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APPLICABILITY OF COLD METAL TRANSFER FOR REPAIR OF DISSIMILAR METAL WELDS IN STAINLESS STEEL PIPING IN NUCLEAR POWER PLANTS

Daniels, Thomas W
http://rave.ohiolink.edu/etdc/view?acc_num=osu1429873704

Abstract Details

2015, Master of Science, Ohio State University, Materials Science and Engineering.
Corrosion resistant butt welds in nuclear coolant loop piping were made with the Ni-base alloy 600 family during their initial construction several decades ago. Time has proved this material selection to be inadequate for resisting stress corrosion cracking from the corrosive primary water solution. Continuing damage from primary water stress corrosion cracking (PWSCC) requires welds to be repaired using structural weld overlays (SWOLs) on the outer pipe diameter to prevent through-thickness breaches. In recent years the nuclear power industry has faced challenges with hot cracking when depositing SWOLs using gas-tungsten arc welding (GTAW). The solidification cracking has been linked to high dilution of the high-Cr Ni-base filler metal from the cast stainless steel base metal pipe. This study investigates the low heat input process called cold metal transfer (CMT) as an alternative to GTAW, and its ability to create low-dilution dissimilar metal SWOLs of filler metal 52M on 304L stainless steel. Bead on plate welds were made to simulate SWOLs on nuclear piping. Experimental procedures included a design of experiment (DOE) to optimize CMT overlay parameters, ThermoCalc™ solidification simulations of diluted 52M with 304L stainless steel, compositional quantification of weld transition regions, and ultrasonic evaluation and mechanical testing of a CMT weld overlay mockup. DOE was used to acquire understanding of the CMT process and optimize bead-on-plate overlay parameters with respect to desired weld characteristics, which included low dilution, good surface quality, moderate toe angle, and defect-free welds. DOE was effective in developing predictive empirical models that were then used to target desired weld dilution, surface quality, geometry, and arc power responses. The resulting optimized CMT weld showed responses that fell within, or near ranges for acceptable criteria predicted by empirical equations. The solidification behavior of the diluted 52M-304L system was simulated with ThermoCalc™ and the Scheil-Gulliver module. Non-equilibrium phase fields were plotted in solidification temperature ranges as a function of dilution to construct a pseudobinary phase diagram. Solidification temperature range and segregation of Nb indicated a highly mixed system is more susceptible to solidification cracking than either base metal or filler metal extremes. CMT and GTAW welds were compared using optical microscopy, scanning election microscopy (SEM), energy dispersive spectroscopy (EDS) and electron probe microanalysis (EPMA). Results from EDS scans traversing weld transition regions showed the compositional transition in CMT welds is much steeper, transitioning from base metal to minimally diluted filler metal composition within a few tens of micrometers. The majority of the transition in CMT welds is contained in the planar growth region of the solidified metal, which lacks the segregation mechanism for solidification cracking. Transition of GTAW welds occurs over a much greater distance into the weld nugget, and within columnar and dendritic regions where segregation and highly diluted compositions coincide. EPMA data of line scans traversing primary dendrite arms support evidence of less potentially detrimental segregation occurring in CMT welds than in GTAW welds. A weld overlay mockup was constructed using CMT weld parameters based on those optimized from the DOE. Phased array ultrasonic testing (PAUT) was performed to evaluate defect occurrence and inspectability of the overlay material. The overlay contained no unintentional defects, while also detecting intentional defects created to simulate cracks, and lack of fusion defects. Mechanical test specimens were extracted from the overlay mockup and subject to transverse side bend and tension tests based on ASME Section IX qualifications to test the overlay’s strength and ductility. The combination of base metal and filler metal was sufficiently ductile to survive transverse side bend testing, which revealed no cracks or open discontinuities. Tension testing showed the weld metal meets and exceeds the minimum strength specification of both the weld metal and base metal. Both mechanical tests should represent a “pass” with respect to Section IX requirements.
Boian Alexandrov (Advisor)
John Lippold (Committee Member)
158 p.
Engineering; Materials Science; Metallurgy; Nuclear Engineering
welding, weldability, dissimilar metal, overlay, CMT, nuclear, high chromium, nickel base

Recommended Citations

Citations

  • Daniels, T. W. (2015). APPLICABILITY OF COLD METAL TRANSFER FOR REPAIR OF DISSIMILAR METAL WELDS IN STAINLESS STEEL PIPING IN NUCLEAR POWER PLANTS [Master's thesis, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1429873704

    APA Style (7th edition)

  • Daniels, Thomas. APPLICABILITY OF COLD METAL TRANSFER FOR REPAIR OF DISSIMILAR METAL WELDS IN STAINLESS STEEL PIPING IN NUCLEAR POWER PLANTS. 2015. Ohio State University, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1429873704.

    MLA Style (8th edition)

  • Daniels, Thomas. "APPLICABILITY OF COLD METAL TRANSFER FOR REPAIR OF DISSIMILAR METAL WELDS IN STAINLESS STEEL PIPING IN NUCLEAR POWER PLANTS." Master's thesis, Ohio State University, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=osu1429873704

    Chicago Manual of Style (17th edition)

osu1429873704
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This open access ETD is published by The Ohio State University and OhioLINK.