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ABSTRACT. Microstructural evolution at the fusion boundary in dissimilar welds between ferritic and austenitic alloys can sig

Specifications	Nelson-10/00 Zaida
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Content	ABSTRACT. Microstructural evolution at the fusion boundary in dissimilar welds between ferritic and austenitic alloys can significantly influence both the weldabil- ity and service behavior of the dissimilar combination. A fundamental investiga- tion was undertaken to characterize fu- sion boundary microstructure and to bet- ter understand the nature and character of boundaries that are associated with cracking in dissimilar welds. In a previ- ous paper, the evolution of the fusion boundary during the onset of solidifica- tion was discussed. In this paper, the na- ture and evolution of the fusion bound- ary and surrounding regions in dissimilar metal welds during subsequent on-cool- ing transformations in the fusion zone and heat-affected zone (HAZ) will be dis- cussed. A model system consisting of a high- purity iron base metal and 70Ni-30Cu (AWS A5.14 ERNiCu-7) filler metal was used to study this behavior. Using this simple Fe-Ni-Cu system, fusion bound- ary microstructures were developed that were analogous to those observed in more complex engineering systems. Transmission electron diffraction analy- sis and orientation imaging microscopy (OIM) revealed the orientation relation- ships between adjacent HAZ and weld metal grains at the fusion boundary were different than the cube-on-cube relation- ship normally observed in similar metal welds. The room temperature fusion boundary in the system studied exhibited grain boundary misorientations consis- tent with common FCC/BCC relation- ships, i.e.,Bain, Kurdjumov-Sachs and Nishyama-Wassermann. A theory de- scribing the evolution of the fusion boundary is proposed and the nature and character of the “Type II” grain boundary is described. Introduction Cracking phenomena associated with welds have been a recurring problem that has received considerable attention by many researchers over the last four decades. It is well accepted that weld- related cracking normally occurs along grain boundaries. Such grain-boundary- related cracking phenomena include weld solidification cracking, weld metal liquation cracking (microfissuring) in multipass welds, HAZ liquation crack- ing, reheat (stress relief) and strain-age cracking, and ductility dip cracking in both the weld metal and HAZ. Unfortu- nately, the materials that exhibit the greatest propensity for these cracking phenomena are those that are vital to the national infrastructure, i.e.,aluminum al- loys, nickel-based alloys and stainless steels. Cladding or dissimilar metal welds have by no means been immune to such failures and exhibit some unique crack- ing phenomena not observed in welds between similar materials. In fact, crack- ing or disbonding along or near the fu- sion boundary in dissimilar ferritic- austenitic welds has been a persistent problem for more than 60 years. Despite the persistence and potential conse- quences, the evolution, nature and role of weld metal interfaces in promoting or mitigating weld-related cracking are not well understood. The implications of boundaries and structures with regard to crack growth rates, fatigue, stress corro- sion cracking, etc., have been researched extensively in the materials science arena. However, in spite of the recurring problems and economic losses, there ex- ists a lack of understanding regarding the role of boundaries and structures in pro- moting or mitigating weld-related crack- ing. Therefore, a fundamental investiga- tion was undertaken to investigate the nature and character of those boundaries and structures near the fusion boundary in dissimilar ferritic-austenitic welds. A previous paper (Ref. 1) addressed the nature and character of the elevated temperature fusion boundary at the onset of solidification. This paper will present in detail the effects of on-cooling trans- formations and the nature of the fusion boundary and surrounding microstruc- ture within the austenitic and alpha fer- rite temperature ranges. WELDING RESEARCH SUPPLEMENT \| 267-s RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT WELDINGRESEARCH SUPPLEMENT TO THE WELDING JOURNAL, October 2000 Sponsored by the American Welding Society and the Welding Research Council Nature and Evolution of the Fusion Boundary in Ferritic-Austenitic Dissimilar Metal Welds — Part 2: On-Cooling Transformations BY T. W. NELSON, J. C. LIPPOLD AND M. J. MILLS On-cooling transformations in the fusion zone and heat-affected zone were studied for their relation to crack initiation KEY WORDS Dissimilar Welds Grain Boundary Crack Growth On-Cooling Phase Transformation Ferritic-Austenitic Weld Cracking T. W. NELSON, formerly with The Ohio State University, is now with Brigham Young Uni- versity in Provo, Utah. J. C. LIPPOLD and M. J. MILLS are with The Ohio State University, Columbus, Ohio.
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