Stress Corrosion Crack Growth Rate Research Articles

ChemInform Abstract: The Stress Corrosion Crack (SCC) Growth Rate in Sensitized Alloy 600 in Thiosulfate Solution.

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The stress corrosion crack growth rate in sensitized alloy 600 in thiosulfate solution

A constant deflection technique incorporating precracked specimen was employed to determine the stress corrosion crack (SCC) growth rate in sensitized Alloy 600 in thiosulfate solution under applied potential conditions. At passive potentials, the intergranular stress corrosion crack (IGSCC) growth rate increased, while the critical stress intensity factor for SCC ( K ISCC ) decreased with increasing potential. At + 500 mV(SCE), in the transpassive potential region, crack growth rate retardation was observed.

Slip-step dissolution and micromechanical analysis to model stress-corrosion crack growth of type 321 stainless steel in boiling mgci2

It is hypothesized that for ductile austenitic stainless steels exposed to boiling MgCl2 solution, the relevant crack propagation mechanism is slip dissolution. This model relates crack advance to oxidation or anodic dissolution that occurs on the bare surface that is created when a thermo-dynamically stable, protective film at the crack tip mechanically ruptured. Based on the model of slip-bare metal dissolution repassivation and crack-tip strain analysis, a theoretical equation of stress-corrosion crack growth rate as a function of crack-tip strain rate and potential for 321 stainless steel in boiling 42 pct MgCl2 solution is proposed. The theoretical prediction shows that when the crack-tip strain rate changes from 10−4 to 10−2 s−1 the crack propagation rate changes from 0.01 to 3 mm/h at the free corrosion potential (−0.35 VSCE). If the crack-tip strain rate is above 10−2/s, the crack propagation rate should correspond to the upper bound determined by the maximum metal dissolution rate. When the crack-tip rate is below 10−4/s, the crack propagation rate is below 0.01 mm/h. The slip-step dissolution model predicted that there exists a critical potentialEc, above which the crack propagation rate is independent on potential, but below which the crack propagation rate decreased with decreasing potential. The theoretical prediction has been verified by slow strain rate tests of 321 stainless steel under potential control (above −0.35 VSCE) in 42 pct MgCl2 solution.

The effect of hydrogen peroxide on the stress corrosion cracking of 304 stainless steel in high temperature water

The effect of hydrogen peroxide on stress corrosion crack growth rate of sensitized 304 stainless steel in high temperature water was investigated using a specially designed testing cell which minimizes the decomposition of hydrogen peroxide. Hydrogen peroxide accelerated crack growth irrespective of dissolved oxygen content level within the test range (20–400 ppb dissolved oxygen). The effect of oxygen and hydrogen peroxide could be characterized using an effective oxygen content [O 2] eff, defined below, [ O 2] eff =[ O 2]+ 1 2 [ H 2 O 2] . Results were interpreted based on existing cracking mechanisms.

Analytical electron microscopy of grain boundaries in high-strength steels

Phosphorus could be detected at prior austenite grain boundaries (PAGB) in high-strength alloy steels quenched and tempered at 500°C when using a VG's HB 501 dedicated field emission STEM but not with a conventional JEOL 4000FX STEM. No phosphorus was detected at PAGB's in the as-quenched materials or away from PAGB's in tempered materials of either type. The grain boundary coverage of phosphorus was, assuming a specimen thickness of 80 nm, 0.7 monolayers for the 3.5NiCrMoV rotor steel and 0.4 monolayers for the AISI 4340 steel. The grain boundary concentration of phosphorus, assuming a specimen thickness of 80 nm and a segregated layer thickness of 1 nm, for the 3.5NiCrMoV rotor steel was 6 wt% and for AISI 4340 4 wt%. Compared to the bulk concentration of about 0.01 wt% this means that the enrichment factor of P to the grain boundaries was several hundred times (610 respectively 370). Our measurements showed no correlation between the stress corrosion crack growth rate and the grain boundary phosphorus concentration. The yield strength, however, decreased after tempering while the phosphorus concentration at the grain boundaries increased.

Effects of Temperature on Crack Growth Rate in Sensitized Type 304 Stainless Steel and Alloy 600

Abstract Effects of temperature on stress corrosion crack growth rates were studied on three heats of sensitized type 304 (UNS S30400) stainless steel (SS) and one heat of sensitized alloy 600 (UNS N06600). Data were obtained using reversed direct current (DC) potential drop crack length monitoring on 25-mm compact-type (CT) specimens in 25°C to 288°C water, usually containing 200 ppb oxygen (O2). Most data were obtained in 0.3 μM sulfuric acid (0.267 μS/cm). In one heat, 0.1 μM hydrochloric acid (HCl) (0.076 μS/cm), 0.5 μM HCl (0.224 μS/cm), and air-saturated water (≈ 8,800 ppb O2 and ≈ 0.5 μS/cm) also were evaluated. All heats and materials had similar temperature dependence and showed a peak in crack growth rate at ≈ 200°C in these water chemistries, which typically was a factor of 30 to 100 times higher than at 288°C or 25°C. Growth rates decreased rapidly above 250°C compared to those between 25°C and 200°C. In many cases, similar rates were observed at 25°C and 288°C. Data were compared with the lit...

