U-bend Specimens Research Articles

Chloride-Induced Stress Corrosion Cracking of Friction Stir-Welded 304L Stainless Steel: Effect of Microstructure and Temperature

Dry storage canisters of used nuclear fuels are fabricated using SUS 304L stainless steel. Chloride-induced stress corrosion cracking (CISCC) is one of the major failure modes of dry storage canisters. The cracked canisters can be repaired by friction stir welding (FSW), a low-heat input ‘solid-phase’ welding process. It is important to evaluate the ClSCC resistance of the friction stir welded material. Stress corrosion cracking (SCC) studies were carried out on mill-annealed base materials and friction stir welded 304L stainless U-bend specimens in 3.5% NaCl + 5 N H2SO4 solution at room temperature and boiling MgCl2 solution at 155 °C. The engineering stress on the outer fiber of the FSW U-bend specimen was ~60% higher than that of the base metal (BM). In spite of the higher stress level of the FSW, both materials (FSW and BM) showed almost similar SCC failure times in the two different test solutions. The SCC occurred in the thermo-mechanically affected zone (TMAZ) of the FSW specimens in the 3.5% NaCl + 5 N H2SO4 solution at room temperature, while the stirred zone (SZ) was relatively crack-free. The failure occurred at the stirred zone when tested in the boiling MgCl2 solution. Hydrogen reduction was the cathodic reaction in the boiling MgCl2 solution, which promoted hydrogen-assisted cracking of the heavily deformed stirred zone. The emergence of the slip step followed by passive film rupture and dissolution of the slip step could be the SCC events in the 3.5% NaCl + 5 N H2SO4 solution at room temperature. However, the slip step height was not sufficient to cause passivity breakdown in the fine-grained SZ. Therefore, the SCC occurred in the partially recrystallized softer TMAZ. Overall, the friction-stirred 304L showed higher tolerance to ClSCC than the 304L base metal.

Rapid evaluation of the critical condition for hydrogen-induced cracking in ultrahigh-strength automotive steel sheets: A semiquantitative investigation based on U-bend specimens

The production of automotive steel plates, which involves pickling, electroplating, and welding among other steps, introduces hydrogen. Additionally, during the service life of components fabricated using such plates, electrochemical corrosion occurs, and hydrogen atoms are introduced. The hydrogen atoms are adsorbed and diffused into the interior of the metal, accumulating at any defects. Automotive parts are commonly fabricated using cold forming techniques. The hydrogen embrittlement sensitivity of ultrahigh-strength automotive sheets (UHSSs) is influenced by the stress, concentration of diffusible hydrogen, and plastic strain resulting from the cold forming process. The standard methods for quantitative studies on the UHSS hydrogen embrittlement sensitivity, such as the constant-load testing and low-strain-rate testing, require a specialized equipment and extended experimental period, making them impractical for original automotive equipment manufacturers. This study focuses on the use of the quenching and partitioning (QP) steel QP1180, which contains residual austenite, and martensite steel MS1500, which has a low deformation ability but high strength, as research objects. The equivalent strain and stress were controlled by adjusting the bending radius and U-bend opening distance, while the diffusible hydrogen concentration was charged through immersion. The fracture time of the U-bend sample was recorded, and the critical diffusible hydrogen concentration for hydrogen-induced cracking was measured. Additionally, the stress distribution during the U-bending process was simulated using the finite-element method. To evaluate the hydrogen embrittlement resistances of QP1180 and MS1500, the area enclosed by the load stress and equivalent strain (bending radius) and fracture time were semiquantitatively determined. Finally, the hydrogen-induced delayed fracture space regions of QP1180 and MS1500 were determined in the three-dimensional space, with equivalent strain, load stress, and diffusible hydrogen concentration as coordinate axes. This provided a convenient and reliable means of determination of the critical conditions for a safe service and rapid evaluation of the hydrogen embrittlement sensitivity of ultrahigh-strength automotive sheets.

Mechanism Study of Stress Corrosion Behavior under Tensile and Compressive Stresses for Welded Joint Used in Nuclear Turbine Rotor.

