Constant Extension Rate Tensile Tests Research Articles

The effect of surface grinding on the stress corrosion cracking initiation of 316LN stainless steel in 600 °C supercritical CO2

The effect of surface grinding on the stress corrosion cracking initiation of 316LN stainless steel in 600 °C supercritical CO2 was studied through constant extension rate tensile test. The ground surface exhibited much higher oxidation resistance than the polished surface as the deformation layer can greatly enhance the diffusion of active elements and promote the formation of more protective oxide scale. Moreover, the intergranular cracking was effectively suppressed on the ground surface as the ingress of oxidant along the original grain boundary was inhibited.

Effect of surface machining on the environmentally-assisted cracking of Alloy 182 and 316L stainless steel in light water reactor environments: results of the collaborative project MEACTOS

Abstract The main objective of the EU-funded project mitigating environmentally-assisted cracking through optimisation of surface condition (MEACTOS) was to gain knowledge on the ability of different surface machining procedures to mitigate environmentally-assisted cracking (EAC) in typical light water reactor structural materials and environments. Surfaces of cold-worked (CW) type 316L austenitic stainless steel and nickel-based weld metal Alloy 182 flat tapered tensile specimens were machined using different processes. EAC initiation susceptibility of these specimens was evaluated using constant extension rate tensile (CERT) tests under simulated boiling water reactor (BWR) and pressurized water reactor (PWR) conditions and assessed using constant load experiments. More than a hundred tests were performed covering about 10 years of autoclave testing time. Only minor or no measurable improvements in EAC initiation susceptibility as a function of surface treatments (grinding or advanced machining) compared to the standard industrial face milling were demonstrated. In most cases, the stress thresholds for EAC initiation determined in constant load tests confirmed the trend obtained from CERT tests. This paper summarises the most important results and conclusions concerning the EAC initiation behaviour for the CW 316L and Alloy 182 under reducing PWR and oxidizing BWR conditions.

Insights into the superior stress corrosion cracking resistance of FeCrAl alloy in high temperature hydrogenated water: The critical role of grain boundary oxidation

The stress corrosion cracking (SCC) initiation behavior of FeCrAl alloy was studied through constant extension rate tensile (CERT) test in high temperature hydrogenated water. Compared with a nuclear-grade austenitic 304 stainless steel (SS), the FeCrAl alloy is much more resistant to SCC initiation. The difference in SCC resistance results from the different oxidation behaviors. 304 SS shows typical preferential intergranular oxidation (PIO) which can degrade the strength of grain boundary. In contrast, FeCrAl is immune to PIO and forms homogeneous nano-grained oxide layer. The original grain boundary vanishes after oxidation, thus is not a weak site for SCC initiation.

Strain rate dependence of the effect of intergranular carbides on the resistance to stress corrosion cracking initiation of alloy 690 in simulated PWR primary water

The stress corrosion cracking (SCC) initiation of alloy 690 in simulated PWR primary water was evaluated in constant extension rate tensile tests with different strain rates. The SCC initiation was inhibited by intergranular carbide precipitation. However, the carbide effect weakened with increasing strain rate. The SCC mitigation effect of carbides mainly arises from the carbides’ ability to impede the transfer of dislocations across grain boundaries and delay the local break-down of surface oxide films. However, dislocations accumulated near the grain boundary can promote intergranular oxidation which induces cracking after the oxide film fails.

Investigation of surface treatment effects on the environmentally-assisted cracking behaviour of Alloy 182 in boiling water reactor environment

ABSTRACT Surface treatments of primary circuit components in light water reactors are regarded as possible ways to mitigate environmentally-assisted cracking (EAC). To date, it is not fully conclusive which surface condition is suitable to reduce the EAC initiation susceptibility. Constant extension rate tensile (CERT) tests were performed by several labs using flat tapered tensile specimens with different surface conditions (ground, industrial face milled, advanced face milled and shot peened), exposed to a boiling water reactor normal water chemistry environment at 288°C. Despite some scatter in the results, the CERT tests revealed that the EAC initiation susceptibility seems lowest for the advanced face milled surface and highest for the shot peened surface. However, it must be emphasised that the differences were moderate and that the surprising behaviour of the shot peened surface can be explained. The mechanical grinding of the surface did not significantly retard EAC initiation compared to industrial face milling.

