Carbide precipitation in welds of two-phase austenitic-ferritic stainless steel

Abstract

Transmission electron microscopy (TEM) and microanalytical chemistry were performed on sensitized samples of duplex welds that exhibited both skeletal ferrite microstructures and lath ferrite microstructures. The objective was to understand why welds with lath ferrite, contrary to a theoretical prediction, are not immune to sensitization. Most of the ferrite-austenite (α-γ) interphase boundaries in the welds with skeletal ferrite were curved and incoherent, while those in welds with lath ferrite were predominantly planar and semicoherent. The density of carbide precipitation on incoherent boundaries was much greater than that on semicoherent boundaries. Carbide precipitates on incoherent boundaries were typically equiaxed, while those on semicoherent boundaries had very high aspect ratios and appeared to form along ledges in the interphase boundary. During sensitizing heat treatments, the chromium-depleted zone on the ferrite side of the interphase region transformed to austenite, causing the α-γ interphase boundary to move into the ferrite region. This markedly increased the width of the chromium-depleted zone in the austenite phase and extended the time of heat treatment required to replenish the zone with chromium. It is proposed that migration of the α-γ interphase boundary, which occurs to a much greater extent in the welds with lath ferrite, is responsible for their unexpected susceptibility to sensitization at 550°C.