Stress corrosion damage containing pitting and cracking was investigated for NiCrMoV steel-welded joint used in nuclear turbine rotor in 3.5% NaCl solution. The U-bend specimen containing tensile face and compressive face was adopted which was conducive to study the effects of stress on pitting and stress corrosion cracking. On tensile surface of U-bend specimen, pit grew in open environment and was converted into crack covered with passivated film. Interestingly, corrosion pit was also found on compressive surface, which might attribute to this location creating enclosed environment causing Cl- to diffuse hardly. Then, pit grew under the occluded oxide crust composed of special crystalline corrosion products. When pit approached to critical size, crack initiated from it. The critical size of pit for crack initiation from pit was 35 μm on tensile surface and 95 μm on compressive surface. The sensitivity of crack initiation in tensile surface was higher than that in compressive surface. Then, cracking on compressive surface was controlled by slip dissolution mechanism, that is, dislocation outcrop generated through plastic deformation during manufacturing and absorbed Cl- in enclosed environment to accelerate metal dissolution and film rupture. Thereupon, the stress corrosion cracking on compressive surface was able to be maintained. The findings compared corrosion damage modes caused by the two kinds of stress and emphasized the nonignorable role of compressive stress on stress corrosion damage.

Effects of environmental parameters on chloride-induced stress corrosion cracking behavior of austenitic stainless steel welds for dry storage canister application

This study investigated the chloride-induced stress corrosion cracking (CISCC) behavior expected to occur in welds of austenitic stainless steel, which are considered candidate materials for dry storage containers for spent nuclear fuel. The behavior was studied by varying temperature, relative humidity (RH), and chloride concentration. 304L-ER308L welded plates were processed into U-bend specimens and exposed to a cyclic corrosion chamber for 12 weeks. The CISCC behavior was then analyzed using electron microscopy. A previous study by the authors confirmed that CISCC occurred in ER308L at 60 °C, 30% RH, and 0.6 M NaCl via selective corrosion of δ-ferrite. When the temperature was lowered from 60 °C to 50 °C, CISCC still occurred. However, when the humidity was reduced to 20% RH, CISCC did not happen. This can be attributed to the retardation of the deliquescence of NaCl at lower humidity, which was insufficient to promote CISCC. Furthermore, increased chloride concentration to 1.0 M resulted in the absence of CISCC and widespread surface corrosion with severe pitting corrosion because of the increase in thin film thickness.

Effects of Micro-Shot Peening on the Stress Corrosion Cracking of Austenitic Stainless Steel Welds

Micro-shot peening on AISI 304 and 316 stainless steel (SS) laser welds was performed to evaluate its effect on the susceptibility to stress corrosion cracking (SCC) in a salt spray containing 10% NaCl at 80 °C. The cracking susceptibility of the welds was disclosed by testing U-bend specimens in a salt spray. Micro-shot peening caused an intense but narrow deformed layer with a nanocrystal structure and residual compressive stress. Austenite to martensite transformation occurred heavily on the top surface of the micro-shot peened welds. SCC microcracks were more likely to be initiated at the fusion boundary (FB) of the non-peened welds. However, fine pits were formed more easily on the micro-shot peened 304 fusion zone (FZ), which was attributed to the extensive formation of strain-induced martensite. The nanograined structure and induced residual compressive stress in the micro-shot peened layer suppressed microcrack initiation in the 304 and 316 welds in a salt spray. Compared with the other zones in the welds in a salt spray, the high local strain at the FB was the cause of the high cracking susceptibility and could be mitigated by the micro-shot peening treatment.

