2003 Volume 43 Issue 5 Pages 706-713
The effect of microstructural and microhardness variations on the formation of hydrogen induced cold cracks (HICC) has been investigated in the multipass weld metals. Multipass weld metal cracking tests were performed with submerged arc welding on 25 mm thick HSLA-100 steels using commercial welding materials. All the cracks observed in the welds were transverse and vertical to the welding direction, and most of them were located in the interior of top layer beads except the final pass. The morphology and preferred location of cold cracks imbedded in the weld metal were studied by scanning acoustic microscopy (SAM) and were confirmed on the sectioned specimens by optical microscopy. Overlapping the two-dimensional crack morphology on the microstructure demonstrated that most of cracks were located in the areas having a structure of columnar grains but not in the areas of recystallized grains. Thus they tended to stop their propagation at the fusion boundaries. Microhardness measurement showed that the recrystallized region had a high hardness and was surrounded by tempered band that had a minimum hardness. Crossing over the tempered band, the hardness increased gradually to a high level. Most of the cracks were densely populated in this region in which the hardness increased gradually. These results indicated that the columnar grain structure would be the most susceptible microstructure to cold cracking and the hardness did not play a major role in developing cold cracks in the present welds. Those cracks were developed predominantly in intergranular mode following the elongated columnar grain boundaries. Based on these results, several suggestions could be made for developing preheat-free welding consumables.
