Microstructural parameter controlling weld metal cold cracking

Abstract

Purpose: Facing with the practical limitation in lowering diffusible hydrogen content, the possible modification of weld microstructure would alleviate the flux cored arc (FACW) weld deposits from the risk of weld metal cold cracking. Therefore, it was aimed to identify and evaluate the effect of weld microstructure on cold cracking susceptibility of FCAW weld metals, and then to give a basic guideline for designing new welding consumables from the microstructural point of view. Design/methodology/approach: In order to figure out the parameter(s) that can quantify the microstructural susceptibility of multi-pass weld deposit, two sets of FCAW deposits having tensile strength of about 600MPa were prepared by controlling the Ni content to allow a sufficient variation in weld microstructure but with little change in weld metal strength. Cold crack susceptibility of those two chemistries was evaluated by ‘multi-pass weld metal cracking test’ at various levels of diffusible hydrogen content. Findings: All of the cold cracks developed were Chevron-type cracks and the occurrence of such cracks was depending on the proportion of grain boundary ferrite as well as the diffusible hydrogen content. In fact, at the same level of diffusible hydrogen, 1.5%Ni wire showed better resistance to cold cracking than the 0%Ni even though that was higher in strength and carbon equivalent. This result could be explained by the difference in grain boundary ferrite content between those two welds based on the microstructural characteristics of Chevron cracking that preferentially propagates along grain boundary ferrite. Research limitations/implications: Detrimental effect of grain boundary ferrite against cold cracking has been addressed for Chevron-type cracks that is commonly developed in the ferritic weld metals having 600MPa strength level so that present result may not be valid for higher strength welds over 700MPa which shows vertical-type cracks. Practical implications: In addition to hydrogen control approach, microstructural modification in a way to reduce the proportion of grain boundary ferrite can be pursued for developing welding consumables with improved resistance to cold cracking. Originality/value: Proportion of grain boundary ferrite was proposed as a parameter that can quantify the microstructural susceptibility of multi-pass weld deposit. This fact can be used for users and manufacturers in selecting and designing welding consumables with better resistance to cold cracking.