Estimating Failure Mechanism of Steel Specimens using Stress Corrosion-Cracking (SCC) Testing Methods: State and Development

Stress Corrosion Cracking is cracking in the material that occurs due to a combination of tensile stress, material sensitivity, and a reactive environment. SCC can cause material failure, so the phenomenon is hazardous for human life safety. It is caused by the crack propagation mechanisms that cannot be controlled, and the failure time is unpredictable. SCC susceptible is experienced by structures operating in a reactive environment such as seawater. If the material structure is in a corrosive seawater environment and receives loads from continuous waves of seawater, the structure will experience failure caused by SCC. The dangerous effects caused by SCC are necessary to research these testing methods and standards. Previous studies have conducted SCC testing using several methods, including the U-Bend specimen, C-Ring specimen, tensile specimen, and slow strain rate testing. However, comprehensive information about these methods is still limited. Therefore, it is necessary to analyze several SCC testing methods on a laboratory scale. This paper references students and future researchers who want to study the SCC phenomenon. Besides, determining the appropriate SCC testing method helps engineers consider the materials used in the design process using more advanced technology.

Evaluating the reliability of PWSCC resistance of TT Alloy 690 and associated welds to the end of anticipated PWR plant life

In order to evaluate the primary water stress corrosion cracking (PWSCC) resistance and possible SCC growth rate (SCCGR) for thermally treated (TT) Alloy 690 to the end of anticipated PWR plant life, including various levels of cold work and associated welds, long-term PWSCC initiation tests using reverse U-bend specimens and SCCGR measurement tests were conducted at 360 °C in simulated PWR primary water. In addition, the possibility of long-range ordering (LRO) and/or short-range ordering (SRO) was evaluated by micro-Vickers hardness measurements, electron diffraction analysis and nano-scale EDS by high resolution TEM. The studied alloys were TT Alloy 690 and a Ni-33at%Cr model alloy after long-term ageing in air at 360, 400, 420 or 475 °C. No SCC was detected on any of the reverse U-bend specimens after testing for up to 45,000 h at 360 °C in simulated PWR primary water. Moreover, SCCGRs for the as-received TT Alloy 690 and with various levels of cold work or its associated welds did not increase when tested at 360 °C in simulated PWR primary water after thermal ageing at 360, 400, 420 or 475 °C in air for up to ∼20,000 h. In the case of the Ni-33at%Cr model alloy, LRO/SRO was clearly identified by electron diffraction analysis after thermal ageing at 475 °C in air. However, no evidence of LRO/SRO was observed in the case of the as-received TT Alloy 690 or after various levels of cold work or in its associated welds by any of the test methods after long-term thermal ageing in air at any temperature. From these test results, it is concluded that TT Alloy 690 and its associated welds in real components of PWRs have extremely high SCC resistance and extremely low SCCGRs should PWSCC be initiated, up to the end of any currently anticipated PWR plant life time.

On the feasibility of duplex stainless steel 2205 as an accident tolerant fuel cladding material for light water reactors

A few tests were performed to evaluate the feasibility of 2205 duplex stainless steel (DSS) as an accident tolerant fuel (ATF) cladding material, including corrosion tests in simulated pressure water reactor (PWR) primary water and high-temperature steam, stress corrosion cracking (SCC) initiation test, mechanical properties test and thermal aging effects. In both 360 °C PWR primary water and 1200 °C steam, DSS 2205 showed weight changes comparable to the high-Cr alloys. After 3163 h of exposure in 400 °C steam, U-bend specimens of DSS 2205 showed no obvious cracks, suggesting that SCC is not likely to be initiated during the service life. Compared to FeCrAl alloys, DSS 2205 showed comparable strength but greater ductility. Though 400 °C thermal aging caused an increase in strength for DSS 2205, total elongation was more than 35% even after 11,061 h aging, greater than other Fe-based ATF alloys. Also, concerns on neutron penalty, irradiation, fabrication and spent fuel storage were discussed. The overall results suggest that DSS 2205 could be considered as a candidate ATF cladding material.