Impact of Chloride on the Environmentally-Assisted Crack Initiation Behaviour of Low-Alloy Steel under Boiling Water Reactor Conditions

Low-alloy reactor pressure vessel steels have a rather low susceptibility to stress corrosion cracking (SCC) in a boiling water reactor (BWR) environment if the high-temperature water contains no anionic impurities. Recent investigations revealed that under oxidizing BWR normal water chemistry (NWC) conditions extremely small amounts of chloride, can cause very high SCC growth rates in these materials. Therefore, the effect of continuous and temporary chloride additions on the crack initiation behaviour was explored by a series of constant extension rate tensile (CERT) and constant load tests in high-temperature water. In an NWC environment, containing ≥2 ppb of chloride, strain-induced corrosion cracking (SICC) initiation occurred briefly after the onset of plastic yielding and at much smaller strains than in high-purity water. On the other hand, under reducing hydrogen water chemistry conditions with up to 700 ppb chloride, no SICC was detected up to very high strains. CERT experiments, with moderate short-term chloride transients before and during the loading, showed that even serious mechanical loading transients, one day after returning to high-purity water, did not result in early SICC initiation.

Influence of hydrogen on deformation and fracture mechanisms in austenitic steel types

In the present work, the hydrogen/material interaction with two austenitic steel types, created via a completely different alloying strategy, is investigated. The two steels include 304L austenitic stainless steel (ASS) and 18Mn-0.6C twinning-induced plasticity (TWIP) steel. Constant extension rate tensile tests are performed to evaluate the influence of hydrogen on the deformation and fracture mechanisms. A reference condition without hydrogen is compared to a seven days electrochemically hydrogen precharged condition. 304L ASS shows transformation to α’-martensite without hydrogen, while hydrogen increases the α’-martensite fraction and additionally enables the transformation to ε-martensite. The fracture surface has a transgranular, quasi-cleavage appearance. The TWIP steel shows deformation twinning without hydrogen, while abundant ε-martensite transformation is observed in the presence of hydrogen. The fracture type is intergranular due to the high cracking sensitivity of the grain boundaries.

Stress corrosion cracking initiation susceptibility of Alloy 182 with different surface treatments

The effects of machining surface preparations on the stress corrosion cracking (SCC) susceptibility in boiling water reactor environment were evaluated for Alloy 182 weld metal. Constant extension rate tensile test, constant load test and electrochemical tests (passivation, current-voltage curves and electrochemical impedance spectroscopy) showed that the surface treatments strongly influence the resistance to SCC initiation. The machining-induced deformation and surface oxide layer from the surface treatments affect the re-passivation behaviour and SCC initiation susceptibility. The SCC susceptibility of the studied surface treatments was found following the sequence of shot peened > industrial surface milled > polished > advanced manufactured surface.

Evaluation of the biodegradation product layer on Mg-1Zn alloy during dynamical strain

Magnesium (Mg) alloys are attractive biodegradable implant materials. The degradation products on Mg alloys play a critical role in the stability of the interface between implant and surrounding tissue. In the present study, the effects of dynamic deformation on the interface layer of biomedical Mg-1Zn alloy were investigated using the constant extension rate tensile tests (CERT) coupled with electrochemical impedance spectroscopy (EIS). The deformation of the Mg-1Zn alloy had an adverse influence on the impedance of the surface degradation layer formed in simulated body fluid that only containing inorganic compounds. However, the surface degradation layer with improved corrosion resistance was obtained for the strained samples tested in protein-containing simulated body fluid. The spontaneous or enhanced adsorption of protein into the degradation product led to a flexible and stable hybrid anti-corrosive layer. A relationship between the dynamic deformation of Mg alloy and the impendence of the degradation layer was established, which demonstrates the necessity for in situ characterisation of the evolution of the surface layer under dynamic condition.

Mechanical Behaviour of Type 316L Austenitic Stainless Steel in Low Pressure Hydrogen Steam Environment

Low pressure superheated H2-steam appears to be an interesting alternative to pressurized water environments, since it is capable of performing accelerated environmentally assisted cracking (EAC) experiments for nickel base and stainless steel alloys. Constant Extension Rate Tensile (CERT) tests were performed with strain rates of 2 × 10−6 or 2 × 10−8 ms−1 at 350, 400, 440 and 480 °C on flat tapered specimens of Type 316L austenitic stainless steel. The tapered shape allows the determination of crack initiation over a range of stresses and strains simultaneously on one specimen and therefore the threshold stress value was obtained. The environment was 6 times more oxidizing than the dissociation pressure of NiO. The acquired mechanical properties are summarized and threshold stresses for EAC crack initiation are evaluated.

