Continuous Cooling Transformation Diagrams of the High Tensile Steel "Wel-ten 55" and Structures and Properties of Heat-Affected Zone by Welding (Report 3)

Relations between the continuous cooling transformation diagrams and the structures and properties of heat-affected zone by welding

Abstract

The rapid heating maximum temperature of the heat-affected zone due to welding decreases with the increase of the distance from the fusion line. But the cooling times, from A₃ transformation point to 500°C, in the cooling process from those various maximum temperatures at the heat-affected zone do not differ so much. However, the cooling time changes extensively by thickness and width of a plate, welding current, proceeding rate of bead and other welding conditions.
Structures and hardnesses at the neighbourhood of the fusion line in the heat-affected zone of a welded joint are similar to those of the specimen, cooled with the same cooling time as those of that part of affected zone, which can be known by the continuous cooling transformation diagram in case of the rapid heating maximum temperature 1300°C. Accordingly, if we obtain such continuous cooling transformation diagram, the maximum hardness and the corresponding 'structure which appear in the heat-affected zone can be supposed by the diagram.
The absorbed energy by impact bend and bend angle of the specimen, used to obtain the continuous cooling transformation diagram in case of the rapid heating maximum temperature 1300°C, show extremely small values when the cooling time is shorter than the critical cooling time Cf. Therefore, it is considered that if the welding is performed so as to cool the neighbourhood of the fusion line in the heat-affected zone with the longer time than the Cf', the resistances of the obtained joint for shock and deformation are large and the joint will be secure. Further, it is desirable to make steels which have necessary mechanical properties and show the critical cooling time C1' being as small as possible.