Visualization of Hydrogen in Stress and Strain Fields Using SIMS

In order to clarify the mechanism of hydrogen embrittlement in high strength steel, it is necessary to evaluate local hydrogen concentration in stress and strain fields where hydrogen embrittlement is considered to occur. There are several ways to visualize hydrogen, but SIMS is advantageous in that it can detect hydrogen directly without reaction. In this study, visualization of hydrogen accumulated in stress and strain field was tried using SIMS. Hydrogen flux intensity in the bent portion of the U-bend specimen, where the stress gradient exists, changed according to the stress gradient, and the hydrogen flux intensity was higher on the outside of the bending than on the inside, with the center of the plate thickness at the boundary. On the shearing surface with strain field, the hydrogen flux intensity had a positive correlation with the stress. On the other hand, it remained constant with respect to the strain. From these observations, the hydrogen partitioning behavior in steel could be visualized semi-quantitatively by using the isotope labeling method with SIMS.

Hydrogen fracture maps for sheared-edge-controlled hydrogen-delayed fracture of 1180 MPa advanced high-strength steels

Hydrogen-induced delayed fracture (HIDF) of 1180 MPa martensitic (MS), dual-phase (DP), and quenched and partitioned (Q&P) advanced high-strength steels was studied using shear-cut U-bend specimens. Fracture maps identified the conditions that caused HIDF. HIDF initiation was largely independent of the bending strain, and was controlled by the sheared edge, creating the critical amount of plastic strain damage required for HIDF. A hydrogen enhanced plasticity (HEP) mechanism caused HIDF. Differing responses to HIDF of the three steels were related to how the microstructures interacted with the stress and strains introduced by shearing. Exploratory work indicated that shear burr arrangement had little influence.

Finite element analysis of edge and cleavage cracks in U-bend specimen using contour integral approach

Abstract The present study reports the finite element analysis of edge and cleavage cracks in the sub-model of U-bend specimen under pure bending loading. Both the edge and cleavage cracks were modeled using contour integral approach. Twenty different cases of edge cracks were analyzed using ABAQUS CAE software to understand their influence in mode-I stress intensity factor (SIF). Additionally, one cleavage crack inspired by a failed U-bend specimen was modeled and analyzed to understand its severity. The crack model was verified with the help of two different empirical equations suggested by Brown and Tada, respectively. The edge and the cleavage cracks exhibit the same degree of severity for equal crack length. It was also observed that von-mises stress does not dictate, but only influence the mode-I SIF value in pure bending condition.

Hydrogen-induced delayed fracture of a 1180 MPa martensitic advanced high-strength steel under U-bend loading

Hydrogen-induced delayed failure (HIDF) of an 1180 MPa martensitic advanced high-strength steel (MS-AHSS) was studied using U-bend tests. HIDF susceptibility was increased by increasing stress, plastic strain and hydrogen content. Fracture initiated in the region of high tensile stress at the apex of the U-bend specimen. Hydrogen-induced fracture propagated in two stages, coinciding with the two stress regions of the specimen. The hydrogen content of the U-bend specimen increased with plastic strain and hydrogen fugacity. Delayed fracture is possible for specimens (and components) that contain substantial damage due to plastic strain and are subjected to a substantial stress.

EXPERIMENTAL STUDY OF CHLORIDE-INDUCED STRESS CORROSION CRACKING (CISCC) FOR AUSTENITIC SUS 304L UNDER THERMAL INSULATION

Chloride-Induced Stress Corrosion Cracking (CISCC) has been frequently reported in the petrochemical industries where it usually causes failure to the austenitic stainless steel structures encased with thermal insulation in a chloride-containing environment. This study aims to analyze and evaluate the risk of chloride concentration and different types of insulation materials on CISCC of Austenitic SUS 304L. The experimental study was carried out for 14 days using U-bend specimens through the Drip Test apparatus, as per ASTM G30 and ASTM C692. U-Bend specimens were installed onto the test rig and shielded with different insulation materials. Sodium chloride (NaCl) salt solutions were periodically dripped onto the specimens, to simulate the wet and dry cycle. Insulation materials used were rockwool, calcium silicate and perlite, whereas the concentrations of NaCl solutions were set at 0.1, 1.0 and 3.5 wt.%. The specimens were inspected usingdye penetration tests (DPT), stereo microscope and optical microscope throughout the study to determine the CISCC susceptibility. Results showed that cracking was observed on the specimen with rockwool insulation at 3.5 wt% NaCl and 90°C. Rockwool has a high water absorption capacity at which saltwater will evaporate when in contact with a hot metal surface, resulting in the build-up of salt deposits of high concentration over time. SCC was not observed on other specimens with different conditions, but salt deposits, general corrosion and pitting corrosion were found. From the study, the thermal insulation of rockwool was found to have the highest tendency to cause SCC, followed by calcium silicate and perlite.Keywords: SUS304L, chloride-induced stress corrosion cracking, insulation materials, U-Bend specimen, drip test

