CORROSION FATIGUE OF STEEL IN VARIOUS AQUEOUS ENVIRONMENTS

Abstract— Corrosion fatigue of three steels namely: AISI 1018, AISI 4340 and 17–7 PH stainless steel are investigated in various environments. Testing mediums chosen are natural seawater, polluted sea water from an industrial site, drinking tap water produced by desalination process, 4% sodium chloride tap water solution and a laboratory air environment. Results obtained for the three steels are consistent in pointing out that pollutants in seawater do not contribute to an acceleration of corrosion fatigue. The aggressiveness of drinking water is attributed to its chemical instability, low alkalinity and bicarbonate content.     

ULTRASONIC FATIGUE OF AN AUSTENITIC STAINLESS STEEL

Abstract This investigation into the fatigue of a commercial austenitic stainless steel (UHB 3 MM) at ultrasonic frequencies (20 and 25 kHz) has been primarily concerned with the initial stages of the fatigue process prior to fatigue crack initiation. Results have been obtained using equipment in which it is possible to superimpose ultrasonic push pull stresses upon a static tensile stress. Microstructural examination of the specimen surface and of the internal deformation rearrangements by means of electron microscopy, has revealed fatigue damage which is comparable to that occurring during conventional, low frequency, fatigue. Thus, in contrast to other investigators, we have obtained evidence of dislocation dipole and deformation band formation together with regularly spaced slip band markings from tests at low amplitudes. Higher amplitude cycling has indicated the formation of dislocation tangles and cells.
The results do not suggest a significant change in fatigue mechanism upon changing the testing frequency into the ultrasonic regime.     

THE EFFECT OF CATHODIC PROTECTION POTENTIAL ON CORROSION FATIGUE CRACK GROWTH RATE OF AN OFFSHORE STRUCTURAL STEEL

Abstract— Duplicate tests have been performed to determine the effect of cathodic protection potential on corrosion fatigue crack growth rate of a modern offshore structural steel, produced by thermo-mechanically controlled processes. The experiments were carried out using compact tension specimens exposed to artificial seawater at 10°C and subjected to constant amplitude loading at 0.35 Hz. Reproducible results showed that the merits of cathodic protection potentials are strongly dependent on stress intensity ratio R and stress intensity range Δ K . It appears that a specific value of cathodic potential may not give comprehensive protection against corrosion fatigue within the spectrum of variable amplitude loading experienced in service. Fractography showed the initiation of secondary cracks on the fracture surface to be associated with the dissolution of calcium sulphide inclusions, regardless of imposed cathodic potential.     

抽油杆用钢的腐蚀疲劳性能分析

对三种抽油杆用钢试样在清水介质中进行了旋转弯曲疲劳试验，测定了这三种钢的腐蚀疲劳极限，绘出了P-S-N曲线，并进行了综合性的分析。测试结果表明，在清水介质条件下它们的疲劳强度值相近且偏低。     

SHORT FATIGUE CRACK GROWTH BEHAVIOUR OF A LOW CARBON STEEL UNDER CORROSION FATIGUE CONDITIONS

The effect of an aqueous chloride environment upon the development and growth of short fatigue cracks from smooth specimen surfaces has been studied under fully reversed torsional fatigue loading conditions. Crack initiation and growth has been monitored using a plastic replication technique enabling a full history of cracking characteristics to be recorded. Corrosion fatigue conditions were achieved by complete immersion in a 0.6 M NaCl solution, of nominal pH value 6.0, with specimens corroding at the free corrosion potential. Variations to these conditions were obtained by the addition of concentrated hydrochloric acid enabling test solution pH values to be altered, typically pH values of 3.5 and 2.0 were obtained. Further information regarding the effects of the environment on the early stages of crack development were obtained by conducting two stage alternate immersion type testing conditions. Evaluation of these effects through previously established Elastic-Plastic Fracture Mechanics models shows that the environment plays a major role during the early stages of microstructure-dominated crack growth particularly as cracks approach major barriers to propagation and at decreasing levels of applied shear stress.     

