Stress corrosion cracking of a 60 MW steam turbine rotor

The paper presents a root cause analysis of a steam turbine rotor blade groove cracking. The scope of analyses included material testing and mechanical integrity calculations. In scope of material testing, fracture microstructure was assessed and basic mechanical property characteristics of the rotor discs were determined. In scope of integrity analyses, the stress fields in the blade grooves were calculated and the possibility of cracking due to different mechanisms was assessed. Both calculations and material tests confirmed the stress corrosion cracking to be the root cause of the rotor failure. This was a basis for proposing the rotor discs repair by overlay welding with a lower strength material and modifications to the groove geometry.     

Microstructure and intergranular stress corrosion cracking susceptibility of a SA508-52M-316L dissimilar metal weld joint in primary water

Correlation of microstructure and intergranular stress corrosion cracking (IGSCC) susceptibility for the SA508-52M-316L dissimilar metal weld joint in primary water was investigated by the interrupted slow strain rate tension test following a microstructure characterization. The susceptibility to IGSCC in various regions of the dissimilar metal weld joint was observed to follow the order of Alloy 52 Mb> the heat affected zone of 316L> the dilution zone of Alloy 52 Mw> Alloy 52 Mw weld metal. The chromium-depletion at the grain boundary is the dominant factor causing the high IGSCC susceptibility of Alloy 52 Mb. However, IGSCC initiation in the heat affected zone of 316L is attributed to the increase of residual strain adjacent to the grain boundary. In addition, the decrease of chromium content and increase of residual strain adjacent to the grain boundary increase the IGSCC susceptibility of the dilution zone of Alloy 52 Mw.     

Investigations on welding residual stresses in penetration nozzles by means of 3D thermal elastic plastic FEM and experiment

Recent discoveries of stress corrosion cracking (SCC) in weldments including penetration nozzles at pressurized water reactors (PWRs) and boiling water reactors (BWRs) have raised concerns about safety and integrity of plant components. It is well known that welding residual stress is an important factor resulting in SCC in weldments. In the present work, both experimental method and numerical simulation technology are used to investigate the characteristics of welding residual stress distribution in penetration nozzles welded by multi-pass J-groove joint. An experimental mock-up is fabricated to measure welding residual stress at first. In the experiment, each weld pass is performed using a semi-circle balanced welding procedure. Then, a corresponding finite element models with considering moving heat source, deposition sequence, inter-pass temperature, temperature-dependent thermal and mechanical properties, strain hardening and annealing effect is developed to simulate welding temperature and residual stress fields. The simulation results predicted by the 3D model are generally in good agreement with the measurements. Meanwhile, to clarify the influence of deposition sequence on the welding residual stress, the welding residual stress field in the same geometrical model induced by a continuous welding procedure is also calculated. Finally, the influence of a joint oblique angle on welding residual stress is investigated numerically. The numerical results suggest that both deposition sequence and oblique angles have effect on welding residual stress distribution.     

An optimum selection strategy of reflective cracking mitigation methods for an asphalt concrete overlay over flexible pavements

Reflective Cracking (RC) has been a daunting challenge in pavement maintenance and rehabilitation (M&R), yet, still, after several decades of research, no exclusive solution prevails. Moreover, RC mitigation methods have shown significant variation in in situ performance. Therefore, a technique tailored to select an effective RC mitigation method is essential for the success of pavement M&R. In this study, a life cycle cost (LCC) and multi-criteria decision-making (MCDM) analyses were conducted to evaluate the effectiveness of currently available RC mitigation methods and to select the optimal method for an asphalt concrete overlay above flexible pavements. The MCDM includes three components: LCC, performance, and materials (recyclability). These criteria determine the selection ranking of each RC mitigation method. In addition, the effects of the priority level including cost, performance, and recyclability on the final decision were evaluated by conducting a series of sensitivity analysis under multiple scenarios; therefore, weight combination of the three criteria were recorded to define the measurements affecting the final decision.     