Environmentally assisted cracking behavior of U-bend specimens of Mg–RE alloys in chloride containing basic solution

Abstract Environmentally assisted cracking (EAC) behavior of two Mg-rare earth (RE) alloys such as Mg–Zn–Gd–Nd–Zr (EV31A) and Mg–Y–Nd (WE43C) alloys was investigated by using U-bend specimens. Open circuit potentials (OCP) of the U-bend specimens were monitored during the EAC tests in 0.1 M NaOH solution with different chloride concentrations at room temperature. EV31A (as-received, and peak aged) and WE43C (peak aged) specimens failed by SCC in 80 ppm chloride containing 0.1 M NaOH solution at OCP. When the EAC initiation occurred, the OCP decreased continuously. Irregular fluctuations of the OCP were observed in the absence of EAC. The OCP versus time profile could be used for monitoring EAC failure of the Mg–RE alloy components in real life service. Applied potentials did not cause cracking of the EV31A alloy in 80 ppm Cl− containing 0.1 M NaOH. Accelerated cracking was observed on the WE43C alloy in peak-aged condition under the applied potentials in the transpassive region when compared to that of OCP condition. Overaging decreased the susceptibility to cracking.

Effect of plastic strain damage on the hydrogen embrittlement of a dual-phase (DP) and a quenching and partitioning (Q&P) advanced high-strength steel

The hydrogen influence on two 1180 MPa advanced high-strength steels was studied using electropolished U-bend specimens in HCl solutions of pH 1 and pH 3. Bending limit diagrams identified conditions for safe use. Hydrogen-assisted fracture occurred only near the bending limit, and was promoted by microvoids and microfractures, introduced by the severe plastic deformation of the U-bend specimens. The hydrogen fracture initiation and early propagation was by the mechanisms designated as hydrogen fracture due to hydrogen enhanced microvoid coalescence (HFMVC). Exploratory work indicated that fractures were critically dependent on the surface state of the specimens.

The effect of sensitization treatment on chloride induced stress corrosion cracking of 304L stainless steel using U-bend test

Chloride induced stress corrosion cracking (CI-SCC) commonly occurs for 300 series of austenitic stainless steels that are widely used in the petrochemical industry. The occurrence of CI-SCC requires certain circumstances such as corrosive environment, tensile stress, and susceptible material. Heat treatment temperature between 550 °C–650 °C followed by cooling process causes the sensitization of austenitic type stainless steel in which a chromium-depleted band is formed adjacent to the grain boundary. In this study, the impact of sensitization on CI-SCC was examined using austenitic stainless steel grade 304L (SS 304L). The specimens were shaped into U-bend as per ASTM G30 and exposed for long-term immersion test. U-bend samples were heat sensitized at 600 °C for 2 h followed by air cooling. Samples were immersed in 0.1 wt%, 1.0 wt% and 3.5 wt% NaCl into the water bath at temperature of ambient (baseline), 70 °C and 90 °C for 42 days. They were monitored weekly for visual examined for crack. Stereo and optical microscopy were used to identify the CI-SCC and morphology of U-bend specimens. In the beginning, pit corrosion was observed and followed by cracks growth initiated from the localized pit. The results indicated that Cl-SCC was observed for all sensitized SS 304L specimens at elevated temperature conditions which was more susceptible than the as-received type specimens. The trend indicated that the higher the temperature or chloride concentration, the shorter the time for crack initiation. There was no CI-SCC for both types of specimens at ambient condition. Two CI-SCC likelihood graphs with respect to chloride concentration versus temperature are plotted based on the test data.

