Compound layer formation on iron-based alloys upon nitriding; phase constitution and pore formation

摘要

The microstructural development associated with the formation of the compound (so-called "white") layer on the surface of ferritic iron and iron-based alloys by nitriding has been presented in a comparative way on the basis of results obtained in the recent past by nitriding model systems. A tentative explanation of the strikingly different layer-growth mechanisms for iron and iron-based alloys has been offered. Attention has been paid in particular to (i) the possible occurrence in the compound layer of "mixed" iron-based nitrides, with the alloying element Me with affinity for nitrogen (e.g. Me = Cr, Al, V) dissolved substitutionally on the iron sublattice of the nitride, and to (ii) the various operative mechanisms for pore formation in the compound layer.