TWI Industrial Member Report Summary 41/1977
By J L Robinson
It has been known for some years that the root regions of MMA welds in thick C-Mn steel plate can display fracture toughnesses markedly lower than weld metal from locations near the surface. The objectives of the work described in this report were to investigate the causes of this phenomenon, and to examine the influence of some procedural and other variables upon the magnitude of the effect. To these ends a number of MMA butt welds were made in plate 68mm to 25mm thick, the major factors studied being degree of restraint, extent of weaving, back gouging and stress relief heat treatment. The fracture toughness of the weldments was evaluated by means of Charpy and COD tests, together with the determination of tensile properties, and the performance of metallographic examinations. It was concluded that strain ageing in root regions, as a result of the thermomechanical effects of subsequent runs, was an important factor reducing root toughness for the particular conditions evaluated. However, dilution of weld metal with parent plate material, and arc energy variations, could also exert a marked influence on root toughness under certain circumstances. The absence of back gouging, low levels of restraint with respect to angular movements, and full weaving, were found to accentuate the differences between root and subsurface properties. Postweld heat treatment, whilst producing overall improvements in toughness, did not eliminate the fall in root regions.