高張力鋼熱サイクル再現部の疲労特性について(第1報)

Abstract

Generally speaking, the fatigue strength of welded joints of high tensile strength steels is not so superior to that of ordinary mild steels, but any reasonable explanation has not been reported yet.There are many factors reducing the fatigue strength of welded joints, for example, undercutting, geometrical effect of reinforcement, residual stresses and microstructural change in weld heat affected zone, etc.Among those many factors, we intended to study the fatigue strength of weld heat affected zone of various kinds of steels range from 60 Kg/mm2 ultimate strength steel to 80 Kg/mm2 ultimate strength steel. In order to reproduce the microstructure of various portions in weld heat affected zone, test specimens were subjected to thermal cycles with synthetic thermal cycle apparatus, seting peak heating ternperatures of these thermal cycles 640, 740, 850, 1000 and 1350°C, corresponding to certain points at varied distances from weld bond, then machined to smooth fatigue specimens.Amsler's high frequency Vibrophores fatigue testing machine was used for evaluating the fatigue strength of all specimens. Fatigue limits of each specimens subjected to thermal cycles are nearly equal or higher than that of base metals in every case, and are proportional to hardeness or tensile strength of various portions in weld heat affected zone.These results mean that the reduction of fatigue limits of butt-weld joints of high tensile strength, steels is not due to the structural change, hardening or softening in weld heat affected zone.