TWI Industrial Member Report Summary 100/1979
By N Bailey and R J Pargeter
Tests to examine the influence of the flux on the strength and toughness of multipass submerged arc welds in 30mm thick mild steel have been made with a range of fluxes and an arc energy of 3kJ/mm. Unalloyed wires were used to give weld Mn contents of approximately 1%, although some additional tests were made to explore the effects of Mn content.
The main influence of flux on strength and toughness is exerted through its influence on the weld metal composition, which in turn influences microstructure and inclusion content. A high inclusion content reduces upper shelf toughness and also the slope of the Charpy V transition curve and hence increases the Charpy transition temperature.
The wire and plate composition play a major part in influencing strength and cleavage resistance. Yield and tensile strength can be related to weld composition simplified to a single value, the carbon equivalent.
Good cleavage resistance can be achieved either with a microstructure containing at least 60% acicular ferrite, when the yield strength will normally exceed 450N/mm2, or by reducing the yield strength as far as possible below that value in combination with a microstructure of upper bainite, side plate structures and coarse grain boundary or massive proeutectoid ferrite. With both types of behaviour, low C, P, Si and N contents appear to be beneficial. Strong tough welds are obtained with basic fluxes, but less strong welds with good cleavage resistance can be achieved with manganese silicate and calcium silicate flux types.
Stress relief heat treatment improved the cleavage resistance of all welds except most of those made with manganese silicate fluxes; this treatment generally reduced strength and increased ductility.