Constant Extension Rate Tensile Research Articles

Tensile and stress corrosion cracking behavior of ferritic–martensitic steels in supercritical water

Tensile and stress corrosion cracking behavior of ferritic–martensitic steels in supercritical water were studied in order to evaluate the suitability of these steels for supercritical water nuclear reactor concept. The ferritic–martensitic steels tested in this study consisted of T91, HCM12A, HT-9, weld T91, and weld HCM12A. A series of constant extension rate tensile (CERT) tests at a strain rate of 3 × 10 −7 s −1 were conducted in supercritical water over a temperature range of 400–600 °C and pressure 24.8 ± 0.07 MPa. CERT tests in argon and in supercritical water with 100 and 300 appb dissolved oxygen also were performed at 500 °C to compare the effect of environment. The results show that HT-9 exhibited the highest yield and maximum stresses, followed by HCM12A, and T91. The reduction in area of T91 is the highest, followed by HCM12A, and HT-9. Temperature has a great effect on tensile behavior of these steels. An increase in test temperature from 400 to 600 °C reduces the yield stress by ∼50%. Both T91 and HCM12A weld steels exhibited a slightly lower yield and maximum stresses than the base steels. Increased dissolved oxygen in the water resulted in a significant reduction of ductility. Fractography showed that all of the specimens exhibited ductile rupture except for HT-9 that showed evidence of intergranular cracking. Intergranular cracking in HT-9 is affected by temperature and oxygen concentration in supercritical water.

Irradiation-assisted stress corrosion cracking of austenitic alloys in supercritical water

Four commercial austenitic alloys, 316L, D9, 690 and 800H were irradiated with 2.0 MeV protons to doses of up to 7 dpa at temperatures of 400 and 500 °C to study the role of irradiation and temperature on stress corrosion cracking in supercritical water. Constant extension rate tensile (CERT) tests were performed on the irradiated specimens in deaerated supercritical water at the same temperature as the irradiation. The results showed that irradiation significantly increased the severity of intergranular stress corrosion cracking in all four alloys. Cracking severity increased with dose and temperature. The severity of cracking correlated with both the hardness and the grain boundary chromium concentration, making attribution to a single irradiation feature difficult. However, a very significant increase in cracking of alloy 690 at 500 °C is likely due primarily to RIS rather than to hardening. Cracking of the irradiated alloys in supercritical water follows the same behavior as in subcritical water, implying that a common mechanism may be controlling.

Localized deformation and IASCC initiation in austenitic stainless steels

Localized deformation may play a key role in the underlying mechanism of irradiation assisted stress corrosion cracking (IASCC) in light water reactor core components. In this study, four austenitic alloys, 18Cr8Ni, 15Cr12Ni, 13Cr15Ni and 21Cr32Ni, with different stacking fault energies were irradiated to 1 and 5 dpa at 360 °C using 3.2 MeV protons. Interrupted constant extension rate tensile (CERT) tests were conducted in a simulated BWR environment to determine IASCC susceptibility. In order to characterize the localized deformation in slip channels and grain boundaries, parallel CERT experiments were also performed in an argon atmosphere. Results show that the IASCC susceptibility of the tested alloys increases with increasing irradiation dose and decreasing stacking fault energy. IASCC tends to initiate at locations where slip channels intersect grain boundaries. Localized deformation in the form of grain boundary sliding due to the interaction of slip channels and grain boundaries is likely the primary cause of the observed cracking initiation.

Selective Internal Oxidation as a Mechanism for Intergranular Stress Corrosion Cracking of Ni-Cr-Fe Alloys

The mechanism of selective internal oxidation (SIO) for intergranular stress corrosion cracking (IGSCC) of nickel-base alloys has been investigated through a series of experiments using high-purity alloys and a steam environment to control the formation of NiO on the surface. Five alloys (Ni-9Fe, Ni-5Cr, Ni-5Cr-9Fe, Ni-16Cr-9Fe, and Ni-30Cr-9Fe) were used to investigate oxidation and intergranular cracking behavior for hydrogen-to-water vapor partial pressure ratios (PPRs) between 0.001 and 0.9. The Ni-9Fe, Ni-5Cr, and Ni-5Cr-9Fe alloys formed a uniform Ni(OH)2 film at PPRs less than 0.09, and the higher chromium alloys formed chromium-rich oxide films over the entire PPR range studied. Corrosion coupon results show that grain boundary oxides extended for significant depths (>150 nm) below the sample surface for all but the highest Cr containing alloy. Constant extension rate tensile (CERT) test results showed that intergranular cracking varied with PPR and cracking was more pronounced at a PPR value where nonprotective Ni(OH)2 was able to form and a link between the nonprotective Ni(OH)2 film and the formation of grain boundary oxides is suggested. The observation of grain boundary oxides in stressed and unstressed samples as well as the influence of alloy content on IG cracking and oxidation support SIO as a mechanism for IGSCC.

