50kg/mm<SUP>2</SUP>級高張力鋼すみ肉溶接部の割れについて

Abstract

It is generally known that the majority of fillet weld cracks are toe cracks, underbead cracks and so on. But we have found an unusual crack in the.fillet welds of 50 kg/mm2 grade high strength steels. The crack occurred at the heel-opposite side of the toe-of fillet weld. Therefore we refer to them by the name of "Heel-Crack" in this paper. We have carried out various experiments to make clear the characteristics of the crack and the causes of its formation, and established a practical preventive method for the crack based on these test results. The test results are summarized as follows:(1) Heel-cracks are located at the heel of fillt welds and they are hardly detectable by inspection methods such as visual observation and magnetic particle inspection.(2) It has been difficult to reproduce the heel-cracks by well-known weld cracking tests such as C.T.S. test and Reeve cracking test. We have developed a Non-restraint T type Cracking Test which can reproduce heel-cracks successfully.(3) Heel-crack is a cold crack which occurs at temperatures below 100°C.(4) Main factors which affect the heel-crack formation are the cooling process of the weld, rigidity and hydrogen in the weld.(5) Many 50 kg/mm2 grade high strength steels are susceptible to the formation of heel-cracks. But semikilled steels seems somewhat less susceptible to heel-cracks than killed steels.(6) Crack sensitivity is increased by increasing the carbon equivalent and/or the maximum hardness at underbead.(7) Heel-crack could be prevented by preheating or controlling the bead length. To prevent the heelcrack, higher preheating temperature and longer weld bead are desirable.