CORROSION FATIGUE OF AN HSLA STEEL

Abstract— Single-pitted specimens of an HSLA steel, were tested in laboratory air and in 1 M NaCl solution to study the influence of a corrosive environment on its fatigue life.
The growth of fatigue cracks and the partitioning of the fatigue life into fatigue crack initiation and fatigue crack propagation were studied by photographing the pit and the cracks developing on it periodically during testing. Non-propagating or dormant surface cracks were not observed in this study. Fractography using SEM showed the locations of fatigue crack initiation. The mechanisms of corrosion fatigue were studied by performing tests in 1 M NaCl at different test frequencies. Corrosion pits proved to be crack initiation sites. Hydrogen embrittlement was found to be unimportant in the corrosion fatigue of HSLA steel in this study. The 1 M NaCl corrosive environment appeared to reduce the fatigue life of this material by a dissolution mechanism. The effect of pit depth was studied by testing specimens having various pit depths. An effect of pit size was apparent. Fatigue life decreased with increasing pit depth. Pit depth, rather than the ratio of pit depth to pit diameter, influenced fatigue behaviour. A non-damaging pit depth was found.     

Fatigue limit evaluation of various martensitic stainless steels with new robust thermographic data analysis

Thermography represents an important tool to study fatigue behaviour of materials.In this work, the fatigue limit of martensitic and precipitation hardening stainless steels has been determined with thermographic methods. Despite their use in corrosive and cryogenic environments, there is a data lack in literature concerning the study of fatigue behaviour.The peculiarity of these materials is the brittle behaviour: therefore, during fatigue tests the characteristic small deformations determine small changes of temperature. Thus, to properly determine the fatigue limit of aforementioned stainless steels, a more accurate setup is necessary in order to correctly detect surface temperature of specimens due to dissipation heat sources.In literature, different procedures have already been proposed to evaluate the fatigue limit from thermal data but very few works lead to an early detection of dissipation process which can obtain a further reduction of overall testing time. The aim of the paper is to propose a new robust thermal data analysis procedure for estimating fatigue limit of stainless steels in automatable way.     

Untersuchungen zur Schwingungsrißkorrosion von unlegierten Stählen in wäßrigen Medien

Although a lot of work is done up to now in investigating corrosion fatigue many questions about the corrosion fatigue process of steel are still open. This applies for instance to the quantitative proportion at the complete damaging process which the corrosion strain component will assume and it applies moreover to special details of the crack initiation stage. The damaging process of corrosion fatigue of steel in the active state is generally supposed to proceed in the following three stages: “formation of pits”, “crack initiation” and “crack propagation”. The objective of the present study was at first to get by statistical means a quantitative assumption about the influence of the corrosive medium and secondly to get information about the contribution of pitting to the crack initiation process. Tension-compression experiments were conducted with specimen from steels Ck 15 and Ck 35 in the normalized state both without a corrosive influence (inert spindle oil) and with corrosive influence (desalted water, 3% NaCl-solution). It was found that the damaging process in its principal course is not affected by the kind of the corrosive medium but is hardly influenced by the extent of the load-amplitude at a given load-frequency.     

Schwingungsrißkorrosionsverhalten des Duplexstahles X 2 CrNiMoN 22 5 3 unter definierten Wärmeübergangsbedingungen

Corrosion fatigue behaviour of duplex stainless steel X 2 CrNiMoN 22 5 3 under heat transfer conditions The corrosion behaviour of metallic components is not only affected by the temperature of the corrosive environment but also by the heat transfer conditions between the heated material and the cooling agent. Therefore the corrosion fatigue behaviour of the austenitic-ferritic stainless steel X 2 CrNiMoN 22 5 3 (german material-number 1.4462) in 3% NaCl-solution is investigated for isothermal conditions and three different heat transfer conditions. The specimens are tested under cyclic tension load (R = s?_u/s?_o = 0) with a frequency off =25 Hz up to N_Grenz 10⁷. The isothermal fatigue strengths are 380 N/mm² for room temperature and 340 N/mm² for a temperature of 70°C. For heat transfer conditions between the sinusoidal loaded specimens and the corrosive agent a new developed testing equipment is presented. The corrosion fatigue strength for a heat flux of 45 KW/m² reaches a value of 410 N/mm², while the improvement relative to the isothermal room temperature strength is lower for higher heat flux values (100 and 150 KW/m²). The better corrosion fatigue behaviour for heat transfer conditions bases on the favourable conditions for the formation of the passive layer. The thickness of the layer is nearly twice as high as for isothermal room temperature corrosion and therefore the crack initiation is delayed. For higher values of heat flux local corrosion attack is found. With that the positive effect on corrosion fatigue strength is diminished.     

