Corrosion fatigue behaviour of a 15Cr‐6Ni precipitation‐hardening stainless steel in different tempers

Systematic fatigue experiments, including both high‐cycle axial fatigue (S–N curves) and fatigue crack growth (FCG, da/dN–ΔK curves), were performed on a precipitation‐hardening martensitic stainless steel in laboratory air and 3.5 wt% NaCl solution. Specimens were prepared in three tempers, i.e. solution‐annealed (SA), peak‐aged (H900) and overaged (H1150) conditions, to characterize the effects of ageing treatment on the corrosion fatigue (CF) resistance. S–N results indicated that fatigue resistance in all three tempers was dramatically reduced by the aqueous sodium chloride environment. In addition, the smooth‐surface specimens in H900 temper exhibited longer CF lives than the H1150 ones, while those in SA condition stood in between. However, for precracked specimens, the H1150 temper provided superior corrosive FCG resistance than the other two tempers. Comparison of the S–N and FCG curves indicated that early growth of crack‐like defects and short cracks played the major role in determining the CF life for smooth surface. The differences in the CF strengths for the S–N specimens of the given three tempers were primarily due to their inherent differences in resistance to small crack growth, as they were in the air environment.     

Environmental effect on the fatigue performance of bare and oxide coated 7075-T6 alloy

Fatigue behaviors of bare and anodic oxide coated 7075-T6 alloy have been investigated in laboratory air and 3.5%NaCI solution environment by using smooth cylindrical specimens. Presence of corrosive attack during fatigue test drastically reduced fatigue performance of the alloy. The deleterious effect was observed to be pronounced at high-cycles fatigue region, where the fatigue strength of the bare specimen was lowered by a factor of 2.9. However, the oxide coated specimens having a thickness of 23 μm showed a modest reduction in fatigue strength. Corrosion fatigue (CF) strength of the bare specimens was predominantly controlled by pitting-induced crack nucleation. Examinations on the surfaces of the corrosion-fatigued and immersed test specimens revealed that cyclic loading stimulated corrosion pit formation during CF tests. Also, corrosion behaviors of both the coated and bare specimen shave been investigated by potentiodynamic test. Despite superior corrosion resistance of coated specimens, fatigue performance was adversely affected under the combined action of corrosion attack and cyclic loading.     

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.     

Effect of acetic acids on the corrosion crack growth of 3.5NiCrMoV steels in high temperature water: synergistic interaction between stress corrosion and corrosion fatigue

Stress Corrosion Cracking (SCC) tests (pH: 3 ~ 5) and Corrosion Fatigue (CF) tests (R = 0.2, 0.1 Hz) were conducted to evaluate the effect of acetic acid on the corrosion crack growth behavior in high temperature water at 150°C. Acetic acid significantly influenced the corrosion fatigue cracking behavior of turbine disc steels in high temperature water. The CF crack growth rates of turbine disc steels increase until the organic acid concentration reaches a critical saturation value (between pH 4 and pH 3) because of the crack tip sharpening. Below the critical value of pH, the CF crack growth rates decreases because of the crack tip blunting. The corrosion fatigue crack growth rate is accelerated by the interaction of the fatigue and the stress corrosion in the test environment. The synergistic interaction should be accounted for in the realistic prediction of the corrosion fatigue life of turbine steel (3.5NiCrMoV steels) in high temperature water of acetic acid solution. With the high temperature corrosion fatigue data obtained in this study, it is possible to assess the life of turbine components in high temperature and high pressure.     

Corrosion fatigue of austempered ductile iron

Corrosion fatigue (CF) behavior has been investigated for an austempered ductile iron (ADI) by conducting systematic fatigue tests at 20 Hz, including both high-cycle fatigue (HCF, S-N curves) and fatigue crack growth (FCG, da/dN-K curves), in air, lubrication oil and several aqueous environments. Results showed the HCF resistance of ADI was dramatically reduced by the given aqueous media, in particular, to a greater extent with a decrease in pH value. However, the given room-temperature aqueous solutions did not exert significantly detrimental effects on the Stage II crack growth compared with an atmospheric environment but an increase in solution temperature caused enhanced Stage II crack growth. Among the given variables of the bulk environment, pH had the greatest influence on HCF response while temperature had the most influence on the FCG of long cracks. In addition, SAE 10W40 lubrication oil provided an inert environment to remove the corrosive effect and enhance the CF resistance of ADI. The overall comparisons indicated the environmental effects would generate more influence on Stage I cracking than on Stage II cracking for the given ADI.     

