Experience with the use of the acoustic emission technique for monitoring scale failure in wet and dry environments

Abstract

A brief survey of the acoustic emission technique for monitoring scale cracking and failure on 2.25–24% Cr steels in wet and dry environments is given. A number of acoustic emission test rigs are described. Some of the more simple test rigs are used for testing small oxidation coupons during isothermal oxidation. More sophisticated rigs have been used for testing full size heat exchanger tubes during thermal cycling.

Most acoustic emission measurements in a wet environment come from testing at temperatures below 650°C. There are examples from Alloy 800 and thermal barrier coatings that were tested at higher temperatures, 900°C and 1100°C, respectively. Through the years acoustic emission tests have been performed in dry air, dry air+10%H₂O, dry air+0.5%SO₂, and Ar+5%H₂+50%H₂O. Consequently, a wide variety of exposure temperatures and atmospheres can be investigated using acoustic emission techniques.

Qualitative acoustic emission results can detect when scale cracking occurs at exposure temperature, where such cracks are produced by growth stress. Acoustic emission signals have been measured during sample cooling, where the signal arises from scale cracking that is caused by the thermal expansion mismatch stress. Measured results have clearly shown that scale cracking caused by both growth stress and thermal expansion mismatch stress are affected by water vapor in the exposure environment. Post-test metallographic investigations show that crack orientation and the oxide scale phases are also affected by the gas composition in the test rig. Additionally the sample mass gain and scale thickness is affected by water vapor content.

Finally, acoustic emission techniques are helpful for understanding the phenomena of breakaway oxidation and spallation/exfoliation.     

Significance of minor alloying additions and impurities on alumina scale growth and adherence in FeCrAl alloys

Abstract

Ultra-high purity Fe–Cr–Al–Y model alloys with controlled additions of impurities such as phosphorus and carbon, and potentially more beneficial elements such as titanium and zirconium have been prepared by induction melting in water-cooled, silver crucibles. 1 mm thick samples were then prepared by hot and cold rolling and annealing prior to cyclic oxidation in air at temperatures in the range 1100–1300°C. Other impurities were kept to a minimum of <10 ppm. scanning electron microscopy, Auger surface analysis and Scanning Transmission Electron Microscopy were used to characterise the samples both before and after oxidation.

Weight gain studies during oxidation showed that the high phosphorus containing alloy went into breakaway very quickly, after only 200 hours at 1300°C, while the Ti and Zr rich samples lasted for 1900 hours and 3300 hours respectively. In some cases, chromium or titanium rich precipitates were found along the alloy grain boundaries, often associated with carbon, while in other cases precipitates were found along the oxide metal interface. Although phosphorus was found at this interface in some of the samples, it was not always present, and may not be the only contributing factor to the premature breakaway failure of the oxides. A complete review of the microstructural evolution of these samples during oxidation will form the main topic of this paper.     

The effect of gas composition and contaminants on scale growth rates of FeCrAl alloys

Abstract

Fe–20Cr–5Al alloys were exposed to several environments crucial for industrial applications. Experiments in a simulated fuel-rich exhaust gas, in combustion gas, in air with additions of SO₂ and HCl, and a N₂–NO mixture were compared to the oxidation kinetics in air.

While the addition of HCl and SO₂ enhances oxide growth via increased spallation, the N₂–NO mixture and the simulated exhaust gas seem to act as a shield gas due to the reduced oxygen partial pressure. As the local geometry can also change the gas composition during oxidation, e.g. inside crevices, results from an experiment with a deep bore are presented, simulating such a situation.     