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”.     

The influence of surrounding environment on the fatigue properties for a high pressure die cast AlSi9Cu3 alloy

The influence of mechanical strength for aluminium castings is often correlated in the literature to the occurrence of cast defects like porosity. However, because aluminium castings in the automotive industry are often used without surface treatment such as painting etc., the influence of corrosion and its effect on fatigue properties are required. Basically a surrounding environment can affect fatigue strength either by enhancing initiation or by increasing fatigue crack propagation properties. In this study, the influence of pre-exposure prior to fatigue testing has been evaluated. This implies that environmental enhancement of fatigue initiation due to corrosion has been studied. Two different environments [seawater acidified test (SWAAT) and Volvo Indoor Corrosion Test (VICT)] often employed for assessment of corrosive properties in the automotive industry have been used for pre-exposure of specimens. Based on experimental results, it is shown that fatigue strength is reduced by approximately 50% for specimens pre-exposed to SWAAT environment, while insignificant influence was found for specimens pre-exposed to VICT environment. The degradation of fatigue strength was found to be due to localized corrosive attacks. Propagation of these corrosive attacks takes place preferably in the eutectic phase and especially at the borderline between primary aluminium dendrites and the eutectic phase.     

Corrosion fatigue behavior of a heat-treated carbon steel and its statistical characteristics

The investigation of fatigue damage in corrosive environments is an important problem, because such environments reduce fatigue strength far below the typical fatigue strength determined in air. In this study, rotating bending fatigue tests of plain specimens in NaCl solution were carried out using a heat-treated 0.45% carbon steel, in order to clarify the physical background of corrosion fatigue damage. The emphasis is to perform the successive observations by the plastic replica method. The results show that corrosion pits are generated at the early stages of cycling, then most of them grow with further cycling until a crack is initiated from each corrosion pit. The initiation of corrosion pits from slip bands is only observed in the case when the stress range is relatively large, in the range of stress under which slip bands are produced in air. After initiation of a crack, a crack propogates by accompanying frequent interaction and coalescence with other cracks. The growth rate of an especially small crack in NaCl solution is larger than that in air. However, the growth rate of a comparatively large crack is smaller in NaCl solution than in air. Moreover, the statistical characteristics of corrosion fatigue behavior were investigated by exhibiting the distributions of crack initiation life and crack length.     

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。     

Fatigue performance of friction stir welded Al2024 alloy in a different corrosive environment

The study examines the corrosion fatigue behavior of friction stir welded Al2024 alloy in the corrosive medium. The fatigue tests are conducted at a stress ratio of -1 in the different corrosive medium. The decrease in the fatigue life of welded joints in the corrosive environment is attributed to an increase in the crack initiation susceptibility in the presence of corrosive media. The fractured surfaces are investigated using the scanning electron microscopy (SEM). The formation of the corrosive compounds was studied using x-ray diffractometry.     

Monitoring of corrosion‐fatigue degradation of grade R4 steel using an electrochemical‐mechanical combined approach

The study deals with degradation and failure of mooring chains by the combined effect of corrosion and mechanical cyclic stresses on steel. Monitoring of the corrosion‐fatigue damage was carried out using electrochemical assessment during the application of the cyclic mechanical loading on grade R4 martensitic steel specimens, immersed in artificial seawater. The spontaneous potential of the steel specimens and Electrochemical Impedance Spectroscopy measurements were performed to assess the corrosion degradation. Tests conducted under fully reversed stress‐controlled loading have shown earlier failure caused by the corrosive environment when compared with the tests performed in ambient air. The corrosion potential response was used to identify the crack initiation and propagation stages.     

Delamination fracture of prestressing steel: An engineering approach

This paper provides a composites engineering approach to explain the stress corrosion behaviour of high-strength prestressing steel wires. To this end, two eutectoid steels in the form of hot rolled bar and cold drawn wire were subjected to slow strain rate tests in aqueous environments in corrosive conditions corresponding to localized anodic dissolution and hydrogen assisted cracking. While a tensile crack in the hot rolled bar always propagates in mode I, in the cold drawn wire an initially mode I crack deviates significantly from its normal mode I growth plane and approaches the wire axis or cold drawing direction, thus producing a mixed mode propagation. In hydrogen assisted cracking the deviation happens just after the fatigue pre-crack, whereas in localized anodic dissolution the material is able to undergo mode I cracking before the deflection takes place. Therefore, a different behaviour is observed in both steels and even in the same steel under distinct environmental conditions. An explanation of such behaviour can be found in the pearlitic microstructure of the steels. This microstructural arrangement is randomly-oriented in the case of the hot rolled bar and markedly oriented under the wire axis direction in the case of the cold drawn wire. Thus both materials behave as composites at the microstructural level and their plated structure (oriented or not) would explain the different time-dependent behaviour in a corrosive environment.     