Role of grain boundary engineering in the SCC behavior of ferritic–martensitic alloy HT-9

This paper focuses on the role of grain boundary engineering (GBE) in stress corrosion cracking (SCC) of ferritic–martensitic (F–M) alloy HT-9in supercritical water (SCW) at 400°C and 500°C. Constant extension rate tensile (CERT) tests were conducted on HT-9 in as-received (AR) and coincident site lattice enhanced (CSLE) condition. Both unirradiated and irradiated specimens (irradiated with 2 MeV protons at 400°C and 500°C to a dose of 7dpa) were tested. Ferritic–martensitic steel HT-9 exhibited intergranular stress corrosion cracking when subjected to CERT tests in an environment of supercritical water at 400°C and 500°C and also in an inert environment of argon at 500°C. CSL-enhancement reduces grain boundary carbide coarsening and cracking susceptibility in both the unirradiated and irradiated condition. Irradiation enhanced coarsening of grain boundary carbides and cracking susceptibility of HT-9 for both the AR and CSLE conditions. Intergranular (IG) cracking of HT-9 results likely from fracture of IG carbides and seems consistent with the mechanism that coarser carbides worsen cracking susceptibility. Oxidation in combination with wedging stresses is the likely cause of the observed environmental enhancement of high temperature IG cracking in HT-9.

Influence of localized deformation on A-286 austenitic stainless steel stress corrosion cracking in PWR primary water

The low cycle fatigue (LCF) behaviour of precipitation-strengthened A-286 austenitic stainless steel was first investigated at room temperature under 0.2% plastic strain control. LCF led to hardening for the first 20 cycles and then to significant softening. LCF-induced dislocation microstructure was characterized using both bright and dark-field imaging techniques in transmission electron microscopy. Cycling softening was correlated with the formation of precipitate-free localized deformation bands. The effect of these precipitate-free localized deformation bands on A-286 stress corrosion cracking (SCC) behaviour in PWR primary water was then examined by means of constant extension rate tensile (CERT) tests at 320 °C and 360 °C. Comparative CERT tests were performed on companion specimens with similar yield stress but pre-fatigued to a few cycles (4–8) or between 125 and 200 cycles. Specimens pre-fatigued to a few cycles with no precipitate-free localized deformation bands exhibited little susceptibility to intergranular SCC (IGSCC). In contrast, the presence of precipitate-free localized deformation bands formed by pre-fatigue to between 125 and 200 cycles strongly promoted IGSCC. The interest of the approach used in this study is to provide insight into the role of localized deformation in irradiation assisted stress corrosion cracking.

Hydrogen effects on the tensile and fatigue properties of Eurofer’97

Hydrogen embrittlement (HE) of Eurofer’97 was investigated by means of constant extension rate tensile (CERT) and fully reversed load-control low cycle fatigue (LCF) tests, run at room temperature and under electrochemical charging before and during specimen deformation. Increasing H content from 1.6 to 5.6 wppm caused increasing ductility loss compared to air behaviour and increasing data scatter from replicated CERT tests in terms of embrittlement indexes and fracture modes. The same trend manifested itself on Eurofer fatigue lifetimes after cyclic loading below the monotonic yield, but to a major degree as frequency was lowered. Fractographic analysis under scanning electron microscope indicated that brittle cracking, which macroscopically involved intergranular (IG) or transgranular (TG) separation, included morphologies closely related to HE phenomena through plastic flow modification and/ or interface decohesion. Correlation of these results with H desorption spectra and microstructural assessments suggested that the origin of mechanical response variability from specimen-to-specimen with similar H levels could arise from material heterogeneity in terms of trap characteristics.

