Harumasa Nakamura

Cold Cracking in Multilayer Welds of 100kg/mm2 Grade Seel

The present study is an application of the NRIM TRC test (Tensile Restraint Cracking test) to cold cracking in multilayer welds of a high-sterngth steel with a strength level of 100 kg/mm2. It was aimed in this study to make clear the cracking behavior and to get practical methods for the prevention of cracking.A TRC specimen was set up in a 1000 tons NRIM TRC tester and pulled after finishing of a test weld. The constant loading at various stress levels was sustained for sufficient time for cracking.There occurred two types of macrocracks in multilayer welds depending on the direction of restraining, that is, transverse and longitudinal cracks in longitudinal and transverse TRC tests, respectively. Those cracks in longitudinal and transverse TRC tests, respectively. Those cracks were considered extensions of microcracks. Cracks, both transverse and longitudinal, were the delayed type due to hydrogen and occurred after an incubation period under a sustained tensile loading which exceeded the critical tensile stress for cracking. The incubation period extended from about 1 to 10 hr, mainly depending on restraining stress and hydrogen content. The critical tensile stress for cracking was raised with an increase of preheating temperature and with a decrease of hydrogen content. The critical tensile stress for longitudinal cracking was slightly lower than that for transverse cracking at the same welding conditions and hydrogen content.Transverse cracks were prevented under the longitudinal tensile stress of 65 kg/mm2 with any of the following methods: (a) preheating at 200°C, (b) 100°C preheating and locally tempering postheating at 500°C, or (c) 150°C preheating and lower temperature postheating for 5 hr at 100°C. The restraining stress of 65 kg/mm2 was a little over the restraining stress in the so-called window restrained test which has been appreciated as fairly reporducible one of actual severe restraint. Very little change of hydrogen content, about 0.2 cc/ 100 g, in deposited metal was decisive to transverse cracking in such high strength steel welds, supposing actual joints with severe restraint.Longitudinal cracks were gradually propagated and then usually caused a brittle fracture throughout the joint. The carck was prevented with preheating at 200°C under the transverse tensile stress of 55 kg/mm2 At the tensile stress of 65 kg/mm2 any crack did not occur with simultaneous use of 150°Cpreheating and lower temperature postheating, 200°C for 1 hr or 100°C for 5 hr.Effectiveness of lower temperature postheating was discussed with an estimation of existing hydrogen content in a weld at the instant of cracking.

Effect of Microstructure on Root Cracking in High Strengh Steel Welds

Effects of microstrure on root cracking of high strength steel welds were discussed in this report, mainly with the use of SH-CCT diagrams of test steels. Critical hardness values for root cracking were also discussed.The main conclusions obtained in this study are as follows :(1) A safety cooling process to prevent root cracking in HAZ should be chosen so that the ferrite and intermediate structure may appear more than about 40% for HT 60, intermediate structure more than about 25% for HT 70, and more than about 10% for HT 80 steels, respectively, when a joint of severe restraint is welded with a low hydrogen type electrode.(2) For a weld metal, it is necessary to prevent root cracking in a severe restraint joint that the ferrite and intermediate structure precipitate more than in HAZ.(3) A root crack in HAZ or weld metal is a mixed fracture of transcrystalline and intercrystalline types.(4) There was found no relationship between root cracking tendencies and Mf temperatures.(5) Retained austenite in HAZ was decomposed with the lapse of time above 250°C, but did not affect the root cracking phenomena.(6) Critical hardness, above which a root crack will surely occur in a joint of severe restraint, were found to be as follows :270 VHN (1 kg) for HAZ of mild steel weldment365 VHN (1 kg) for HAZ of HT 50 weldment400 VHN (1 kg) for HAZ of HT 60 weldment405 VHN (1 kg) for HAZ of HT 70 weldment415 VHN (1 kg) for HAZ of HT 80 weldment290 VHN (1 kg) for weld metal of HT 50 and HT 60 weldments330 VHN (1 kg) for weld metal of HT 70 and HT 80 weldments