Role of Carbides in Stress Corrosion Cracking Resistance of Alloy 600 and Controlled-Purity Ni-16% Cr-9% Fe in Primary Water at 360°C

Abstract

Intergranular stress corrosion cracking (IGSCC) of two commercial alloy 600 (UNS N06600) conditions (heat-treated atlow temperature [600LT] and at high temperature [600HT]) and two controlled-purity Ni-16% Cr-9% Fe alloys (carbondoped mill-annealed [CDMA] and carbon-doped thermally treated [CDTT]) were investigated using constant extension rate tensile (CERT) tests in primary water (0.001 M lithium hydroxide [LiOH] + 0.01 M boric acid [H 3 BO 3 ]) with 1 bar (100 kPa) hydrogen overpressure at 360°C and 320°C. Heat treatments produced two types of microstructures in the commercial and controlled-purity alloys: one dominated by grain-boundary carbides (600HT and CDTT) and one dominated by intragranular carbides (600LT and CDMA). CERT tests were conducted over a range of strain rates and at two temperatures with interruptions at specific strains to determine the crack depth distributions. Results showed IGSCC was the dominant failure mode in all samples. For the commercial alloy and controlled-purity alloys, the microstructure with grain-boundary carbides showed delayed crack initiation and shallower crack depths than did the intragranular carbide microstructure under all experimental conditions. Data indicated a grain-boundary carbide microstructure is more resistant to IGSCC than an intragranular carbide microstructure. Observations supported the film rupture/slip dissolution mechanism and enhanced localized plasticity. The advantage of these results over previous studies was that the different carbide distributions were obtained in the same commercial alloy using different heat treatments and, in the other case, in nearly identical controlled-purity alloys. Observations of the effects of carbide distribution on IGSCC could be attributed more confidently to the carbide distribution alone rather than other potentially significant differences in microstructure or composition. Crack growth rates (CGR) increased with increasing strain rate according to a power law relation with a strain rate exponent between 0.4 and 0.64. However, CGR measured in m/unit strain decreased with increasing strain rate, indicating an effect of environment or creep. Temperature dependence of CGR was consistent with the literature.

The effects of grain boundary carbide density and strain rate on the stress corrosion cracking behavior of cold rolled Alloy 690

“…So the crack growth in unit time increases with strain rate due to increasing strain applied while that in unit strain decreases with strain rate due to decreasing test time. The work of Was and Lian [9] on Alloy 600 in 360°C primary water confirms this deduction and the results from Boursier et al [23,24] …”

“…Rios et al [30] did U-bend tests in 360°C primary water and found that 1 h sensitization at 700°C increased the initiation time of Alloy 600. Was and Lian [9] reported that the crack length per unit area and maximum crack depth of Alloy 600 after SSRT tests in 360°C primary water decreased with increasing grain boundary carbide coverage and the mitigating effect of grain boundary carbide on SCC was also revealed in Ni-16Cr-9Fe-xC alloys under similar environments [10,11]. Very limited results about the effects of grain boundary carbide density on the SCC initiation of Alloy 690 have been reported due to the difficulty in cracking this material.…”

“…It is recognized that semi-continuous grain boundary carbides induced by thermal treatment can enhance the SCC resistance of Alloy 600 in caustic environment [6] and reducing environment [7][8][9][10][11] although the intergranular carbides reportedly increased the susceptibility to SCC in oxidizing environment [12]. For Alloy 690, the beneficial effect of carbides in caustic has also been found in U-bend tests [13] although it was not revealed in C-ring tests due to insufficient stress applied [14].…”

Corrosion aspects of Ni–Cr–Fe based and Ni–Cu based steam generator tube materials

“…[6]. Was and Lian [27] have examined the roles of carbides in SCC resistance of Alloy 600 and controlled purity Ni-16%Cr-9%Fe in primary water at 633 K. A grain boundary carbide microstructure has been found to be more resistant to IGSCC as compared to an intragranular carbide microstructure and crack growth rate has been found to increase with increasing strain rate [27]. Angeliu et al [28] have investigated repassivation and crack propagation of Alloy 600.…”

Stress and temperature dependence of creep in Alloy 600 in primary water

“…Large precipitates are known to act as athermal obstacles together with dislocations contributing to the internal stress. [14] However in this alloy, intragranular carbides do not affect creep [19] or hardening [20] by virtue of their extremely low volume fraction and incoherency. The internal mations: (1) the flow stress is proportional to the strain in Figure 6 and (2) the internal stress is proportional to the flow stress ( int ϭ C flow ).…”