Tempering precipitates of steel 1.25Cr-0.5Mo and their effects on its hydrogen attack resistance

Studies were made on the tempering precipitates and their effects on hydrogen attack (HA) resistance of the medium - temperature HA - resistant Steel 1.25Cr-0.5Mo. It was shown by TEM of carbon extraction replicas that, with increasing tempering temperature, more carbides were precipitated and coarsened. When tempered at 520°C and 620°C, almost only M₃C was precipitated. When tempered at 670°C, M₃C was the main precipitate with a small amount of M₂₃C₆. While tempered at 710°C, M₂₃C₆ was the main precipitate with a small amount of M₃C. It was observed by SEM that ,after charging hydrogen, samples tempered at 620 °C were seriously attacked by hydrogen, which resulted in great drop of strength and hardness. While no apparent HA phenomenon was observed for samples tempered at 520°C, 670°C and 710°C, but rupture strength of samples tempered at 520°C was greatly decreased. It is related that large quantity of unstable M₃C precipitated along grain boundaries, and the C-rich and Cr-poor regions for the sample tempered at 620°C appeared, resulted in the HA degradation of the material.