Embrittlement and aging at 475 °C in an experimental ferritic stainless steel containing 38 wt.% chromium

The aging at 475 °C of an experimental ferritic stainless steel containing 38 wt.% chromium was studied. The physical changes occurring at this temperature were followed with the aid of Mössbauer spectroscopy, measurements of Charpy V-notch toughness and hardness, metallography and corrosion testing. The hardness seems to be most strongly affected by the α → αFe + α″Cr spinodal decomposition reaction, but the Charpy toughness is controlled by processes that occur before substantial spinodal decomposition has taken place. The rapid deterioration in toughness at 475 °C is most likely caused by the pinning of dislocations by very-fine-scale complexes of chromium, carbon and nitrogen atoms that precipitate from the supersaturated ferrite. The increase in hardness was also accompanied by an increased tendency towards twinning during deformation. The rate of corrosion of the material in selected FeCl3 and H2SO4 environments was not measurably affected by the aging treatments. Removal of the interstitial elements by a pre-aging treatment accelerated the spinodal decomposition of the alloy markedly. An explanation for this effect is propsed.