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Effect of grain boundary ferrite on susceptibility to cold cracking in high-strength weld metal

Due to the practical limitations of lowering the diffusible hydrogen content of flux-containing welding consumables, it is now felt that modification of the weld microstructure would alleviate the risk of weld metal cracking in multi-pass weld deposits. Thus, this study aimed to identify and evaluate the effect of the weld microstructure on the cold cracking susceptibility of FCAW weld metals and then to provide a basic guideline for designing new welding consumables from a microstructural point of view. In order to identify the parameter(s) that can quantify the microstructural susceptibility of multi-pass weld deposit, two sets of FCAW wires with tensile strength of about 600 MPa were prepared by controlling the Ni content to allow sufficient variation in the weld microstructure, but with little change in weld metal strength. The cold crack susceptibility of those two chemistries was evaluated by a multi-pass weld metal cracking test at various levels of diffusible hydrogen content. All of the cold cracks developed were Chevron-type, and the occurrence of such cracks depended upon the proportion of grain boundary ferrite (%GF) as well as the diffusible hydrogen (HD) content. In fact, at the same level of HD, higher Ni (1.5%Ni) wire showed better resistance to cold cracking than lower Ni (0%Ni) wire even though the latter was stronger and also higher in carbon equivalent. This result could be explained solely by the difference in grain boundary ferrite content between those two welds since Chevron cracking preferentially initiates at and propagates along grain boundary ferrite. Therefore, we propose the use of a value of %GF as a parameter to quantify the microstructural susceptibility of ferritic multipass weld deposit with a strength level of about 600 MPa. It was further suggested that, in addition to the hydrogen control approach, microstructural modification in the form of reducing the %GF can be pursued to develop welding consumables with improved resistance to cold cracking.