CRACK NUCLEATION AND PROPAGATION IN BLADE STEEL MATERIAL

AbstractStress corrosion cracking and corrosion fatigue of 12 Cr steel in sodium chloride solution has been investigated. Tests have been performed in air at room temperature and in aqueous solution with 22% NaCl at 80°C. The influence of corrosion pits on crack nucleation has been investigated. On fracture surfaces tested in environment (22% NaCl solution), crack initiation was observed in correspondence with corrosion pits; in this case fatigue life can be described using a fracture mechanics approach. The ΔK value for crack nucleation from a pit in rotating bending fatigue tests is very low in air (about 3 MPa√m). The results of slow strain rate tests on smooth specimens show that there is a threshold stress intensity, KISCC, of about 15 MPa√m and a plateau in stress corrosion crack growth rate of about 10‐5mm/s.

Static and cyclic crack growth of low alloy steels in high-oxygen water simulating HDR test conditions

Abstract Steel samples of reactor pressure vessel and piping steels from the German HDR programme have been tested in high oxygen water at different temperatures simulating HDR test conditions. The specimens have been exposed to sequences of static and cyclic loading or to purely cyclic loading. During the tests, threshold stress intensity values for stress corrosion cracking and crack growth rates with various cyclic loading parameters were determined. Extensive fracture surface and oxide layer investigations were also performed. Water chemistry parameters such as dissolved oxygen content, pH and conductivity were continuously monitored during the tests. Finally, the measured laboratory water chemistry parameters were compared to those measured in the HDR plant during full scale testing of components and the relevance of the results for normally operating plants is discussed.

Life assessment of nuclear LP rotor due to SCC

The stress corrosion cracking (SCC) of nuclear LP rotors has been a worldwide problem since the late 1960s. It has been considered the major factor for maintaining the reliability of turbines for nuclear power plants. Since the problem is not exceptional to the local nuclear power plants, some efforts have been undertaken in order to mitigate or resolve the SCC problem. The main activities in this study were focused on the survey and determination of SCC growth rates, critical crack length calculation by finite element methods (FEM), and comparison of life assessment models. Examples of deterministic and probabilistic life assessment for a local steam rotor disc with SCC cracks are presented.

Deriving SCC Initiation Times and Growth Rates from Posttest Fractrographic Analysis

Abstract This exploratory effort demonstrates the feasibility of determining crack initiation times and growth rates by posttest analysis of the fracture surfaces of stress corrosion cracking (SCC) test specimens. The fracture surface topography analysis (FRASTA) technique was applied to constant extension rate test (CERT) specimens of type 304 (UNS S30400) stainless steel fractured in several aqueous environments. Initiation times and propagation rates were determined for intergranular stress corrosion cracks in specimens tested at 288°C in clean oxygenated water containing a small amount of H2SO4. Cracking appeared to begin predominantly at the specimen's surface in clean water, but in the specimen's interior in the H2SO4-contaminated water.

Structure, mechanical properties, and stress corrosion behaviour of high strength spray deposited 7000 series aluminium alloy

A 7000 series high strength aluminium alloy was produced by spray deposition via the Osprey process. This alloy, designated AlZn11MgCu, exhibits an excellent combination of fracture toughness and strength in both the peak aged T6 and overaged T7 conditions. The yield strength and fracture toughness at room temperature are 700 MN m−2 and about 40 MN m−3/2 in the T6 temper, and 500 MN m−2 and 75 MN m−3/2 in the T7 temper, respectively. The exceptional high fracture toughness is attributed to the absence of coarse constituents and oxides. The microstructure is homogeneous, free of segregation, and has a fine grain size. The hardening precipitates in the T6 temper are mainly η′, and in the T7 temper they are η (MgZn2), resulting in a different fracture behaviour; in the T6 temper the fracture mode is microshearing and in the T7 temper the fracture mode is ductile failure. Both T6 and T7 conditions show a stress corrosion crack growth rate higher than that of comparable alloys, 7075-T651 and 7075-T7351.MST/1308

Structure, mechanical properties, and stress corrosion behaviour of high strength spray deposited 7000 series aluminium alloy

