Nature and evolution of the fusion boundary in ferritic-austenitic dissimilar metal welds - Part 2: On-cooling transformations

Microstructural evolution at the fusion boundary in dissimilar welds betwee n ferritic and austenitic alloys can significantly influence both the welda bility and service behavior of the dissimilar combination. A fundamental in vestigation was undertaken to characterize fusion boundary microstructure a nd to better understand the nature and character of boundaries that are ass ociated with cracking in dissimilar welds. in a previous paper, the evoluti on of the fusion boundary during the onset of solidification was discussed. In this paper, the nature and evolution of the fusion boundary and surroun ding regions in dissimilar metal welds during subsequent on-cooling transfo rmations in the fusion zone and heat-affected zone (HAZ) will be discussed. A model system consisting of a high-purity iron base metal and 70Ni-30Cu (A WS A5.14 ERNiCu-7) filler metal was used to study this behavior. Using this simple Fe-Ni-Cu system, fusion boundary microstructures were developed tha t were analogous to those observed in more complex engineering systems. Tra nsmission electron diffraction analysis and orientation imaging microscopy (OIM) revealed the orientation relationships between adjacent HAZ and weld metal grains at the fusion boundary were different than the cube-on-cube re lationship normally observed in similar metal welds. The room temperature f usion boundary in the system studied exhibited grain boundary misorientatio ns consistent with common FCC/BCC relationships, i.e., Bain, Kurdjumov-Sach s and Nishyama-Wassermann. A theory describing the evolution of the fusion boundary is proposed and the nature and character of the "Type II" grain bo undary is described.