Carbide precipitation in austenitic stainless steel carburized at low temperature

Low-temperature gas-phase carburization can significantly improve the surface mechanical properties and corrosion resistance of austenitic stainless steel by generating a single-phase “case” with concentrations of interstitially dissolved carbon exceeding the equilibrium solubility limit by orders of magnitude. Upon prolonged treatment, however, carbides (mostly χ, M5C2) can precipitate and degrade the properties. High-resolution and spatially resolved analytical transmission electron microscopy revealed the precise carbide–austenite orientation relationship, a highly coherent interface, and that precipitation only occurs when (i) the carbon-induced lattice expansion of the austenite has reached a level that substantially reduces volume-misfit stress and (ii) diffusional transport of nickel, chromium, and iron – enhanced by structural defects – can locally reduce the nickel concentration to the solubility limit of nickel in χ-carbide.