高張力鋼熔接変質部の冷却状況と顕微鏡組織および硬度(第2報)

Abstract

To make clear the effects of welding conditions on the micro structure and hardness in welding heat-affected zone of high tensile Mn-Si steel, authors have studied cooling phenomena at that zone in the neighbourhood of fusion line.According to the first report, there was no variation in cooling curves and hardnesses unless the specimens were smaller than 150 mm in width under reported conditions. Therefore, we have continued the investigation on the effect of thickness and welding conditions using certain larger specimens and got some relations between cooling curve of heat-affected zone and the microstructure of the part with maximum hardness.Each effect of plate thickness, proceeding rate of bead and welding current is significant, but the decrease of bead length does not change the cooling curve seriously, if it exceed sixty or seventy percent of plate length. The electrode type has also a considerable effect on the cooling curve.Critical cooling curves, which start to allow each appearance of intermediate transformation structure, ferrite and pearlite, have been decided with microscopic investigation (Fig. 9).And authors proposed that the welding conditions should be selected suitably in the light of above mentioned curves of a given steel, in order to prevent an occurrence of very hardened part in the heat-affected zone by welding.