Evaluation of stress corrosion cracking and hydrogen embrittlement in an API grade steel

Stress corrosion cracking (SCC) and hydrogen embrittlement (HE) of pipeline steels in contact with soil was investigated. Different soils were prepared in order to determine their physical, chemical and bacteriological characteristics. Slow strain rate testing was carried out by using aqueous extracts from soil samples and NS4 standard solution. Stress vs. strain curves of API 5L grade X46 steel were obtained at different electrode potentials (E_corr, 100 mV below E_corr and 300 mV below E_corr) with 9 × 10⁻⁶ s⁻¹ and 9 × 10⁻⁷ s⁻¹ strain rate. In addition, the hydrogen permeation tests were carried out in order to evaluate the susceptibility of hydrogen penetrates into theses steels. The results demonstrated the incidence of cracking and their dependence on the potential imposed. In that case, cracking occurred by stress corrosion cracking (SCC) and the hydrogen embrittlement (HE) had an important contribution to cracking initiation and propagation. Cracking morphology was similar to the SCC reported on field condition where transgranular cracking were detected in a pipeline collapsed by land creeping. It was important to point out that even under cathodic potentials the material showed the incidence of secondary cracking and a significant reduction of ductility.     

Treatment of residual stress in failure assessment procedure

The effect of residual stress on component failure has been investigated using the distributions from current failure assessment procedures, and a residual stress profile simple to apply with less conservatism has been proposed for the weld geometries of T-plate and tubular T-joint. The stress intensity factors (SIFs) in the two weld geometries under various types of loads have been calculated using the Green’s function method. The Green’s functions were determined not only for the T-plate but also for the tubular T-joint with the built-in ends. The use of a linear (bending) stress profile, derived from an analysis of measured residual stress distributions in T-plate and tubular T-joints, has been examined. The profile was validated with experimentally measured residual stress distributions in two materials, a high strength and medium strength ferritic steel and two geometries, a T-plate joint and a tubular T-joint for crack lengths up to half the plate or pipe thickness. Whereas the recommended residual stress distributions are geometry and material specific, it is shown that a simplified linear bending profile provides a possible guideline, applicable to a range of materials and geometries, where detailed information on weld procedures or residual stress profiles are unavailable.     

磁屏表面裂纹分析

本文对磁屏外表面的轴向开裂进行了分析。结果表明，该表面开裂为应力腐蚀开裂。其原因是磁屏表面存在较大的切向拉应力，从而在酸洗过程中产生了应力腐蚀开裂。对冲压后的磁屏采用消除应力退火，随后再酸洗，未发生表面轴向开裂。     

铸铁表面激光熔敷层的抗裂性和耐磨性

采用免预热多道搭接的激光熔敷工艺，在铸铁表面制备出抗裂耐磨的激光熔敷层，研究了激光功率和扫描速度对熔敷层抗裂性的影响。适当的激光功率和扫描速度可以使铁基熔敷层的裂纹率最低．随着Ni含量的增加，熔敷层内奥氏体相体积分数增加；K可促进共晶碳化物团球化，熔敷层抗裂性增强．通过对铸铁表面激光熔敷层裂纹的萌生与扩展的动态观察，揭示了熔敷层开裂的微观机制．裂纹易在粗大渗碳体区或石墨与奥氏体的界面处萌生，奥氏体可明显抑制裂纹的扩展．Ti含量增加使熔敷层中原位自生的TiC的体积分数增大，熔敷层磨损质量损失减少，耐磨性增强．     

Some effects of yield strength on the stress corrosion cracking behaviour of low alloy steels in aqueous environments at ambient temperatures

The present paper describes the results of a literature review on the effects of material yield strength on the threshold stress corrosion cracking (SCC) condition known as K_1SCC, of a series of low alloy steels in various aqueous environments at ambient temperatures. It has been shown that an increase in yield strength reduced the value of K_1SCC and that this effect was more significant over the lower yield strength range 600–1200 MPa; at higher yield strength levels the effects of yield strength were much less evident. A review of the results indicated that SCC transgranular fracture was seen at yield strength values below around 1200 MPa while above this value the fracture path was exclusively intergranular in nature. Note that the transition point between transgranular and intergranular SCC coincided with the point at which yield strength effects were much reduced. A series of models reported in the literature, which attempted to explain the effects of yield strength on K_1SCC in terms of a fracture mechanics framework were examined with the introduction of a critical distance concept. Finally, the predictions of a grain size effect on the point at which the fracture path changed from transgranular to intergranular were taken from one model and used to show that the large data scatter in the region where transgranular fracture was operative, viz., the lower yield strength region of 600–1200 MPa, could be the result of grain size variations between the different reported studies.     

