CRACK NUCLEATION AND PROPAGATION IN BLADE STEEL MATERIAL

Stress 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, K_ISCC, of about 15 MPa√m and a plateau in stress corrosion crack growth rate of about 10^-5mm/s.     

Stress corrosion cracking of AISI 321 stainless steel in acidic chloride solution

The stress corrosion cracking (SCC) of AISI 321 stainless steel in acidic chloride solution was studied by slow strain rate (SSR) technique and fracture mechanics method. The fractured surface was characterized by cleavage fracture. In order to clarify the SCC mechanism, the effects of inhibitor KI on SCC behaviour were also included in this paper. A study showed that the inhibition effects of KI on SCC were mainly attributed to the anodic reaction of the corrosion process. The results of strain distribution in front of the crack tip of the fatigue pre-cracked plate specimens in air, in the blank solution (acidic chloride solution without inhibitor KI) and in the solution added with KI measured by speckle interferometry (SPI) support the unified mechanism of SCC and corrosion fatigue cracking (CFC).     

流量计导压管断裂失效分析

某化工厂工艺管线上的316不锈钢材质的孔板流量计导压管断裂,导致介质泄漏发生火灾。为查明其失效原因,对断裂的仪表管进行成分、硬度、金相、断口形貌和腐蚀产物分析,确认仪表管发生断裂的原因是在安装应力、震动和环境中Cl元素的共同作用下,先发生了应力腐蚀形成裂纹源,裂纹达到门槛值后又以疲劳形式扩展,最终导致开裂。     

Stress corrosion cracking of maraging steel weldments

Abstract

The stress corrosion characteristics of 18 wt-%Ni (MDN-250) maraging steel and its weldments made under different welding conditions have been investigated. The threshold stress intensity factor K _ISCC in stress corrosion conditions has been determined in 3.5 wt-%NaCl environment for the base metal and weldments. The fractured surfaces were analysed to study the types of fracture during stress corrosion cracking in base and weld metals. Fracture toughness tests were carried out and the results obtained from these tests have been compared with K _ISCC values.     

Effects of various environments on fatigue crack growth in Laser formed and IM Ti–6Al–4V alloys

The fatigue crack growth behaviors of Laser formed and ingot metallurgy (IM) Ti–6Al–4V alloys were studied in three environments: vacuum, air and 3.5% NaCl solution. Taking the Unified Fatigue Damage Approach, the fatigue crack growth data were analyzed with two intrinsic parameters, stress intensity amplitude ΔK and maximum stress intensity K_max, and their limiting values ΔK^* and . Fatigue crack growth rates da/dN were found increase with stress ratio R, highest in 3.5% NaCl solution, somewhat less in air and lowest in vacuum, and higher in IM alloy than in Laser formed one. In 3.5% NaCl solution, stress corrosion cracking (SCC) was superimposed on fatigue at R=0.9 for where K_max>K_ISCC, the threshold stress intensity for SCC. This and environment-assisted fatigue crack growth were evidenced by the deviation in fatigue crack growth trajectory (ΔK^* vs. curve) from the pure fatigue line where . Furthermore, the fractographic features, identified along the trajectory path, reflected the fatigue crack growth behaviors of both alloys in a given environment.     

Zusammenwirken von Spannungsrißkorrosion und Ermüdung bei der Schwingungsrißkorrosion eines hochfesten Stahles

Conjoint Action of Stress Corrosion Cracking and Fatigue on Corrosion Fatigue of a High Strength Steel The corrosion fatigue characteristics of a high strength, martensitic steel in 0.5 n NaCl solution is investigated with regard to the fatigue and stress corrosion cracking behaviour of the material. Test parameters are stress ratio and frequency, testing is carried out with fracture mechanics methods, the crack surfaces are examined fractographically. An analysis of the results reveals that corrosion fatigue in high strength steel is caused by fatigue or by stress corrosion cracking, depending on the kinetics of the two processes. Fatigue and stress corrosion cracking do not act cumulative or additive. Instead, the kinetically faster process causes crack advance. The crack growth characteristics are interpreted with respect to the fractographic appearance of the crack surfaces. Corrosion fatigue cracks propagate either intergranular relative to the prior austenite grain boundaries as stress corrosion cracks do or transgranular like fatigue cracks, depending on the crack growth rates of the two processes. Fatigue and stress corrosion cracking do not interact, at least in a measurable degree, because of the different crack path of the two fracture processes. Results can be assessed quantitatively with the “process competition model”.     

