TWI Industrial Member Report Summary 67/1978
By D J Abson, R E Dolby and P H M Hart
Since the presence of a high proportion of acicular ferrite confers on weld metals a high resistance to cleavage fracture, it is important to understand the factors controlling the formation of this microstructural constituent. Experiments have therefore been carried out to investigate the effect on acicular ferrite formation of changes in weld metal oxygen and sulphur levels and hence total inclusion volume fraction in submerged arc welds. Oxygen levels ranged from 0.011 to 0.060% and sulphur levels from 0.006 to 0.029%.
The results are consistent with the hypothesis that inclusions in weld metal can influence the nucleation of ferrite. The effect of the changes in oxygen content was much more marked than those produced by the changes in sulphur level. A decrease in oxygen content from 0.030 to 0.011% virtually eliminated the nucleation of acicular ferrite and thus produced a substantially bainitic structure instead of a predominantly acicular ferrite one. An increase in the oxygen content from 0.030 to about 0.060% yielded predominantly side plate structures when the high oxygen level was produced from a calcium silicate medium silica flux. In contrast, oxygen levels of 0.06% produced from an alumina flux resulted in little change, i. e. acicular ferrite was still extensively formed.
It is concluded that the inclusion type and size distribution was substantially more important than the total inclusion volume fraction in influencing ferrite nucleation.