The environmental degradation behavior of FeNiMnCr high entropy alloy in high temperature hydrogenated water

Co-free high entropy alloy (HEA) 28%Fe-27%Ni-27%Mn-18%Cr shows good irradiation resistance and could be a potential candidate for structural material in nuclear power industry. In this work, the environmental degradation behavior of this HEA was investigated in high temperature hydrogenated water. This HEA shows extensive intergranular cracking after constant extension rate tensile test, indicating that it is very susceptible to stress corrosion cracking (SCC) initiation. The microstructure analysis indicates that Mn and Cr diffuse outwards along the grain boundary during oxidation, inducing grain boundary migration. The Cr flux promotes the formation of Cr-enriched oxide layer above the grain boundary which prevents intergranular oxidation when the sample is unstressed. However, Mn is unstable in the oxide and tends to dissolve into the solution, making the oxide porous. The porous oxide fractures readily when stressed, inducing SCC initiation.

Effects of surface treatments on environmentally-assisted cracking susceptibility of Alloy 182 in BWR environment

The effects of four machining surface preparations on the environmentally-assisted cracking (EAC) initiation and early crack growth were evaluated for Alloy 182 flat tapered specimens exposed to a simulated boiling water reactor (BWR) environment under constant extension rate tensile test conditions. Results showed that the apparent EAC critical threshold stress for initiation was highly dependent on the surface treatment. Detailed microstructural characterization revealed the nature of the machining-induced deformation associated with the four surface treatments. Surface treatments affected the surface oxidation during exposure in the simulated BWR environment. The machining-induced deformation structure affected the EAC initiation susceptibility.

Environmentally assisted cracking in the low pressure superheated hydrogen steam

Abstract Low pressure superheated H2-steam appears to be an interesting alternative to pressurized water environments, since it is capable of performing accelerated environmentally assisted cracking (EAC) experiments for nickel base and stainless steel alloys. Constant Extension Rate Tensile (CERT) tests were performed with displacement rates of 2×10-6 or 2×10-8 ms-1 at 350, 400, 440 and 480 °C on flat tapered specimens of Type 316L austenitic stainless steel. The tapered shape allows the determination of crack initiation over a range of stresses and strains simultaneously on one specimen and therefore the threshold stress value was obtained. The environment was 6 times more oxidizing than the dissociation pressure of NiO. The acquired mechanical properties are summarized and threshold stresses for EAC crack initiation are evaluated.

IASCC of neutron irradiated 316 stainless steel to 125 dpa

The irradiated microstructure and irradiation-assisted stress corrosion cracking (IASCC) susceptibility were determined for neutron-irradiated cold-worked (CW) 316 stainless steel tested in 320 °C simulated PWR primary water with various damage levels of 46.9, 67.4, and 125.4 dpa. The size and density of dislocation loops and precipitates were found to vary little over the dose range 46.9–125.4 dpa and dislocation loops were found to control the irradiation hardening. The %IG in constant extension rate tensile (CERT) tests and the stress to initiate a crack in four-point bend tests were consistent in establishing that the IASCC susceptibility was essentially unchanged across the dpa range. GB oxidization was observed prior to crack initiation, and localized deformation and triple junctions where local stresses were high were preferred crack initiation sites. IASCC cracks initiated preferentially on grain boundaries oriented nearly normal to the applied stress. An apparent threshold stress threshold for IASCC was determined to be ~45% of the irradiated yield strength for neutron-irradiated CW 316 SS between 46.9 and 125.4 dpa.

Acceleration of Environmentally Assisted Cracking Initiation of the Type 316L Steel in High-Temperature Water and Hydrogenated-Steam Vapor Environments

Environmentally assisted cracking of solution-annealed Type 316L austenitic stainless steel with two different surface treatments (polished versus ground) was investigated using the constant extension rate tensile test methodology in high-pressure water at 350°C and low-pressure H2-steam vapor at 350°C, 400°C, 440°C, and 480°C, while maintaining electrochemical corrosion potentials in the NiO stability regime. Flat tapered specimens were used to indentify the threshold stress for cracking (∼400 MPa) under these environmental conditions. Intensive oxidation and typical intergranular cracks were observed to initiate from the polished surface at 400°C and 440°C, whereas wavy cracks initiated on the ground surface. The results indicate that the ultrafine-grained layer formed adjacent to the ground surface effectively suppressed intergranular crack initiation under these test conditions. The slow loading under H2-steam vapor at 400°C under the oxidizing condition (NiO) was found to be a suitable high-accelerated test to study early stages of the cracking.

Study of Crack Initiation of 15-15Ti Austenitic Steel in Liquid PbBi

The austenitic steel 15-15Ti is being considered as one of the candidate materials for internal structural components of future heavy liquid metal (HLM) nuclear systems. This work studies the steel compatibility with liquid PbBi. Constant extension rate tensile (CERT) tests of tapered specimens were used to study sensitivity to liquid metal embrittlement (LME) and crack initiation. The taper creates a variation of stress along the gauge length which allows the identification of the stress and strain for the crack appearance. Testing was performed in air and in PbBi with 10−6 to 10−12 wt % oxygen content at 300 °C. Post-test observation by scanning electron microscopy (SEM) highlighted the crack morphology. An evaluation of the environmental effect on the crack initiation is presented.