Corrosion Fracture of Structural Metallic Materials: Effect of Electrochemical Conditions in Crack

Abstract: The work is a compressed review based on the summarised results and the original approach for study of corrosion crack growth, taking into account local electrochemical conditions in the crack tip, which was developed at the Karpenko Physico‐Mechanical Institute of NASU. The model scheme of the pre‐fracture zone in the corrosion crack tip, which can be defined by the local values of pH of solution, electrode potential of metal E and stress intensity factor K_I is proposed. For its realisation, the special method and testing equipment for corrosion crack growth study and local electrochemical measurements in the crack were developed. The variation of the electrochemical conditions in corrosion cracks was studied, and it has been found that some stabilised levels of the pH and E values can be achieved in the tip of a non‐propagating and a propagating crack under static and cyclic loading during of exposure time. On this ground, the method for forecasting of the threshold stress intensity factor K_ISCC under stress corrosion cracking was proposed using these characteristic values of pH and E. This method was also adopted for the determination of the threshold stress intensity factor K_th under corrosion fatigue. The special method for determining corrosion fatigue crack growth rate diagrams based on consideration of extreme electrochemical conditions in the crack tip was developed. It has been proven that such diagrams reflect the extreme influence of the environmental factor on corrosion fracture of material, and they may be recommended as the base for the remaining lifetime calculation of the structural elements exploited under environmental conditions.     

Failure analysis of TP304H tubes in the superheated steam section of a reformer furnace

The cracking failure of TP304H tubes in the superheated steam section of a reformer furnace was analyzed. Through the analysis of macro-appearance, micro-appearance of specimens with cracks, metallurgical structure of specimens from an intact pipe section and cracked pipe section, energy spectrum detection of fracture surface, residual stress measurement, and investigation of the service medium, the cracking mode was described as the stress corrosion cracking (SCC) of austenitic stainless steel. In this case, the materials in the heat affected zone were sensitized by inappropriate welding technology. This together with the higher pH value in the steam due to the failure of a gas-liquid separator led to the final cracking of the reformer furnace tube. So the inappropriate welding technology and the failure of the gas-liquid separator were the main factors in this fracture accident.     

Corrosion Induced Explosion of a High-Pressure Fire-Extinguishing Gas Cylinder

The failure of a high-pressure fire-extinguishing cylinder was investigated. Failure was induced by internal surface corrosion and stress corrosion cracking (SCC) due to condensation of carbonic acid. In internal surface, especially the area near the bottom of the exploded cylinder, severe corrosion was characterized by local pits. SCC initiated from these local corrosion pits was observed by metallurgical analysis. Microstructure of the failure cylinder near the internal surface consisted of multiple-banded structure and the banded structure could accelerate local corrosion initiation and propagation. The corrosion products built up on the fracture surface were primarily ferrous carbonate (FeCO₃). The determination of moisture in fire-extinguishing gas was also examined.     

Fatigue failure of a Slickline wire induced by corrosion pits

A Slickline wire failed after nearly 400 h of service. In order to find out the main causes and the sequence of this failure, a detailed analysis was carried out on a fracture fragment of this component. This analysis revealed that the operation of the wire produced a series of superficial discontinuities, comprising corrosion pits, fatigue cracking and wear grooves, that provided several stress raisers which served as the initiation point for the failure. Additionally, the manufacturing process introduced some longitudinal cracks that helped in the propagation of the final fracture. Finally, due to the presence of dimples in the last portion of the failure, it could be concluded that the ductility of the material was not compromised.     

