Transient and steady state crack growth kinetics for stress corrosion cracking of a cold worked 316L stainless steel in oxygenated pure water at different temperatures

The stress corrosion cracking (SCC) growth kinetics for a cold worked 316L stainless steel was continuously monitored in high purity water at different temperatures and dissolved oxygen (DO) levels under a K (or K_max) of 30 MPa m^0.5. The total SCC test time was more than 8000 h to make sure the steady state crack growth rate under each test condition could be reached. Crack growth rate (CGR) increases with increasing temperature in the range 110-288 °C. A typical intergranular-cracking mode is identified. Depending on the previous test condition, especially the temperature, three kinds of crack growth kinetics, i.e., increasing with testing time then becoming steady, being constant during the whole period, or decreasing with test time then becoming steady, are identified and discussed. Time-dependent and testing history-dependent crack growth modes were confirmed in two series of tests in 2 ppm DO and 7.5 ppm DO pure water. The apparent activation energies are calculated and compared with other data in different environments under different applied loading levels for understanding the cracking mechanism.     

A probe into low-temperature stress corrosion cracking of 304L stainless steel by scanning vibrating electrode technique

Scanning vibrating electrode technique (SVET) that is a reliable method for local measuring of electrochemical potential at the sample surface is used to study stress corrosion cracking of 304L stainless steel in 0.1?M NaCl+HCl solution at room temperature. SVET maps showed a periodic emergence and receding as well as a forward mobility of an active anodic front. This behaviour could indicate a step-wise and discontinuous crack propagation mechanism. SEM images revealed the presence of a side crack adjacent to the main crack which confirms the presence of an expanded anodic front in SVET maps.     

Inhibition effect of metal cations to intergranular stress corrosion cracking of sensitized Type 304 stainless steel

Inhibition effect of metal cation to intergranular stress corrosion cracking (IGSCC) of sensitized Type 304 stainless steel has been investigated by slow strain rate technique (SSRT) with a dynamic crack observation system. The SCC tests were conducted in fully-deaerated aqueous solutions containing various metal sulfates of 10⁻⁵ kmol/m³ at 95 °C. Metal cations of Na⁺, Ca²⁺, Mn²⁺ and Zn²⁺ were selected in this study, and were characterized by a hardness based on the hard and soft acids and bases (HSAB) concept. As a result, it was found that a hard metal cation in the test solution increased crack initiation time and decreased mean crack initiation frequency. In other words, metal cations with larger hardness have an ability of suppressing initiation process of the IGSCC. On the other hand, apparent mean crack velocity was independent of the hardness of metal cation. The finding that the metal cations with large hardness inhibit initiation of the IGSCC was able to be rationally explained on the basis of the passive film model combined with the HSAB concept.     

堆内构件304L焊接件在除氧和氯离子环境中的应力腐蚀开裂研究

采用高温高压慢应变速率拉伸试验方法(SSRT),研究了在除氧环境下不同浓度氯离子对304L焊接件在模拟一回路高温高压硼锂水介质中氯致应力腐蚀开裂的影响。结果表明:当氯离子浓度为1 mg/L时,其各项力学性能指标与惰性氮气空白样一致,表明在低浓度氯离子除氧条件下,304L焊接件没有应力腐蚀敏感性。随着氯离子含量(1~50mg/L)的增加,304L焊接件应力腐蚀敏感指数变化呈增加趋势。断口侧面没有观察到明显的裂纹,氯离子浓度越低,断口缩颈现象越明显,表明主要是机械断裂。氯离子浓度较高时,棱角部分出现微裂纹源,可能与棱角部分应力更为集中相关。所有样品的断裂位置都在焊缝和热影响区,在焊缝和热影响区发生了巨大的形变,离断口越近,变形越严重。焊接部位是304L堆内构件中薄弱环节,应该成为应力腐蚀开裂和其他性能检测的重点部位。     

Effects of water chemistry and loading conditions on stress corrosion cracking of cold-rolled 316NG stainless steel in high temperature water

The effects of electrode potential, stress intensity factor and loading history on stress corrosion cracking growth of a cold-rolled 316NG stainless steel in 288 °C pure water were investigated. Crack branching and intergranular stress corrosion cracking along random grain boundaries were observed by electron-back scattering diffraction. A strong dependence of crack growth rate on stress intensity factor is observed. A single-cycle overloading produced a retarded transient cracking growth period. The mild inhibiting effect of decreasing electrode potential on crack growth of cold-rolled 316NG SS is analyzed based on the interaction between crack tip mechanics and crack tip oxidation kinetics.     

