Effect of metallurgical transformations on weld residual stresses — application to E690 steel grade

Abstract

Many authors have discussed the effect of welding residual stresses on the life properties of welds. As it has been demonstrated that usual welds have residual stresses at the level of the yield strength of the base material, many codes or standard assume this point for the design of welded structures. Because residual stresses are linked to thermal cycles and metallurgical transformation in HAZ as well as in weld metal, few authors have nevertheless demonstrated that residual stresses can be lower than expected. For the demonstration of this phenomenon on HY80 to HY130, previous workers have used TEKKEN tests instrumented with strain gauges. The same kind of experiment has been used by CLI to understand the effect of the combination of different filler materials and weld metal ranging from HY80 to HY130 with matching and undermatching combinations. A comparison with martensitic stainless steels has been also performed. According to these results, it can be shown that the chemical composition of the weld metal can lead to complete change of restrain cycles on TEKKEN tests. When the weld was sufficiently alloyed to have martensite or lower bainite in weld metal, residual stresses have been the lower. As the tests have been performed on steels already used for the fabrication of JACK-UPs, a comparison of the lab results to industrial one's was easy. For this purpose, a rack delivered for the fabrication of Jack-Up legs has been assessed for residual stresses. As expected, the residual stresses measured with drilling hole method were far lower than the yield strengths of weld metal or base material (E 690 type / SUPERELSO 702). These results that comply with CLI experience on residual stresses on high-strength steels have then confirmed the tendencies established on laboratory tests. It is then demonstrated that use properties of high-strength steels, especially when sufficiently alloyed, are to be higher than expected because the residual stresses are much lower than the assumed level.