COMPOUND LAYER GROWTH AND COMPOUND LAYER POROSITY OF AUSTENITE PLASMA NITROCARBURISED NONALLOYED STEEL

Compound layer growth and compound layer porosity of an austenite plasma nitrocarburised slowly cooled AISI 1020 steel sample were studied by controlling the plasma nitrocarburising atmosphere with nitrogen, hydrogen and methane content in the following proportions: 25 vol.% N-2, 73.25 vol.% H-2, 1.75 vol.% CH4 at 600 degrees C. During the initial stage of plasma nitrocarburising, highly porous surface structures are formed similar to those described as ''the open structure of a piece of coral'' observed in salt bath nitrocarburising. For a 9 min treated sample a triple layer composed of cementile, compound layer and nitrogen austenite is formed, which further develops into epsilon iron nitride containing a compound layer and the dissociated nitrogen austenite sublayer. Porosity is mainly nucleated and grows at the grain boundaries, the carbon-enriched region and places where alpha-iron forms. The structures of quenched and annealed samples were also examined. Pure epsilon compound top layer can be retained after quenching. This layer is unstable at this gas composition and transforms to gamma', alpha-iron (and Fe3C) containing epsilon during slow cooling. Annealing enhances the core property of the plain carbon steel by transforming the martensite and residual nitrogen austenite into bainite.