New Developments with C–Mn–Ni High Strength Steel Weld Metals — Part A. Microstructure

The relationship between alloying content, microstructure and properties has been studied for high strength steel weld metals with 7 to 9 wt. % nickel. Neural network modelling suggested that manganese reductions lead to large impact toughness increases and that nickel must be added in a controlled manner with respect to manganese in order toincrease impact toughness. Carbon additions were predicted to enhance yield strength at limited loss to impact toughness. Following these predictions, shielded metal arc welding was used to produce experimental welds with manganese at 0.5 or 2 wt. %, nickel at 7 or 9 wt. % while carbon was varied between 0.03 and 0.11 wt. %. Based on Themo-Calc calculations and the observed segregation behaviour, it was concluded that the weld metals solidified completely as austenite. With high resolution microscopy the microstructure was found to be a mixture of predominantly upper bainite and a coarse grained, previously not documented, constituent characterised to be coalesced bainite in dendrite core regions, with mainly martensite present in interdendritic regions for high manganese weld metals. Manganese reductions were found to promote upper bainite while carbon additions were found to promote martensite. A constitutional diagram was constructed that summarises microstructure as a function of manganese and nickel contents. In Part B of this work, the mechanical properties are presented and discussed in relation to the microstructure and the neural network predictions .