On the hot cracking of HSLA steel welds: Role of epitaxial growth and HAZ grain size

Centerline hot cracking is one of the major defects observed during welding of HSLA steels. While extensive efforts have been devoted to the control of weld process parameters, less attention has been paid to the influence of the starting microstructure. In the present work, the effects of the initial base metal composition and grain size, as well as of the Ni/Cu ratio of the filer metal, on the susceptibility centerline cracking, were investigated. T-type joint configuration, commonly used in the transportation and construction industries, were considered for the welding trials and the weld joints were characterized using laser confocal and electron microscopies, including chemical (EDS) and electron backscattered diffraction analysis (EBSD). Epitaxial growth was identified as the dominant solidification pattern according to thermodynamic calculations and the lattice coincidence of the delta-ferrite in HAZ and the weld metal. Results indicated that regardless of the effect of filler metal composition, hot cracking occurred at the centerline of all the more alloyed base metal welds. A correlation was established between the base metal grain size and the development and interlocking of primary and secondary dendrite arms. The grain growth kinetics of the base metals in the HAZ region was correlated to the thermal stability of microalloyed carbides and nitrides. No specific relationship between the Ni content of the filler metal and the centerline hot cracking was observed in this investigation.