Spannungsrißkorrosion in hochreinem Wasser von 3½ % NiCrMoV-Vergütungsstählen für Dampfturbinen-Scheiben und -Wellen

Stress Corrosion Cracking in High Purity Water of 3½ % NiCrMoV – Quenched and Tempered Steel for Steam Turbine Discs and Shafts In recent years intergranular stress corrosion cracking has occured world-wide in the shrink-fitted discs of low pressure turbine rotors made of low alloy steels. Only in a few cases steam impurities such as NaOH, Na₂CO₃, Na₂SO₄, H₂S, or NaCl, which initiate SCC, could be found. The stress corrosion cracking behaviour of the turbine disc steel 26 NiCrMoV 14 5 with a yield strength of approx. 850 N/mm² was examined under special corrosion conditions. Gaseous and other impurities of the water, which lead to higher conductivity can initiate stress corosion cracks and increase the stress corrosion crack velocity insignificant. Stress corrosion crack initiation can be prevented by shifting the pH-value and the free corrosion potential in the region of passivity. Unfavourable crevice conditions must be avoided. Solutions are shown, how to prevent stress corrosion cracking of steam turbine discs.     

Die Scheiteldruckprüfung korrosionsbeanspruchter Hoch- und Höchstdruckrohre zur Untersuchung des Bauteil-Rißwachstumsverhaltens unter Modus II-Schwingungsrißkorrosion

External Crown Fatigue Loading of High and Ultrahigh Pressure Tubes Subjected to Corrosion – a Highly Informative Test Predicting the Crack Growth Behaviour of Tubular Components under Mode II Corrosion Fatigue Conditions Stressing high and ultrahigh pressure tubes by external static or fatigue loads has been qualified as a convenient method to simulate the load case “internal pulsating pressure” by analysing the stress state of thick walled tubes when loaded by internal pressure and external crown loads, respectively. The results of different analytical calculations were compared with that of a Finite-Element-Computation demonstrating, for tubes with nominal pressures in the range of 325–3600 bar, an excellent correspondence. Tests with 86 tube cuttings of steel X 6 CrNiMoTi 17 12 2 (W.-No. 1.4571; ASTM UNS S 31635; BS 320531) showed the following results: In air, pulsating pressures of 325 bar (corresponding to the maximum allowable operating pressures) are sustained indefinitely. Under mode II-corrosion fatigue in 0,1 N H₂SO₄ (30°C) failure occures after 3,8 · 10⁷ mode cycles. A twentyfold H₂SO₄ concentration will lower the number of cycles to fracture to one tenth of this value without leaving mode II. Under mode II corrosion fatigue crack growth will propagate faster in radial direction than in air, so that leak-before-break under internal pressure will be likely. Crack growth rates in radial direction increased with increasing acid concentration so that the probability for leak-before-break will further increase. Highest priority for the surveillance strategy of components loaded in mode II CF has, however, the prove that crack initiation in this environment is commencing much earlier than in air, and definitely earlier than found for compact specimens tested in a mode II pulsating fatigue or rotating bending test.     

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.     

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.     

ELASTIC–PLASTIC ANALYSIS OF THE FATIGUE LIMIT FOR A MATERIAL WITH SMALL FLAWS

Abstract Fatigue tests were carried out on mild steel with small cracks for which linear fracture mechanics is not effective. A fatigue limit criterion, based on the cyclic plastic zone size at a crack tip being a material constant at the fatigue limit, can effectively evaluate the effects of crack length and stress ratio. Regarding flaws as cracks, the theory gives fatigue limit values close to those obtained in experiments on specimens with natural defects, such as surface roughness, micro-shrinkage cavities, inclusions etc. The effect of water corrosion was also investigated.     

Einfluß der Temperatur und der Umgebungsbedingungen auf das Ermüdungsverhalten metallischer Werkstoffe

Influence of Temperature and Environmental Conditions on Fatigue Behaviour of Metallic Materials The corrosion fatigue behaviour of metallic materials is influenced by environmental conditions. With increasing temperature a similar diminishing of fatigue strength can be observed as in the usual tensile properties. Air and water vapour decrease the number of cycles to fracture because they accelerate the crack propagation by adsorptive and reactive processes at the crack tip. By the effect of pure hydrogen or hydrogen sulfide and mixtures from both appears a marked increasing of crack propagation rate in fracture-mechanical test pieces alternating loaded at low frequency (1 Hz). Corrosive mediums influence also the formation of cracks. A mechanical electrochemical failure mechanism leads to a total loss of fatigue strength. Depending on performance of materials in an aggressive solution active or passive corrosion fatigue occurs. With regard to the form of appearance the two kinds differ in characteristic manner.     

