EFFECT OF NITROGEN ON TOUGHNESS AND STRAIN AGE EMBRITTLEMENT OF FERRITIC STEEL WELD METAL

Investigations were carried out to evaluate the influence of dissolved nitrogen in ferritic steel weld metal on its toughness and strain agi ng behaviour through fracture mechanics as well as conventional impact testing approaches. Manual metal are C-Mn steel weld metals with nitr ogen contents between 80 and 210 ppm (wt) were investigated under four different post-weld conditions, namely, (i) as welded, (ii) stress re lieved, (iii) artificially strain aged, and (iv) artificially strain a ged and stress relieved. Quantitative metallography and low load micro hardness studies of microphases were an integral part of these investi gations. The results demonstrate the highly detrimental effect of nitr ogen on the toughness behaviour of C-Mn steel weld metal, particularly under strain aging conditions. This is substantiated by the decrease of acicular ferrite content with the accompanying increase in primary ferrite and ferrite with second phases in the microstructures. Also, t here is a distinctive increase in acicular ferrite microhardness. Post -weld stress relieving heat treatment under these conditions results i n only a marginal improvement in toughness and shifts the fracture beh aviour from brittle to ductile or quasiductile only for low nitrogen w eld metals. Comparing the results of the crack tip opening displacemen t and Charpy tests, it is observed that both methods show the influenc e of nitrogen on toughness behaviour to have the same form but the mag nitudes of the effect measured are different, the results obtained usi ng the fracture mechanics method appearing very conservative.