Thermal Aging Effects on Fusion Boundary of Dissimilar Metal Weldments in Primary Coolant System of Nuclear Power Plant

Abstract

In the primary coolant system of the pressurized water reactor, austenitic Ni-base weld metals are deposited on low-alloy steel to form dissimilar metal weld joints. An example is the weld joint between head penetration nozzle (austenitic Ni-base alloy) and reactor pressure vessel (low-alloy steel). Leakage issues from Alloy 82/182 Ni-base weld metal have frequently been reported since 2000. In addition, the cumulative number of cracked welds increased with the operation time of the pressurized water reactors. Recently, the Ni-base weld metal was replaced from Alloy 82/182 (about 15 wt.% of Cr) into Alloy 52/152 (about 30 wt.% of Cr), and the incident of cracking in Alloy 52/152 has not been reported since the replacement. Recently, the cracks, which approach fusion boundary of Ni-base weld metal / low-alloy steel, were reported. To investigate the effects of (1) weld metals and (2) long-period thermal aging in operating PWRs on the integrity of the fusion boundary, a program of microstructural characterization, mechanical evaluations, corrosion testing and crack growth rate measurements was performed. Two different weld metals were compared: (a) Alloy 182, Ni-base weld metal, and (b) Alloy 152, Ni-base weld metal with a higher chromium content than the Alloy 182. The effect of thermal aging on microstructure evolution, mechanical property and resistances on corrosion and cracking was studied by applying heat treatments to the as-welded mock-up samples. Corrosion testing and crack growth rate measurements, in a simulated primary coolant environment of pressurized water reactors, were performed to rank the resistances to corrosion and cracking. Microstructural examination represents several M23C6 precipitates formed at the fusion boundary of Ni-base weld metal / low-alloy steel at the as-welded state because of the high solubility of C in the low-alloy steel. Nucleation and growth of the precipitates occurred with thermal aging heat treatments. Alloy 152, which has the higher Cr content compared to Alloy 182, had more Cr rich precipitates at the fusion boundary, compared to the one of Alloy 182. Based on the results of corrosion testing and crack growth rate measurements, the corrosion rates and crack growth rates were dependent on the number of the precipitates. The fusion boundary of the Alloy 152 / low-alloy steel is highly susceptible to cracking compared to the one of the Alloy 182 / low-alloy steel and can be weakened with thermal aging heat treatment. In addition, the fusion boundary showed lower resistance to cracking than Ni-base weld metals of Alloy 182 and Alloy 152.