Microstructural Characterization of SCC Crack Tip and Oxide Film for SUS 316 Stainless Steel in Simulated PWR Primary Water at 320°C

Recent studies on stress corrosion cracking (SCC) behaviors of austenitic stainless steels in hydrogenated high-temperature water show that low potential SCC (LPSCC) can occur on cold-worked SUS 316 stainless steel (hereinafter, 316SS). In this study, oxide films and crack tips on cold-worked 316SS exposed to hydrogenated high-temperature water were characterized using analytical transmission electron microscopy (ATEM), grazing incidence X-ray diffraction (GIXRD) and Auger electron spectroscopy (AES) in order to study the corrosion and SCC behaviors of these films and crack tips. A double layer structure was identified for the oxide film after a constant extension rate tensile (CERT) test. The outer layer was composed of large particles (0.2–3 μm) of Fe3O4 and the inner layer consisted mainly of fine particles (~10 nm) of FeCr2O4. In addition, nickel enrichment was identified at the metal/oxide interface. Particles of Fe3O4 were also identified on the crack walls. These results indicate that the same electrochemical reactions had occurred inside and outside the crack. The crack tip area was filled with corrosion products of a chromium-rich oxide. In addition, nickel enrichment was observed at the crack tip. The formation of the nickel-enriched phase indicates that a selective dissolution reaction of iron and chromium occurred at the front of the LPSCC crack.

Oxidation induced intergranular cracking and Portevin–Le Chatelier effect in nickel base superalloy 718

The mechanical behaviour of nickel base superalloy 718 was investigated by means of constant extension rate tensile (CERT) tests performed at 5×10 −7 s −1, in air and under secondary vacuum, in the temperature range 400–600°C. Below 470°C, the Portevin–Le Chatelier (PLC) effect occurs and leads to shear fracture. Above 500°C, the PLC effect disappears and oxidation-induced intergranular cracking takes place. The transition from serrated flow to continuous plastic flow occurs abruptly in a narrow temperature domain (470–500°C) and coincides with a transition from ductile to intergranular cracking. Thus, the interactions between oxidation and plasticity at the crack tip were investigated by means of a specific CERT test conducted at 500°C up to 4% plastic strain and at 470°C up to fracture in air. Results show that the local plastic strain rate may control the oxidation induced intergranular cracking process.

Simulación del efecto de la irradiación mediante el trabajado en frío y los tratamientos térmicos en dos aceros inoxidables austeníticos

In the present study, annealed type 304 SS was cold worked and heat treated to simulate irradiation hardening, ductility loss and grain boundary segregation. Constant Extension Rate Tensile (CERT) tests were conducted to reproduce Irradiation Assisted Stress Corrosion Cracking (IASCC) in BWR (Boiling Water Reactor) environment.

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 at low 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 [H3BO3]) 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 a...

Hydrogen sulphide stress corrosion cracking of weld overlays for desulfurization reactors

Weld overlays of austenitic stainless steels on 2.25Cr-Mo steel were made by the shielded metal arc welding process. Single layer weld overlays (SLWO) of 3 mm thick were made by using either E309 (A-type specimen) or E347 (B-type specimen) electrodes and double layer weld overlays (DLWO) were made by depositing 1 mm of E309 and then 2 mm of E347 on the steel (C-type specimen). Constant extension rate tensile (CERT) tests were performed to investigate the fracture characteristics of the weld overlay specimens in air and in H 2S solution. Cracks were initiated at the fusion boundaries between the stainless steel deposit and 2.25Cr-Mo steel substrate, and then propagated normally to loading direction into the steel substrate and stainless steel deposit. This procedure was repeated for all types of specimens in the as-welded condition. The results revealed the untempered narrow diluted zone (DZ) and heat-affected zone (HAZ) were susceptible to hydrogen embrittlement (HE). However, the high susceptibility of the HAZ to HE could be reduced significantly in all specimens by tempering at 690 °C for one hour. The susceptibility to cracks in the DZ was lower for the A-type specimens than the B-type and C-type specimens. C-type specimens had the declined tensile strength, which could be attributed to the extensive dilution in the first layer deposit, resulting in a high susceptibility to HE.

