The influence of precipitated austenite on hydrogen embrittlement in 5.5Ni steel

This work was undertaken to test the influence of precipitated austenite on transgranular hydrogen embrittlement in 5.5Ni steel. Prior work has shown that the mechanism of transgranular hydrogen embrittlement in this steel is interlath separation. Since the austenite that forms during the tempering of 5.5Ni steel precipitates on the martenite lath boundaries, it was hypothesized that the austenite might have a beneficial effect. The experimental results show, however, that the precipitated austenite decreases the toughness in the presence of hydrogen. The apparent mechanism is straightforward. The precipitated austenite transforms to martensite in the strain field ahead of the crack tip. Interlath cracks appear at the periphery of the fresh martensite particles. They are apparently caused by the volume change that accompanies the martensite transformation, which imposes a tension across the lath boundary. The interlath cracks link together to form the macroscopic fracture.