Mechanism of upper-nose temper embrittlement in Mn–Ni–Mo A533 gr. B steel

Abstract

The mechanism of upper-nose temper embrittlement, which occurs upon long-time tempers at elevated temperature in some low-alloy steels, has been studied in the case of a Mn–Ni–Mo A533 gr. B steel. It was shown to be primarily due to the formation of very coarse Mo-rich M₂₃C₆ carbides in the grain boundaries which fracture upon plastic straining of the matrix and thus initiate premature cleavage cracking by serving as Griffith flaws. A quantitative analysis of the concomitant evolution of all the other microstructural parameters showed that, as a whole, it had a beneficial action on toughness, partially compensating for the embrittlement provoked by the coarse grain-boundary carbide formation.