Corrosion fatigue short crack growth behaviour in a high strength steel

The influence of an aggressive environment (0.6 M, aerated NaCl solution) on short fatigue crack initiation and growth behaviour has been studied. The study involved three major test series, namely: air fatigue, corrosion fatigue, and intermittent air fatigue/corrosion fatigue. The above tests carried out under fully reversed torsional loading conditions at a frequency of 5 Hz, showed that it was the non-metallic inclusions which took part in crack initiation resulting from debonding at metal matrix/inclusion interface and pitting of inclusions in both air and corrosove environments, respectively. Short fatigue crack growth results in these two environments obtained by using plastic replication technique, indicated a large effect of microstructure i.e. prior austenite grain boundaries. The stage/stages at which the environmental contribution was dominant has been discussed by considering the results achieved from intermittent tests. However, the mechanisms involved in corrosion fatigue short crack growth have also been described in the light of results obtained from futher investigations carried out by conducting corrosion fatigue tests under applied cathodic potential conditions and tests on hydrogen pre-charged specimens under air fatigue and uniaxial tension conditions.     

Temperature‐dependent characteristics of DH36 steel fatigue crack propagation

DH36 steel is a widely used material in marine engineering. The fatigue crack propagation rates of DH36 steel at low temperatures have a crucial influence on the fatigue strength of structures operating in polar environments. The objective of this paper is to investigate the fatigue crack propagation behaviour of DH36 steel at low temperatures (?60°C to 20°C) by carrying out tensile tests and fatigue crack propagation tests of DH36 steel, in order to obtain the fatigue crack propagation behaviour of DH36 steel. The influence of the elastic modulus on the crack length measured by the compliance method is considered. On the basis of the Paris law, the crack propagation rate at different temperatures is investigated. The results and the observed failure modes indicated that fatigue ductile‐to‐brittle transition (FDBT) occurred as the temperature was lowered.     

Small fatigue crack initiation and growth behavior of high-strength low alloy steel in various environments

Fatigue tests under rotating bending and reversed torsion were carried out in air, distilled water and 3% saltwater, using smooth specimens of high-strength low alloy steel (Cr-Mo steel). The initiation and growth behavior of small fatigue cracks in each environment were evaluated based on detailed observations, and the effects of corrosive environment were also discussed. The fatigue strength decreased with increasing aggressiveness of test environment. The decreases in corrosive environment were due to earlier fatigue crack initiation. From the observed locations at which small fatigue cracks began, it was considered that the crack initiation was primarily governed by hydrogen embrittlement in distilled water and also affected by corrosive dissolution in 3% saltwater. The validity of the application of linear fracture mechanics for small fatigue cracks was established. The growth rates of small fatigue cracks were higher than for large through cracks, and not accelerated by the corrosive environment. Moreover, fatigue life in the corrosive environment was estimated by using the crack growth characteristics in air.     

Corrosion fatigue life prediction of a steel shaft material in seawater

Corrosion fatigue behaviour of a medium strength structural material was studied in air and in 3.5% NaCl solution. Emphasis was placed on the study of corrosion pit formation and the development of cracks from pits. Pitting and crack propagation were quantified throughout the fatigue loading thereby allowing a model to be developed that included the stages of pitting and the pit-to-crack transition in order to predict the fatigue life. The results showed that a large number of corrosion pits with small size form at a very early stage in the fatigue lifetime. The number of pits and subsequent cracks was found to be higher at higher stress levels leading to multiple crack development and coalescence. When compared to air, fatigue life in a corrosive environment was significantly reduced at low stress levels due to pitting damage, indicating a dominant role of corrosion over that of mechanical effects. The corrosion fatigue model proposed shows good agreement with the experimental test data at lower stress levels but predicts more conservative lifetimes as the stress increases. Kitagawa–Takahashi diagram was produced for both test environments where it is indicated that the fatigue limit can be eliminated in a corrosive environment.     