Failure of a 17-4 PH stainless steel sailboat propeller shaft

In this study, a 17-4 PH precipitation hardening stainless steel propeller shaft failed in use when installed in a sailboat working in a marine environment. Failure analysis was conducted on the propeller shaft. Results indicate that the failure was caused by the fracture of the propeller shaft by torsional fatigue and stress corrosion cracking (SCC). SCC progressed transgranulary in the martensitic matrix.     

Form und Mechanismen von Rissen und Brüchen unter gleichzeitiger korrosiver und dynamischer Beanspruchung mit unterschiedlichen Frequenzen

Form and Mechanisms of Cracks and Fractures Caused by Simultaneous Corrosive and Dynamic Stress under Various Frequencies Distinction of crack nucleation and crack propagation under variable frequencies and electrolytic corrosion. Relationship between crack nucleation and frequency due to differing sliding mechanisms. Crack propagation under decreasing frequencies causes transition from corrosion fatigue to cyclic stress corrosion cracking and finally stress corrosion cracking can be observed. Coordination of that transition with the increasing influence of the K_ISCC-value as determinant of the crack propagation. The influence of frequency on high temperature materals in hot gas atmosphere is mainly determined by creep mechanisms during top-load phase and the gas-metal-reactions. Accordingly the life time increases with growing load frequency.     

Failure of aircraft propeller assembly

This paper analyses the causes of the incident of a Cessna trainer whose propeller was separated due to the cracking of the propeller blade hub during the take off roll. Beach marks and fatigue striations, typical of fatigue cracks, were observed on the fracture surface and corrosive oxides were detected in the center of beach marks that are considered to be the crack origin. The stress acting on the fracture surface under a corrosive environment forms corrosive oxides, such as mud cracks. By analyzing the fractography and metallography of the failed parts, it is found that the propeller blade hub nucleated stress corrosion cracking (SCC) as a result of residual stress and corrosive environment and the SCC was the cause of the fatigue crack. Moreover, a fatigue crack reaches its critical length by repeated cyclic stress, which occurs during the rotation of the propeller blade and then, the rest of the fracture occurred instantaneously.     

某酸奶厂配料罐304不锈钢夹套底部材料的失效分析

某酸奶厂二次配料罐304不锈钢夹套底部出现开裂现象,影响了工厂的正常运行。通过观察裂纹宏观形貌、裂纹金相特征、断口形貌以及断口腐蚀产物能谱分析及应力腐蚀敏感性试验等分析了开裂原因。结果表明:配料罐夹套开裂是由奥氏体不锈钢的应力腐蚀造成的;根据事故发生的原因,提出了返修和消除缺陷的方法。     

Boiler Stack Economizer Tube Failure

A metallurgical evaluation was performed to investigate the failure of a type 304 stainless steel tube from a boiler stack economizer. The tube had three distinct degradation mechanisms: pitting corrosion, chloride stress corrosion cracking, and fatigue fracture. The primary failure mechanism for the tube was fatigue fracture, but the other mechanisms may have eventually caused a tube failure in the absence of fatigue. This paper details the visual, SEM/EDS, and metallographic examinations used to determine that these failure mechanisms were each present in the same tube.     

On the mechanism of intergranular stress corrosion cracking of sensitized stainless steel in tetrathionate solution

Intergranular stress corrosion cracking (SCC) mechanism in sensitized stainless steel (Type 304) was investigated experimentally. A tetra-thionic potassium (K₂S₄O₆) chemical solution was used to mimic polythionic acid SCC which the most aggressive SCC type. During the SCC test, the steel specimen was subjected to three-point bending with constant strain at room temperature, and simultaneous monitoring of acoustic emission and corrosion potential were employed to monitor SCC initiation and progression. At the early stage, transient phenomenon of local anodic dissolution was observed. Upon initiation of SCC, passivation film fracture and dissolution of metal at specimen surface take place. Through microscopic observation of SCC tip, it was found that the SCC tip advanced along the grain boundary with further mechanical loading. This suggested that the stress component plays a significant role of SCC propagation, in addition to the effect of the localized metal dissolution along Cr-depleted grain boundaries.     