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).     

Evaluation of stress corrosion resistance and corrosion fatigue fracture behavior of ultra-high-strength P/M Al–Zn–Mg alloy

Quasi-static tensile tests in air and slow strain rate tests (SSRTs) in a 3.5% NaCl solution were conducted in an ultra-high-strength P/M Al–Zn–Mg alloy fabricated through powder metallurgy. Attention is also paid to fatigue strength and fatigue crack growth behavior in laboratory air and in a 3.5% NaCl solution. The alloy has extremely high strength of about 800 MPa. However, elongation at break remains small, at about 1.3%. The final fracture occurs by a macroscopically flat crack normal to the tensile axis, with little reduction in area and little shear lip on the periphery of a smooth sample. However, it fails microscopically in a ductile manner, with dimples. Dimple size is less than 1 μm, because the grain size of the alloy is extremely small. Strengthening mechanisms operating in the alloy are: small grains, sufficient metastable η′ phase in a matrix, and intermetallic compound acting as a fiber reinforcement. The SSRT strength in a 3.5% NaCl solution decreases slightly at a very low strain rate, that is smaller than those observed in aluminum alloys sensitive to stress corrosion. This means that the crack initiation resistance to stress corrosion is superior. However, under cyclic loading, the corrosion fatigue strength becomes lower than that conducted in air, because pitting corrosion on a sample surface acts as a stress concentrator. Crack initiation site of quasi-static and fatigue failure of the alloy is at inclusions, and hence, it is essential to decrease inclusions in the alloy for the improvement of the mechanical properties. Fatigue crack resistance of the alloy is inferior to conventional Al–Zn–Mg alloys fabricated by ingot metallurgy, because the fatigue fracture toughness, or ductility, of the alloy is inferior to other Al alloys, and intergranular cracking promotes crack growth. However, no influence of 3.5% NaCl solution on corrosion fatigue crack growth is observed, although an investigation is required into whether stress corrosion crack growth occurs or not, and at the same time, and of corrosion fatigue crack growth behavior at lower stress intensity. The fracture surface and crack initiation sites are closely examined using a high-resolution field emission type scanning electron microscope, and the fracture mechanisms of the alloy are discussed.     

FATIGUE CRACK GROWTH RATES IN Al–Li ALLOY, 2090. INFLUENCE OF ORIENTATION, SHEET THICKNESS AND SPECIMEN GEOMETRY

.The fatigue crack growth (FCG) behaviour of the aluminium–lithium (Al–Li) alloy 2090-T84 has been investigated from a series of constant amplitude FCG tests. The influence of in-plane orientation (L–T, T–L, L–T + 45°) and sheet thickness (1.6 and 6  mm) on the FCG rates for the rolled product has been examined. In general, the T–L orientation possesses superior FCG resistance for both thicknesses and the 6  mm thick sheet material showed marginally improved FCG resistance when compared to the 1.6  mm thick material, for all orientations. Closure-corrected FCG data suggests that much of the difference between the L–T and T–L orientation for the 6  mm thick sheet arises from differences in crack closure levels. When comparing the crack closure levels for C(T) and M(T) specimens, a significant difference is shown as ΔK increases. Fatigue crack growth rates for ΔK less than 15  MPa m were significantly higher in the M(T) specimens compared to the C(T) specimens. Compared with other factors examined the influence of specimen geometry appears to be a dominant factor.     

Engineering property comparisons of 7050-T73651, 7010-T7651 and 7010-T73651 aluminium alloy plate

A comparison of some engineering properties of 7050-T73651, 7010-T7651 and 7010-T73651 plate has been made. The properties investigated were strength, stress corrosion resistance, fracture toughness and fatigue crack propagation resistance under flight simulation loading.

