Analysis of underbead cracking in underwater wet weldments on A516 grade 70 steel.

Abstract

The use of underwater weldments on U.S. Naval Vessels is highly desirable due to the ability of performing repairs without costly dry dock expenses. The primary problem with underwater wet weldments is underbead cracking in the heat affected zone (HAZ). The fundamental factors causing underbead cracking in underwater wet weldments using a shielded metal arc welding (SMAW) process are high quench rates, slag inclusions, diffusible hydrogen levels and porosity. The weld metal analysis included use of optical and scanning microscopy as well as microhardness testing. Three weld samples made at 5 deg C, 12 deg C, and 25 deg C water temperature were analyzed in this thesis. HAZ underbead cracking was present in all three welds analyzed although the 5 deg C sample was the only weld that exhibited extensive cracking whereas the 25 deg C sample only had cracking near the upper 50% of the weld passes. Crack origination in all three samples near the cap was evident and was most likely due to small levels of bead tempering at this location. This thesis addresses the mechanisms of the cracking as well as the effects of diffusible hydrogen, cooling rates, and water temperatures on wet weldments