Constant Extension Rate Tests Research Articles

Analysis of environmentally assisted crack initiation of Alloy 182 in simulated light water reactor primary water with the engineering initiation model EngInit

ABSTRACT Environmentally assisted cracking of nickel-base alloys has been observed in the primary coolant of light water reactors and is known as primary water stress corrosion cracking. In respect of crack growth, disposition curves are available for integrity analysis. However, in respect of initiation, there is no generally accepted approach. Here, a methodology is proposed to derive a usage factor towards initiation. Reaching a usage factor of 1 means that initiation is incipient. How far one is away from a usage factor of 1 tells us about the margin towards initiation. The key function in the approach is the rate at which the usage factor increases. It is proposed to be a function of strain rate and stress. Data from constant extension rate tests with flat, tapered tensile specimens have been used to parameterise the model. The parameterised model is subsequently used to predict initiation in a tensile-loaded, large-notch specimen.

Determination of Critical Hydrogen Concentration and Its Effect on Mechanical Performance of 2200 MPa and 600 HBW Martensitic Ultra-High-Strength Steel

The influence of hydrogen on the mechanical performance of a hot-rolled martensitic steel was studied by means of constant extension rate test (CERT) and constant load test (CLT) followed with thermal desorption spectroscopy measurements. The steel shows a reduction in tensile strength up to 25% of ultimate tensile strength (UTS) at critical hydrogen concentrations determined to be about 1.1 wt.ppm and 50% of UTS at hydrogen concentrations of 2 wt.ppm. No further strength degradation was observed up to hydrogen concentrations of 4.8 wt.ppm. It was observed that the interplay between local hydrogen concentrations and local stress states, accompanied with the presence of total average hydrogen reducing the general plasticity of the specimen are responsible for the observed strength degradation of the steel at the critical concentrations of hydrogen. Under CLT, the steel does not show sensitivity to hydrogen at applied loads below 50% of UTS under continuous electrochemical hydrogen charging up to 85 h. Hydrogen enhanced creep rates during constant load increased linearly with increasing hydrogen concentration in the steel.

Effect of Film-Forming Amines on the Acidic Stress-Corrosion Cracking Resistance of Steam Turbine Steel

This work evaluates the effect of film-forming amines (FFA) on the acidic stress-corrosion cracking (SCC) resistance of NiCrMoV turbine steel. Contact angle measurements show an increased hydrophobicity of the surface when coating the steel with oleyl propylene diamine (OLDA). According to potentiodynamic measurements and post-mortem scanning electron microscopy (SEM) analysis, anodic dissolution and hydrogen embrittlement still occur when the steel is FFA coated. In situ constant extension rate testing (CERT) in acidic aqueous environment at elevated temperature of FFA-coated steel shows a ductility gain compared to non-coated steel, explained by a decrease in both corrosion rate and hydrogen uptake.

Mechanistic interpretation on acidic stress-corrosion cracking of NiCrMoV steam turbine steel

The present work evaluates the acidic stress-corrosion cracking (SCC) of NiCrMoV steam turbine steel. Between a pH of 4.2 and 7, iron oxidises to Fe2O3, whereas iron dissolution (Fe2+) and hydrogen proton (H+) reduction are favoured at lower pH. The corrosion rate increases with the H+ concentration via a linear relationship as well as by a temperature increase. During in-situ constant extension rate testing, both the ductility and strength losses, caused by anodic dissolution and hydrogen embrittlement, increase with a higher acetic acid concentration. Hydrogen embrittlement causes larger embrittled zones with higher acetic acid concentrations as well as stress-corrosion cracking.

Evaluation of the active mechanism for acidic SCC induced mechanical degradation: A methodological approach

This work evaluates the mechanical degradation of Dual Phase steel due to acidic stress-corrosion cracking (SCC). The SCC susceptibility is studied by in-situ constant extension rate testing, followed by post-mortem fractographic evaluation, while the corrosion rate is determined by electrochemical measurements. Elevating the temperature results in a decrease in both strength and ductility. Organic acids addition further leads to a significant ductility loss, whereas oxygen removal yields a ductility gain. The corrosion rate shows a linear relationship with the H+ concentration. Between a pH of 4.2–7, anodic dissolution occurs, whereas hydrogen absorption takes place at lower pH, causing hydrogen-assisted stress-corrosion cracks.

