Metallurgical Design Rules for High-Strength Steel Weld Metals

A review of a dilatometric analysis of selected Fe-C-Mn high-strength steel shielded metal arc weld metals showed that balanced Ti, B, Al, O, and N additions reduced the austenite-to-ferrite transformation-start (TS) temperature. These microalloy additions must match the following aim levels for composition control: Ti at 400 ppm (0.04 wt-%), B at 40 ppm (0.004 wt-%), Al at 200 ppm (0.020 wt-%), O at 400 ppm (0.04 wt-%), and N preferably below 80 ppm (0.008 wt-%) to ensure effective deoxidation, form complex inclusions, and distribute them to enable development of highly fracture-resistant refined weld metal microstructures. It may be wiser to avoid the rich and lean ends for these microalloy additions, except N, which should be held at the lean end, preferably much below 80 ppm (0.008 wt-%). The balanced Ti, B, Al, O, and N additions offered nearly a 100°C shift in lowering the Charpy V-notch (CVN) test temperature for either 28 or 100 J absorbed energy. Dilatometric evaluations of reheated weld metals showed that 1) the balanced Ti, B, Al, O, and N additions lowered the actual TS temperature by about 60°C compared to the calculated austenite-to-ferrite transformation temperature obtained from the constitutional equation; 2) N with more than 100 ppm (0.010 wt-%) effectively nullified the beneficial effects of Ti, B, and Al additions in lowering the transformation temperature; and 3) at N content much below 80 ppm (0.008 wt-%), both a lower TS temperature and a narrow start-to-finish (TS–Tf) temperature range helped in achieving exceptional weld metal CVN impact toughness.