Bogdan Alexandreanu

Combined effect of special grain boundaries and grain boundary carbides on IGSCC of Ni–16Cr–9Fe–xC alloys

Abstract Susceptibility to intergranular stress corrosion cracking in Ni–16Cr–9Fe– x C alloys in 360°C primary water is reduced with increasing fraction of special grain boundaries, i.e. coincident site lattice boundaries (CSLB) and low angle boundaries, and grain boundary carbides. Intergranular stress corrosion cracking (IGSCC) was investigated using interrupted constant extension rate tensile tests in a primary water environment at 360°C. Thermal–mechanical treatments were used to increase the fraction of special boundaries from approximately 20–25% to between 30 and 40%. In a carbon-doped heat, further heat treating was used to precipitate grain boundary carbides preferentially on high-angle boundaries (HAB). Orientation imaging microscopy was used to determine the relative grain misorientations and scanning electron microscopy (SEM) was used to identify specific grain boundaries after each interruption. After each strain increment, the same regions in each sample were examined for cracking. Results showed that irrespective of the microstructure condition, CSLBs always cracked less than HABs. Results also showed that IGSCC is reduced with increasing solution carbon content, and for the same amount of carbon in solution, the addition of grain boundary carbides reduced IGSCC still further. The best microstructure was the one consisting of an enhanced CSLB fraction and chromium carbides precipitated preferentially on high-angle boundaries.

The effect of grain boundary character distribution on the high temperature deformation behavior of Ni–16Cr–9Fe alloys

The objective of this work was to test the Thaveeprungsriporn model for the dependence of creep rate on the coincident site lattice (CSL) fraction. The model attributed the large reduction in creep rate in alloys with a high population of CSL boundaries to the greater difficulty of extrinsic grain boundary dislocation (EGBD) absorption at coincident site lattice boundaries (CSLBs) vs. high angle boundaries (HABs). Ease of EGBD absorption was assessed by measuring the annihilation rates of EGBDs in both CSL-related and HABs following an anneal at 360 °C. Results showed that EGBDs are annihilated at HABs at a rate that is on average three times that at CSLBs, implying a grain boundary diffusion coefficient in CSLBs that is 12 times lower than that in HABs. The expectation that a reduction in EGBD absorption would lead to greater matrix hardening was investigated using nano-hardness measurements. Results showed that the hardness in the vicinity of CSLBs is greater than that near HABs, and the grain-averaged hardness increases with the fraction of contiguous CSLBs. Further, strain hardening is greater in CSL-enhanced samples than in reference, solution annealed samples. These results taken together substantiate the hypothesis that CSLBs impede dislocation absorption into the grain boundary, thereby increasing lattice hardening and internal stress in the sample, resulting in a reduced creep rate.

The stress corrosion cracking behavior of alloys 690 and 152 WELD in a PWR environment.

Alloys 690 and 152 are the replacement materials of choice for Alloys 600 and 182, respectively. The latter two alloys are used as structural materials in pressurized water reactors (PWRs) and have been found to undergo stress corrosion cracking (SCC). The objective of this work is to determine the crack growth rates (CGRs) in a simulated PWR water environment for the replacement alloys. The study involved Alloy 690 cold-rolled by 26% and a laboratory-prepared Alloy 152 double-J weld in the as-welded condition. The experimental approach involved pre-cracking in a primary water environment and monitoring the cyclic CGRs to determine the optimum conditions for transitioning from the fatigue transgranular to intergranular SCC fracture mode. The cyclic CGRs of cold-rolled Alloy 690 showed significant environmental enhancement, while those for Alloy 152 were minimal. Both materials exhibited SCC of 10−11 m/s under constant loading at moderate stress intensity factors. The paper also presents tensile property data for Alloy 690TT and Alloy 152 weld in the temperature range 25–870°C.Copyright

A priori determination of the sampling size for grain-boundary character distribution and grain-boundary degradation analysis

Abstract The objective of this work was to formulate a simple model that can be used for assessing the statistical significance of the grain-boundary character distribution in a material. Increased interest in the control of the grain-boundary character distribution to influence grain boundary properties has led to numerous studies on the characterization of grain-boundary type without regard to the statistical significance of the results. Specifically, the model was developed to determine, a priori, the number of boundaries that need to be characterized such that the fraction of boundaries of a particular type is known to within a specific fractional error. The inclusion of experimental error in the model accounts for the misidentification of boundary type in the characterization process. The model also addresses the statistical significance of boundary degradation by boundary type. This is a more restrictive application of the same formalism in that a low probability of degradation (e.g. cracking, corrosion and cavitation) on a scarce boundary type may result in very few measurements of degraded boundaries and, hence, poor statistics. The objective of the model is again to determine, a priori, the number of boundaries that need to be characterized for the result to have statistical significance. The model is applied to two sets of data obtained on nickel-based alloys to show, firstly that, for a coincident site lattice boundary fraction larger than 0.2 to be known with a fractional error less than 0.10, a minimum of 500 boundaries need to be characterized and, secondly, the number of grain boundaries that need to be characterized to provide statistical significance in the comparison between the Brandon criterion and the Palumbo et al. criterion for correlation with grain-boundary cracking.