THE AIR FATIGUE AND CORROSION FATIGUE OF A 13% Cr TURBINE BLADE STEEL

Abstract— Reasons for data scatter from fatigue tests on a high strength 13% Cr steel are discussed and data is presented for corrosion fatigue of this steel in a condensing steam environment. Rotating bend tests were performed at 50 and 3 Hz and tension-tension fatigue tests at 0.016 Hz and R =0.1. Some specimens were periodically inspected to identify sites of fatigue crack initiation. The role of inclusions in initiating fatigue cracks was investigated and it was found that the population of larger inclusions in the steel matched that at fatigue crack initiation sites. Comparison of fatigue lives from polished and abraded specimens indicated that there is little point in producing highly polished surfaces for this steel to try and improve fatigue life unless the inclusion content can be reduced in size and number. Corrosion fatigue data is presented indicating the magnitude of the reduction of fatigue life caused by condensing pure steam and condensing chlorinated steam.     

FRETTING FATIGUE OF AN AUSTENITIC STAINLESS STEEL IN SEAWATER

Abstract— Fretting fatigue tests of an austenitic stainless steel used for a propeller tail shaft were carried out in seawater and in air. In seawater, fretting significantly reduced the fatigue strength, however, the fretting fatigue lives at higher levels of stress were longer than those in air. The tangential force coefficient (defined as the ratio of the frictional force amplitude and the contact load) in seawater was much lower than that in air and varied in the range from 0.3 to 0.5 during the fretting fatigue tests. The lower tangential force coefficient in seawater seems to be the main reason for the longer fretting fatigue life in seawater. The prediction of fretting fatigue life was made on the basis of elastic-plastic fracture mechanics, where the frictional force between the specimen and the contact pad was taken into consideration. The predicted fatigue lives agreed well with the experimental results in both air and seawater.     

CRACK INITIATION AND PROPAGATION BEHAVIOUR OF A HEAT-TREATED CARBON STEEL IN CORROSION FATIGUE

Abstract— When estimating fatigue damage quantitatively it is important to clarify its physical basis. In this study, rotating bending fatigue tests of a heat-treated 0.45% carbon steel were carried out in 3% NaCl solution, in order to clarify the physical basis of corrosion fatigue damage from successive observations of plastic replicas. The results show that corrosion pits are generated during 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 observed only in the case when the stress range is relatively large, and in the range of stress for which slip bands are produced in air. After initiation of a crack, the crack propagates by frequent interactions 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.     

THE EFFECT OF CATHODIC POLARIZATION ON CORROSION FATIGUE OF A HIGH STRENGTH STEEL IN SALT WATER

Abstract— Corrosion fatigue crack growth rates in high strength steel are often increased when a large cathodic polarization is applied. The corrosion fatigue mechanism in this case is generally considered to be due to hydrogen embrittlement. In the present study the crack growth process was carefully monitored by taking replicas from initially smooth specimens of a high strength steel under fully reversed push-pull loading while: (1) exposed to laboratory air, (2) immersed in a 0.6 M sodium chloride (NaCl) solution at open circuit potential (OCP) and (3) with an applied cathodic potential of —1250 mV (SCE). It is shown that the effect of cathodic polarization is dependent on the applied stress level and the nature of the cracking process, which in turn, is related to the sue of the crack. For stress levels at or below the in-air fatigue limit, failure did not occur for cathodically polarised specimens despite the number of loading cycles being 10 times that of the lifetime of identical tests in solution at the open circuit potential. At stress levels above the in-air fatigue limit the reduction in fatigue endurance caused by the presence of the corrosive environment can be partially recovered through cathodic polarization. The role of non-metallic inclusions in the cracking process under various exposure conditions is discussed, and a cracking mechanism is proposed.     

AN UNDERSTANDING OF FATIGUE THRESHOLDS THROUGH THE INFLUENCE OF NON-METALLIC INCLUSIONS IN STEEL

Abstract —In recent years the low fatigue growth rate regime has been the subject of extensive study revealing its complex and often seemingly inconsistent nature. The value of alternating stress intensity at threshold ΔK_Th) is sensitive to a range of variables which include mean stress, stress history, monotonie and cyclic yield strength, grain size and environment. A degree of coherency may be achieved by interpreting the data in terms of a crack closure mechanism which is a function of surface roughness. Experiments, in which the surface roughness (φ) has been altered by the introduction of non-metallic inclusions, give insight into the significance of crack closure in this regime.     