Environmentally assisted cracking of T91 ferritic-martensitic steel in heavy liquid metals

AbstractIn order to advance material development for future nuclear systems, an insight into the cracking conditions of T91 ferritic-martensitic steel in heavy liquid metals (HLM) is provided. The paper critically reviews previous experimental data and summarizes them with new results. The new testing of T91 steel was performed in contact with slow flow and static HLM to study crack initiation, especially in liquid PbBi eutectic at 300°–350°C and Pb at 400°C with about 1 × 10−6 wt.% of oxygen. Pre-stressed coupons were exposed to the liquid metals for up to 2000 h. Constant extension rate tests (CERTs) were performed in the liquid metals to accelerate cracking development. Under static conditions, the testing resulted in oxidation without any crack observation. Under the CERT ones, the T91 steel showed a tendency to crack initiation in PbBi, while in Pb, cracks were not initiated even when the oxide layer was broken. Moreover, the environmentally assisted crack initiated at the maximum load and continued to grow under further loading without unstable failure. Both previous and new data have confirmed that high stress and plastic strain are pre-conditions for the environmentally assisted cracking of T91 in static HLM. It indicates that in the systems utilizing continuous oxygen control of HLM, the LME/EAC of the T91 could develop only in the beyond design load conditions. Further testing is necessary to address the HLM flow speed effect.

Influence of microstructural anisotropy on the hydrogen-assisted fracture of notched samples of progressively drawn pearlitic steel

In this study, fracture surfaces of notched specimens of pearlitic steels subjected to constant extension rate tests (CERTs) are analyzed, in an environment causing hydrogen assisted fracture. In order to obtain general results, both different notched geometries (to generate quite distinct stress triaxiality distributions in the vicinity of the notch tip) and diverse loading rates were used. The fracture surfaces were classified in relation to four micromechanical models of hydrogen-assisted micro-damage. To this end, fractographic analysis in each fracture surface was carried out with a scanning electron microscopy. Generated results increase the number of micromechanical models found in the scientific literature.

Hydrogen effects on mechanical performance of nodular cast iron

Abstract The KBS-3 method for long-term disposal of spent nuclear fuel is designed with an external self-standing copper shell, which provides the most important barrier against corrosion and escape of radionuclides, and an internal nodular cast iron insert, which provides the load-bearing structure against external loads. The material intended for the load-bearing insert is ferritic nodular cast iron EN 1563 grade EN-GJS-400-15U. In this paper, hydrogen uptake and sensitivity to hydrogen-induced cracking of the cast iron were studied using tensile testing under continuous electrochemical charging in 1 N H2SO4 solution. Hydrogen uptake was measured by using the thermal desorption method. It was found that the hydrogen desorption profile manifests three distinct peaks at initial locations of 400, 500, and 700 K with a heating rate of 6 K/min. Plastic deformation results in a remarkable increase of the 400 K peak, which indicates hydrogen uptake during deformation. In the constant extension rate tests (CERT) and the constant load tests (CLT), electrochemical hydrogen charging reduced markedly the elongation to fracture and time to fracture, respectively. In CLT, hydrogen charging increased dramatically the creep rate at the applied load of about 0.7 yield stress. Ligaments between the graphite nodules exhibit brittle cleavage facets in the presence of hydrogen, while the ligaments show a characteristic ductile appearance of shear and small dimples when testing in air or distilled water. The obtained results are discussed in terms of the known mechanisms of hydrogen-induced cracking and the role of the graphite nodules in the embrittlement of ductile cast iron.

