MICROSTRUCTURAL CHARACTERIZATION OF AS-CAST HIGH-NITROGEN FE-15CR-15NI ALLOYS

A base composition of Fe-15Cr-15Ni was melted in a hot-isostatic-pressure furnace under nitrogen pressures from 0.1-150 MPa (1-1500 atm) to produce materials with nitrogen concentrations from 0.04-3.6 wt%. Microstructural characterization of the as-cast materials was completed using optical microscopy, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy techniques, The phases present in the materials were austenite, nitrides (Cr2N and CrN), and martensite. All of the materials solidified with primary austenite dendrites. Dendritic solidification of CrN occurred in interdendritic regions of the austenite in alloys containing between 0.6 and 1.9 wt% N. Cellular precipitation of Cr2N occurred in alloys with intermediate nitrogen concentrations (0.6 and 1.0 wt%). The orientation relationship between the Cr2N and austenite, expressed as {111}gamma//{0001}Cr2N and [110]gamma//[- 1100]Cr2N, was confirmed. Precipitates of CrN with both lamellar and disc morphologies formed in the austenite. Both had a cube-cube orientation relationship with the austenite. Only about 10% of the 3.6 wt% N material solidified as primary austenite. The remainder of the material solidified as a eutectic according to the reaction L --> gamma + CrN. All of the austenite in the eutectic region subsequently transformed to martensite. Increasing volume fractions of martensite formed with increasing bulk nitrogen concentrations in the materials due to alloy depletion of austenite caused by nitride formation.