Trapping Sites of Diffusible Hydrogen in Deposited Weld Metals associated with Hydrogen Delayed Fracture

Hydrogen-delayed fracture (HDF) was analyzed from the deposited weld metals of 600 MPa, 800 MPa, and 1000 MPa flux-cored arc welding wires. Two types of the deposited weld metal were used such as the rutile weld metal and alkali weld metal, for each strength level. Constant loading test and thermal desorption spectrometry (TDS) analysis were conducted on the hydrogen pre-charged specimens electrochemically for 72 h. Even though the welds consisted of the complicated microstructure, electron back-scattered diffraction (EBSD) was able to determine the grain boundary by the examination of grain orientation. The effects of microstructures such as acicular ferrite, grain-boundary ferrite, and low-temperature-transformation phase on the time-to-failure and amount of diffusible hydrogen were analyzed. The fracture time for hydrogen-purged specimens in the constant loading tests decreased as the grain size of acicular ferrite decreased. Analyzing the TDS results, the HDF was determined by the amount of diffusible hydrogen in the deposited weld metal. The major trapping sites for diffusible hydrogen were the grain boundaries, as determined by calculating the activation energies for hydrogen detrapping. The inclusions mainly made of Ti and Mn complex oxides were defined to be a trapping site of non-diffusible hydrogen. As the strength was increased and alkali weld metal was used, the resistance to HDF was decreased.