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4報)

Abstract

14mm steel plates (Wel-ten 50) and 20 mm steel plates (Wel-ten 55) of high tensile Mn-Si steels were used. The single bead was deposited on these plates and the change, by preheating with flame, of cooling curve about the heat-affected zone (the neighbourhood of fusion line), was investigated. According to this result, the cooling curve after welding shows a more gradual slope as the preheating temperaiure rises to about 350°C.The flatness of deposited metal, penetration, width of heat-affected zone and austenitic grain size increass as the preheating temperature rises. The hardness of welded part decreases considerably with preheating. The maximum hardness of the heat-affected zone for the Wel-ten 55 steel plate with 20 mm thickness is very high in case of no preheating. In this case, martensite appears on about 45% area of the part of the maximum hardness and the hardness is about 312 in Vickers hardness number, but when preheating is done it becomes remarkably lower value and is 210 in case of about 350°C preheating.Judging by the cooling curves after welding and the existence or non-existence of constituents in microstructure of heat-affected zone, two fields., showing existence of constituents, divided by a critical cooling curve are clarified. Further., the micro-Vickers hardness of each consituent was measured.