Grain Boundary Composition and Effects on Environmental Degradation

Radiation hardening and radiation-induced chromium (Cr) depletion were related to intergranular stress corrosion cracking (IGSCC) response among various stainless steels (SS). Available data on neutron-irradiated materials were analyzed and correlations developed between fluence, yield strength, grain-boundary Cr concentration, and cracking susceptibility in high-temperature water environments. Large heat-to-heat differences in the critical fluence (0.2 neutrons/cm2 to 2.5 x 1021 neutrons/cm2) for IGSCC were documented. Variability often was consistent with yield strength differences among irradiated materials. IGSCC correlated better to yield strength than to fluence for most heats, suggesting a possible role for radiation-induced hardening (and microstructure) on cracking. However, isolated heats revealed a wide range of yield strengths (450 MPa to 800 MPa) necessary to promote IGSCC which could not be explained by strength effects alone. Grain-boundary Cr depletion qualitatively explained differences in IGSCC susceptibility for irradiated SS. Examination of measured Cr content vs SCC showed that all materials showing IG cracking had some grain-boundary depletion (> 2%). Grain-boundary Cr concentrations for cracking (< ~ 16 wt%) were in good agreement with results from similar SCC tests on unirradiated type 304 SS (UNS S30400) with controlled depletion profiles. Heats that prompted variability in the yield strength correlation were accounted for by differences in their interfacial Cr contents. Thus, certain SS apparently were more resistant to cracking, even though they had significant radiation-induced Cr depletion. Cr depletion was believed to be required for SCC of irradiated SS, but susceptibility in this study was modified by other microchemical and microstructural components.

Radiation hardening and radiation-induced chromium depletion are related to intergranular stress corrosion cracking (IGSCC) response among various stainless steels. Available data on neutron-irradiated materials have been analyzed and correlations developed between fluence, yield strength, grain boundary chromium concentration and cracking susceptibility in high-temperature water environments. Large heat-to-heat differences in the critical fluence (0.2 to 2.5 n/cm2) for IGSCC are documented. In many cases, this variability is consistent with yield strength differences among irradiated materials. IGSCC correlated better to yield strength than to fluence for most heats suggesting a possible role of the radiation-induced hardening (and microstructure) on cracking. However, isolated heats reveal a wide range of yield strengths from 450 to 800 MPa necessary to promote IGSCC which cannot be understood by strength effects alone. Grain boundary chromium depletion is found to qualitatively explain differences in IGSCC susceptibility for irradiated stainless steels. Examination of measured chromium contents versus SCC shows that all materials showing IG cracking have some grain boundary depletion (≥2%). Grain boundary chromium concentrations for cracking (below ~16 wt%) are in good agreement with similar SCC tests on unirradiated 304 SS with controlled depletion profiles. Heats that prompt variability in the yield strength correlation, are accounted for by differences in their interfacial chromium contents. Certain stainless steels are more resistant to cracking even though they have significant radiation-induced chromium depletion. It is proposed that chromium depletion is required for IASCC, but observed susceptibility is modified by other microchemical and microstructural components.

A coupled diffusion and cohesive zone model for intergranular stress corrosion cracking in 316L stainless steel exposed to cold work in primary water conditions

The Inter Granular Stress Corrosion Cracking (IGSCC) is a dominant damage mechanism of the austenitic stainless steel. The primary circuit piping of RBMK type reactors is produced from austenitic stainless steel 08×18H10T. Defects in welded joints of pipes with nominal diameter of 300 mm were detected during In-service inspections [1]. Metallographic investigations defined that crack growth mechanism is IGSCC. The appearance of defects increases the probability of RCS piping failures of these pipes. A leak or break in RCS piping is not acceptable from safety and political (society risk) points of view. According this the evaluation of these cracks is very important for safe operation of this type reactor. The procedures for IGSCC crack evaluation consist of two parts. The first part is determination of the acceptable crack size for the component with crack, and the second part is the crack growth calculation. The acceptable flaw size provides information about the largest flaw size which component can tolerate without failure with accepted safety factors. The crack growth calculation determines how long does it take for the existing crack to reach the maximal acceptable size. The results of these calculations (acceptable crack size and crack growth) determine the further inspection schedule of the components with crack. The objective of this paper is the evaluation of the IGSCC defects detected during In-Service inspection in the primary circuit piping which outside diameter of piping is 325 mm, the wall thickness – 16 mm. Detected cracks were evaluated using method R6 [2]. The IGSCC crack growing analysis was performed using methodology presented in document [3]. The prognosis results were compared with crack data detected during In-service inspection. According analysis results were determined that the IGSCC defects detected during In-service inspection can be left without repairing for 1.5 years operation.

