Microstructural stability of Ta minor-alloying HR3C stainless steel at 973 K

Abstract

The present work investigates the influence of minor-alloying element Ta on the microstructural evolution and mechanical properties of HR3C ASS (Fe-25Cr-22Ni-0.4Nb-0.18N-0.07C, wt. %) at elevated temperatures. Alloy ingots are prepared in a vacuum arc furnace and then hot-rolled, solid-solutioned at 1423 K/0.5 h, stabilized at 1173 K/0.5 h, and finally aged at 973 K for different hours (25–408 h). The microstructure and precipitated phases in different heat-treatment states are characterized with optical microscopy (OM), scanning electron microscopy (SEM), X-ray fluorescence spectrometer (XRF), electron probe microanalysis (EPMA), and transmission electron microscope (TEM), respectively. The experimental results show that Ta minor-alloying can suppress the precipitation of brittle σ and Cr23C6 phases effectively, as evidenced that only a few of σ and Cr23C6 particles appear on the grain boundaries after 408 h aging, exhibiting a high microstructural stability. It is attributed to the amount of Nb and Ta elements, which satisfies the composition rule that the molar ratio of (Nb + Ta)/C is 1/1. In addition, the micro-hardness of Ta-modified alloys doesn't vary significantly with the aging time, which are consistent with the high microstructural stability. The strengthening mechanisms of stabilized alloys are also discussed, which demonstrate that the precipitation strengthening of (Nb, Ta)C carbides is dominant in Ta-modified alloys.