It was found that both 7050 and 7010 are high strength deep hardenable alloys with only minor differences in crack tolerance properties. The fracture toughness of both alloys is equivalent, while 7050 possesses slightly better resistances to stress corrosion cracking and fatigue crack propagation under flight simulation loading.     

Fatigue crack initiation and growth on a steel in the very high cycle regime with sea water corrosion

This paper is devoted to the effect of corrosion on the gigacycle fatigue strength of a martensitic-bainitic hot rolled steel used for manufacturing offshore mooring chains for petroleum platforms. Smooth specimens were tested under fully reversed tension between 10⁶ and 10¹⁰ cycles in three testing conditions and environments: (i) in air, (ii) in air after pre-corrosion and (iii) in air under real time artificial sea water flow. The fatigue strength at greater than 10⁸ cycles is reduced by a factor more than five compared with non-corroded specimens. Fatigue cracks initiate at corrosion pits due to pre-corrosion, if any, or pits resulting from corrosion in real time during the cyclic loading. It is shown that under sea water flow, the fatigue life in the gigacycle regime is mainly governed by the corrosion process. Furthermore, the calculation of the mode I stress intensity factor at hemispherical surface defects (pits) combined with the Paris-Hertzberg-Mc Clintock crack growth rate model shows that fatigue crack initiation regime represents most of the fatigue life.     

The influence of microstructure on the fatigue crack growth rate in marine steels in the Paris Region

This paper presents a study on the effect of microstructure on the fatigue crack growth (FCG) rate in advanced S355 marine steels in the Paris Region of the da/dN versus ΔK log–log plot. The environments of study were air and seawater (SW), under constant amplitude sinewave fatigue loading. Fundamentally, three phenomena (crack tip diversion, crack front bifurcation and metal crumb formation) were observed to influence the rate of FCG. These phenomena appear to be a function of the material microstructure, environment and crack tip loading conditions. The three factors retarded the crack growth by reducing or redistributing the effective driving force at the main active crack tip. A crack path containing extensively the three phenomena was observed to offer strong resistance to FCG. In SW, the degree of the electrochemical dissolution of the microplastic zone appears to be an additional primary factor influencing FCG in the steels.     

Corrosion fatigue performance of pre-split steel wires for high strength bridge cables

The first parametric investigation about corrosion fatigue (CF) behaviour of pre-split high-strength galvanized steel wires was conducted for perfecting the design and maintenance of modern bridge cable structures. Counting the cycle number to failure presents that lower fatigue endurance always correlates with higher stress amplitude, while decrease in load ratio and/or increase in cyclic load frequency significantly prolongs the CF life. Electron fractography indicates that the fatigue crack growth rate of the steel wires is lower in air with the presence of tyre tracks on the fracture surface, and faster in aggressive media resulting from anodic dissolution and crack opening displacement at the crack tips. Longer CF endurance of bridge cable steel wires can be expected through ideal thermo-mechanical treatment after the successive cold drawing, for a significant benefit on corrosion resistance and microstructure improvement.     

中碳高强度钢腐蚀疲劳研究

文中研究了35CrMo 及40CrNiMo 钢淬火不同温度回火时,在3.5%NaCl 盐雾介质中的腐蚀疲劳行为以及35CrMo 钢在3.5%NaCl 水溶液,0.1NHCl+3.5%NaCl 水溶液和空气中的腐蚀疲劳行为。结果表明,40CrNiMo 钢随回火温度升高,盐雾介质中腐蚀疲劳抗力增加,裂纹止裂倾向也增大。5.5Hz 时随介质的 pH 值降低,裂纹扩展加速。电镜断口观察表明,盐水介质腐蚀疲劳是以氢脆机制为主,盐雾介质疲劳是以阳极溶解机制为主。     

Very high cycle fatigue of a high strength steel under sea water corrosion: A strong corrosion and mechanical damage coupling

