Influence of carbon, manganese and nickel on microstructure and properties of strong steel weld metals Part 2 -: Impact toughness gain resulting from manganese reductions

Two experimental high strength steel weld metals were produced with 7 wt-% nickel and either 2 or 0.5 wt-% manganese. Neural network predictions that it is advantageous to reduce the manganese concentration in high nickel alloys have been confirmed, with impact energy increasing from 32 to 113 J at -40 degrees C. High resolution microstructural investigations showed that both weld metals contained mainly martensite at interdendritic regions and predominantly bainite at dendrite core regions, as a consequence of manganese and nickel segregation. In the high manganese weld metal significant amounts of coarse grained coalesced bainite formed whereas mainly upper bainite was seen with 0.5 wt-% manganese. Reducing manganese content increased the transformation temperature, promoting fine upper bainite at the expense of coarse coalesced bainite. Increased toughness was attributed to the finer grain size of bainite constituents and a more effectively tempered microstructure.