DUAL PRECIPITATION STRENGTHENING EFFECT OF COPPER AND NIOBIUM IN HIGH-STRENGTH STEEL WELD METAL

Experimental work to produce high strength steel welds with systematic additions of copper (up to 3.5 wt.pct.) and niobium (up to 0.45 wt.pc t.) at 3.6 kJ mm(-1) (90 kJ in(-1)) heat input was carried out. Copper additions were found to precipitation strengthen the high strength st eel weld metals. An estimated yield strength of 900 MPa (130 ksi) can be achieved in single-pass welds with approximately 2.8 wt.pct. of cop per addition. Nevertheless, multipass welding over the copper-enhanced weld deposit resulted in nonuniform weld metal properties across the weld beads. Niobium additions did not provide as powerful a strengthen ing effect in the single-pass high heat input welds as the copper addi tions. At approximately 3.2 wt.pct. copper and 0.06 wt.pct. niobium in the weld deposits, the two elements provided the required strengtheni ng effect with thermally stable precipitates to produce high strength steel weld metals that exhibited acceptable properties in high heat in put, single- and multi-pass conditions. The combined effect of copper and niobium in high strength steel weld metal derives from the combina tion of two kinds of precipitates with distinct precipitation reaction kinetics and thermal stability.