Effect of microstructures on SCC of steel: Field failure analysis case study and laboratory test result

Based on continuum mechanics approach, the first step on the modeling of the stress corrosion cracking (SCC) is to understand the morphology of the cracks. Later on, it is followed by other considerations such as the chemical interaction, and electron diffusion. This paper investigates the morphology of the SCC behavior of steels with the main purpose of developing the basis of computational modeling. It also aims to give the basic sense for practical engineers in the fields as well. It mainly focuses on the morphology of the SCC on three basic common microstructures of steel: austenite, ferrite–pearlite, and martensite. The objects were taken both from real field works and from accelerated laboratory tests. The field samples were extracted from actual failed parts at known operating conditions and lifetimes. The laboratory test was an immersion in sodium hydroxide solution. The experimental parameters were controlled in such away to simulate accelerated field failures with all three microstructures. The crack depth and behavior of the stress corrosion cracking (SCC) were then analyzed after immersion test and subsequently, the mechanism of stress corrosion cracking was studied with the focus on their morphological observations. The result shows that each microstructure produces unique shape that can be used as a basis for computational model creation. Field engineers can also take advantage of the results for materials selection.     

Investigation of stress corrosion cracking of low-alloy steel in water

For four low-alloy steels with a wide range of tensile strength, the dynamical processes of the nucleation and propagation of stress corrosion cracking (SCC) in water with various polarization conditions and in a 0.1 N K₂Cr₂O₇ solution were traced with an optical microscope. The results show that if the tensile strength of the steel is higher than a critical value that is different in different polarization conditions and K_IK_ISCC, the plastic zone in front of a loaded crack tip is enlarged with time i.e. the delayed plastic deformation occurs in all the environments used. The nucleation and propagation of SCR will proceed when this delayed plastic deformation develops to a critical condition. Neither anodic and cathodic polarization nor the inhibitor can change the feature of the delayed plasticity and the nucleation and propagation of SCC in water.In all the environments used, K_ISCC is increased and $\text{da}\text{dt}$ is decreased with the decrease in the strength of the steel. K_ISCC is increased and da/dt is decreased with the anodic polarization and the addition of the inhibitor, but the cathodic polarization has the opposite effect.     

Effect of alloying additions onK ISCC of ultrahigh strength NiSiCr steel

The effect of alloying additions viz. cobalt, molybdenum, cerium and a combination of cobalt and molybdenum, on theK _ISCC of NiSiCr steel in 3·5% NaCl aqueous solution was studied. Addition of cobalt to NiSiCr steel resulted in an increase in theK _ISCC whereas molybdenum addition decreased theK _ISCC. Cerium addition did not affect theK _ISCC while the combination of cobalt and molybdenum resulted in an increase in theK _ISCC although not as much as in the case of cobalt addition. The effect of alloying elements onK _ISCC could be attributed to their effect on the critical fracture stress and yield strength.     

Effect of hydrogen on stress corrosion cracking of copper

The effects of hydrogen on electrochemical behavior and susceptibility of stress corrosion cracking (SCC) of pure copper were studied. SCC susceptibility of pure copper in a 1 M NaNO₂ solution was increased by pre-charged hydrogen. The effect of hydrogen on the susceptibility is more obvious in the low stress region due to the longer fracture time, which resulted in a longer time for more hydrogen to diffuse toward the crack tip. Synergistic effects of hydrogen and stress on corrosion and SCC processes were discussed. The results showed that an interaction between stress and hydrogen at the crack tip could increase the anodic dissolution rate remarkably.