This paper is focused on the effect of sea water corrosion on the gigacycle fatigue strength of a martensitic–bainitic hot rolled steel R5 used for manufacturing off-shore mooring chains for petroleum platforms in the North Sea. Crack initiation fatigue tests in the regime of 10⁶ to 10¹⁰ cycles were carried out on smooth specimens under three different environment conditions: (i) without any corrosion (virgin state) in air, (ii) in air after pre-corrosion, and (iii) in-situ corrosion-fatigue under artificial sea water flow. A drastic effect of sea water corrosion was found: the median fatigue strength beyond 10⁸ cycles is divided by 5 compared to virgin state specimens. The crack initiation sites were corrosion pits caused by pre-corrosion or created during corrosion-fatigue under sea water flow. Furthermore some sub-surface and internal crack initiations were observed on specimens without any corrosion (virgin state). Crack propagation curves were obtained in mode I in air and under sea water flow. Calculation of the stress intensity factor at the tip of cracks emanating from hemispherical surface pits combined with the Paris–Hertzberg–Mc Clintock crack growth rate model showed that fatigue crack initiation period represents most of the fatigue life in the VHCF regime. Additional original experiments have shown physical evidences that the fatigue strength in the gigacycle regime under sea water flow is mainly governed by the corrosion process with a strong coupling between cyclic loading and corrosion.     

Fatigue behavior of AISI 347 stainless steel in various environments

The aim of this study is to investigate the influence of environmental factors, including pH, chloride ion, and pitting inhibitor, on the fatigue properties of AISI 347 stainless steel. Systematic fatigue tests, including both high-cycle fatigue (HCF, S-N curves) and fatigue crack growth (FCG, da/dN-K curves), have been conducted in air and several aqueous environments. Results showed the HCF strength was markedly reduced in an acid solution and in a chloride-containing solution, as compared to the air value. An addition of pitting inhibitor could restore the HCF strength in salt water back to the level in atmospheric air by preventing the formation of corrosion pits and decreasing the corrosion rate. However, the corresponding stage II FCG rates in all given environments were almost equivalent. These results indicated that the variation of chemistry in bulk environment exerted more influence on the fatigue crack nucleation than on the extension of long fatigue cracks.     

Crack propagation of pipeline steel exposed to a near-neutral pH environment under variable pressure fluctuations

This investigation attempts to understand the corrosion fatigue crack propagation behavior of pipeline steels exposed to near-neutral pH environments. The fatigue loading was designed to simulate the underload-type variable amplitude pressure fluctuations found during pipeline operation. The effects of amplitudes (R ratios) of underload and minor cycles were investigated. It has been found from this investigation that the crack growth rate is enhanced significantly through load interaction of the variable amplitude fatigue. The acceleration factor is found to be up to 2.7 and 5.3 for tests in air and in the near-neutral pH solution, respectively. The crack growth rate decreases with R ratios of underload and minor cycles for tests both in air and in near-neutral pH environments. The latter could enhance crack propagation by a factor of up to 11, as compared with the crack growth rate in air. The critical R ratio of minor cycles at which the minor cycles do not contribute to crack propagation through load interaction was determined to be as high as 0.982, which is much lower than the threshold determined by constant amplitude fatigue. This critical R ratio could be utilized to demarcate stress corrosion cracking and corrosion fatigue, and should be incorporated as one of the design principles for components/structures subjected to variable amplitude cyclic loading.     

压水堆控制棒驱动机构CANOPY密封焊缝服役环境与失效

综述了国际上对PWR核电站控制棒驱动机构密封焊缝(CSWs)结构、载荷、应力状态、失效分析的研究。研究认为CSWs的结构设计易造成高溶解氧服役环境,在CSWs死水区空腔存在微量氯离子污染的可能,导致较大应力腐蚀倾向。目前研究确认的失效模式有穿晶应力腐蚀(TGSCC)、沿晶应力腐蚀(IGSCC)、SCC+点蚀。计算出CSWs死水区空腔理论浓度可达230×10~(-6),分析认为溶解氧和低浓度水平的氯污染是可引起上述腐蚀的环境因素;CSWs应力腐蚀裂纹扩展模式与材料敏化、服役溶液环境、初始应变、应力状态等因素相关;点蚀可能成为应力腐蚀的起源也可发展成为独立的破坏形式。     

