TWI Industrial Member Report Summary 70/1978
By N Bailey and R J Pargeter
Tests to examine the influence of the flux on the strength and toughness of two-pass submerged arc welds in 30mm thick mildsteel have been made with a range of fluxes. Unalloyed wires were used to give weld Mn contents of approximately 1%. Some additional tests were made to explore the effects of Mn content and also of section thickness.
The main influence of flux on strength and toughness is exerted through its influence on composition, which in turn influences microstructure and inclusion content. A high inclusion content reduces upper shelf toughness and also the slope of the Charpy traniition curve and hence increases the Charpy transition temperature.
Proof and tensile strength were proportional to composition, expressed as the carbon equivalent. No effect of flux on strength or ductility properties was found. Stress relief heat treatment at 600°C reduced proof and tensile strength by 70N/mm2 and increased elongation and reduction of area by ~5%.
Although regression analysis of toughness results did not provide useable equations, the results suggested that Si, C, P and N were generally harmful and Mn beneficial. Welds with a high resistance to cleavage contained grain boundary ferrite, upper bainite or interphase carbides, side plate structures and coarse acicular ferrite, none exceeding 50% of the total microstructure.
Basic fluxes gave the best resistance to cleavage andtearing, i.e. low COD and Charpy transition temperatures with high upper shelf energies. Alumina and manganese silicate flux welds also gave good cleavage resistance but resistance to tearing was less and Charpy transitions higher. Calcium silicate flux types gave indifferent COD and Charpy transitions except that the cleavage resistance was usefully improved by heat treatment.
Stress relief heat treatment generally reduced both COD transition temperature and yield strength and sometimes Charpy transition temperature.