Papers by Keyword: Nb Precipitation

Abstract: The unavoidable phase transformation upon cooling makes a direct observation of austenite microstructure impossible. One of the methods to investigate the high temperature condition of austenite is to use a model alloy, which would not transform to martensite on quenching. In the present work the recrystallisation and grain growth of austenite during thermo-mechanical processing (TMP) were studied for three Ni-30Fe-Nb-C alloys containing 0.33, 0.85 and 1.42 wt% Nb. The austenite microstructures were characterised using optical and scanning electron microscopy, and energy dispersive X-ray spectroscopy. The austenite recrystallisation kinetics was investigated with respect to the precipitate distribution variations. A relative strength of the grain boundary pinning effect from solute Nb and precipitated particles is discussed.

Abstract: Effects of aging treatment on high temperature strength of Nb added ferritic stainless steels for automotive parts were investigated. Hot tensile tests were carried out at 700 °C after the aging at 700 °C for different aging times using Gleeble 1500. High temperature strength of all steels decreased as the aging time increased. In Nb free steels, the reduction in high temperature strength is mainly due to grain growth. On the other hand, in Nb added steels, the reduction in high temperature strength occurred by Nb precipitation. It was observed that Fe2Nb (Laves phase), Nb(C,N) and Fe3Nb3C were precipitated out during the aging at 700 °C in Nb added steels. The coarsening rate of Fe2Nb was higher than that of Nb(C,N). Fine Fe2Nb precipitates formed during at the early stage of aging contributed to high temperature strength in 0.01C-0.38Nb steel. However, coarse Fe2Nb particles formed during the aging were very detrimental to high temperature strength. The coarsening of Fe2Nb was relatively retarded by adding Mo.