Effect of repeated-tempering induced martensite on the microstructural evolution in 17–7 PH stainless steel

Abstract

Single long-time tempering and triple tempering processes have been applied to 17–7 PH stainless steel (chemical composition: Fe-0.09C-16.4Cr-7.7Ni-0.9Al, wt%) to investigate the effects of the stability of austenite on subsequent martensitic transformation and aging hardening. The as-received steel was composed of 0.55 austenite and 0.45 martensite (volume fraction). Scanning electron microscopy images showed that the volume fraction of martensite (up to 0.98) in triple-tempered specimens ((0.5 h @ 760 °C) × 3) was about 0.15 higher than that (0.83) in single-tempered ones ((1.5 h @ 760 °C) × 1). The results clearly indicate the effectiveness of the triple-tempering process in progressively improving martensitic transformation. Direct observation of the austenite/martensite interface has provided evidence that the preceding martensitic transformation induces accommodation strain into the adjacent austenite, which brings about the mechanical stabilization of austenite but will facilitate the succeeding martensitic transformation after three consecutive tempering operations. The relevant microstructural evolution has been explored and discussed. Furthermore, subsequent aging at 565 °C led to precipitation hardening associated with the formation of NiAl nanoparticles. The triple-tempered specimen yielded the higher peak hardness value (424 HV) in 20 min, significantly faster than the single-tempered specimen, which took 90 min to reach the lower one (357 HV). The corresponding number density of NiAl nano-precipitates and dislocation densities of martensite at various aging times have been examined by transmission electron microscopy to elucidate the benefits of the triple tempering process for precipitation strengthening.