| Abstract Scope |
INTRODUCTION
Since the inception of nuclear power generation, nickel-chromium alloys and welding products have been employed for construction and repair of internals wetted by primary water. For pressurized water reactors (PWR’s), approximately 30% Cr was found necessary to resist primary water stress-corrosion-cracking (PWSCC). The welding community has assigned the numerical identity for these products as 52. 52, the first 30%Cr product from the 1990’s is subject to solidification cracking (SC), ductility dip cracking (DDC) and other welding related issues such as oxides and inclusions. 52 was largely replaced by 52M in the early 2000’s, and it was the overwhelming choice for repair and fabrication for over a decade until further research showed the DDC-resistance advantages of 52MSS. The discovery that the addition of 3-5% Mo with about 2%Nb promoted the formation of serpentine (tortuous) grain boundaries was introduced as 52X-H in about 2008 and was perfected (heat number NX79W1UK) in 2010. 52MSS was shown to be superior to 52M in overall cracking resistance but rare and small solidification cracks persisted in 52MSS due to the formation of small amounts of laves phase. Additional research which began in September 2012 (with the experimental melt HV1648), explored the replacement of portions of Nb with Ta. Ta additions were found to reduce laves phase and decrease the solidification temperature range (STR). The product to be described in this presentation and paper is represented by HV1673 and VX131WXW and is identified as 52MSS-Ta.
52MSS-Ta, when modeled in JMatPro® using Scheil Predictions, has no theoretical laves phase and exhibits slightly better SC resistance than 52M along with excellent DDC resistance.
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EXPERIMENTAL PROCEDURES
Development and Research for alloys and welding products have been conducted using modeling methods such as thermodynamic modeling with optimization techniques. These techniques are often supplemented with multi-wire machine GTAW to test weldability. Small variations can be tested easily and quickly without the need for experimental melts that require hot and cold working ingots to wire. The discovery and development of DDC-resistant 30% Cr wires in 2008 was made possible by the addition of Nb and Mo to 52M. This improved performance was realized and proven using multi-wire machine GTAW in conjunction with strain-to-fracture (STF) testing. Welds made with 0.0, 0.75% 1.87%, 3.04%, and 4%Mo with 2.0-2.5%Nb were fabricated and showed that DDC tendency was a function of %Mo at about 2.0 Nb using the basic 52MSS composition.
Testing of modeling-proposed alloys for improved solidification cracking (SC) resistance is performed by transvarestraint (TVT) and confirmed by longitudinal varestraint testing (LVT). Once the concept of using a combination of Nb and Ta was decided upon, the optimum combination needed to be determined. The initial work was performed using thermodynamic modeling which predicted phase percentages of gamma, eutectic, laves and other phases across multiple analyses. Based on these results, three experimental melts were fabricated with varying amounts of Fe, Nb, Ta, and C.
Strain-to-fracture and transvarestraint testing were performed on the melts to determine the best performance. Heat HV1673 was chosen due to outstanding TVT and STF test results. Based on these results, a production melt has been made and is currently in testing. We expect to have results by September 2020.
DISCUSSION
Numerous performance targets have been proposed for nuclear welding products. The highest profile targets are resistance to DDC and SC. These are most important because governing specifications for nuclear equipment fabrication and repair call for no cracks or linear indications. Our chosen experimental melt, HV1673, with 3% Ta and 0.6 Nb provides optimum values for each of these targets.
An additional target is increased high temperature tensile (HTT) strength at 360°C and above. Based on previous testing, we expect our production heat to exhibit HTT of approximately 80Ksi (550MPa) at 360°C, a 10% increase over that of 52M.
Another pair of related targets is enhanced wetting and bead cleanliness that will improve the clarity of ultrasonic testing (UT). As the populations of nitrides, carbonitrides and oxides is reduced, the transparency of weld metal is improved when inspected ultrasonically. Our newest production melt was processed by special methods to reduce nitride and oxide inclusions.
We expect the near 30% chromium content to provide excellent resistance to PWSCC with very low crack growth rates (CGR) when tested in autoclaves with specially formulated primary water.
A final consideration is resistance to long range ordering (LRO). Our production melt, VX131WXW contains 6% Fe to improve resistance to LRO in accordance with prediction of the Ni2Cr stability range using JMatPro® version 10.2. Recent Thermocalc® work indicates that compositions with 30%Cr, with additions of Mo, Nb, and Ta exhibit “C-curve” behavior above and to the right of the regions susceptible to LRO under typical PWR operating conditions.
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CONCLUSIONS
Minimization of laves phase coupled with optimum composition produces outstanding SC resistance.
Optimum combination of Nb and Mo with other controls impart resistance to DDC.
30%Cr provides resistance to PWSCC and other compositional controls provide low CGR in PWR environments.
Special processing procedures of composition reduce populations of inclusions, improves weld bead-wetting and improves clarity of UT inspection capability.
Control of Cr, Nb, Ta, Mo and Fe provide resistance to long range ordering (LRO)
Current chosen composition produces about 10% greater RTT (room temperature tensile) and HTT strength over those of 52M. |