Effect of corrosive medium on fatigue crack growth behaviour and fracture in high martensite dual phase steel

Fatigue crack growth (FCG) behaviour in both near-threshold and higher stress intensity range (ΔK) in intercritically annealed dual-phase (DP) steel containing martensite between 32% and 76% in ferrite has been studied in 3·5% NaCl solution. It is shown that the amount of martensite content in dual phase steel has a significant effect on threshold (ΔK _th) values and FCG rates. Higher content of martensite in ferrite leads to higher threshold values and lower FCG rates. Further, ΔK _th is much higher in 3·5% NaCl solution as compared to that in laboratory air. Fractography studies reveal that in the near-threshold region, fracture surfaces are characterized mainly by intergranular cracking in corrosive (3·5% NaCl solution) environment. Higher threshold values in 3·5% NaCl solution is attributed to the higher crack closure induced by rougher fracture surface and by the strong wedge effects of corrosion products.     

Mechanisms for corrosion fatigue crack propagation

ABSTRACT The corrosion fatigue crack growth (FCG) behaviour, the effect of applied potential on corrosion FCG rates, and the fracture surfaces were studied for high‐strength low‐alloy steels, titanium alloys, and magnesium alloys. During investigation of the effect of applied potential on corrosion FCG rates, polarization was switched on for a time period in which it was possible to register the change in the crack growth rate corresponding to the open‐circuit potential and to measure the crack growth rate under polarization. Due to the higher resolution of the crack extension measurement technique, the time rarely exceeded 300 s. This approach made possible the observation of a non‐single mode effect of cathodic polarization on corrosion FCG rates. Cathodic polarization accelerated crack growth when the maximum stress intensity (K_max) exceeded a certain well‐defined critical value characteristic for a given material‐solution combination. When K_max was lower than the critical value, the same cathodic polarization, with all other conditions (specimen, solution, pH, loading frequency, stress ratio, temperature, etc.) being equal, retarded or had no influence on crack growth. The results and fractographic observations suggested that the acceleration in crack growth under cathodic polarization was due to hydrogen‐induced cracking (HIC). Therefore, critical values of K_max, as well as the stress intensity range (ΔK) were regarded as corresponding to the onset of corrosion FCG according to the HIC mechanism and designated as K_HIC and ΔK_HIC. HIC was the main mechanism of corrosion FCG at K_max > K_HIC (ΔK > ΔK_HIC). For most of the material‐solution combinations investigated, stress‐assisted dissolution played a dominant role in the corrosion fatigue crack propagation at K_max < K_HIC (ΔK < ΔK_HIC).     

The failure of an armour faced conveyor connector

The combined use of modern metallurgical techniques for fracture examination, laboratory test data and fracture mechanics calculations allows metallurgical failures to be examined in a quantitative manner. Complex load histories and environments can result in more than one sub-critical cracking mechanism occurring in a component. Quantitative understanding of the rate determining cracking process is a necessary prerequisite to rectifying the problem. The following case study describes a connector from an armour faced conveyor which failed in service. The failure investigation involved fractography, stress analysis, material property evaluation and fracture mechanics calculations. Fractographic evidence indicated a stress corrosion failure mechanism. Calculations of critical crack sizes showed that stress corrosion cracking alone could not account for the fracture. It was concluded that the failure was due to a sequence of three cracking processes which preceded unstable ductile fracture. Firstly, frictional heating caused rubbing or quench cracks typically 0.5–1 mm deep. Secondly, corrosion fatigue cracks grew several millimetres allowing the third fracture process, stress corrosion cracking, (SCC) to initiate and grow. In the situation described here, this process was much faster than corrosion fatigue. The influence of defect size due to rubbing cracks and the influence of K_ISCC have been compared with the corrosion fatigue life of the component. An increase in K_ISCC and hence critical defect size for SCC has been shown to increase the corrosion fatigue life of the component by a large factor. A change in design would also alleviate the problem of SCC by reducing the static stress, which is the driving force for SCC.