Magnetite-accelerated stress corrosion cracking of alloy 600 in water containing 100 ppm lead oxide at 315 °C

The purpose of this work is to verify the effect of magnetite coupled with Alloy 600 on stress corrosion cracking (SCC) behavior in water containing 100 ppm PbO at 315 °C. SCC tests were conducted using U-bend specimens partially covered with magnetite, simulating a condition wherein magnetite was deposited on steam generator tubes. The maximum crack depths of the Alloy 600 specimens coupled with magnetite were 20–68% greater than those of the uncoupled specimens for a given test time. The coupling of magnetite to Alloy 600 led to the fast growth of oxide films, resulting in the formation of relatively Cr-depleted inner oxides with defects. The corrosion potential of Alloy 600 was 90 mV lower than that of magnetite at 80 °C, indicating that the corrosion of Alloy 600 could be stimulated by galvanic coupling with magnetite. The accelerated stress corrosion and altered film properties were discussed from the viewpoint of a galvanic coupling effect.

An Evaluation Method for Hydrogen Embrittlement of High Strength Steel Sheets Using U-bend Specimens

An evaluate method for hydrogen embrittlement property of high strength steel sheet has been proposed in this study. To take into consideration of the effect of plastic strain in addition to the effects of applied/residual stress and diffusible hydrogen, U-bend specimens have been adopted because steel sheets for automobiles are usually used after press forming into various parts. After U-shape bending, the specimen was loaded using a bolt. The proposed evaluation method is based on the measurement of critical hydrogen content or critical hydrogen charging condition for hydrogen embrittlement fracture at given stress and strain conditions. The hydrogen charging current density was increased in step-wise manner until cracking was observed, and cracking was detected by optical observation and by monitoring voltage between the sample and a counter electrode. The critical hydrogen contents for specimens with varied applied stress were obtained by means of thermal desorption spectroscopy. For the critical hydrogen content, both the hydrogen contents in strained portion of the specimen and no-strained portion were measured. The former is affected by introduced dislocations caused by straining and the latter is thought to be proportional to the hydrogen fugacity. Both critical hydrogen contents tended to be decreased slightly when the applied stress was relatively high.

Corrosion and stress corrosion cracking initiation of grade 304 and 316 stainless steels in activated Methyl Diethanol Amine (aMDEA) solution

This study concerns corrosion and stress corrosion cracking (SCC) initiation of type 304 and 316 austenitic stainless steels in activated Methyl Diethanol Amine (aMDEA) solution. Corrosion and stress corrosion cracking behavior of stainless steels are less known in aMDEA solution. In this regard constant deflection U-bend specimens were used for micro-crack initiation tests. Cyclic polarization test were performed to evaluate corrosion performance of 304 and 316 stainless steels. Results showed that increase in aMDEA solution concentration from 30 to 50 wt% had minor effect on corrosion and crack initiation of 304 and 316 stainless steels. Increase in temperature from 25 to 70 °C raised the corrosion rate near two folds. Grade 304 stainless steels was more susceptible to SCC crack initiation compared to 316 stainless steel, since longer cracks were formed. This was explained by the SCC film rupture mechanism.

A Study on the Effect of Surface Grinding on Chloride Induced Pitting Initiation of Austenitic Stainless Steel 304 under Thermal Insulation at Ambient Temperature

The investigation was carried out to study the effect of surface grinding on chloride induced stress corrosion cracking (CISCC) of austenitic stainless steels 304 at ambient temperature condition. The U-bend tests with thickness 13 mm as per ASTM G 30 were used to investigate the effect of surface grinding on chloride induced stress corrosion cracking (CISCC) of austenitic stainless steel 304 in a corrosive atmosphere containing sodium chloride at ambient temperature. At a high level of tensile residual stress had developed the pits on U-bend specimen surface at ambient temperature with the presence of low to high chloride concentration level. The experimental results recommend that develop a proper metallurgical fabrication criteria, specification, and procedures for pressure vessels to avoid a recurrence in future.