引張拘束割れ試験(TRC試験)による高張力鋼溶接部のルート割れにおよぼす拘束外力の影響について

Abstract

Effects of transverse restraining force on root cracking of high strength steel welds were studied with NRIM TRC tester, that is, Tensile Restraint Cracking Tester which was developed by the National Research Institute for Metals. In the TRC test, two pieces of a butt welded specimen is tensile-loaded transverse to the weld line and kept at an arbitrary constant value until cracking occurs. In the present tester, two pieces, 210×120 mm, test welded as long as 120 mm, can be loaded up to a maximum of 20 tons. Since the root cracking in this investigation was exclusively of cold cracking, type, the tensile loading was done 3 minutes after the finish of test welding in a Y-groove joint. The results obtained with high strength steel welds of tensile strength level of 60 to 80 kg/mm2 are as follows :(1) The root cracking of high strength steel welds in this study was of cold cracking and delayed failure type. Effect of hydrogen embrittlement in welds was obvious.(2) A cold cracking was generally initiated after a certain incubation period and finished after an additional period. The incubation period was the smaller, the greater the tensile restraining stress which is defined as the mean stress in weld metal, that is, the load divided by the longitudinal sectional area transverse to the direction of loading.(3) There was a critical stress below which root cracking did not occur. The value of the critical stress seems to depend on ductility of weld metal, hydrogen content in weld zone and welding condition. The critical stress was increased by raising the preheating temperature until the value reached the yield stress of the weld metal at a critical value of preheating temperature. With thecr itical preheating, it was observed that no root cracking occurred in modified Lehigh Restraint Cracking Specimen tested with the same welding condition.(4) For a T-1 steel weld, welded at room temperature and kept tensile stress free in the TRC tester for 20 minutes and more after welding, both the critical tensile stress and the incubation period were considerably increased, which seems to show that the amount of diffusible hydrogen in weld metal is an essential factor in root cracking of high strength steel welds.(5) The latest theory of hydrogen embrittlement in steels which had been proposed by Troiano and others seems to be valuable in explaining the mechanism of root cracking in high strength steel welds.