Effect of Al upon Age-Hardening of 17-7 PH Steel, and Nature of its Age-Hardening (On the Nature of Age-Hardening of 17-7 PH Steel, II)

In the previous paper, the phase changes in 17-7 PH steel were determined. In the present paper, the effect of Al upon its age-hardening characteristics was studied, and its nature was clarified.The age-hardening of alloys free from δ(0∼1.2%Al) is enhanced markedly with Al content owing to an increase of α′ precipitates. In alloys with δ alone (>4%Al), however, α′ precipitates so rapidly that it cannot be suppressed even by water-quenching after solution treatment, and hence no appreciable age-hardening is observed on subsequent aging. Thus, the maximum hardening is obtained in alloys which consist of αM and a small amount of δ with about 1.5%Al corresponding to the Al content of the standard 17-7 PH steel. In commercial 17-7 PH steel with about 0.07%C, M23C6 carbide also precipitates from the matrix during aging, which, however, seems unlikely to be responsible for the age-hardening of this steel, because its amount is negligibly small.Considering the results of the present and previous experiments, the remarkable age-hardening of 17-7 PH steel was attributed to the precipitation of α′ from αM.