Cooling Curve, Microstructure and Hardness in Welding Heat-Affected Zone of High Tensile Steel (Report 5)

Effects of Postheating

Abstract

The welded parts of 14 mm steel plates (Wel-ten 50) and 20 mm steel plates (Wel-ten 55) were postheated respectively when about 70 sec or 40 sec elapsed after welding with a torch travelling at intervals of the definite distance, about 14 cm or 8 cm, behind the electrode tip. Furthermore, 14 mm steel plate (Wel-ten 50) was investigated by varying the postheating time. Those results are summarized as follows :
1) The microstructures of the heat-affected zone in the neighbour hood of fusion line, in deposited and postheated specimen, are estimated practically from the temperature-time fields divided by the critical cooling curves. The peak temperature range of. postheating which causes the spheroidization of carbide and of postheating which produces new structure by austenitization were clarified.
2) The flatness of deposited metal, the width of heat-affected zone and the austenitic grain size change remarkably with preheating as mentioned in the report 4. In the case of postheating, when the distance between the electrode-tip and torch-tip is smaller, such effects of preheating also appear in some degree. If the distance is shortened extremely, the welding arc becomes unstable, the slag is scattered and the pits appear on the surface of bead.
3) The maximum hardness of the heat-affected zone for the Wel-ten 55, 20 mm steel plate becomes remarkably lower with postheating. If the welded joint is postheated at about 650-750°C, the spheroidization of carbide occurs remarkably and the hardness becomes lower.