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

Abstract

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.

Retaining Mechanical Properties of GMA-Welded Joints of 9%Ni Steel Using Experimentally Produced Matching Ferritic Filler Metal

“…A ductile impact fracture surface was also obtained for the post-weld heat-treated joint ( Figure 18 b) due to its WM’s tempered martensitic structure with more stable retained austenite. The effect of PWHT is in agreement with a previous study on SMAW with 9%Ni steel [ 29 ]. Tensile and impact properties are influenced not only by the microstructure of the fusion zone, but also by the degree of its mechanical mismatching depending on the type of the filler metal.…”

“…Weld soundness was confirmed using a radiographic test and no internal welding defects were detected. The microstructure of the WM is a cast dendritic austenitic structure ( Figure 5 a), typical to that of the used austenitic filler metal [ 29 ]. The microstructure of the HAZ is a coarse-grained martensitic structure and the prior austenite grain boundaries can be seen ( Figure 5 b).…”