Effect of non-conventional heat treatment of API X60 pipeline steel on corrosion resistance and stress corrosion cracking susceptibility

PurposeThe purpose of this paper is to study the effect of heat treatment (HT) applied to an API X60 steel in corrosion resistance and stress corrosion cracking (SCC) susceptibility through slow strain rate tests (SSRT) in NS4 solution and congenital water (CW) to assess external and internal SCC, respectively.Design/methodology/approachAPI X60 steel was heat treated at a temperature of 1,200°C for 30 min followed by water quenching. Specimens from this steel were machined according to NACE TM 198. SSRT were performed in a constant extension rate tests (CERT) machine at room temperature at a strain rate of 1 × 10–6s–1. For this purpose, a glass cell was used. Corrosion behavior was evaluated through polarization curves (PCs).FindingsThe SCC index obtained from SSRT indicates that the steel heat treated could be susceptible to SCC in CW and NS4 solution; the mechanism of SCC was hydrogen embrittlement. Thus, CW may promote the SCC phenomenon in pipelines. HT improves the steel corrosion resistance. Higher corrosion rate (CR) was observed when the steel is exposed to CW. The corrosion process in X60 steel shows that the oxidation reaction in the anodic branch corresponds to an activation process, and the cathode branches reveal a diffusion process.Originality/valueThe purpose of the heat treatment applied to X60 steel was to generate a microstructure of acicular ferrite to improve the corrosion resistance and SCC behavior.

Stress Corrosion Cracking of Aeronautical Alloys Al6061-T6

For the aeronautical industry corrosion has always been an important problem, especially within the aircraft maintenance, falling into great expenses to study and prevent the formation of it on some elements that are indispensable for the aircraft’s service. The external stresses or the existing loads joined with the atmospheric environment can promote degrading mechanisms that combined cause cracking on the material by means of tension and early failures. This phenomenon, called Stress Corrosion Cracking (SCC), produce a great loss of mechanical strength, without evidence of the material’s damage and loss, but it can produce fast mechanical fracture and catastrophic failures. In this study the objective was to evaluate the used in aeronautical industry, Al6061-T6, to identify which one presents a better resistance to SCC in salt, caustic and acid environments. Mechanical tests were performed using low speeds on a CERT (Constant Extension Rate Test) machine, varying the environment, its pH and concentration. Simultaneously, the electrochemical noise tests were also performed to determine the corrosion rate and link the Stress-Deformation plots with the electrochemical patters recorded, giving results more complete and accurate to the behavior of the materials when in service.

Hydrogen effects on mechanical properties of 18%Cr ferritic stainless steel

Role of microstructure in susceptibility of 18Cr ferritic stainless steel to hydrogen embrittlement was studied. Specimens of the studied steel were charged with hydrogen electrochemically from 0.1N H2SO4 solution under controlled cathodic potential providing a homogeneous hydrogen distribution over the specimen cross-sections. Thermal desorption spectroscopy analyses were carried out investigating the uptake, trapping and diffusion of hydrogen in the ferritic stainless steel.Microstructural change caused by heat-treatment at 1050°C and 1200°C associated preferably with grain size growth from 18µm to 65µm and 349µm, respectively, resulting in significant degradation of the mechanical properties of the studied steel. The effect of the grain size growth on hydrogen susceptibility was studied with constant extension rate test (CERT) performed under continuous hydrogen charging. It is found that hydrogen has a remarkable effect on the elongation to fracture of the Fe-Cr ferrite: in the presence of H elongation to fracture of the steel reduces up to 75% compared to the H-free counterpart. In general, the hydrogen sensitivity of the mechanical properties increases with increase of the mean grain size of the studied ferritic stainless steel. However, the detailed analysis reveals a complicated, non-linear behavior of the hydrogen sensitivity. Scanning electron microscopy (SEM) of the fracture surfaces of the tensile specimens tested during continuous hydrogen charging reveals a quasi-cleavage fracture surface morphology. Hydrogen-induced cracking in the studied 18Cr ferritic steel was studied using electron backscatter diffraction (EBSD) analysis from the side surfaces of the tensile tested specimens.