Effects of loading mode and water chemistry on stress corrosion crack growth behavior of 316L HAZ and weld metal materials in high temperature pure water

The stress corrosion cracking (SCC) growth rates of 316L weld heat-affected zone (HAZ) and weld metal materials in high temperature pure water at 288 °C were measured using contoured double cantilever beam (CDCB) specimens and an alternating current potential drop (ACPD) in situ crack-length monitoring system. The effects of loading mode and dissolved oxygen and hydrogen on crack growth rate (CGR) were experimentally quantified. Typical intergranular SCC was found in the HAZ specimen and interdendritic SCC was identified in the weld metal specimen. The HAZ specimen and the weld metal specimen showed quite a similar response to the applied loading modes and the water chemistry, even though their absolute CGR values were different. The crack growth rates under trapezoidal loading were moderately higher than those under constant loading by several tenths percent. Switching the water chemistry from the oxygen-bearing water to the hydrogen-bearing water drastically decreased the electrochemical potential and the crack growth rate, and vice versa. A time-lag period for crack growth was observed after switching the water chemistry back to the oxygen-bearing water, where the crack growth rate was low even the dissolved oxygen concentration and the electrochemical potential had become high. Strain hardening and the resultant uneven distribution of deformation contribute to the enhanced intergranular SCC growth behavior in the HAZ area. The crack growth kinetics is analyzed based on the deformation/oxidation interaction at the crack tip, considering the importance of the electric-charge transfer, mass transport kinetics and the crack tip strain rate.     

Impact of temperature excursion on stress corrosion cracking of stainless steels in chloride solution

The impact of a temperature excursion on the subsequent stress corrosion crack growth at the normal operating temperature has been investigated for 321 stainless steel (UNS32100) and 316L stainless steel (UNS31603) using precracked compact tension specimens. Although the data are preliminary the indication is that once crack growth has initiated in 321 SS at the elevated temperature, 130 °C in this study, the crack growth may be sustained at the lower temperature (40 °C), at least over the exposure time of about 700 h. However, the growth rate of 316L SS at the lower temperature was significantly lower than for 321 SS and tended to zero after 2000 h. For the 316 SS a temperature transient should not impact on structural integrity, provided it is short in duration.     

Monte Carlo simulation of stress corrosion cracking on a smooth surface of sensitized stainless steel type 304

Stress corrosion cracking (SCC) on a smooth surface of structural metal materials occurs by initiation and coalescence of micro cracks, subcritical crack propagation and multiple large crack formation or final failure under combination of material, stress and corrosive environment. In this paper, a Monte Carlo simulation of the SCC process is proposed based on stochastic properties for micro crack initiation and concepts in fracture mechanics for crack coalescence and propagation. The procedure is as follows: The possible number of grain-sized micro cracks which can be initiated is set for a given space and initiation times for all cracks are assigned by random numbers based on exponential distribution. Sites and sizes of cracks are assigned by uniform random numbers and normal random numbers, respectively. Coalescence and propagation of cracks are determined based on fracture mechanics. The emphasis in the model is put on the influence of semi-elliptical surface cracks. Numerical simulations are carried out based on the results of creviced-bent-beam tests for sensitized stainless steel type 304 under high-temperature and high-purity water containing dissolved oxygen and the influence of micro crack initiation rate and coalescence condition on the simulation results is discussed.     

Influence of local stress on initiation behavior of stress corrosion cracking for sensitized 304 stainless steel

To investigate the influence of local stress on initiation behavior of stress corrosion cracking (SCC) for sensitized Type 304 stainless steel, cracking process during constant load SCC test was monitored and recorded with an in situ crack observation system. The changes in number of cracks, sum of crack length and cracked area on the specimen surface with test time were identified from the cracking images analyzed by image processing. In the SCC tests, many cracks were initiated and coalesced on the surface, and the coalescence of cracks played an important role to primary crack growth. The influence of applied stress on crack initiation was different from that on crack growth. In addition, there was a difference between influences of stress on incubation period to crack initiation and crack initiation rate. Due to these differences, a stress of 0.8S_y was thought to cause relatively many cracks compared with 0.5S_y and 1.3S_y (S_y = 200 MPa). Through quantitative estimation of distribution in local stress around a crack by finite element analysis method, it was deduced that the crack initiation is influenced not only by bulk stress applied at the end of the body, but also by local stress formed around pre-existing cracks. According to pre-existing cracks, stress enhancement accelerates the crack growth, while the stress relaxation causes the suppression of new crack initiation. Based on the experiment and analysis results, three types of growth process were suggested, which are caused by propagation itself, by new crack initiation at vicinity of the crack tip, and by coalescence of approaching cracks. Then, it was concluded that, in order to predict/simulate the cracking behavior of this SCC system, the influence of local stress on the crack initiation should be taken into account.     

