Weldability aspects of high yield strength Q & T steels

A specially designed, highly restrained speciamen test (60º groove angle) was successfully used to study root run weld metal solidification cracking origins and tendencies. Thirty six submerged arc welds were made on two High Yield Strangth Quenched and Tempered base steels (HY100 and Q2N0, with two low alloy wires (LINDE 95 and OERLIKON S3 NiMo 1), two basic fluxes (OP41TT and OP121TT), and six welding parameters combination at three energy inputs, viz: 1.9, 2.8 and 4.1 KJ/mm. The weld metal solidification cracking phenomenon showed up to be extremely complex and basically dependent on chemical composition, solidification (primary) structures, microsegregation, flux type, dendrites size and orientation, and base metals stress state. The most resistant welds to this defect were those made using Q2N base steel, OP121TT flux and OERLIKON S3 NiMo 1 wire. A combination of the following properties are thought to have induced this result: high carbide-to-ferrite elements forming in the base metal; flux promoting weld centerline equiaxed structure and less microsegregation; wire chemical composition, with high Mn/Si ratio, inducing also less microsegragation at the weld centerline; lower carbon and nickel combined contetens in presence of also lower phosphorus and sulphur contents. The Q2N greater ability to relieve the strain around the weld pool at high temperature is another possibility, albeit not practically demonstrated. The welding paramenters main influence on solidification cracking tendency was found to be through change in centreline solidification macrostructure and dendrite size, but the stress fields within and around the weld pool are also thought to play a significant role. The dendrite size holds a strong relationship with weld bead geometric factors, such as weld bead height-to-width ration and, principally, external area-to-perimeter ratio. The dendrite size measured on the weld bead longitudinal section must be corrected, for it depends on the angle between the dendrite growth direction and the weld ...