Axial fatigue of a gas-nitrided quenched and tempered AISI 4140 steel: effect of nitriding depth

Fatigue testing under fully reversed axial loading (R=?1) and zero‐to‐tension axial loading (R= 0) was carried out on AISI 4140 gas‐nitrided smooth specimens. Three different treatment durations were investigated in order to assess the effect of nitriding depth on fatigue strength in high cycle fatigue. Complete specimens characterization, i.e., hardness and residual stresses profiles (including measurement of stabilized residual stresses) as well as metallographic and fractographic observations, was achieved to analyse fatigue behaviour. Fatigue of the nitrided steel is a competition between a surface crack growing in a compressive residual stress field and an internal crack or ‘fish‐eye’ crack growing in vacuum. Fatigue life increases with nitriding depth until surface cracking is slow enough for failure to occur from an internal crack. Unlike bending, in axial fatigue ‘fish‐eye’ cracks can initiate anywhere in the core volume under uniform stress. In these conditions, axial fatigue performance is lower than that obtained under bending and nitriding depth may have no more influence. In order to interpret the results, special attention was given to the effects of compressive residual stresses on the surface short crack growth (closure effect) as well as the effects of internal defect size on internal fatigue lives. A superimposed tensile mean stress reduces the internal fatigue strength of nitrided steel more than the surface fatigue strength of the base metal. Both cracking mechanisms are not equally sensitive to mean stress.     

Effects of particle size on fatigue crack initiation and small crack growth in SiC particulate-reinforced aluminium alloy composites

Fatigue crack initiation and small crack growth were studied under axial loading using powder metallurgy 2024 aluminum-matrix composites reinforced with SiC particles of three different sizes of 5, 20 and 60 μm. The 5 and 20 μm SiC_p/Al composites exhibited nearly the same fatigue strength as the unreinforced alloy, while the 60 μm SiC_p/Al composite showed a significantly lower fatigue strength due to its inferior crack initiation resistance that could be attributed to interface debonding between particles and the matrix. Small crack growth behaviour was different depending on stress level. At a low applied stress, the addition of SiC particles enhanced the growth resistance, particularly in the composites reinforced with coarser particles, while at a high applied stress, the 60 μm SiC_p/Al composite showed a considerably low growth resistance, which could be attributed to interaction and coalescence of multiple cracks. In the 5 μm SiC_p/Al composite, small cracks grew avoiding particles and thus few particles appearing on the fracture surfaces were seen, particularly in small crack size region. In the 20 and 60 μm SiC_p/Al composites, they grew along interfaces between particles and the matrix and the number of particles appearing on the fracture surfaces increased with increasing crack size or maximum stress intensity factor.     

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.     

Spannungsrißkorrosion in hochreinem Wasser von 3–3 ½% NiCrMoV-Vergütungsstählen für Dampfturbinen-Scheiben und -Wellen. Teil I

SCC in High Parity Water In recent years intergranular stress corrosion cracking has occured world-wide in the shrink-fitted discs of low pressure turbine rotors made of low alloy steels. Only in a few cases steam impurities such as NaOH, Na₂CO₃, Na₂SO₄, H₂S or Nacl, which initiate SCC, could be found. To clarify the SCC-behaviour experiments on turbine disc steels with different chemical compositions an yield strength were performed in high purity water. The results show, that chemical composition has no effect on the crack initiation. Under high purity water conditions no crack initiation due to stress corrosion cracking is observed on the steel with a yield strength of 850 N/mm². On the steel with a yield strength of 1250 N/mm² which is not used in service, crack initiation occurs in pure water. But if sharp cracks already exist, crack propagation occurs in both cases. The investigations showed, that stress corrosion cracking of turbine discs can be prevented by a good water chemistry with a cation conductivity less than 0.2 μS/cm (μmho/cm).     

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.     

Corrosion fatigue behaviour and microstructural characterisation of G20Mn5QT cast steel in 3.5‐wt% NaCl solution

Corrosion fatigue behaviour and microstructural characterisation of G20Mn5QT cast steel are investigated in simulated seawater. Fractography is performed by using scanning electron microscopy (SEM). The macroscale fracture surface and microstructure of the failed specimen are acquired including the crack initiation, crack propagation, and pitting evolution. The maximum cyclic stress (S) versus number of cycles to failure (N) curves is derived by three‐parameter fatigue curve method. Fatigue life is predominantly controlled by the corrosion pitting‐induced crack initiation when tested in simulated seawater at lower stress levels. As the maximum cyclic stress is less than 185 MPa, the chloride ion erosion is the main influence factor, which affects the fatigue failure of the G20Mn5QT cast steel in simulated seawater.     