A 7000 series high strength aluminium alloy was produced by spray deposition via the Osprey process. This alloy, designated AlZn11MgCu, exhibits an excellent combination of fracture toughness and strength in both the peak aged T6 and overaged T7 conditions. The yield strength and fracture toughness at room temperature are 700 MN m−2 and about 40 MN m−3/2 in the T6 temper, and 500 MN m−2 and 75 MN m−3/2 in the T7 temper, respectively. The exceptional high fracture toughness is attributed to the absence of coarse constituents and oxides. The microstructure is homogeneous, free of segregation, and has a fine grain size. The hardening precipitates in the T6 temper are mainly η′, and in the T7 temper they are η (MgZn2), resulting in a different fracture behaviour; in the T6 temper the fracture mode is microshearing and in the T7 temper the fracture mode is ductile failure. Both T6 and T7 conditions show a stress corrosion crack growth rate higher than that of comparable alloys, 7075-T651 and 7075-T7351.MST/1308

Stress corrosion crack growth behavior of a low alloy steel in 3.0% NaCl solution

Stress corrosion crack growth behavior of a low alloy steel in 3.0% NaCl solution was studied. The investigation also examines the influence of initial stress intensity factor (Ki) and overload plastic zone size on the incubation period and stress corrosion crack growth rate. Compact tension specimens with (TL) orientation prepared from SAE 4140 steel in hardened and tempered condition were used. To examine the influence of overload, initially fatigue precracked specimens were overloaded to various levels, namely 10, 25, and 50% more than initial stress intensity levels (Ki) at two different (Ki) levels (Ki= 28 and 46 MPa√m) and unloaded. Stress corrosion crack growth behavior of these specimens was studied after reloading them to their initial stress intensity levels (i.e., Ki = 28 and 46 MPa√m, respectively) and keeping them submerged in 3.0% NaCl solution.

Effects of Stress on Cracking Velocity and Critical pH Value inside Stress Corrosion Cracks

Abstract The effects of stress on corrosion velocity and stress corrosion crack growth rate for the system 18-8 austenitic stainless steel (SS) (sensitized)/Cl− were studied by using linear polarization and slow strain rate techniques in simulated occluded cell solutions of different crack propagation stages with different pH values at 90°C. The stress—elongation curves, crack length—time curves, and corrosion rate—pH curves all showed a critical pH value (≥1.8), which is higher than that found previously (≅1.3) for unstressed specimens. The actual stress corrosion cracking (SCC) velocity was several orders of magnitude higher than the corrosion rates of stressed specimens, indicating that mechanical failure induced by hydrogen embrittlement is the dominating factor for SCC velocity.

Influence of Overload Plastic Zone Size on Stress Corrosion Crack Growth Behavior of a Low Alloy Steel in 3.0% NaCl Solution

Abstract An investigation was carried out to examine the influence of overload plastic zone size on stress corrosion crack growth behavior for a low alloy steel. The investigation also examines the influence of initial stress intensity factor on stress corrosion crack growth behavior. Compact tension specimens with TL orientation prepared from SAE 4140 steel in the hardened and tempered condition were used. Two pertinent environments were chosen: deionized water at room temperature (23°C) and 3.0% NaCl solution at room temperature (23°C). The specimens were initially precracked in fatigue and loaded to different levels of stress intensity factors. Crack growth behavior of these specimens was studied by keeping them submerged in deionized water and 3.0% NaCl solution to establish a base line. To examine the influence of overload plastic zone size on stress corrosion crack growth behavior, the precracked specimens were overloaded to various load levels (i.e., 5, 10, 25, and 50% more than their initial stress intensity level [Ki]) and then unloaded. The crack growth behavior of these specimens was then monitored in a similar fashion after reloading to the original initial stress intensity level. The influence of initial stress intensity level on the incubation period for the growth of the crack and on the stress corrosion crack growth rate was also examined. The results of the present investigation demonstrate that the incubation period for the growth of stress corrosion cracking increases with an increase in overload plastic zone size. The crack growth rate at a constant initial stress intensity level was found to be lower in salt water solution than in deionized water. The incubation period for the growth of stress corrosion cracking was found to decrease with an increase in initial stress intensity level. Once the crack has grown beyond the introduced overload plastic zone, the crack growth rate was found to resume its normal rate. However, for the similar overload plastic zone size, the reduction of the effective crack growth rate was found to be greater in deionized water than in salt solution. This reduction in effective crack growth rate in overloaded specimens can be attributed to the presence of residual compressive stresses in the crack tip region in overloaded specimens.

