Understanding Mechanical Properties of Novel High Strength Steel Weld Metals Through High-Resolution Microstructural Investigations

Shielded metal arc welding and submerged arc welding were used to prepare experimental weld metals with variations in nickel, manganese and carbon contents. The weld metals contained Ni between 6.6 and 10.5 wt. %, Mn between 0.5 and 2.0 wt. % while carbon was varied between 0.03 and 0.11 wt. %. Mechanical properties were found to be sensitive to alloying content. However once the optimum level of Mn, Ni and C are chosen, impact toughness greater than 60 J at 100°C, was possible to achieve in combination with a yield strength over 900 MPa. Utilizing the recently developed possibilities of high-resolution field emission gun scanning electron microscopy, correlations could be made between the microstructure and mechanical properties. Large amounts of coarse grained coalesced bainite was associated with moderate strength and toughness while a combination of upper and lower bainite led to high impact toughness with relatively high strength. A fine distribution of martensite and coalesced bainite gave high strength and toughness. Finally it was also confirmed for a range of Ni, Mn and C levels, that coalesced bainite forms at compositions where the martensite and bainite-start temperature are narrowly separated.