The Microstructure and Microscopic Mechanical Performance of Welded Joint for 9%Ni Steel Using Nickel-Based Filler Metal

Shielded metal arc butt welded joints for 9%Ni steel using nickel-based filler metal were analyzed by optical microscopy, scanning electron microscope, as well as transmission electron microscopy. The nanoindentation method was used to study the correlation between the structure and micromechanical properties of the fusion welded joint. The results show that there is a microstructural evolution from melted deposit to 9%Ni base steel. A significant coarse lath martensite-bainite mixture has been obtained in 9%Ni steel close to the fusion boundary while the retained austenite distributing in grain boundary of lath martensite has reduced to an undetectable level. In fusion boundary, an intermetallic layer has been observed which corresponds to the epitaxial growth of weld metal. The analysis of stress-strain behavior using nanoindentation shows that the heat-affected zone of coarse grains exhibits ductility loss and quantitative plastic deformation failure. The fusion boundary has the lowest value of yield stress while the coarse grained heat affected zone has the maximum value of yield stress.