Repassivation kinetics and stress corrosion of mild steel in phosphate solutions

Stress corrosion crack growth rates have been measured for mild steel in a 1 N sodium phosphate solution adjusted to pH 6, using slow strain rate tensile and cantilever bend tests. It was found that de-aeration of the test solutions significantly reduced the stress corrosion susceptibility. Repassivation kinetics have been measured using the potential step and drop-weight straining wire techniques. Calculations of crack growth rates using the film rupture-dissolution mechanism agree well with the observed crack velocities. The effect of de-oxygenation can be explained in terms of its influence on the passive film properties.

Influence of retrogression and reaging treatments on the strength and stress corrosion resistance of aluminium alloy 7075-T6

The influence of retrogression and reaging (RRA) treatments on high strength aluminium alloy 7075 was studied using hardness and tensile testing and by measuring the stress corrosion crack growth rate using double-cantilever-beam specimens. Transmission electron microscopy was used to study the microstructures at both the matrix and the grain boundary. The RRA treatments result in a significant improvement of the stress corrosion resistance of aluminium alloy 7075-T651 without sacrificing its maximum strength. The crack velocity decreases to a level comparable with that of the T73 structure as a result of the optimum RRA treatments. The initial strength decrease during the retrogression treatment is mainly due to a dissolution of small η′ particles. The coarsening of grain boundary precipitates occurs during the retrogression treatment. The steady state crack velocity decreases logarithmically with increase in the volume fraction of grain boundary precipitates.

Influence of austenitizing temperature on stress corrosion in 4330m steel—The role of impurity segregation in stress corrosion cracking of high strength steel

Measurements of the threshold stress intensity for stress corrosion cracking (SCC), KISCC, and crack growth rate,da/dt, in distilled water were made, respectively, on bolt-loaded WOL and precracked three-point-bending specimens of a 4330M steel. A significant improvement of resistance to SCC was obtained by increasing quenching temperature and it is due to a reduction of segregated impurities of P and S at prior austenite grain boundaries. Intergranular cracking tendency increases with inter-granular concentration of impurities and the fracture mode changes from intergranular separation along prior austenite grain boundaries to transgranular quasi-cleavage as the segregated impurity becomes low enough. The combined effects of hydrogen and intergranular impurities on reducing intergranular cohesion and the time for approaching the critical concentration of hydrogen are dis-cussed in terms of a dynamic model which takes into account the accumulation of hydrogen ahead of a moving microcrack.

Clean steels for steam turbine rotors—their stress corrosion cracking resistance

This work presents the results of a comprehensive study concerning stress corrosion crack growth rates in steam turbine rotor steels exposed to hot water. The effects of stress intensity, temperature, and dissolved gases in the water have been investigated. Special attention has been given to the influence of impurities and alloying elements in the steel such as P, S, Mn, Si, Mo, and Ni, and to the effect of yield strength and fracture toughness on the growth rates of stress corrosion cracks. The results of this study clearly show that there exists a threshold stress intensity of about 20 MNm−3/2 above which the invariably intergranular stress corrosion cracks grow at a constant, stress-independent velocity. This plateau stress corrosion crack growth rate isnot affected by the oxygen and carbon dioxide concentration in the water. The temperature and the yield strength of the steel have a strong influence on the growth rate of stress corrosion cracks. In contrast, there isno effect of the steel composition within the range investigated, neither of the impurity elements such as P and S, nor of the major alloying elements such as Mn, Si, Mo, and Ni. Steels with low fracture toughness due to temper embrittlement do not exhibit faster stress corrosion crack growth rates in water than nonembrittled steels. No direct relationship between intergranular temper embrittlement and intergranular stress corrosion crack growth in water can be demonstrated.

Stress corrosion cracking of nuclear reactor pressure vessel and piping steels

This paper presents an extensive investigation of stress corrosion cracking of nuclear reactor pressure vessel and piping steels exposed to hot water. Experimental fracture mechanics results are compared with data from the literature and other laboratories. Thus a comprehensive overview of the present knowledge concerning stress corrosion crack growth rates is provided. Several sets of data confirm that ‘fast’ stress corrosion cracks with growth rates between 10 −8 and 10 −7 m/s and threshold stress intensities around 20 MN m − 3 2 can occur under certain conditions. However, it appears possible that specific environmental, mechanical and metallurgical conditions which may prevail in reactors can result in significantly lower stress corrosion crack growth rates. The presently known stress corrosion crack growth rate versus stress intensity curves are discussed with emphasis on their usefulness in establishing safety margins against stress corrosion cracking of components in service. Further substantial research efforts would be helpful to provide a data base which permits well founded predictions as to how stress corrosion cracking in pressure vessels and piping can be reliably excluded or tolerated. It is emphasized, however, that the nucleation of stress corrosion cracks (as opposed to their growth) is difficult and may contribute substantially to the stress corrosion free service behavior of the overwhelming majority of pressure vessels and pipes.