The dependency of the crack growth rate on the loading pattern and temperature in stress corrosion cracking of strain-hardened 316L stainless steels in a simulated BWR environment

The stress corrosion cracking (SCC) growth rate of a warm-rolled (WR) 316L stainless steel contoured double cantilever (CDCB) specimen was measured in high purity water at various temperatures and under various loading patterns. An alternating current potential drop (ACPD) technique was used to monitor the crack growth kinetics throughout the tests. The fracture surface exhibited typical intergranular SCC characteristics. Depending on the test conditions, three kinds of crack growth kinetics, i.e., increasing with time then becoming steady, being constant during the whole period, decreasing with time then becoming steady, were identified and are described. The steady state crack growth rate (CGR) values are used to quantify the effects of the loading pattern and the environmental temperature. A moderate increase in the crack growth rate was encountered by employing periods of unloading and reloading to form a trapezoidal loading pattern and the enhancement factor was found to depend on the holding time and the times for unloading and reloading. It was found that the crack growth is thermally activated; however, the apparent activation energy is not constant but seems to be greater at higher temperatures. Several types of temperature-dependent crack growth kinetics are proposed based on the rate-determining step for the crack growth. The present experimental results can be rationalized by considering multiple element processes such as aqueous mass transport and solid-state mass transport in the crack growth. The cracking mode, the temperature dependence of the crack growth rate, and the transient crack growth behavior for WR 316L SS after changing the environmental temperature are quite similar to those for a cold-worked(CW) 316L SS tested in the same environment, despite their different absolute crack growth rate values. The effect of yield strength on CGR is more significant at lower temperatures and the apparent activation energy for the crack growth rate seems to be lower in the material with a higher yield strength. Time-lag crack growth behavior was found at points during several test steps on WR 316L SS, for example, just after in situ pre-cracking and after increasing or decreasing the temperature, which is quite consistent with the results obtained with CW 316L SS. The importance of in situ monitoring of the crack growth for obtaining steady state crack growth rates is emphasized, especially for those steps for which a nonlinear crack growth period occurs after changing the test condition.     

Understanding the effect of nitrogen in austenitic stainless steel on the intergranular stress corrosion crack growth rate in high temperature pure water

Understanding the effect of nitrogen content on the crack growth rate (CGR) due to intergranular stress corrosion cracking (IGSCC) in high temperature (288 °C) pure water, in non-sensitised and strain-hardened stainless steel (SS) type 304 LN was the focus of this study. Non-sensitised SS containing two different levels of nitrogen (0.08 and 0.16 wt.%) in the solution annealed condition was strain-hardened by cross-rolling at 200 °C (warm rolling). It has earlier been reported that SS with a higher nitrogen level in the warm rolled condition has a higher CGR in high temperature pure water. Tensile testing was carried out using both the SS in the warm rolled as well as in the solution annealed condition at 288 °C. Samples were prepared for transmission electron microscopy (TEM) from the warm rolled SS and from the tensile tested (at 288 °C) specimens. TEM studies indicated that twinning and shear band formation were the major modes of deformation due to rolling at 200 °C and these feature were observed to terminate at grain boundaries, leading to regions of higher strain and stresses at grain boundaries. Higher nitrogen SS has higher grain boundary strain and stresses making the grain boundary regions more susceptible to IGSCC, resulting in higher CGR values. At 288 °C dislocation entanglement and cross-slip were the predominant modes of deformation.     

Passivity breakdown and stress corrosion cracking of stainless steel

Passivity breakdown of stainless steel is an important initial process for starting stress corrosion cracking. It was found that small amount of impurities in environments affects the initiation process, but do not affect the propagation process of SCC. The environmental effect on the initiation process is rationally explained by introducing “bound water model” of passive film and HSAB (Hard and Soft Acid and Base) rule. Background of the issue including bound water model and HSAB rule was discussed.     

Microstructural characteristics of the oxide scale formed on 304 stainless steel in oxygenated high temperature water

The microstructural characteristics of oxide scale formed on type 304 stainless steel in oxygenated high temperature water have been investigated. From outer to inner layer, the oxide scale consists of faceted spinel particles, irregularly shaped hematite particles and a compact layer of nano-sized spinels. Some outmost spinels formed on top of other particles are depleted in Cr, while the hematite particles tightly embedded into the inner layer contain more Cr in the inner than in the outer part. The inner nano-sized oxide grow inwards directly from the bottom of outer particles. The related oxidation mechanism is discussed.     