SMALL FATIGUE CRACKS IN AN AUSTENITIC STAINLESS STEEL

The present study concerns nucleation and growth of small surface cracks during the low-cycle fatigue of a nitrogen-containing austenitic stainless steel. Metallographic replicas as well as longitudinal sectioning were used to record the developing crack pattern on the specimen surface. The influence of grain size and nitrogen content is considered. Small surface cracks are observed after about 10% of the fatigue life. The nucleation of cracks continues until about half of the lifetime, when the crack density saturates. This saturation phenomenon is related to the local unloading effect of growing cracks.
The mean crack length increases continuously as a power-law until specimen failure. However, small grains and a low nitrogen content amplify the effect of crack–grain boundary interactions resulting in an intermediate retardation in growth.
At high nitrogen contents, the crack growth characteristics are very much related to the slip bands formed. This results in a more simultaneous growth of cracks, a more jagged feature of the cracks introducing a higher roughness-induced crack closure effect, and, consequently, better fatigue properties.     

THE CRITERION OF WELDING COLD CRACKING OF HSLA STEEL

本文通过PRRC 试验,研究了焊接接头防止冷裂纹的临界应力,局部预热附加热应力与预热和后热规范的关系,提出了国产低合金高强钢焊接接头的冷裂敏感系数判据,然后在实际工程中对所建立的判据进行了验证与应用。     

EFFECT OF ENVIRONMENT ON THE BEHAVIOR OF FATIGUE CRACK GROWTH IN AN AS-ROLLED DUAL-PHASE STEEL

本文研究了一种热轧双相钢在空气和3.5%NaCl 溶液中的疲劳裂纹扩展行为,这种双相钢在空气中具有较好的疲劳裂纹扩展阻力。在3.5%NaCl 溶液中的门槛值比空气中高,这是裂纹内腐蚀产物的楔子作用所致。随着应力比的增加,门槛值降低,并符合应力比对门槛值影响的竞争模型。在较高△K 下,扩展速率符合Paris 公式。在3.5%NaCl 溶液中的腐蚀为活性腐蚀。     

FATIGUE LIFE DISTRIBUTION AND GROWTH OF CORROSION PITS IN A MEDIUM CARBON STEEL IN 3%NaCl SOLUTION

Abstract— Statistical fatigue tests have been conducted on a structural medium carbon steel, S45C, in room air and in 3%NaCl solution, using five cantilever-type rotary bending fatigue testing machines which were specially manufactured for the purpose of the present study. Fatigue life distribution was examined at three and five stress levels in air and in 3%NaCl solution, respectively, and twenty specimens were allocated to each stress level. In room air, it was found that fatigue life distributions followed the three-parameter Weibull distribution, which were closely related to fracture morphology. In 3%NaCl solution, they also followed the Weibull distribution, but the scatter in fatigue life was smaller in comparison to that in air. It is suggested that the decrease in the scatter of fatigue life may be attributed to a smaller fraction of crack initiation life in 3%NaCl solution. The growth of corrosion pits was investigated using a laser microscope. The distribution of corrosion pit depths followed the log-normal distribution, and the corrosion pit depths increased with increasing time or the number of cycles. It was found that the growth of corrosion pits was accelerated by stress cycling and the depths increased with increasing stress level. Based on these results, a growth law of corrosion pits, including the effect of stress cycling, is proposed.     

FATIGUE BEHAVIOR OF A RAIL STEEL

The fatigue behavior of a hot-rolled, control-cooled, plain carbon eutectoid rail steel has been characterized. The data include monotonic and cyclic stress-strain curves, low cycle fatigue data and near-threshold fatigue crack growth rate behavior in air and in vacuo. The effects of environment and mean stress on the near-threshold fatigue crack growth rates of rail steel are significant. At a low stress ratio (R), ΔK_o is lower in vacuum (7 MPa √m) than in moist air (10 MPa √m). At high R, ΔK_o is higher in vacuum (6 MPa √m) than in air (4 MPa √m). The beneficial effect of moist air on FCGR at low ΔK and low R is attributed to an increase in closure due to fracture surface roughness and oxide film.     