Chemical-mechanical failure of oxide scales on 9% Cr steels in air with H2O

Abstract

The oxidation behaviour of 9% Cr steels P91 and Nf616 has been investigated at 650°C in dry air and in air with water vapour, where particular attention was given to breakaway failure. Additional emphasis was given to the quantitative characterisation of the kinetics of chromium depletion in the metal subsurface zone resulting from scale growth, CrO₂H₄ evaporation, and scale cracking and healing, with scale cracking being monitored by acoustic emission measurements. While in dry air the steels show protective oxidation behaviour up to 10000 h, breakaway oxidation may occur already after 100 h in humid environments, which was correlated with the stronger Cr-depletion and the development of intrinsic oxide scale growth stresses exceeding a critical value, in the case of water vapour containing air. In the paper the different parameters that are responsible for breakaway oxidation were identified and discussed with regard to the role of water vapour in the environment. As a conclusion it turns out that breakaway is not a consequence of Intrinsic Chemical Failure (InCF) but of a Mechanically Induced Chemical Failure (MICF).     

In situ ESEM investigations of the oxide growth on hot work tools steel: effect of the water vapour

Abstract

Tempered martensitic steel modified AISI H11 is used in forging processes where tool failure can result from the combination of thermo-mechanical and chemical damage. A better knowledge of the oxidation mechanisms in this material could be useful for a better appreciation of its service behaviour and lifetime. The low chromium content of this Fe–Cr type steel allows the development of mainly Fe_2–xCr_xO₃ oxides with corundum structure and leads to enhanced oxidation in the presence of water vapour.

In situ FEG–ESEM images show the scale microstructural modifications during high temperature exposure, as well as the lateral growth of oxide particles. Together with GIXRD, SEM/EDS and SIMS analysis, FEG–ESEM also allows assessment of the H₂O effect on oxidation behaviour during high temperature exposures (600 and 700°C). Water vapour induces either pores or crystallites size increase, favours faceted oxides particles with enhanced density at the highest partial pressure. At this microscopic scale, anisotropic growth of crystallites is observed, and size expansion rates are found to be linear and characteristic of each individual particle.

Temperature acts principally on oxide film microstructure. Whatever the environment, homogeneous scale growth is observed at 600°C whereas the steel surface is heterogeneously covered by oxides at 700°C.     

Characterisation of oxide scales developed on high temperature resistant alloys in pure steam environments

Abstract

Steam oxidation of heat exchanger tubes and pipe work is of growing interest as research into the improvement of power plant efficiencies shows the need for much higher steam temperatures and pressures. This paper reports on the characterisation of the oxide scales grown during the steam oxidation of four alloys (T23, T92, TP347HFG and Inconel 740) in atmospheric pressure steam at four temperatures (600, 650, 700 and 750°C) for periods of 250, 500 and 1000 h. Three methods have been employed in analysing these scales: reflected light optical microscopy, scanning electron microscopy with energy dispersive X-ray analysis and X-ray diffraction.

The thickness, composition, morphology and spalling behaviour of the oxides differed with alloy composition, exposure times/temperatures and sample shapes. The ferritic steels exhibited the most severe oxidation, with the scales formed on these typically being triple-layered: an inner layer of Fe – Cr spinel, central layer of magnetite and outermost layer of haematite. However, the amount of haematite formed changed with the exposure time/temperature, alloy and sample orientation. In comparison TP347HFG and Inconel 740 showed significantly slower oxidation, with generally thin oxide scales (<5 µm) developing even at the highest exposure temperatures, though TP347HFG started to form some nodular growths after 1000 h exposure at the two higher temperatures.     

The effect of grain size and composition on the fracture toughness of Ti–Al–Nb alloys undergoing stress-induced martensitic transformation

The effect of grain size and composition on the fracture toughness of Ti–Al–Nb alloys in β solution-treated condition was investigated. The fracture toughness of the alloys was found to increase with an increase in grain size initially, reach a maximum and subsequently decrease with further increase in grain size. This trend was attributed primarily to the effect of grain size on the enhancement of fracture toughness due to stress-induced martensitic transformation (SIMT) at the crack tip, which in turn can be related to the effect of grain size on trigger stress for SIMT. Alloys containing higher Al and Nb showed a higher toughness for the same grain size, which was also explained in terms of effect of composition on the trigger stress.     