Effect of laser peening on fatigue performance in 300M steel

This study investigated and quantified the ability of laser peening to extend fatigue life and mitigate common fatigue crack nucleation mechanisms including foreign object damage and stress corrosion cracking in high strength, 300M steel. Residual stress was measured and fatigue tests were completed for as‐machined, shot peened and laser peened coupons tested under a variety of conditions including in a corrosive environment and with simulated foreign object damage. The results are compared to available reference data studying the application of another emerging surface treatment, low plasticity burnishing, to the same material (300M steel) and in similar coupons. Laser peening resulted in large stress‐life improvements in each condition, with a 54% increase in fatigue strength in an air environment for laser peened coupons at a lifetime of one million cycles. Additional fatigue testing in the 300M program showed that laser peening mitigated fatigue strength losses and improved the lifetimes (relative to as‐machined, air environment) for coupons subject to either a corrosive environment or simulated FOD. Performance of laser peened coupons under all conditions proved to be better than reference data for coupons treated with shot peening or low plasticity burnishing.     

Effect of surface roughness on low-cycle fatigue life of Cr---Mo---V steel at 550 °C

Low-cycle fatigue tests have been carried out on Cr---Mo---V steel specimens with two different modes of surface roughness at 550 °C. The fatigue life of specimens with a rough surface is approximately half of that of specimens with a smooth surface over the plastic strain range investigated. From interrupted tests, it is seen that low-cycle fatigue behaviour is largely influenced by the crack initiation process even for a high strain range and the decrease of fatigue life in specimens with a rough surface is mainly due to a reduction in the number of cycles for crack initiation.     

Corrosion fatigue testing: the combined effect of stress and high temperature corrosion

Abstract

A corrosive environment can have a detrimental effect on the fatigue life of a material due to a change in failure mechanism. Attempts have been made to replicate this change on nickel-base superalloy CMSX-4 cast in the <001> orientation. Fatigue testing in air, of this material typically produces a fracture on an angle of approximately 55° which is consistent with the fracture having propagated on a {111} slip plane. The aim of the research was to fatigue test in a corrosive environment with the purpose of producing a crack/fracture which deviated from the typical angle and thus confirm that the corrosive environment had affected the fatigue mechanism. It was concluded that the change in mechanism to high temperature corrosion fatigue was associated with a reduced load application rate together with precorroding the test specimens to trigger the initiation of the corrosion fatigue mechanism.     

Recent developments in ultrasonic fatigue

The recently increased interest in very high cycle fatigue properties of materials has led to extended use and further development of the ultrasonic fatigue testing technique. Specimens are stimulated to resonance vibrations at ultrasonic frequency, where the high frequency allows collecting lifetime data of up to 10¹⁰ cycles and measuring crack propagation rates down to 10^?12 m per cycle within reasonable testing times. New capabilities and methods of ultrasonic testing and outstanding results obtained since the year 1999 are reviewed. Ultrasonic tests at load ratios other than R = ?1, variable amplitude tests, cyclic torsion tests and methods for in situ observation of fatigue damage are described. Advances in testing at very high temperatures or in corrosive environments and experiments with other than bulk metallic materials are summarized. Fundamental studies with copper and duplex steel became possible and allowed new insights into the process of very high cycle fatigue damage. Higher cyclic strength of mild steels measured at ultrasonic frequency because of plastic strain rate effects are described. High‐strength steels and high‐alloy steels are less prone to frequency influences. Environmental effects that can lead to prolonged lifetimes in some aluminium alloys and possible frequency effects in titanium and nickel and their alloys are reviewed.     

Micromechanical Modelling of Time-Dependent Stress-Corrosion Behaviour of High-Strength Steel

This paper provides micromechanical bases to explain the time-dependent stress corrosion behaviour of high-strength prestressing steel wires. To this end, two eutectoid steels in the form of hot rolled bar and cold drawn wire were subjected to slow strain rate tests in aqueous environments in corrosive conditions corresponding to localized anodic dissolution and hydrogen assisted cracking. While a tensile crack in the hot rolled bar always propagates in mode I, in the cold drawn wire an initially mode I crack deviates significantly from its normal mode I growth plane and approaches the wire axis or cold drawing direction, thus producing a mixed mode propagation. In hydrogen assisted cracking the deviation happens just after the fatigue precrack, whereas in localized anodic dissolution the material is able to undergo mode I cracking before the deflection takes place. Therefore, a different time-dependent behaviour is observed in both steels and even in the same steel in distinct environmental conditions. An explanation of such behaviour can be found in the pearlitic microstructure of the steels. This microstructural arrangement is randomly oriented in the case of the hot rolled bar and markedly oriented in the wire axis direction in the case of the cold drawn wire. Thus both materials behave as composites at the microstructural level and their plated structure (oriented or not) would explain the different time-dependent behaviour in a corrosive environment.