CORROSION FATIGUE FRACTURE-MODE MAPS OF LOW ALLOY STEELS

The fatigue fracture features of low alloy steels in aqueous environments are investigated. It is revealed that microstructures, compositions and mechanical variables will affect the fracture mode. A series of fracture-mode maps of structural steels are established and a corrosion-fatigue-mechanism map of 15 Mn dual-phase steel is developed. The importance of fractography and fracture-mode is emphasized.     

A FATIGUE-CRACK-GROWTH-BASED ANALYSIS OF TWO-STEP CORROSION FATIGUE TESTS

Abstract A method for the analysis of two-step fatigue level sequences is proposed and compared with experimental results. Two-step loading tests of the aluminum alloy 2017-T4 in 3% sodium chloride solution have been carried out in conjunction with a replica technique used to monitor the growth of fatigue cracks. Fatigue cracks were nucleated at corrosion pits 10–15 μm in size, and crack growth rather than crack initiation was found to take up the major portion of the fatigue lifetime in these tests. The results could therefore be analyzed on the basis of the following constitutive relation for fatigue crack growth. da/dN=A(ΔK_eff-ΔK_effth)² This analysis was simplified since the influence of transients in the crack growth rate induced upon change in load level was found to be minimal. The approach provides a rational basis for dealing with load-sequence effects.     

EFFECT OF WAVEFORM ON CORROSION FATIGUE CRACK GROWTH

Abstract A study was made on the effects of stress rise time T ₁, maximum stress holding time T ₂, stress decreasing time T ₃ and minimum stress holding time T ₄ of a cycle on fatigue crack growth for a low alloy carbon steel in 3% NaCl solution. Measurements of the effective stress intensity range ratio U and observations of crack tip response were performed to clarify the causes of waveform effects.
The results were summarized as follows; T ₁ had a strong accelerating effect due to corrosive dissolution of the fresh surfaces of the crack which were formed during T ₁. The crack growth rate was enhanced as T ₁ increased and reached a constant value (about 3 times that in air) after T ₁= 10s. The crack growth rate at low Δ K , however, decreased as T ₁ increased more than T ₁= 1 s. T ₂, T ₃ and T ₄ decreased the crack growth rate. The extent of decrease not only depended on the period of T ₂ (or T ₃, T ₄), but also on Δ K and T ₁. A previously derived crack growth law which considered waveform and frequency effects, is also valid to a first approximation for the present results.     

AN ASSESSMENT OF A MICRO-MECHANIC MODEL OF HYDROGEN-INDUCED STRESS CORROSION CRACKING, BASED ON A STUDY OF AN X65 LINE PIPE STEEL

Abstract— The susceptibility of an X65 line pipe steel to hydrogen-induced stress corrosion cracking (SCC) is investigated. SCC tests on the steel are carried out in three environments of different aggressivity based on a NACE TM-01-77 solution with dissolved gaseous H₂S. The threshold stress intensity factor is calculated for each environment using the multiple specimen technique. The steel is tested in the as-received condition and after homogenisation. Permeation tests are carried out on specimens of the same steel in the same environment. The relationship between threshold stress intensity factor and hydrogen concentration is obtained. The results from these studies are used to generate the parameters in the micro-mechanic model of Akhurst and Baker. The validity and physical significance of the model is assessed.     

FRACTOGRAPHIC ANALYSIS IN THE UNDERSTANDING OF CORROSION FATIGUE MECHANISMS

Fractographic analyses have been used to explain the cyclic crack growth behaviour of A533-B, Ducol W30, a C-Mn steel and type 304 stainless steel in simulated light water reactor environments at ambient temperature. Fractographic observations have offered an explanation for anomalous crack growth behaviour and have also indicated where micro structural or environmental variables dominate in producing certain fracture modes and crack growth rates. An understanding of the operative corrosion fatigue mechanisms has been formulated through these fractographic analyses. Environmental crack growth in the ferritic steels has been described by a model involving both anodic dissolution and hydrogen embrittlement. Conditions where only one of these mechanisms would dominate have been identified and limits to their effect postulated. A crystallographic mode of failure observed in the austenitic type 304 stainless steel has also been explained by a selective dissolution process.