Hydrogen embrittlement and segregation of impurities in type 10MnNi2Mo pressure vessel steel

Abstract

This paper summarizes the findings of a study in which the brittle fracture resistance and hydrogen embrittlement resistance of a 10MnNi2Mo steel were investigated both in the quenched and tempered state, and after a subsequent heat treatment that simulated the thermal cycling in the heat-affected zone around a weld joining thick-walled components. This simulating treatment included two alternative stress relieving procedures, at 650 and 580°C respectively. The simulating treatment caused marked coarsening of the prior austenitic grains; had no substantial effect on the static and dynamic fracture toughness, nor on the transition temperatures; but reduced the resistance against hydrogen embrittlement, and during the development of hydrogen-induced cracks produced some intercrystalline facettes. A supplementary study of surface segregation, by Auger electron spectroscopy on the free surfaces of the specimens, indicated that this incidence of intercrystalline failure may be at least partly ascribed to the pronounced segregation of phosphorus during stress relieving at both of the temperatures employed. This made an interesting comparison with 20Mn steel, where the simulating treatment profoundly reduced both the fracture and notch toughness levels but led to favourable resistance to hydrogen embrittlement. The fact that no intercrystalline failures were detected in hydrogen-charged 20Mn steel is ascribed in part to the lower observed segregation activity at the stress relieving temperatures, in part to the resultant microstructure of this non-alloyed steel.