Stress Corrosion Cracking Life Research Articles

Probabilistic Fracture Mechanics Simulation of Stress Corrosion Cracking Using Accelerated Laboratory Testing and Multi-Scale Modeling

A propagation approach to manage stress corrosion cracking (SCC) is justified for modern high-strength alloys that exhibit low-subcritical crack growth rates, and is enabled by maturing probabilistic fracture mechanics and mechanism-based hydrogen damage modeling. A computer program, SCCrack, is developed based on stress intensity (K) similitude and fast Monte Carlo analysis to predict SCC life distributions from input distributions of variable material crack growth rate (da/dt vs. K), starting crack size, environment, and stress. Growth rates are obtained using an accelerated experiment that exploits precision electrical potential measurement of crack propagation, small crack size, and slow-rising K loading. Data are leveraged by fundamental models of SCC threshold (KTH) and H-diffusion limited Stage II da/dt, built on the hydrogen environment assisted cracking mechanism. With these inputs, SCCrack enables multi-scale “What if?” simulations of the effects of alloy-environment-precorrosion-stress variable...

Microstructure and corrosion behaviour of gas tungsten arc welds of maraging steel

Superior properties of maraging steels make them suitable for the fabrication of components used for military applications like missile covering, rocket motor casing and ship hulls. Welding is the main process for fabrication of these components, while the maraging steels can be fusion welded using gas tungsten arc welding (GTAW) process. All these fabricated components require longer storage life and a major problem in welds is susceptible to stress corrosion cracking (SCC). The present study is aimed at studying the SCC behaviour of MDN 250 (18% Ni) steel and its welds with respect to microstructural changes. In the present study, 5.2 mm thick sheets made of MDN 250 steel in the solution annealed condition was welded using GTAW process. Post-weld heat treatments of direct ageing (480 °C for 3 h), solutionizing (815 °C for 1 h) followed by ageing and homogenizing (1150 °C for 1 h) followed by ageing were carried out. A mixture of martensite and austenite was observed in the microstructure of the fusion zone of solutionized and direct aged welds and only martensite in as-welded condition. Homogenization and ageing treatment have eliminated reverted austenite and elemental segregation. Homogenized welds also exhibited a marginal improvement in the corrosion resistance compared to those in the as-welded, solutionized and aged condition. Constant load SCC test data clearly revealed that the failure time of homogenized weld is much longer compared to other post weld treatments, and the homogenization treatment is recommended to improve the SCC life of GTA welds of MDN 250 Maraging steel.

SCC Life Estimation Based on Cracks Initiated from the Corrosion Pits of Bolting Material SCM435 Used in Steam Turbine

Life estimation was performed for the stress corrosion cracking (SCC) that occurs in deaerated and wet hot pure steam at the bottoms of the threads of bolts made of SCM435 (equivalent to AISI 4137) used in steam turbine. SCC is believed to occur when corrosion pits are formed and grow to critical size, after which SCC is initiated and cracks propagate until the critical fracture toughness value is reached. Calculations were performed using laboratory and field data. The results showed that, for a 40 mm diameter bolt with 0.2% offset strength of 820 MPa, the critical crack depth for straight-front cracks was 5.4 mm. The SCC life depends on the lubricant used; the SCC life estimated from this value is approximately 70 000 hours when graphite is used as a lubricant.

Intergranular stress corrosion cracking behavior of niobium-added Type 308 stainless steel weld overlay metal in a simulated BWR environment

Type 308 stainless steel weld metal as an internal cladding of reactor pressure vessels for boiling water reactors is subject to postweld heat treatment during fabrication and can suffer sensitization depending on carbon and ferrite contents. This sensitization can be avoided by using niobium-added Type 308 weld metal (specified as Type 308 NbL) which was developed for one-layer overlay welding application. In the present study, stress corrosion cracking (SCC) behavior of heat-treated Types 308 and 308NbL weld metals in oxygenated high temperature pure water was evaluated by slow strain rate test and U-bend tests with and without crevice. Every test showed that Type 308NbL weld metals were highly resistant to SCC compared to ordinary Type 308 weld metals. In single U-bend test, one-layer overlay weld metals of Type 308NbL produced by electroslag welding process using wide strip electrodes were crack free over 23,000 h. The U-bend test data of ordinary Type 308 weld metals were successfully analyzed by an SCC reaction model. Using this analysis, the SCC life margin for Type 308NbL over ordinary Type 308 weld metals, expressed as a ratio of respective times to SCC initiation, was estimated to be about 36.

