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Development of a High Chromium Ni-Base Filler Metal Resistant to Ductility Dip Cracking and Solidification Cracking

Hope, Adam T, Hope
http://rave.ohiolink.edu/etdc/view?acc_num=osu1461175457

Abstract Details

2016, Doctor of Philosophy, Ohio State University, Welding Engineering.
Many nuclear reactor components previously constructed with Ni-based alloys containing 20 wt% Cr have been found to be susceptible to stress corrosion cracking. The nuclear power industry now uses high chromium (~30wt%) Ni-based filler metals to mitigate stress corrosion cracking. Current alloys are plagued with weldability issues, either solidification cracking or ductility dip cracking (DDC). Solidification cracking is related to solidification temperature range and the DDC is related to the fraction eutectic present in the microstructure. It was determined that an optimal alloy should have a solidification temperature range less than 150°C and at least 2% volume fraction eutectic. Due to the nature of the Nb rich eutectic that forms, it is difficult to avoid both cracking types simultaneously. Through computational modeling, alternative eutectic forming elements, Hf and Ta, have been identified as replacements for Nb in such alloys. Compositions have been optimized through a combination of computational and experimental techniques combined with a design of experiment methodology. Small buttons were melted using commercially pure materials in a copper hearth to obtain the desired compositions. These buttons were then subjected to a gas tungsten arc spot weld. A type C thermocouple was used to acquire the cooling history during the solidification process. The cooling curves were processed using Single Sensor Differential Thermal Analysis to determine the solidification temperature range, and indicator of solidification cracking susceptibility. Metallography was performed to determine the fraction eutectic present, an indicator of DDC resistance. The optimal level of Hf to resist cracking was found to be 0.25 wt%. The optimal level of Ta was found to be 4 wt%. γ/MC type eutectics were found to form first in all Nb, Ta, and Hf-bearing compositions. Depending on Fe and Cr content, γ/Laves eutectic was sometimes found in Nb and Ta-bearing compositions, while Hf-bearing compositions had γ/Ni7Hf2 as the final eutectic to solidify. This study found that the extra Cr in the current generation alloys promotes the γ/Laves phase eutectic, which expands the solidification temperature range and promotes solidification cracking. Both Ta-bearing and Hf-bearing eutectics were found to solidify at higher temperatures than Nb-bearing eutectics, leading to narrower solidification temperature ranges. Weldability testing on the optimized Ta-bearing compositions revealed good resistance to both DDC and solidification cracking. Unexpectedly, the optimized Hf-bearing compositions were quite susceptible to solidification cracking. This led to an investigation on the possible wetting effect of eutectics on solidification cracking susceptibly, and a theory on how wetting affects the solidification crack susceptibility and the volume fraction of eutectic needed for crack healing has been proposed. Alloys with eutectics that easily wet the grain boundaries have increased solidification crack susceptibility at low volume fraction eutectics, but as the fraction eutectic is increased, experience crack healing at relatively lower fraction eutectics than alloys with eutectics that don’t wet as easily. Hf rich eutectics were found to wet grain boundaries significantly more than Nb rich eutectics. Additions of Mo were also found to increase the wetting of eutectics in Nb-bearing alloys.
John Lippold (Advisor)
Antonio Ramirez (Advisor)
Boian Alexandrov (Committee Member)
148 p.
Engineering; Materials Science
Weldability; Solidification Cracking; Ductility Dip Cracking; Alloy Design; Wetting; Eutectics; Nickel-based;

Recommended Citations

Citations

  • Hope, Hope, A. T. (2016). Development of a High Chromium Ni-Base Filler Metal Resistant to Ductility Dip Cracking and Solidification Cracking [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1461175457

    APA Style (7th edition)

  • Hope, Hope, Adam. Development of a High Chromium Ni-Base Filler Metal Resistant to Ductility Dip Cracking and Solidification Cracking. 2016. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1461175457.

    MLA Style (8th edition)

  • Hope, Hope, Adam. "Development of a High Chromium Ni-Base Filler Metal Resistant to Ductility Dip Cracking and Solidification Cracking." Doctoral dissertation, Ohio State University, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=osu1461175457

    Chicago Manual of Style (17th edition)

osu1461175457
603
© 2016, all rights reserved.
This open access ETD is published by The Ohio State University and OhioLINK.
Release 3.2.12