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In-Service Performance
4.09.3.1 Environmentally Assisted Cracking
Welds and their heat-affected zones have long been known to be the areas of concern for environmentally assisted cracking (EAC) because of their propensity
for as-fabricated flaws, high residual stresses, elevated plastic strains, chemical heterogeneity, and microstructural differences relative to base metals (Chapter 5.04, Corrosion and Stress Corrosion Cracking of Ni-Base Alloys; Chapter 5.05, Corrosion and Stress Corrosion Cracking of Austenitic Stainless Steels; Chapter 5.06, Corrosion and Environmentally-Assisted Cracking of Carbon and Low-Alloy Steels; Chapter 5.02, Water Chemistry Control in LWRs; and Chapter 5.08, Irradiation Assisted Stress Corrosion Cracking). Common EAC concerns in the nuclear industry include corrosion fatigue of low-alloy steels, hydride-induced
cracking of zirconium alloys, and stress corrosion cracking of corrosion-resistant structural alloys. Specifically, stress corrosion of austenitic stainless steel65-68 and nickel-alloy welds and their heat — affected zones has been a topic of considerable research.9,66,69—72 In austenitic stainless steels, sensitization — and welding-induced plastic strains are the key factors in stress corrosion resistance.
In nickel-alloy welds, the bulk chromium concentration, the solidification segregation, and the as-fabricated plastic strain are critical factors for understanding their stress corrosion performance. The stress corrosion cracking growth rate of nickel — alloy filler metals in high-temperature, high-purity water is shown as a function of their bulk chromium concentration in Figure 18(a). Note the strong decrease in stress corrosion crack growth rate at bulk chromium levels near 22 wt%. This decrease is likely associated with a change in crack tip oxide from NiO-type to a more stable spinel (NiCr2O4) or corundum (Cr2O3) structure.73-75 However, the bulk chromium concentration does not explain the extreme resistance of EN52 weld metal compared to other ^30 wt% bulk chromium alloys (Figure 18(a)). Consideration of how solidification segregation affects the grain boundary chromium concentration is critical to understanding the stress corrosion resistance of the high-alloy weld metals.76 Specifically, niobium — and molybdenum-bearing alloys tend to deplete the solidification grain boundaries in
chromium, while the Nb — and Mo-free EN52 grain boundaries are enriched in chromium as shown in the graph in Figure 18(b).
In Alloy 600 heat-affected zones, the increased susceptibility to SCC in high-temperature deaerated water is due, in large part, to the lack ofintergranular chromium carbides.77,78 Figure 19(a) shows a crosssection of a stress corrosion crack grown in an Alloy 600 heat-affected zone and the flat grain boundary topography (GBT) in the HAZ, which is an indication of a low degree of intergranular chromium carbide precipitation.78 Additionally, Figure 19 shows the different chromium concentration profiles in the HAZ and base metal (Figure 19(b)), the increased strain in the weld and HAZ relative to the base metal (Figure 19(c)), and the transmission electron micrographs of the grain boundaries in the HAZ (showing sparse (M7C3- and M23C6-type carbides) versus the large continuous Cr7C3 carbides in the unaffected base metal (Figure 19(d)). The diffraction patterns in Figure 19(d) identify the M23C6 (left) and M7C3 (right) carbides.
Stress corrosion crack growth rate predictions for Alloy 600 heat-affected zones are shown in Figure 20, which illustrates the strong temperature dependence as well as the effects of the applied stress intensity factor and the electrochemical potential. Figure 20 is based on eqn [1], which describes the crack growth rate of A600-type alloys exposed to high-temperature, high-purity water,77 in which A0, », m, b, x0, and c are the
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