Susceptibility Evaluation to the Stress Corrosion Cracking in Biofuels

The objective of this research is to evaluate the susceptibility of stress corrosion cracking (SCC) to the effect of adding water in different concentrations of 0.5, 5 and 10% in ethanol-gasoline blends up to 95 and 85% of ethanol, in addition to an inert media for API 5L-X52 steel, commonly used in the transport of hydrocarbons, by constant extension rate test (CERT). Extension rate range was 10-6 s-1, simultaneously, the potential and current noise were examined by the Electrochemical Noise technique where the registered fluctuations have correspondence with the dissolution of the material. Additionally, it was observed that tests with low content in % (V) of water present the less satisfactory results, as well as secondary cracks in the samples analyzed by SCC.

Environmentally Assisted Cracking of Alloy 7050-T7451 Exposed to Aqueous Chloride Solutions

The stress corrosion cracking (SCC) behavior of 7050-T7451 plate material was investigated in short-transverse direction performing constant load and constant extension rate tests. Smooth and notched tensile specimens were permanently immersed in substitute ocean water and in an aqueous solution of 0.6 M NaCl + 0.06 M (NH4)2SO4. Alloy 7050-T7451 exhibited high SCC resistance in both synthetic environments. However, conducting cyclic loading tests, environment-induced cracking was observed. Applying a sawtooth waveform, notched tensile specimens were strained under constant load amplitude conditions at constant displacement rates ranging from 2 × 10−6 to 2 × 10−4 mms−1. The stress ratio R = σ min/σ max was 0.1 with maximum stresses of 300 and 400 MPa. When cyclically loaded in substitute ocean water, notched specimens failed predominantly by transgranular environment-induced cracking. Striations were observed on the cleavage-like facets. The number of cycles-to-failure decreased with decreasing displacement rate. A slope of 0.5 was obtained by fitting the logarithmic plot of number of cycles-to-failure vs nominal loading frequency, indicating a hydrogen embrittlement mechanism controlled by diffusion.

Dynamic Strain Aging of Nickel-Base Alloys 800H and 690

The objective of the current investigation is to characterize the dynamic strain aging (DSA) behavior in alloys 800H and 690. Constant extension rate tests were conducted at strain rates in the range of 10−4 s−1 to 10−7 s−1and temperatures between 295 K and 673 K (22 °C and 400 °C), in an argon atmosphere. Maps for the occurrence of serrated flow as a function of strain rate and temperature were built for both alloys. The enthalpy of serrated flow appearance of alloy 800H was found to be 1.07 ± 0.30 eV.

Effect of microstructural anisotropy on stress corrosion cracking of hot rolled duplex stainless steels

Anisotropy in stress corrosion cracking of lean grade UNS S32101 and standard grade UNS S32205 hot rolled duplex stainless steels was evaluated in this study. Microstructures were characterised with stereology methods in three orthogonal orientations to quantify anisotropy in phase distribution. Constant extension rate tests were conducted in an autoclave at 2×10−6 s−1 in chloride containing alkaline sulphide solution at 170°C. The maximum crack lengths and average crack densities were measured for the rolling longitudinal and transverse longitudinal orientations after stress corrosion cracking tests. Results indicate that stress corrosion crack initiation and growth are a strong function of the microstructure in each orientation. Maximum crack length and average crack density were maximum for the rolling longitudinal orientation. Crack initiation and growth were favoured along the transverse direction. Chlorides enhanced anisotropy in crack growth behaviour by facilitating crack initiation and crack coalescence along phase boundaries, which lead to more severe stress corrosion cracking in the alkaline sulphide environments that were studied.

Hydrogen embrittlement susceptibility of tempered 9%Cr–1%Mo steel

The influence of the subsurface hydrogen activity on the hydrogen embrittlement (HE) susceptibility of a tempered 9%Cr–1%Mo ferritic-martensitic steel (T91) has been studied by constant extension rate tests (CERT) performed under cathodic charging during straining at 20 °C. Changes in the hydrogen activity on the surface were obtained by varying the cathodic current density imposed during a tensile test completed to fracture. The broken samples were observed by scanning electron microscopy (SEM). The results show a strong dependence of the extent of the brittle zone with the subsurface hydrogen activity. This effect could be attributed to an enhancement of the apparent diffusion coefficient of hydrogen when increasing H activity. Increasing H activity could enhance both the trapping rate on pre-existing and strain-induced traps and H dragging by moving traps such as dislocations.

