Environment-Assisted Intergranular Cracking

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.

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.

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.

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.

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.

Over the last few years, an intergranular stress-corrosion cracking (IGSCC) mechanism has been observed for carbon steel pipelines and piping, particularly in in-situ thermal upstream production. All the cracks manifested on the bare (uncoated) outside diameter (OD) surface of the steel, and all were associated with wet mineral wool insulation. Common features include an operating temperature between 70°C and 160°C, intergranular cracking morphology, and exposure to wet mineral wool insulation with vintage post-2003. Recently, a similar IGSCC was observed in pressure vessel grade A516 Gr 70N steel. The potential of having this mechanism manifest in pressure vessels is of some concern, as the span of susceptible equipment goes beyond upstream production assets but to the larger oil and gas industry that includes upgrading, refining, terminals and distribution, and perhaps extending to other industries where the conditions listed above exist. This paper summarizes the investigation of the external IGSCC found on two carbon steel pressure vessels, with the intent of increasing the awareness of the industry with this new damage mechanism.

Fracture mechanics investigation regarding the effects of cracks on the structural integrity of a BWR-core shroud

Environmental degradation of metallic alloys is shown to depend on its local microstructure and microchemistry in many instances. Equilibrium and non-equilibrium processes can produce significant changes to the grain boundary composition and promote intergranular stress corrosion cracking (IGSCC). Austenitic stainless steels are used as an example to illustrate equilibrium effects of Gibbsian segregation and second-phase precipitation as well as non-equilibrium effects of quench-induced and radiation-induced segregation. Interfacial Cr concentration is shown to be the dominant material variable promoting IGSCC of austenitic stainless steels in oxidizing environments. Cracking susceptibility is a direct function of the boundary Cr content regardless of depletion width. However, grain boundary Cr depletion does not adequately explain IGSCC in stainless steels strengthened by cold work or neutron irradiation. Significant interfacial enrichment of Cr, Mo and B are often present in annealed stainless steels which may play a role in the IGSCC of cold-worked materials and delay IASCC to higher radiation doses. Impurity segregants (e.g., P) can promote IG hydrogen-induced cracking but do not have a strong detrimental effect on cracking in high-temperature water environments.

Laboratory testing to produce intergranular stress corrosion cracking (IGSCC) of Alloy 600 steam generator tubing in PWR primary water environments can require lengthy exposure times, even at a temperature of 360°C (680°F). Recent testing of Alloy 600 in hydrogen-containing steam at 400°C (750°F) has shown the characteristic IGSCC phenomenology, but with a significant acceleration in the IGSCC initiation kinetics. The IGSCC initiation kinetics in this environment were about an order of magnitude faster than the kinetics for high purity, hydrogen-containing water at 360°C. The IGSCC behavior depends both on the steam pressure/density and the hydrogen partial pressure. The role of microstructural variables including grain boundary carbide precipitation and heat treatment are similar in water and steam.

A physicomechanical model for occurrence of fractures of austenitic steels upon intergranular stress corrosion cracking (IGSCC) is proposed on the basis of an analysis of experimental data on the susceptibility of irradiated austenitic steels to IGSCC during contact with the coolant of the primary circuit of a water-moderated water-cooled power reactor (VVER). A durability evaluation method for units of vessel internals by the criterion of the initialization of IGSCC is developed that allows for calculations under a timevarying stress-strain state of the unit and a damaging radiation dose.

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.

The effect of pH on the intergranular stress corrosion cracking (IGSCC) susceptibility of sensitized AISI 304 SS in deaerated solutions at 250°C, where , was investigated using potentiostatic slow strain rate tests. A critical potential above which IGSCC occurs has been determined as a function of pH. It was found that depends on pH and decreases abruptly to values below the corrosion potential of the alloy in deaerated solutions at pH's lower than the pH of pure water . No critical potential was identified in the presence of indicating that IGSCC, enhanced by additions, occurs even at high cathodic overpotentials. The combined effect of pH and potential on the IGSCC behavior is discussed in terms of a previously advanced model involving anodic dissolution of the chromium‐depleted grain boundaries, in which the solubility of corrosion products modified by local acidification within the cracks seems to play a substantial role.

This paper evaluates the resistance to intergranular stress-corrosion cracking (IGSCC) in the boiling water reactor of several grades and compositions of austenitic stainless steel castings. A specific objective of this work was to predict the performance of Alloy Casting Institute CF3, CF3A, and CF8 castings in nuclear applications. Full-size welded pipe tests were performed on 10.16-cm-diameter (4 in.) centrifugal castings and small-specimen tests were performed on isothermally treated static and centrifugal castings. The environment for both series of tests consisted of oxygenated high-purity water at 288°C (550 ± 10°F). Axial loads were applied to the full-size pipes in a trapezoidal time-load wave shape with the peak stress exceeding the 288°C (550°F) yield strength of the material tested. For the small-specimen tests, either a constant-extension-rate test technique, a constant-load test technique, or a variable-load test technique was utilized. Both the pipe test and small-specimen tests consistently produce IGSCC in susceptible material such as sensitized AISI-304 stainless steel. Test results clearly indicate the influence of carbon content and δ-ferrite in determining IGSCC susceptibility of austenitic stainless steel castings in the aforementioned environment. Both low carbon content and the presence of critical quantities of ferrite in CF3, CF3A, and CF8 cast material are shown to promote high resistance to IGSCC. In contrast, AISI-304 with essentially zero ferrite (comparable carbon content to CF8) fails by IGSCC when tested similarly.

Intergranular stress corrosion cracking (IGSCC) of two commercial alloy 600 conditions (600LT, 600HT) and controlled- purity Ni-18Cr-9Fe alloys (CDMA, CDTT) were investigated using constant extension rate tensile (CERT) tests in primary water (0.01M LiOH+0.01M H{sub 3}BO{sub 3}) with 1 bar hydrogen overpressure at 360{degrees}C and 320{degrees}C. Heat treatments produced two types of microstructures in both 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 show that in all samples, IGSCC was the dominant failure mode. For both the commercial alloy and the 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. This data indicates that a grain boundary carbide microstructure is more resistant to IGSCC than an intragranular carbide microstructure. Observations support both the film rupture/slip dissolution mechanism and enhanced localized plasticity. The advantage of these results over previous studies is 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. Therefore, observations of the effects of carbide distribution on IGSCC can more confidently be attributed to the carbide distribution alone rather than other potentially significant differences in microstructure or composition.