Methodology for formulating predictions of stress corrosion cracking life

This paper presents a methodology for formulating predictions to evaluate the stress corrosion cracking (SCC) potential of each light-water reactor component, where an index is introduced as a life index or F index. The index denotes the SCC time ratio of a given SCC system to be evaluated against a reference SCC system. The life index is expressed by the products of several subdivided life indexes, which correspond to each SCC influencing factor. Each subdivided life index is constructed as a function containing the influencing factor variable, obtained by analyzing experimental SCC life data. The methodology was termed the subdivided factor method. Application of the life index to SCC life data and field data showed that it was effective for evaluating the SCC potential, i.e. the SCC life. Accordingly, the proposed methodology can potentially describe a phenomenon expressed by a function which consists of the variables of several influencing factors whether there are formulae which unite as a physical model or not.

Ａｌ‐Ｚｎ‐Ｍｇ‐Ｃｕ‐Ｌａ系押出合金の引張強度特性と耐応力腐食割れ性に及ぼすＣｒ，ＭｎおよびＭｇ添加の影響

Two groups of the extruded high strength Al-Zn-Mg-Cu-La system alloys, including Al-10 wt.%Zn-1 wt.%Mg-1.5 wt.%Cu-3 wt.%La with various Cr and Mn contents and Al-8 wt.%Zn-1.2 wt.%Cu-4.0 wt.%La with various Mg contents, were developed as the mechanical materials with a good stress corrosion cracking (SCC) resistance. In these alloys, the effects of the additional elements were investigated on the mechanical properties and SCC life. The main results are as follows;(1) The second phase particles which contained Zn and La were observed in the matrix, and dispersed homogeneously. The size of the particles was 5 to 10μm. For the alloys with Cr, dispersoids including Cr were also observed in the matrix. Their size was over 10μm.(2) The maximum value of 568MPa in tensile strength was observed in Al-8 wt.%Zn-1.2 wt.%Cu-4.0 wt.%La with 2.5 wt.%Mg, which was nearly equal to the commercial aluminum alloys7075-T73 and 7475-T6.(3) The excellent resistance of SCC was obtained in these alloys. Their SCC lives were over 15 times as long as that of Al-8 wt.%Zn-1 wt.%Mg master alloy. In conclusion, we developed extruded Al-high Zn-low Mg system alloys with high strength and good SCC resistance.

腐食防食小特集 高力Ａｌ‐８Ｚｎ‐１Ｍｇ系押出合金の引張特性と耐応力腐食割れ性

Tensile properties and stress corrosion cracking (SCC) resistance of Al-8wt.% Zn-1wt.% Mg system alloys with various Cu and La contents were investigated. The influences of additional elements on mechanical properties and SCC life were discussed on the basis of experimental results. The results obtained in the present work are as follows;(1) Additions of elements of Cu and La were effective to decrease the grain size of those materials. In particular, the grain sizes in alloys containing La contents of more than 1.7wt.% were small at 20μm, comparing with that in the master alloy.(2) For the present alloys containing La, the second phase particles, consisting of Zn and La, have the capacity of hydrogen storage. These particles were between 5 and 10μm in size, and were dispersed in the matrix homogeneously.(3) Tensile properties were improved with increasing Cu or La content. For the alloys with Cu and La, the tensile strength was from 450 to 500MPa, and the elongation between 14 and 17%.(4) The alloys containing La had high resistance to SCC. Their SCC lives were about 15 times as long as that of the master alloy. In conclusion, the alloys added Cu more than 0.6wt.% and La 4.0wt.% were much superior to the master alloy in both the tensile properties and the SCC resistance.

