Abstract
The delayed hydrogen cracking test was performed to evaluate the hydrogen-assisted cracking (HAC) susceptibility of Grade T24 steel base metal and the simulated coarse-grained heat-affected zone (CGHAZ). The base metal did not fail after testing for up to 672 h. In contrast, the CGHAZ sample failed after about 2 h when charged from all four sides, and 4 h when charged only from the internal diameter (ID) surface. The higher HAC resistance of the base metal compared to the CGHAZ was due to the microstructure difference. The tempered bainitic-martensitic microstructure in the base metal was more resistant to HAC compared to the untempered martensite microstructure in the CGHAZ. Fractography analysis indicated the decarburized zone on the ID surface delayed the development of the critical hydrogen concentration in the CGHAZ, thus improving the HAC resistance. The HAC cracking initiated with an intergranular fracture, then transitioned to quasi-cleavage and microvoid coalescence. The fracture behavior was explained using Beachem’s model.
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