Effects of austenitisation heat treatment on the fracture resistance and temper embrittlement of MnMoNi steels

Abstract

The resistance to ductile and brittle fracture of four experimental melts of MnMoNi steel containing varying levels of sulphur, copper and phosphorus has been examined as a function of austenitisation heat treatment, with and without subsequent ageing at 500°C following tempering at 650°C. Fracture resistance was assessed by Charpy impact tests, fracture modes were studied using the scanning electron microscope, grain boundary segregation was quantified from Auger spectroscopy, and boron distribution determined by boron autoradiography. The results indicate that austenitisation heat treatment strongly influences the ductile-brittle transition temperature (DBTT) and upper shelf fracture energy (USE) in the quenched and tempered condition. The subsequent susceptibility to temper embrittlement is also markedly affected, high austenitisation temperatures being detrimental in all respects. Phosphorus segregation has been shown to occur during air cooling from tempering and during isothermal ageing, the degree of segregation increasing with austenitisation temperature, resulting in an increase in DBTT and a reduction in USE. Changes in DBTT and USE on isothermal ageing have been attributed to phosphorus segregation in all four composition melts. Microstructures susceptible to embrittlement have also shown enhanced levels of boron or boron-containing particles at prior austenite grain boundaries.