New Developments with C–Mn–Ni in High Strength Steel Weld Metals — Part B. Mechanical Properties

Microstructure and properties have been studied for high strength steel weld metals with variations in carbon, manganese and nickel. Based on neural network modelling, experimental welds were made using shielded metal arc welding with manganese at 0.5 or 2.0 wt. % and nickel at 7 or 9 wt. %. Additional welds were made where carbon was varied between 0.03 and 0.11 wt. %. Generally there was very good agreement between the recorded mechanical properties and the neural network predictions. A combination of high nickel and manganese was positive for strength but very negative for impact toughness while manganese reductions lead to large impact toughness increases. Carbon additions up to 0.11 wt. % were found to increase yield strength to 912 MPa while still maintaining toughness at over 60 J at –100 °C. Increasing contents of manganese and nickel resulted in a significant lowering of Ac1 thereby contributing to less good impact toughness as a consequence of less tempering in reheated regions. Mechanical properties of the weld metals could be rationalised in terms of their relative amounts of the different microstructural constituents. Martensite provided the highest strength and reasonable toughness. Upper and lower bainite contributed to very good toughness and high strength. The least beneficial properties were found in the high manganese variants due to a combination of primarily coarse grained coalesced bainite and martensite. This combination of microstructural constituents resulted in poor toughness, relatively high yield strength and the highest tensile strength. The optimal combination of strength and impact toughness were found at 0.5 manganese and 7 nickel with intermediate carbon levels resulting in an easily tempered, fine scale mixture of upper and lower bainite together with some martensite.