Low carbon martensitic stainless steels, called supermartensitic stainless steels (SMSS), have widely been applied for flowlines transporting corrosive oil and gas. However, intergranular stress corrosion cracking (IGSCC) in girth welds of the steel has become a major concern recently. A number of research works have been conducted to clarify the mechanism and to establish the measures. The most likely mechanism of IGSCC is chromium depletion on grain boundary because it is observed in lean grade SMSS, which contains no molybdenum. However, neither depletion nor carbide of chromium has been detected so far in 2-3% Mo-added high grade SMSS, although the steel suffers IGSCC under some conditions. In order to clarify the mechanism in high grade steel, effects of chemical components on IGSCC resistance were investigated Simulated HAZ specimens with various conditions of thermal cycles were tested in hot acid environments by U-bend SCC test. The test results showed that reduction of carbon and addition of titanium are effective to improve the resistance to IGSCC. Both would inhibit to form chromium carbide that causes to chromium depletion. Therefore, it suggests that the mechanism of IGSCC in high grade SMSS is chromium depletion, as in lean grade.

Large-diameter Type 304 stainless steel pipe weld heat-affected zone (HAZ) was investigated to determine the rate at which low temperature sensitization (LTS) can occur in weld HAZ at nuclear reactor operating temperatures and to determine the effects of LTS on the initiation and propagation of intergranular stress corrosion cracks (IGSCC). The level of sensitization was determined with the electrochemical potentiokinetic reactivation (EPR) test, and IGSCC susceptibility was determined with constant extension rate tests (CERT) and actively loaded compact tension (CT) tests. Substructural changes and carbide compositions were analyzed by electron microscopy. Weld HAZ was found to be susceptible to IGSCC in the as-welded condition for tests conducted in 8-ppm-oxygen, high-purity water at 288 °C. For low oxygen environments (i.e., 288 °C/0.2 ppm O2 or 180 °C/1.0 ppm O2), IGSCC susceptibility was detected only in weld HAZ that had been sensitized at temperatures from 385 °C to 500 °C. Lower temperature heat treatments did not produce IGSCC. The microscopy studies indicate that the lack of IGSCC susceptibility from LTS heat treatments below 385 °C is a result of the low chromium-to-iron ratio in the carbide particles formed at grain boundaries. Without chromium enrichment of carbides, no chromium depleted zone is produced to enhance IGSCC susceptibility.

This paper describes the intergranular and transgranular stress corrosion cracking of Type 304 stainless steel in high purity water. Numerous authors have previously reported on various aspects of this stress corrosion cracking (SCC) problem(1-6). Temperatures of 150°, 200°, 250°, and 290°C and dissolved oxygen concentrations of 5 ppb to 32 ppm have been investigated. Intergranular stress corrosion cracking (IGSCC) was observed at temperatures above 150°C and oxygen concentrations approximately in excess of 100 ppb. Transgranular stress corrosion cracking (TGSCC) was observed at all test temperatures and oxygen concentrations approximately less than 100 ppb.

The intergranular stress corrosion cracking (IGSCC) in heat affected zone (HAZ) of weldable super martensitic stainless steels has been reported. The Cr-carbide precipitation surrounded by Cr-depletion zone was discussed as the cause of IGSCC for Ti-free martensitic stainless steels. The Cr-depletion along grain boundary in the vicinity of the surface oxide layer on HAZ formed by welding was also reported and the initiation mechanism was discussed based on the Cr-depletion of the surface area. The improvement of SCC resistance by Post Weld Heat Treatment (PWHT) has been recognized. PWHT can be effective to reduce the IGSCC susceptibility because of the healing for Cr-depletion (sensitized zone and surface area). It has been also proposed that the effect of PWHT on the propagation of IGSCC can be explained by the enrichment of molybdenum at the grain boundary, which has the positive effect for enhancing the corrosion resistance, and/or by the elimination of the harmful effect of phosphorus segregation to be drained from the grain boundary to the molybdenum enriched phase. In this study, the molybdenum precipitation behavior along grain boundary by PWHT of super martensitic stainless steels is determined in detail. The quantitative analysis by the extracted residue is carried out with the varied combination of temperatures and durations of PWHT. The relation between the IGSCC susceptibility and the molybdenum precipitation along grain boundary is discussed.