Isolation of carbon and grain boundary carbide effects on the creep and intergranular stress corrosion cracking behavior of Ni−16Cr−9Fe−xC alloys in 360 °C primary water

The influence of carbon and grain boundary carbides on intergranular stress corrosion cracking (IGSCC) of controlled-purity Ni−16Cr−9Fe−xC alloys in 360 °C primary water was investigated using constant load tensile (CLT) and constant extension rate tensile (CERT) tests. The CLT test results confirmed that carbon in solution decreases the creep rate by several orders of magnitude, while grain boundary carbides serve to increase the creep susceptibility. Although carbon increases the work hardening rate, it is demonstrated, using the Bailey-Orowan creep model, that the primary effect of carbon in solution is to delay the recovery process of climb at the grain boundary, thereby reducing the creep rate. Grain boundary carbides produce a negligible contribution to the internal stress and may increase the creep rate by acting as dislocation sources. Grain boundary carbide precipitation increases IGSCC resistance in 360 °C primary water containing 0, 1, and 18 bar hydrogen, providing the highest overall resistance to both environmentally induced creep and cracking. The magnitude of the beneficial effect of grain boundary carbides is extremely sensitive to hydrogen overpressure, with the largest influence observed for 1 bar hydrogen. The detrimental effect of hydrogen on IGSCC shows consistencies with aspects of both film rupture/slip dissolution and hydrogen embrittlement models.

Importance of Molybdenum on Irradiation-Assisted Stress Corrosion Cracking in Austenitic Stainless Steels

Constant extension rate tensile (CERT) specimens were irradiated in the core of a commercial operating boiling-water reactor (BWR). Subsequent to irradiation, CERT testing was performed in a test loop attached to the reactor water cleanup system in the same BWR, using reactor water at a high flow rate. Testing was performed in BWR normal water chemistry (NWC) and hydrogen water chemistry (HWC). Type 316 (UNS S31600) stainless steel (SS) was much less susceptible to irradiation-assisted stress corrosion cracking (IASCC) than type 304 SS (UNS S30400) in NWC and HWC. High-purity (HP) type 304 SS was more susceptible to IASCC than commercial purity (CP) type 304 SS in NWC at intermediate fluence (∼ 1 × 1021 n/cm2, E > 1 MeV). Results suggested chromium depletion resulting from radiation-induced segregation (RIS) was an operating mechanism in NWC and may have been the primary radiation effect at fluences < 3 × 1021 n/cm2 (E > 1 MeV). At higher fluences, another mechanism supervenes. Detailed microchemical analysis was conducted on unirradiated archive material and irradiated material. Two complementary techniques were used: field emission gun scanning transmission electron microscopy (FEGSTEM) and Auger electron spectroscopy (AES). The unirradiated materials showed nonequilibrium grain-boundary segregation of chromium, molybdenum, and phosphorus. Irradiation to intermediate fluences (∼ 1 × 1021 n/cm2, E > 1 MeV) resulted in further enhancements of silicon and phosphorus at the grain boundaries. In the CP materials, molybdenum and chromium remained enhanced at the grain boundaries, with chromium depletion on either side of the grain boundary. Chromium was depleted significantly in the HP material. At higher fluences (∼ 1 × 1022 n/cm2, E > 1 MeV) chromium was depleted significantly in the CP type 304 SS. The presence of molybdenum correlated with retention of the chromium level at the grain boundary during irradiation up to a certain fluence. For the HP type 304 SS, which contained virtually no molybdenum, the chromium level was lower than for the CP heats.

Hydrogen sulphide stress corrosion cracking of 2.25Cr-Mo steel weldments

Constant extension rate tensile (CERT) tests were performed to investigate the fracture characteristics of the A387 steel weldments in an H 2S charging environment. The heterogeneous (HG) welds using a stainless steel ER309-16 as a filler metal displayed inferior mechanical properties. In the as-welded condition, the heat-affected zone (HAZ) with coarse-grained structures initiated the crack. The propagation proceeded within the diluted region along the fusion boundary, owing to the formation of martensite in that region. Both the untempered HAZ and the diluted region revealed a high susceptibility to hydrogen embrittlement. The precipitation of Cr-carbides along the fusion boundary was found after tempering at 690 °C for 1 h, creating a more favorable path for the crack growth. The homogeneous (HM) welds, which were made by using the equivalent composition alloy as a filler metal (ER9016-B3), revealed that the coarse-grained HAZ of the as-welded specimen caused premature failure in H 2S. The tempered HM welds showed somehow improvement in strength and ductility in H 2S, but the crack still initiated at the coarse-grained HAZ of a weld.

