Hydrogen Assisted Cracking in a Microalloyed Steel Subjected to a Rapid Thermal Cycle at High Temperature

In a martensitic-bainitic microalloyed steel, the effect of hydrogen on fatigue crack growth was studied using rotary bending fatigue tests. The steel was subjected to a rapid thermal cycle to get a microstructure similar to that which would be formed within the coarse-grained heat-affected zone of a weld. Crack growth was monitored as a function of the number of cycles applied during fatigue tests on three types of specimens: 1) those without hydrogen charge, 2) those charged with hydrogen and 3) those charged with hydrogen which was then discharged through low-temperature heat treatment. All types showed persistent slip marks, and cracks propagated along high-shear-stress planes. In the presence of hydrogen, crack growth was affected by microstructural defects caused by the hydrogen charging process, and the persistent slip marks developed in an area closer to the crack tip and crack path. On the contrary, without hydrogen, crack growth occurred perpendicular to the applied force, and the persistent slip marks were fewer in number and further from the crack tip and crack path. This indicates that the plasticity increased (i.e., the damage that occurred in the presence of hydrogen matched the hydrogen-enhanced local plasticity mechanism).