Effect of Temperature on the Loss of Ductility of S-135 Grade Drill Pipe Steel and Characterization of Corrosion Products in CO2 Containing Environments

Evaluation of loss of ductility of the API S-135 grade drill pipe steel was studied at different temperatures from 298 K to 448 K (25 °C to 175 °C) in CO2 containing solution using a constant extension rate test in connection with a high-temperature/high-pressure autoclave. The effect of temperature on the composition and morphology of corrosion product layers of this steel were also investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Results indicated the change of loss of ductility with temperature and the maximum loss of ductility was observed at 448 K (175 °C). XRD studies of the specimens revealed the formation of iron carbonate on the surface. SEM study of the surface of the specimens showed a rhombohedric crystalline iron carbonate layer above 373 K (100 °C), while no FeCO3 was detected at temperatures below 373 K (100 °C). Also, the crack size and depth of the cracks increased with temperature.

Stress Corrosion Cracking of Pure Copper under Possible Nuclear Fuel Waste Management Conditions

This study investigated the effect of nitrite/chloride and ammonia/chloride concentrations, on the stress corrosion cracking (SCC) behaviour of oxygen free phosphorous doped (OFP) copper. Constant extension rate tests (CERTs) were performed at room temperature using a compact tension specimen. The findings are summarized as follow: A 1:1 ratio of nitrite to chloride suppressed SCC, as did an overabundance of chloride. The susceptibility of OFP copper to SCC in a deaearated 0.1 mol•L 1 nitrite solution appeared to decrease and the steady-state potential values became more negative with increasing chloride concentration. A 1:1 ratio of ammonia to chloride suppressed SCC, as did an overabundance of chloride. The addition of chloride in concentrations ≤0.5 mol•L 1 had little effect on the SCC behaviour. High chloride concentrations appeared to increase the uniform corrosion of OFP copper, which coincided with a blunting of the crack tip.

On the stress corrosion cracking behaviour of 6XXX series aluminium alloys

Abstract The stress corrosion cracking behaviour of 6061 and 6013 sheet materials was investigated in naturally aged and peak-aged heat treatment conditions. Constant deformation, constant load and constant extension rate tests were carried out using aqueous solutions of 0.6 M NaCl and 0.6 M NaCl + 0.06 M NaHCO3. The microstructure of the alloys was examined using transmission electron microscopy. Potentiodynamic polarisation measurements were performed in aerated and deaerated aqueous chloride and/or bicarbonate solutions to determine corrosion and breakdown potentials. Under constant deformation and constant load conditions, the alloys 6061 and 6013 stressed in transverse direction were resistant to stress corrosion cracking in both T4 and T6 tempers. The constant extension rate testing technique indicated environment-induced cracking with alloy 6061-T4, whereas deterioration in ductility of the other alloys was caused by pitting and intergranular corrosion. Sensitivity to intergranular corrosion did not cause stress corrosion cracking in 6013-T6 sheet.

The effect of acetic acid on the stress corrosion cracking of 3.5NiCrMoV turbine steels in high temperature water

The effects of acetic acid on the stress corrosion cracking (SCC) susceptibility of ASTM A470/471 turbine disk/rotor steels (3.5NiCrMoV) were evaluated. Thus, this study investigated the possibility of corrosion damage to turbine components due to acidified, early condensates containing acetic acid, which is a common impurity in steam cycle systems. Constant extension rate tests (CERTs) were carried out in aqueous solutions containing acetic acid at various concentrations (pH 3–5) at 150 °C to ascertain the susceptibility in terms of strain to failure and reduction in area upon fracture. Acetic acid significantly affects the SCC susceptibility of the turbine steels with the susceptibility increasing with increasing acetic acid concentration, except at the very highest concentration at which excessive dissolution led to crack blunting. The SCC susceptibility test results were analyzed using the CEFM (Coupled Environment Fracture Model), and it is shown that this model accounts for the experimentally–observed relationships between the SCC growth rate and the concentration of the acetic acid in terms of the conductivity of the solution.