Environmentally-Assisted Cracking of Type 316L Austenitic Stainless Steel in Low Pressure Hydrogen Steam Environments

A low pressure, superheated hydrogen-steam system has been used to accelerate the oxidation kinetics while keeping the electrochemical conditions similar to those of the primary water in a pressurized water reactor. The initiation has been investigated using a Constant Extension Rate Tensile (CERT) test. Tests were performed on flat tapered specimens made from Type 316L austenitic stainless steel with strain rates of 2×10-6 and 2×10-8 ms-1 at room temperature and at an elevated temperature of 350 °C. R = 1/6 was chosen as a more oxidizing environment and R = 6 was selected as a more reducing environment, where the parameter R represents the ratio between the oxygen partial pressure at the Ni/NiO transition and the oxygen partial pressure. Different exposures (1 day and 5 days) prior to loading were investigated post-test evaluation by scanning electron microscopy.

Microstructural Investigation of LME Crack Initiated in Ferritic/Martensitic Steel T91 Loaded in Liquid Lead-Bismuth Eutectic at 300 °C.

Liquid lead-bismuth eutectic (LBE) is one of the candidate materials for advanced nuclear systems. The structural materials used in contact with LBE are selected according to the resistance to liquid metal corrosion, irradiation embrittlement, and compatibility with the coolant. However, simultaneous presence of mechanical strain and LBE environment can induce liquid metal embrittlement (LME) in these materials. In this study, a specimen from candidate ferritic-martensitic steel T91 was tested by Constant Extension Rate Tensile (CERT) test exposed to PbBi environment with oxygen concentration 6 × 10−6 wt % at 300 °C up to rupture. Post-test examination using scanning electron microscopy (SEM) showed a deep crack indicating features of LME in the plastic strained region of the tested specimen. Further investigations focused on characterization of the fracture path and microstructure determination using focused ion beam (FIB) and energy dispersive X-ray spectrometry/electron backscatter diffraction (EDX/EBSD). This observation revealed that the dominant LME failure mode of the observed crack is translath or transgranular and the crack stopped at the high-angle grain boundary. The role of oxides in the crack initiation is discussed.

Initiation of LME crack in ferritic martensitic steel in liquid lead-bismuth

This work is aimed at fundamental understanding of the T91 steel cracking mechanisms in liquid PbBi eutectic. In particular, the crack initiation was studied by means of Constant Extension Rate Tensile (CERT) tests with flat tapered specimens in PbBi at 300 °C with oxygen content 3 × 10−7 - 6 × 10−6 wt. % and in air, up to rupture and maximum load. The tapered specimens were meant to create a uniform variation of stress along the gauge length and the threshold stress of the crack initiation was determined. The performance of the steel was affected by the environment. The threshold stress decreased and cracking character changed in PbBi when compared to the testing in air. Post-test examinations carrying out with SEM equipped with FIB found shallow cracks from broken oxide having ductile propagation and a grain-size-deep crack of inter-lath martensite path in the plastic strain region. The cracking stress-strain conditions and the role of oxide rupture causing wetting, as the cracking precursors, are discussed.

Insights into the stress corrosion cracking of solution annealed alloy 690 in simulated pressurized water reactor primary water under dynamic straining

The intergranular attack near both stagnant and active stress corrosion crack (SCC) tips of solution annealed alloy 690 were characterized after constant extension rate tensile tests in simulated pressurized water reactor (PWR) primary water containing 18 cc H2/kg H2O. In both cracks, an intergranular oxide, composed of NiO and Cr2O3, was observed beyond the crack tip with an adjacent Cr depleted grain boundary migration zone. The stagnant crack has a compact Cr2O3 covering the crack tip and the adjacent grain boundary migration zone is deep and free of oxidation, while the active crack has porous penetrative oxide extending into the migration zone. The ability to form a compact Cr2O3 film at the crack tip is important to the IGSCC resistance and may be dictated by the efficiency of Cr diffusion up the migrated grain boundary. When a compact Cr2O3 film does not form at the crack tip, the Cr depleted grain boundary migration zone is subject to penetrative oxidation via the inward diffusion of oxygen, making the zone susceptible to crack propagation. Thus, even a high Cr content cannot guarantee the immunity of a Ni base alloy to SCC during dynamic straining.