Welding Research

SEPTEMBER 2006 -s 200 ABSTRACT. The relationship between alloying content, microstructure, and properties has been clarified for highstrength steel weld metals with 7 to 9 wt-% nickel. Neural network modeling suggested that manganese reductions and nickel additions in a controlled manner with respect to manganese lead to increased 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-% and nickel at 7 or 9 wt-%, while carbon was varied between 0.03 and 0.11 wt-%. Based on ThermoCalc calculations and the observed large nickel and manganese segregation to dendrite boundary regions, it was concluded that the weld metals solidified completely as austenite. High-resolution microscopy was used to characterize the microstructure. In high-manganese weld metals, a mixture of predominantly upper bainite and a coarse-grained constituent, characterized to be coalesced bainite was found in dendrite core regions. Mainly martensite was present at interdendritic regions. Manganese reductions were found to promote upper bainite while carbon additions were found to promote martensite. A constitutional diagram was constructed that summarizes 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.