Martin M. Morra

Effect of Rising and Falling K Profiles on SCC Growth Rates in High-Temperature Water

Effects of rising and falling stress intensity factor (K) profiles on the stress corrosion cracking (SCC) growth rates of stainless steel and nickel alloys has been studied in high-temperature water. Sophisticated test control software was used that changes loading (P) based on crack length (a) to achieve a specific K trajectory by controlling dKlda, not simply dP/dt. The majority of SCC problems develop adjacent to welds, which have a complex residual stress profile versus wall thickness. This, coupled with the dependence of K on crack length, causes K to change as the crack grows, not per se with time (t). The effect of the rate of change in K on crack tip strain rate and the associated crack growth rate is discussed, along with the repercussions to understanding and dispositioning SCC response.

Microstructure And SCC of Alloy X-750

The effect of microstructure, stress intensity factor, corrosion potential and water purity on stress corrosion crack growth rate behavior of Alloy X-750 was investigated in 288 °C water. This material was provided by a utility in the form of an unused stabilizer support bracket. Alloy X-750 has exhibited SCC in field components, and this study was designed to examine its microstructure and SCC response in some detail to determine the suitability of Alloy X-750 for long term reliable service in BWRs.

SCC and Fracture Toughness of XM-19

The effect of stress intensity factor, cold work, corrosion potential and water purity on the stress corrosion crack (SCC) growth rate behavior of as-received and as-received plus 19.3% cold worked XM-19 was investigated in 288 °C BWR water. For 19.3% cold rolled XM-19, high to very high crack growth rates were consistently observed at high corrosion potential, largely independent of heat or orientation. As received XM-19 exhibited SCC growth rates ~5–10X slower than cold worked XM-19, but these rates are still considered high. For all materials and conditions, low corrosion potential conditions reduced the growth rates by about an order of magnitude, and somewhat more if impurities were present in the water. The SCC growth rates for both conditions of XM-19 were somewhat higher than the equivalent conditions of 18-8 stainless steels, such as Types 304/304L and 316/316L. Higher growth rates tend to be observed at higher yield strength, and XM-19 has an elevated yield strength from nitrogen-strengthening; incomplete annealing in the as-received material can also increase the yield strength. The J-R fracture resistance of 19.3% cold rolled XM-19 and as-received XM-19 in multiple orientations and with replicates was evaluated in 288 °C air. The data show a significant effect of crack orientation in the plate (the rolling plane coincides with the plane of the plate), consistent with the inhomogeneous nature of the microstructure. The fracture resistance of as-received XM-19 was good, but the 19.3% cold rolled XM-19 specimens exhibited low toughness, to the extent that many tests were invalid. Fracture resistance in 80–288 °C water environments was not evaluated, but is relevant to LWR components. Irradiation of this heat of XM-19 is in progress at the Idaho National Laboratory Advanced Test Reactor.

SCC of Alloy 152/52 Welds Defects, Repairs and Dilution Zones in PWR Water

Extensive SCC growth rate measurements have been performed on Alloy 690 and its weld metals in the past, and this paper focuses on SCC growth rate evaluation of Alloy 52/152 welds with a variety of defects and/or weld repairs, and in the dilution zone. Ductility dip cracking dominated the weld defects, and weld repair mockups were fabricated by EPRI Charlotte to be 20% or 50% excavation and repair, as well as welds with a refuse pass every layer. Only low and very low SCC growth rates were observed in all cases. Studies on weld dilution zone effects of varying Cr content were evaluated using welds created with variable ratios of dual-filler-wire feel, which permits definitive SCC growth rate measurements in a homogenous weld without the ambiguity of having the crack front in undefined composition of an actual weld dilution zone.

Assessing Residual Strains in Nuclear Power Reactor Internal Components Weld Mockups of Nickel Alloys Using EBSD

The most common failure mode of commercial nuclear reactor internal components is environmentally assisted cracking (EAC). Environmentally cracking is promoted by the simultaneous effects of three affecting variables, namely (1) presence of tensile stresses and strains, (2) and aggressive environment and (3) a susceptible microstructure. If one or more of these affecting variables are removed, cracking will not occur or propagate. For example, EAC susceptibility and growth rate will be minimized if the residual tensile stresses or strains are controlled. Similarly, EAC may be minimized by controlling the chemistry of the weld metal, for example by increasing the content of chromium in the weld metal. The objective of the current research was to complete laboratory weldments of 76 mm thick plates of Alloy 600 using weld metal with three different compositions. Electron backscatter diffraction (EBSD) results of cross sections of the weld showed that approximately 10 to 15% of residual plastic strain existed at near the root of the weld and in the middle section (at approximately 38 mm); however, lower residual plastic strain existed near the top of the weld (last passes).Copyright

Effect of Rising and Falling K Profiles on SCC Growth Rates in High Temperature Water

Effects of rising and falling stress intensity factor (K) profiles on the SCC growth rates of stainless steel and nickel alloys has been studied in high temperature water. Sophisticated test control software was used that changes loading (P) based on crack length (a) to achieve a specific K trajectory by controlling dK/da, not simply dP/dt. The majority of SCC problems develop adjacent to welds, which have a complex residual stress profile vs. wall thickness. This, coupled with the dependence of K on crack length, causes K to change as the crack grows, not per se with time (t). The effect of “K-dot” on crack tip strain rate and the associated crack growth rate is discussed, along with the repercussions to understanding and dispositioning SCC response.Copyright