PRECIPITATION BEHAVIOR OF A NITROGENATED SURFACE-LAYER OF 1.4462 DUPLEX STAINLESS-STEEL

A duplex stainless steel sheet containing masses of 22%Cr-5.5%Ni-3%Mo was nitrogenated at 2 bar partial pressure of N-2 at 1200 degrees C fo r 5 hours. Three zones, with different microstructures and nitrogen co ntents, were formed: a fully austenitic nitrogen rich zone, a transiti on zone composed of an austenitic matrix containing islands of remaini ng ferrite and a nitrogen enriched duplex inner zone. The exposures of the nitrogenated material at temperatures below the nitrogenation tem perature caused precipitation of epsilon-(Cr,Fe)(2)N hexagonal nitride and of sigma phase. The nitride precipitation occurred both continuou sly and discontinuously. TTT diagrams for the austenitic external zone and for the inner ferritic-austenitic zone have been proposed.