Low-Temperature Annealing: A Process to Mitigate Irradiation-Assisted Stress Corrosion Cracking

Abstract Low-temperature (400°C to 500°C) annealing (LTA) treatments were conducted on type 304 (UNS S30400) stainless steel (SS) that had been irradiated to fast neutron fluences from 1.74 × 1021 n/cm2 to 2.95 × 1021 n/cm2 (E [energy] > 1 MeV). The purpose of these treatments was to restore the stress corrosion cracking (SCC) resistance of the unirradiated material. Certain treatments that were conducted at sufficiently high temperatures and for sufficient times were successful in restoring part or all of the preirradiation SCC and hardness properties. Complete restoration of the SCC properties was observed before restoration of the preirradiation hardness. SCC resistance was measured in constant extension rate tensile (CERT) and constant-deflection tests. The effectiveness of a given LTA treatment was found to depend upon the type of SCC test.

Grain Boundary Composition and Irradiation-Assisted Stress Corrosion Cracking Resistance in Type 348 Stainless Steel

Abstract Scanning transmission electron microscopy (STEM) analyses, in-reactor swelling mandrel tests, and laboratory constant extension rate tensile (CERT) tests were conducted on nine custom type...

Use of a Constant Deflection Test to Evaluate Susceptibility to Irradiation-Assisted Stress Corrosion Cracking

Abstract A constant deflection test was developed to evaluate the susceptibility of irradiated stainless steel (SS) to irradiation-assisted stress corrosion cracking. The test was shown useful in supplementing constant extension rate tensile or slow strain rate tensile, constant load, and swelling mandrel tests that have been used in the past. Preliminary data showing the susceptibility of commercial-purity type 304 SS (UNS S30400) as a function of stress, strain, and fast neutron fluence (E > 1 MeV) were presented.

Effects of grain boundary chemistry on the Intergranular Cracking Behavior of Ni-16Cr-9Fe in High-Temperature Water

An experimental program is conducted to determine the role of carbon, chromium, and phosphorus on the intergranular (IG) cracking behavior of Ni-16Cr-9Fe in 360 °C argon and water. Both constant extension rate tensile (CERT) tests and constant load tensile (CLT) tests are used to determine the susceptibility to IG cracking. Results show that carbon in solution strongly suppresses IG cracking behavior through an increased resistance to power-law creep, which promotes failure by the formation and linkup of grain boundary voids. The mechanical deformation at 360 °C is very time dependent, with slower extension rates resulting in greater IG cracking and lower elongation due to the longer time afforded the creep process. Although creep-induced grain boundary fracture is dominant in both water and argon, there is a substantial environmental enhancement in water. Grain boundary carbides do not appear to play a primary role in the grain boundary deformation process. In both environments, addition of P to Ni-16Cr- 9Fe improves the IG cracking resistance, but chromium depletion has no effect. Results imply that carbon in solution plays a critical role in strengthening and increasing resistance to creep- induced grain boundary void formation and fracture.

Effects of grain boundary chemistry on the intergranular cracking behavior of Ni-16Cr-9Fe in high-temperature water

An experimental program is conducted to determine the role of carbon, chromium, and phosphorus on the intergranular (IG) cracking behavior of Ni-16Cr-9Fe in 360 °C argon and water. Both constant extension rate tensile (CERT) tests and constant load tensile (CLT) tests are used to determine the susceptibility to IG cracking. Results show that carbon in solution strongly suppresses IG cracking behavior through an increased resistance to power-law creep, which promotes failure by the formation and linkup of grain boundary voids. The mechanical deformation at 360 °C is very time dependent, with slower extension rates resulting in greater IG cracking and lower elongation due to the longer time afforded the creep process. Although creep-induced grain boundary fracture is dominant in both water and argon, there is a substantial environmental enhancement in water. Grain boundary carbides do not appear to play a primary role in the grain boundary deformation process. In both environments, addition of P to Ni-16Cr-9Fe improves the IG cracking resistance, but chromium depletion has no effect. Results imply that carbon in solution plays a critical role in strengthening and increasing resistance to creepinduced grain boundary void formation and fracture.