The effect of prior deformation on stress corrosion cracking growth rates of Alloy 600 materials in a simulated pressurized water reactor primary water

The effect of prior deformation on stress corrosion cracking (SCC) growth rates of Alloy 600 materials in a simulated pressurized water reactor primary water environment is studied. The prior deformation was introduced by welding procedure or by cold working. Values of Vickers hardness in the Alloy 600 weld heat-affected zone (HAZ) and in the cold worked (CW) Alloy 600 materials are higher than that in the base metal. The significantly hardened area in the HAZ is within a distance of about 2-3 mm away from the fusion line. Electron backscatter diffraction (EPSD) results show significant amounts of plastic strain in the Alloy 600 HAZ and in the cold worked Alloy 600 materials. Stress corrosion cracking growth rate tests were performed in a simulated pressurized water reactor primary water environment. Extensive intergranular stress corrosion cracking (IGSCC) was found in the Alloy 600 HAZ, 8% and 20% CW Alloy 600 specimens. The crack growth rate in the Alloy 600 HAZ is close to that in the 8% CW base metal, which is significantly lower than that in the 20% CW base metal, but much higher than that in the as-received base metal. Mixed intergranular and transgranular SCC was found in the 40% CW Alloy 600 specimen. The crack growth rate in the 40% CW Alloy 600 was lower than that in the 20% CW Alloy 600. The effect of hardening on crack growth rate can be related to the crack tip mechanics, the sub-microstructure (or subdivision of grain) after cross-rolling, and their interactions with the oxidation kinetics.     

Role of nitrite addition in chloride stress corrosion cracking of a super duplex stainless steel

This paper presents the role of addition of nitrite ions in susceptibility of a super duplex stainless steel, SAF 2507 to stress corrosion cracking (SCC) in chloride environment, which has a particular industrial relevance. Slow strain rate testing (SSRT) in 30 wt.% MgCl₂ solution established SCC susceptibility, as evidenced by post-SSRT fractography. However, the addition of nitrite has interesting influence. At their lower concentrations, nitrite additions seem to decrease SCC susceptibility, whereas, at a higher concentration, it has an accelerating effect on SCC. Attempts have been made to understand this behaviour on the basis of the role of nitrite in passivation and pitting characteristics of SAF 2507 in chloride solution.     

Crack initiation model for sensitized 304 stainless steel in high temperature water

To investigate the initiation behavior of stress corrosion cracking (SCC) for sensitized Type 304 stainless steel in high temperature water, a constant load SCC test method combined with in situ crack observation technique was employed. The in situ crack observation system allowed us to detect small cracks of at least 100 μm. As a result, a fracture time decreased with an increase in an applied stress. The first cracks were observed at most 3 h before the specimen was fractured under all the stress conditions. After that, many cracks were initiated in a short time to fracture. The fracture was caused by coalescence of multiple cracks rather than by growth of some primary cracks. The simulation model for surface crack initiation was newly developed using a Monte Carlo method, which was based on damage mechanics and stress analysis around the existing cracks. The simulation could represent the empirical results of changes in the crack distribution and the cumulative number of cracks during the SCC tests. It was concluded, therefore, that the crack initiation process should be considered in simulating the life prediction of the material in this SCC system.     

Sensitivity of stress corrosion cracking of stainless steel to surface machining and grinding procedure

An investigation has been undertaken to establish the effect of surface preparation method on the susceptibility of a 304 stainless steel to stress corrosion cracking under simulated atmospheric corrosion conditions. MgCl₂ was deposited onto four-point bend specimens, which were then placed in a chamber with a relative humidity of 45% and temperature of 60 °C. These test conditions were designed to reflect external exposure of stainless steel components in industrial plant, including nuclear reactor components, situated in a coastal region, but with the severity of the exposure conditions enhanced to allow discrimination of the effect of surface preparation in a short timescale (up to 1500 h). Four surface preparation methods were evaluated: transverse grinding, longitudinal grinding, transverse dressing using an abrasive flap wheel, and transverse milling. For each case, surface topography, surface defect mapping, near-surface microhardness mapping, residual stress and electron back-scattered diffraction measurements were undertaken. Stress corrosion cracks were observed for the ground and milled specimens but not for the dressed specimens, with cracks apparently originating at corrosion pits. The density of cracks increased in the order: transverse ground, milled and longitudinal ground, with the cracks notably much smaller in length for the transverse ground condition. The propensity for cracking could be linked to the high residual stress and apparent nanocrystalline microstructure at the surface. There was a greater propensity for pitting to initiate at local defect sites on the surface (laps, deeper grooves). However, the tendency was not overwhelming, suggesting that other factors such as more general roughness or the distribution of MnS inclusions had an influence, perhaps reflecting the severity of the environment.     

Effect of nickel ion from autoclave material on oxidation behaviour of 304 stainless steel in oxygenated high temperature water

Characteristics of the oxide films formed on 304 stainless steel exposed to 290 °C oxygenated water in a nickel-lined autoclave were examined. The oxides evolve from dominating irregularly shaped hematite to faceted spinels with increasing immersion time. The surface layer of oxide film is first Cr-enriched and then Ni-enriched as immersion time increases. The oxides nucleate by solid-state reactions with selective dissolution of Fe and Ni, and then grow up through precipitation of cations from solution. Nickel ions dissolved from the nickel lining could promote the stability of NiFe₂O₄ spinel and influence the oxidation behaviour of 304 stainless steel significantly.