Effect of hydrogen on fracture morphology of hydrogen assisted cracking in steel weldments.

The interaction of hydrogen and residual stress in a susceptible microstructure in a steel weldment promotes hydrogen assisted cold cracking (HACC). Hydrogen is commonly believed to accumulate in triaxially stressed regions during an incubation period in which the concentration of hydrogen builds up and eventually exceeds a critical amount, resulting in the opening up and propagation of cracks through the weldment. For high hydrogen electrodes, cracking in the weldment can occur in a very short period of time as the supply of hydrogen is abundant. The fracture morphology can be the result of quasi-cleavage (QC) or microvoid coalescence (MVC) or a combination of both. In the present work the microstructures and fracture morphologies of cold cracks developed in weldments produced by deposition of high and low hydrogen electrodes onto 550 MPa (80 ksi) base plate steels were investigated to elucidate the mechanism of fracture with respect to possible hydrogen accumulation sites and the structural sources of crack initiation and easy propagation. Weldments from low hydrogen electrodes exhibited delayed cracking which was found to initiate in the heat affected zone (HAZ) by intergranular (IG) fracture. Cracking in the weld metal of high hydrogen cellulosic electrodes was found to be both transgranular and intergranular (or intercolumnar, Ic). In the case of Ic fracture, the crack path was along the prior austenite grain boundaries in the HAZ and through the veins of allotriomorphic ferrite decorating the columnar grain boundaries in the weld metal. In contrast, the transgranular cracking was mainly quasi-cleavage for the HAZ and ductile for the weld metal, consisting of small coalesced microvoids. Fine non-metallic inclusions were present in the fusion boundary and were associated with the initiation of the microvoids ahead of the advancing crack tip.