Characterization of nitrided 3% chromium steel

Abstract

Four microconstituents have been characterized in a gas-nitrided commercial steel, BS 722M24. There is an outermost layer, the compound zone, consisting of iron nitrides plus, when nitriding at 570°C, cementite. Below 570°C there is a hard nitrided zone consisting of finely dispersed CrN with somewhat coarser CrN which has replaced cementite at prior martensite lath boundaries in the quenched and tempered structure. The carbon displaced from such dissolved cementite either moves deeper into the steel to form a carbide-enriched zone beneath the nitrided zone or migrates to prior austenite grain boundaries within the nitrided layer where it forms cementite. This grain-boundary constituent is what has previously been referred to as ‘subsurface nitride’.