Analysis of stress corrosion cracking life based on reaction model

The stress corrosion cracking (SCC) lives of Type 304 stainless steel, Inconel alloy 600, and niobium stabilized, improved alloy 600 were obtained by an uniaxial constant load in high temperature pure water with 8 ppm dissolved oxygen, and were analyzed based on a reaction model and a certain correlation between SCC life parameters. The load dependency of SCC life was also discussed using the model and an expression in the form of a hyperbolic tangent function. The results showed that the analysis based on the model was useful for the quantitative evaluation of the SCC life of materials.

３．５ＮｉＣｒＭｏＶ鍛鋼の応力腐食割れ寿命分布の解析

The stress corrosion cracking (SCC) life time of turbine rotors in actual nuclear and fossil power plants was analyzed by a statistical method and was estimated to obey an exponential probability distribution. Then, the laboratory accelerated test condition to exhibit such an exponential probability distribution was investigated using 3.5NiCrMoV steel forgings which are being used most widely as a material for low pressure turbine rotors. The exposure of double U-bend specimens in liquid/gas interface of undeaerated 30% NaOH aqueous solution was found to be most fitted for the purpose. Under this condition, the effects of alloying elements and mechanical strength/microstructure on SCC life were studied. The reduction of P content and/or the addition of a small amount of Nb in steel and the reduction of mechanical strength were effective to prolong the SCC life of turbine rotors.

アルミニウム合金７０７５圧延材の応力腐食割れと腐食疲労に関する研究

Stress corrosion cracking (SCC) and corrosion fatigue (CF) experiments were performed on the test specimens in three directions (L, LT, ST) of rolled 7075 aluminum alloy. The results obtained are as follows.(1) The SCC life time of the test specimen in ST direction could be estimated from the applied stress, the tensile strength, and the activation energy which is determined by changing the temperature of corrosive solution and immersing the test specimen in LT direction.(2) There was a combined effect of SCC and CF in CF experiments for the test specimen in LT direction, because the total accumulated damage factor was smaller than one when the linear damage summation method was applied to the experimental result of CF at 5min. stress hold time. This effect was confirmed on the fracture surface of the test specimen.(3) However, the effect was not observed either in the CF experimental result or on the fractograph for the specimen in L direction tested at room temperature and 30min. stress hold time, nor for the specimen in ST direction tested at 70°C and 5min. stress hold time.

高温純水中における鋭敏化304ステンレス鋼の粒界応力腐食割れ寿命の確率分布

高温純水中における鋭敏化304ステンレス鋼の粒界応力腐食割れ寿命の確率分布

濃厚塩化マグネシウム水溶液中におけるオーステナイト・ステンレス鋼の応力腐食割れに及ぼす窒化物および炭化物形成元素の影響

Effect of less than one per cent of aluminum, titanium, vanadium, zirconium, niobium or uranium and heat treatment on the stress corrosion cracking (SCC) of 18Cr-20Ni or 18Cr-16Ni steel was investigated in boiling magnesium chloride solution at 151°C.The results indicate that the above alloying elements have no appreciable influence on the SCC resistance of stable austenitic stainless steel if they do not form nitride or carbide. The addition of aluminum, titanium, zirconium, niobium or uranium improved the SCC resistance of the nitrogen contaminated steel, possibly by decreasing the nitrogen content in solid solution, while vanadium, a weak nitride former, showed no effect. If carbide is precipitated, the SCC life is lowered since the carbon content in solid solution is decreased. It was indicated that aluminum, which forms nitride but does not react with carbon in solid solution, and uranium could be useful alloying elements in improving the SCC resistance of the air-melted stable austenitic stainless steel which might be contaminated with nitrogen.