Localized deformation induced IGSCC and IASCC of austenitic alloys in high temperature water

Grain boundary properties are known to affect the intergranular stress corrosion cracking (IGSCC) and irradiation assisted stress corrosion cracking behavior of austenitic alloys in high temperature water. However, it is only recently that sufficient evidence has accumulated to show that the disposition of deformation in and near the grain boundary plays a key role in intergranular cracking. Grain boundaries that can transmit strain to adjacent grains can relieve stresses without undergoing localized deformation. Grain boundaries that cannot transmit strain will either experience high stresses or high strains. High stresses can lead to wedge-type cracking and sliding can lead to rupture of the protective oxide film. These processes are also applicable to irradiated materials in which the deformation can become highly localized in the form of dislocation channels and deformation twins. These deformation bands conduct tremendous amounts of strain to the grain boundaries. The capability of a boundary to transmit strain to a neighboring grain will determine its propensity for cracking, analogous to that in unirradiated metals. Thus, IGSCC in unirradiated materials and IASCC in irradiated materials are governed by the same local processes of stress and strain accommodation at the boundary.

Cyclic and SCC Behavior of Alloy 690 HAZ in a PWR Environment

The objective of this work is to determine the cyclic and stress corrosion cracking (SCC) crack growth rates (CGRs) in a simulated PWR water environment for Alloy 690 heat affected zone (HAZ). In order to meet the objective, an Alloy 152 J-weld was produced on a piece of Alloy 690 tubing, and the test specimens were aligned with the HAZ. The environmental enhancement of cyclic CGRs for Alloy 690 HAZ was comparable to that measured for the same alloy in the as-received condition. The two Alloy 690 HAZ samples tested exhibited maximum SCC CGR rates of 10−11 m/s in the simulated PWR environment at 320°C, however, on average, these rates are similar or only slightly higher than those for the as-received alloy.

SCC Behavior of Alloy 690 HAZ in a PWR Environment

The objective of this work is to determine the cyclic and stress corrosion cracking (SCC) crack growth rates (CGRs) in a simulated PWR water environment for Alloy 690 heat affected zone (HAZ). In order to meet the objective, an Alloy 152 J-weld was produced on a piece of Alloy 690 tubing, and the test specimens were aligned with the HAZ. The environmental enhancement of cyclic CGRs for Alloy 690 HAZ was comparable to that measured for the same alloy in the as-received condition. The two Alloy 690 HAZ samples tested exhibited maximum SCC CGR rates of 10−11 m/s in the simulated PWR environment at 320°C, however, on average, these rates are similar or only slightly higher than those for the as-received alloy.Copyright

Cyclic and SCC Behavior of Alloy 152 Weld in a PWR Environment

Alloys 600 and 182 are used as structural materials in pressurized water reactors (PWRs) and have been found to undergo stress corrosion cracking (SCC). Alloys 690 and 152 are the replacement materials of choice for Alloys 600 and 182, respectively. The objective of this work is to determine the crack growth rates (CGRs) in a simulated PWR water environment for Alloy 152. In order to meet the objective, specimens made from a laboratory-prepared Alloy 152 double-J weld in the as-welded condition were tested. For the SCC CGR measurements, the specimens were pre-cracked under cyclic loading in a primary water environment, and the cyclic CGRs were monitored to determine the transition from the fatigue transgranular fracture mode to the intergranular SCC fracture mode. The environmental enhancement of cyclic CGRs for Alloy 152 was minimal; nevertheless, the transition from transgranular to intergranular cracking was successful. Weld samples tested from the single heat of Alloy 152 exhibited SCC CGR rates of 10−11 m/s in the simulated PWR environment at 320°C, which is only about an order of magnitude lower than typical for Alloy 182.Copyright

SCC Behavior of Alloy 152 Weld in a PWR Environment

The objective of this work is to determine the crack growth rates (CGRs) in a simulated PWR water environment for Alloy 152. In order to meet the objective, specimens made from a laboratory-prepared Alloy 152 double-J weld in the as-welded condition were tested. For the SCC CGR measurements, the specimens were pre-cracked under cyclic loading in a primary water environment, and the cyclic CGRs were monitored to determine the transition from the fatigue transgranular fracture mode to the intergranular SCC fracture mode. The environmental enhancement of cyclic CGRs for Alloy 152 was minimal; nevertheless, the transition from transgranular to intergranular cracking was successful. Weld samples tested from the single heat of Alloy 152 exhibited SCC CGR rates of 10−11 m/s in the simulated PWR environment at 320°C, which was about an order of magnitude lower than typical for Alloy 182.

Effect of Residual Stress on Crack Growth Specimens Fabricated From Weld Metal

Slitting method residual stress measurements (Hill Engineering and UC Davis) and finite element weld simulation (US Nuclear Regulatory Commission) have been conducted in order to evaluate both the residual stress intensity factor and residual stress profiles for two compact tension coupon blanks. The two compact tension coupon blanks were provided by Argonne National Lab (ANL) and are similar to coupons used in ongoing stress corrosion cracking (SCC) studies in weld metal. The experimental data and finite element results are in reasonable agreement, showing similar trends in calculated residual stress profiles. Results from the work document the effect of specimen size and location on residual stress profiles, and could be used to determine the degree to which residual stresses affect crack growth measurements made in similar coupons.© 2011 ASME