The effects of austenitization temperature on the high temperature ductility of Fe-P-S Alloys

Abstract

The tensile ductility of high purity iron, containing 20 wt ppm S and 40 to 280 wt ppm P, was measured at 700 °C after austenitization at various temperatures. All samples were cooled from the austenitization temperature and reheated to the test temperature in the same way. The shape of the ductility (as measured by percent reduction of area at fracture) vs austenitization temperature curve was found to depend strongly on phosphorus levels: For 40 wt ppm P a deep ductility minimum, caused by grain boundary cavitation and intergranular failure, was found after austenitizing at 1100 °C; for 180 wt ppm P a less pronounced minimum was found; but for 220 and 280 wt ppm P the minimum disappeared completely and the samples showed 95 pct or greater ductility and necked almost to a point. It is proposed that the low ductilities and intergranular failure in the low P samples are caused by the segregation of S to oxide-ferrite interfaces in the grain boundaries, resulting in a weakened interface and easy cavity nucleation. The cavitation decreases for austenitization treatments above 1100 °C because of desorption of S from the boundaries. No mechanism is proposed for austenitization temperatures below 1100 °C. It is suggested that P additions improve the ductility by competing with S for sites on the oxide-ferrite interface, thereby reducing the amount of segregated S.