Mechanische Verträglichkeit bei Verbundwerkstoffen – Naßkorrosion

Mechanical compatibility of metallic composites . Corrosion in aqueous media. As a main condition for corrosion in aqueous media between components of a composite material, both components have to be touched by an invironmental wed atmosphere or of a fluid. If this condition is given contact corrosion can take place with any composite, independent of the mechanical compatibility of the components. Crevice corrosion is possible too if mechanical incompatibility (or extern loads) causes cracks between the components; stress corrosion cracking is possible as well. Stainless steel vires, for instance, can not reinforce the composit if attached by SCC. The other kinds of corrosion mostly dammage the matrix, which also reduces properties of the composit. Examples are given of possible kinds of corrosion for some fiber reinforced metallic composites and for metal laminates.     

Lokale Last- und Eigenspannungsverteilungen an rißbehafteten Oberflächen nach Korrosionsermüdung von vergütetem Ck 45

Distributions of Loading and Residual Stresses in Cracked Surfaces of Corrosion Fatigued Quenched and Tempered Steel Ck 45 Corrosion fatigue in the active state leads to a great number of short cracks and corrosion pits in the surface of metallic materials. This paper deals with the resulting distributions of loading and residual stresses in the case of bending fatigue. The material investigated was the quenched and tempered CBN-ground steel Ck 45. Results presented clearly indicate that stress distributions around cracks and corrosion pits are very inhomogeneous. Corrosion pits lead to a relaxation of manufacturing induced residual stresses. In the loaded state as a consequence of notch effects, maximum stresses are observed, which explains the importance of corrosion pits as crack initiation sites.     

FATIGUE STRENGTH AND FRACTURE MECHANISMS OF CERAMIC- SPRAYED STEEL IN AIR AND A CORROSIVE ENVIRONMENT

Abstract— In order to investigate the fatigue strength and fracture mechanism of ceramic-sprayed steel, rotary bending fatigue tests were conducted at room temperature in air and 3% NaCl solution using specimens of a medium carbon steel (S45C) with sprayed coating layers of Ni-5% A1 (under-coating) and chromia (top-coating). The results obtained are discussed based on observations of fatigue cracks and experimental data on specimens subjected to individual treatments during the ceramic spraying process. It was found that at a very early stage of fatigue life, cracks were initiated at the interface between under- and top-coating layers, and grew rapidly into the ceramic-sprayed layer. However, these cracks did not propagate continuously into the substrate, and the final failure was led by the growth of a crack newly initiated at the surface of the substrate steel. Thus, the fatigue strength of the ceramic-sprayed steel in air could be evaluated due to the property of the substrate. The corrosion fatigue strength of ceramic- sprayed steel was improved when compared to that of the substrate steel. However, the coating layer contained many pores, through which NaCl solution was supplied from the specimen surface to the substrate. Corrosion pits were formed at the interface between the under-coating and the substrate. Subsequently, cracks initiated from the pits and grew into the substrate. Tests were also conducted on specimens whose pores were closed by a shielding treatment. In this case, NaCl solution was supplied to the substrate by cracks initiated in the top-coating layer. The shielding treatment was effective at low stress levels where fatigue life was more than 10⁷ cycles, while it had little effect on improving corrosion fatigue strength at higher stress levels because of the many cracks initiated in the top-coating layer.     

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.     

Maßnahmen gegen Schwingungsrißkorrosion an nichtrostenden Stählen

Methods to Combat Corrosion Fatigue Stainless Steels In chemical works substantial damage to moving components made of stainless steels is caused by corrosion fatigue. The very slight conditions for attack, which are sufficient to cause such corrosion, and the appearance of the surface of the fracture result in such damage frequently being attributed to purely mechanical causes and consequently being combatted with ineffective measures. Fracture analysis and remedial measures are discussed for a range of typical examples. Rotating bending fatigue tests are used to show that corrosion fatigue in the passive state of the steels can be suppressed by increasing the chromium content. Chloride ion contents up to 0.1% showed no appreciable effect. Finally the possible measures against corrosion fatigue are discussed.     

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.     

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.