An inverse method for evaluating weld residual stresses via fatigue crack growth test data

This paper presents an inverse method for calculating the thermal residual stresses in welded specimens via measured fatigue crack growth rates. Firstly, fracture-mechanics superposition law has been used to extract the stress intensity factor due to residual stress contribution from measured crack growth rate. Secondly, a so-called B matrix has been established by performing finite element analysis. Residual stress distribution is then determined by solving linear algebraic equations relating the B matrix and residual stress intensity factors obtained from crack growth test data. The inverse method has been validated by a well-established residual stress distribution and corresponding stress intensity factor, and then applied to an M(T) sample in 2024-T3 alloy with a longitudinal weld. Agreement with the measured residual stresses is reasonably good and reasons for certain differences between the calculated and measured are discussed.     

Investigation of stress corrosion cracking behavior of super 13Cr tubing by full-scale tubular goods corrosion test system

In this paper, a self-built device called “full-scale tubular goods corrosion test system” was used to test a 6 m length super 13Cr tubing (with coupling) to study its corrosion performance in spent acid. The specimen fractured at the tubing and was investigated by visual inspection, optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), and mechanical test. It was the joint function of tensile force (78.6% actual yield strength), inner pressure (70 Mpa) and spent acid that induced stress corrosion cracking (SCC) of the tubing at 120 °C. Three different areas were found on the fracture surface, including crack initiation area, crack expansion area, and final fracture area. The fracture initiated from the “X” shape corrosion cracks which were evolved from small corrosion pits. The reduction of ductility and toughness may also facilitate SCC of the tubing.     

Challenges in modelling the evolution of stress corrosion cracks from pits

The evolution of stress corrosion cracks from pits is important in many industrial applications but continues to be a challenge in both measurement and prediction. Life prediction in these circumstances has to account for pit growth kinetics, the conditions for the transition from pits to cracks, and the growth rate of cracks in the short and long crack domain. An example of importance is the performance of steam turbine rotors in power generation. Although stress corrosion failures are comparatively rare, the consequences can be severe and occasionally catastrophic. Consequently, considerable effort is being focused on evaluating the effect of operational variables on pitting and crack growth and in developing an improved basis for structural integrity assessment. A preliminary mathematical model based on deterministic equations with statistically variable input parameters was developed for simulating the evolution of the pit depth distribution at different exposure times, and the transformation to stress corrosion cracks was based on the Kondo criteria. The model predicted some features of the damage well but recent novel measurements of the evolution of stress corrosion cracks from pits combined with finite element analysis of the strain distribution suggest that the transition from a pit to a crack is more complex than can be accounted for by the Kondo approach.     

Effect of pre-strain and two-step aging on microstructure and stress corrosion cracking of 7050 alloy

A novel heat treatment procedure that combines the pre-strain with a two-step aging was proposed to improve both the strength and stress corrosion cracking (SCC) resistance of the high strength 7050 aluminum alloy. The heat treatment included a post-quenching pre-strain of 5%, a high-temperature aging at 200 °C for several minutes, and a subsequent low-temperature aging at 120 °C for 24 h. The yield strength of the samples aged at 200 °C for 0.5 and 1 min was higher than that of the T6 sample. The SCC resistance of these samples was improved compared to the T6 sample, due to the enlargement in the size and the inter-particle distance of the grain boundaries precipitates and the decreases in the number of the GP zone. Especially, the sample aged at 200 °C for 5 min exhibited a similar SCC resistance and much higher elongation compared the T76 sample.     

Modelling of mode-I stable crack growth under hydrogen assisted stress corrosion cracking

The paper presents a new strategy based on combined analytical and finite element (FE) solution to hydrogen assisted stress corrosion crack growth. The diffusion process is solved analytically through both one-and two-dimensional modelling. These solutions are adopted with two-dimensional FE based cohesive zone model of crack extension study. The results fit well with published experimental data and show improvement over the predictions by full FE approach. The new solution approach helps to reduce time required for simulation/computation. The study has produced a relationship between concentration dependent reduction in cohesive strength and plastic strain rate.     

Failure analysis of stress corrosion cracking in heat exchanger tubes during start-up operation

The cracking failure of a new heat exchanger during first start-up operation has been analyzed. Through the investigation of the operating history of the equipment, analysis of the chemical compositions of tube material and corrosion products, metallographic test of specimens with cracks, the cracking mode can be described as the Stress Corrosion Cracking (SCC) of austenitic stainless steel. This kind of cracking was induced by the chloride in high temperature steam and tensile stress. The residual tensile stress due to seal expansion has been proved by numerical calculation. The pre-heating steam which was polluted by the catalyst with chloride is the main reason for the tube cracking in this case.     

