Effects of alloying additions and austenitizing treatments on secondary hardening and fracture behavior for martensitic steels containing both Mo andW

The effects of alloying additions and austenitizing treatments on secondary hardening and fracture behavior of martensitic steels containing both Mo a nd W were investigated. The secondary hardening response and properties of these steels are dependent on the composition and distribution of the carbi des formed during aging (tempering) of the martensite, as modified by alloy ing additions and austenitizing treatments. The precipitates responsible fo r secondary hardening are M2C carbides formed during the dissolution of the cementite (M3C). The Mo-W steel showed moderately strong secondary hardeni ng and delayed overaging due to the combined effects of Mo and W. The addit ion of Cr removed secondary hardening by the stabilization of cementite, wh ich inhibited the formation of M2C carbides. The elements Co and Ni, partic ularly in combination, strongly increased secondary hardening. Additions of Ni promoted the dissolution of cementite and provided carbon for the forma tion of M2C carbide, while Co increased the nucleation rate of M2C carbide. Fracture behavior is interpreted in terms of the presence of impurities an d coarse cementite at the grain boundaries and the variation in matrix stre ngth associated with the formation of M2C carbides. For the Mo-W-Cr-Co-Ni s teel, the double-austenitizing at the relatively low temperatures of 899 to 816 degreesC accelerated the aging kinetics because the ratio of Cd(Mo + W ) increased in the matrix due to the presence of undissolved carbides conta ining considerably larger concentrations of (Mo + W). The undissolved carbi des reduced the impact toughness for aging temperatures up to 510 degreesC, prior to the large decrease in hardness that occurred on aging at higher t emperatures.