Austenitic stainless steel (SS) used as reactor core component materials becomes susceptible to SCC in high temperature water when irradiated to high neutron fluences. This phenomenon, termed as irradiation assisted stress corrosion cracking (IASCC), has been distinguished from intergranular stress corrosion cracking (IGSCC) which usually occurs of thermally sensitized SS in high temperature oxygenated water. Although the IASCC is phenomenologically quite similar to the IGSCC, its mechanism still remains unknown. In this work both solution annealed and thermally sensitized Type 304 SSs were irradiated to 3x1019 n/cm2(E>1MeV) at 290 °C in the Japan Material Test Reactor (JMTR). The irradiated as well as the unirradiated specimen materials were tensile tested in argon gas and in high temperature water with different dissolved oxygen (DO) concentrations by slow strain rate tensile (SSRT) SCC tests at 290°C. The increase in the yield stress of Type 304 SS due to neutron irradiation was within the range of dispersion of published data for the solution annealed Type 304 SS irradiated to higher neutron fluences in the ATR and commercial BWR. The irradiated sensitized material was more susceptible to SCC at 290 °C than the unirradiated sensitized material in high temperature water with more than 0.2 ppm DO. When at <0.01 ppm DO and in argon gas, the irradiated sensitized SS revealed a small fraction of intergranular cracking (IG), but the unirradiated sensitized SS did not. A comparison of the result in this work with published data indicated that the sensitized SS irradiated to 3x1019 n/cm2 was more susceptible to SCC than the solution annealed SS irradiated to over 1022 n/cm2.

Pressure vessel nozzle repair

Ultrasonic detection and sizing of intergranular stress corrosion cracks (IGSCC) in nuclear reactor cooling systems is a difficult practical problem due to the complicated geometry of these defects and to the variety of other reflectors (e. g., welds) which produce competing ultrasonic indications. The use of models of scattering from such defects can help in improving physical insight into ultrasonic scattering from IGSCC’s and may ultimately be of use in defining inspection protocols and signal processing algorithms which can lead to improved inspection reliability and discrimination between IGSCC’s and other geometrical reflectors. This paper will discuss the application of a model of ultrasonic scattering from a simple Y-shaped crack based upon the Kirchhoff approximation. In this model, the ultrasonic beam is approximated by a Gaussian profile which includes the effects of diffraction and allows calculation of the full ultrasonic radiation pattern which may be used, for example, to simulate a scanned inspection. Included in this paper will be a brief description of the model and a presentation of simulated IGSCC results. Comparisons of the model to experimental measurements will then be addressed followed by an application to the problem of IGSCC sizing based upon the dB-drop and PAT (pulse-arrival-time) techniques.

Effects of thermal history on sensitization behavior and Charpy impact property of type 316L and 316 stainless steels for applications in a fired heater

The influence of oversized solute additions on radiation-induced changes and post-irradiation intergranular stress corrosion cracking behavior in high-purity 316 stainless steels

Because of their resistance to high temperature corrosion by hydrogen sulfide, austenitic stainless steels are commonly used for equipment in desulfurization processes. However, these materials are subject to sensitization (chromium carbide precipitation) from exposure in the temperature range of 454-871°C (850-1600°F) during fabrication or operation. Sensitization makes the equipment susceptible to failure from intergranular stress corrosion cracking caused by polythionic acid. Polythionic acid is formed by the interaction of sulfidic scale, moisture, and oxygen at ambient temperature. The stress necessary to produce cracking is usually present as residual stress from fabrication.

The intergranular stress corrosion cracking (IGSCC) susceptibility of surface finished type 304 and 304L stainless steels has been studied using a constant load tensile specimen in 288 °C water containing 26 ppm dissolved oxygen. The study was to define conditions to prevent IGSCC in piping used in a boiling water reactor (BWR). The results are as follows: 1) IGSCC susceptibility of type 304 stainless steel increased markedly by surface finishing, such as grinding, when in a sensitized condition; increasing surface roughness shortened the time to failure. 2) Type 304 stainless steel specimens, which are solution heat treated, do not show IGSCC susceptibility even if they are ground. 3) Type 304L stainless steels do not show IGSCC susceptibility even if they are ground and sensitized at 621 °C for 2 h.