Physical Metallurgy of Steel Weldability

The concept of hardenability used in the welding field differs from that used in the heat treatment. It represents the hardened depth after quenching in the heat treatment while it is related with the welding condition for fully hardened heat-affected-zone (HAZ) in the welding. The effect of steel chemical composition on hardenability is expressed by a multiplying factor for the former and by carbon equivalency for the latter. The metallurgical relation between these two factors can be clarified by a heat conduction analysis of a quenched round bar. Steel weldability means susceptibility to hydrogen-assisted cold cracking which mostly occurs in the welding of high strength steels. Many carbon equivalents with different coefficients have been proposed to assess the cold cracking susceptibility which is affected greatly by the hardness at HAZ. Since the HAZ hardness is interactively determined by the carbon content and hardenability, carbon equivalency for assessing susceptibility to cold cracking must consider this interactive effect. Toward the international standardization of the guideline for the avoidance of cold cracking, the methods based on different carbon equivalency are discussed.