructures are studied in detail by transmission electron microscopy. The initial martensite packets control the high-angle boundary grain structure after normal tempering, thus contributing to the yield strength through an ordinary Petch-Hall relation for the packet size. The Petch-Hall coefficient for packets seems to decrease on tempering if extensive recovery and subgrain formation occur. Subgrain boundaries and dislocations within subgrains contribute substantially to the yield strength and account for the high strength of this type of martensitic steel. Softening on tempering is mainly a result of recovery through which the original lath structure is destroyed and the initially high dislocation density is reduced. The addition of 0.3%Mo strongly retards recovery and hence softening on tempering. This pronounced influence of Mo appears to be caused by some solid solution interaction probably resulting in retardation of dislocation climb.