Simulation of hydrogen assisted stress corrosion cracking using the cohesive model

This paper investigates the effect of hydrogen diffusion on stable crack propagation by using numerical finite element simulations based on the cohesive model. The model with its two common parameters, cohesive strength, T₀, and critical separation, δ₀, and its two additional parameters for stress corrosion cracking, the effective diffusivity, D_eff, and a material parameter, α, which represents the reduction of the cohesive strength, is described. This model is then employed to predict the stable crack propagation in C(T) specimens made from a high strength structural steel which were tested under hydrogen charging conditions in rising displacement tests using various deformation rates. It is shown that, in general, the prediction of stable crack propagation is promising, but may be further improved by the use of a more sophisticated diffusion equation. Finally, the influence of variations of the effective diffusivity and the cohesive strength reduction on the thus simulated crack growth resistance curves is studied.     

The influence of biaxial loading on branching of a dissolution driven stress corrosion crack

Stress corrosion cracking occurs due to the synergistic interaction between mechanical load and corrosion reactions. In this study, branching during anodic dissolution driven crack growth is studied using an adaptive FE procedure. The crack has an inherent blunt tip due to the dissolution, and the growth is treated as a moving boundary problem with a strain-assisted evolution law. Simulations are performed with different degrees of load biaxiality. It is found that increasing biaxiality promotes branching. No conditions for when branching takes place are found. Crack growth rates for branches are investigated, and it is found that, after an initial acceleration, constant growth rates can be reached, as well as decreasing speed and eventual arrest. The influence of T-stresses and perturbations sensitivity are discussed.     

An analysis of stress uniformity for concrete-like specimens during SHPB tests

As far as concrete-like brittle materials are concerned, whether stresses have been uniformly distributed along the specimen thickness before fracture occurs is a key problem with regard to the validity of SHPB data. By using characteristics method of wave propagation, the SHPB tests for cement mortar which is respectively regarded as the so-called viscoelastic material and the elastic material are numerically studied in the present paper, in order to see how the stress distribution is during both loading and unloading processes. It is found that different rise-times τ_s of incident waves evidently influence the stress uniformity, and the optimum rise-time looks like τ_s/t_L = 2, either shorter or longer rise-time will result in worse situation in stress uniformity; the stress amplitude has no influence on the stress uniformity, but markedly influences the value of strain and average strain rate in specimens. The error found in the determination of dynamic stress–strain curves of rate-dependent brittle materials by the traditional SHPB technique, either by three-wave method or two-wave method, is actually unacceptable, although the dynamic fracture stress can be correctly determined if the requirement of stress uniformity is satisfied before the specimen fails.     

Probabilistic assessment of weld fatigue damage for a nonlinear combination of correlated stress components

Estimation of fatigue damage for welds which are subjected to multiple stress components is addressed. Each component is represented by a separate stress cycle distribution of the Weibull type, and the effect of correlation between the different components is addressed. Three different types of bivariate Weibull distributions are considered. These are of the following categories: (i) The Nataf model (ii) The NKR model and (iii) The Plackett model. Comparison is made between the fatigue damage obtained by application of the different models for different levels of correlation and with different SN-curve exponents. Extension from the bivariate case to higher dimensions is also briefly discussed.     

Threshold stress intensity factor and crack growth rate for stress corrosion cracking of simulated heat affected zone in caustic solution

The threshold stress intensity factor for stress corrosion cracking (K_Iscc) of the simulated heat affected zone (HAZ) of mild steel in caustic solution has been determined using circumferential notch tensile (CNT) technique. The HAZ microstructure produced upon manual metal arc welding of grade 250 steel was simulated over a length of 35 mm of CNT specimens, using a thermo-mechanical simulator. Inter- and trans-granular stress corrosion cracking has been confirmed using a scanning electron microscope. The results presented here validate the ability of CNT technique for the determination of K_Iscc of HAZ and Base metal. Crack growth rates have also been determined using CNT technique. Further, the effect of microstructures on K_Iscc and crack growth rate is discussed in the present study. The determined K_Iscc of Base metal and simulated heat affected zone in 30% caustic solution is 24 and 45 MPa m^1/2, respectively.