Abstract
The notch toughness after aging and the structural changes during aging have been investigated on 12Cr-12Co-Ni-Mo maraging stainless steels containing 3∼6%Ni and 2∼5%Mo.
The maximum hardness was obtained by aging at 510∼560°C for 240 min, although the aging temperature was slightly varied with Ni content. On the other hand, a drastic decrease in notch toughness for low Ni or Mo alloys was caused by aging at 450∼500°C, lower than the aging temperature at which a maximum hardness was attained.
In the case of isothermal aging at 480°C, the decrease in notch toughness was observed in all the alloys after a certain incubation period determined by the chemical composition. The incubation period became shorter with decreasing Ni or Mo content.
The alloys, in which the σ phase was precipitated by solution heat treatment at 850°C, exhibited very poor toughness after aging at 450∼500°C, since an incubation period became shoter by the depletion of Mo.
Intermetallic compound R phase has been identified by X-ray diffraction of the residues obtained by dissolving the alloys aged at a temperature higher than 480°C, but no close correlation has been found between precipitation of the R phase and embrittlement.
The embrittlement is thought to be closely related to the formation of Ni- and Co-rich zone and the diffusion of impurities enhancing the temper embrittlement by low temperature aging.
