Influence of C, Mn and Ni Contents on Microstructure and Properties of Strong Steel Weld Metals — Part II. Impact Toughness Gain from Manganese Reductions

Two experimental high strength steel weld metals were produced with 7 wt. % nickel and manganese at 2 or 0.5 wt. %. Neural network predictions that Mn reductions increase toughness were confirmed with impact energy increasing from 32 to 113 J at –40 °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 weld metal. Reducing manganese increased the transformation temperature promoting formation of upper bainite with a fine grain size and dramatically reduced the amount and size of coalesced bainite. Increased toughness was attributed to the finer grain size of bainite constituents and a more effectively tempered microstructure.