TWI Industrial Member Report Summary 68/1978
By D J Abson
In two series of tandem submerged arc welds deposited at an arc energy of 4. 9kJ/mm in 25mm thick C-Mn steel, the influence of variations in carbon content and in silicon content on the microstructure, hardness and absorbed Charpy energy was investigated in order to gain a clearer understanding of the effect these elements have on weld metal microstructures and toughness. Variations in carbon content from 0.12 to 0.19% were achieved by carburising a prepared groove prior to welding and the variations in silicon content from 0.23 to 0.57% by welding parent material of differing silicon content.
For weld metal containing ~ 1.35%Mn, ~ 0.16%Mo, ~ 0.23%Si and <0.005%Nb, an increase in carbon content from 0.12 to 0.19% reduced the grain boundary proeutectoid ferrite content and both refined the acicular ferrite and gave an increase in the proportion of it. In spite of these microstructural changes, there was a decrease in Charpy toughness which was attributed to an increase in the proportion and in the non-uniformity of distribution of pools of retained phase, and also to an increase in hardness, part of which was attributable to an unintentional increase in nitrogen content.
For weld metal containing ~ 1.26%Mn, 0.17%Mo, 0.15%C and <0.005%Nb, an increase in silicon content from 0.23 to 0.57% produced a small increase in the proportion of acicular ferrite and a refinement of the ferrite laths. From measurement of the weld thermal cycle, it was noted that the temperature range for the transformation of y to a was lowered with an increase in silicon. An increase in hardness from 209 to 225HV5 and an increase in the temperature for an absorbed Charpy energy of 35J, from -53 to -43ºC were observed.