TWI Industrial Member Report Summary 531/1995
Laser and electron beam (EB) welding can offer significant advantages in joint completion rate over conventional arc methods, but a problem with these low oxygen power beam processes is the difficulty of producing steel welds with good fracture toughness. The microstructure of a steel weld is critical in determining its mechanical properties. Although weld metal microstructural development of medium and high oxygen (>100ppm) welding processes (e.g. submerged arc and manual metal arc) has been studied extensively, comparatively little work has been carried out in the field of low oxygen weld metals (e.g. laser and EB).
This report describes phase III of a programme which aims to further the understanding of the microstructural development (and hence how to optimise the toughness) of low oxygen steel weld metal. Phases I and II investigated the effects of Al, Ti and V on the microstructure of low oxygen TIG and low oxygen laser weld metal. This work showed that to produce a fine-grained microstructure the Al/O ratio must be less than unity, and a trace of Ti must be present. Vanadium was also shown to promote, but not to be necessary for, the formation of a fine-grained microstructure. In this third phase, EB and laser weld metals have been studied, in terms of the effects of Al, Ti, V and cooling rate on transformed microstructure. In addition, CTOD tests on selected laser welds were carried out.