The influence of the moisture content of the atmosphere on alumina scale formation and growth during high temperature oxidation of PM2000

Abstract

Preliminary studies have been undertaken on cyclic and isothermal oxidation at 1,300°C of thin (125 μm) samples of commercial ODS alloy PM2000 for up to 350h in two different oxidising environments; dry and moist air. Scanning electron microscopy (SEM) and electron microprobe analysis (EPMA) have been used to study the influence of such environments on alumina scale formation and growth. Initial mass gain observations showed that the alumina scale, which formed on the samples oxidised in air+2.5vol% H₂O grew faster in the early stages of oxidation than in the case of dry air. However the SEM analysis revealed that the scale morphologies in both dry air and air+2.5vol% H₂O were similar. In both cases the scales consisted of equiaxed grains at the scale–gas interface with Ti-rich particles in the outermost part of the scale. The major factor for the total scale failure, the formation of non-protective iron oxide, is the depletion of Al levels to a critical value, below which no protective alumina scale can form; and this occurred slightly faster in moist air than in dry air     

The influence of laboratory test procedures on scale growth kinetics and microstructure during steam oxidation testing

Abstract

Laboratory exposures cannot reproduce all the features present in service conditions. The experimentalist is faced with the conflict between increasing the complexity of laboratory tests to replicate service more closely and keeping testing costs low by maintaining a simple procedure. The influence of various experimental parameters, which can be controlled in the laboratory, on the steam oxidation response of materials is reviewed and recommendations for best practice are proposed.     

The effect of residual stresses arising from laser shock peening on fatigue crack growth

Residual stresses have in the past been introduced to manipulate growth rates and shapes of cracks under cyclic loads. Previously, the effectiveness of shot peening in retarding the rate of fatigue crack growth was experimentally studied. It was shown that the compressive residual stresses arising from the shot peening process can affect the rate of crack growth. Laser shock peening can produce a deeper compressive stress field near the surface than shot peening. This advantage makes this technique desirable for the manipulation of crack growth rates. This paper describes an experimental program that was carried out to establish this effect in which steel specimens were partially laser peened and subsequently subjected to cyclic loading to grow fatigue cracks. The residual stress fields generated by the laser shock peening process were measured using the neutron diffraction technique. A state of compressive stress was found near the surface and tensile stresses were measured in the mid-thickness of the specimens. Growth rates of the cracks were observed to be more affected by the tensile core than by the compressive surface stresses.     

非对称复合膜支撑层阻力对水蒸气渗透影响的研究

利用阻力模型,采用聚砜／硅橡胶非对称复合膜体系比较了支撑层阻力对氮殷、氢５气和水蒸气渗透性能的影响,发现支撑层阻力的存在大大了和低了水蒸气的渗透速率,某些情况下支撑层阻力占总阻力的９０％以上,同时,讨论了支撑层孔隙率、孔径、致密层厚度等膜结构参数对支撑层阻力及膜不脱湿过程的影响。     

The effect of micro-structure on the oxidation behavior of a Ni-based superalloy in water vapor plus oxygen

The oxidation behavior of a Ni-based superalloy with polycrystalline, single-crystalline (SC) and nanocrystalline (NC) structures was studied at 1000 °C in water vapor (20.12 vol.%) plus oxygen. The oxidation behavior of the SC alloy was similar to that of the cast alloy, while nanocrystallization increased the corrosion resistance. The oxides scale on the SC alloy and cast alloy consisted of external TiO₂, Cr₂O₃ and internal Al₂O₃, while only external Al₂O₃ scale formed on the NC coating in water vapor plus oxygen. Meanwhile, the morphologies of oxides scale on three samples are significantly different from one another. The effect of micro-structures on the oxidation behavior of this Ni-based superalloy is discussed.     

Breakdown of the protective oxide on 11 % Cr steel at high temperature in the presence of water vapor and oxygen,the influence of chromium vaporization

Abstract

We report on the effect of water vapor and oxygen on the oxidation of a ferritic/martensitic 11 % Cr steel (CrMoV11 1). The influence of pH₂O, exposure time, gas velocity and temperature was investigated. The samples were exposed to dry O₂, O₂+10 or 40 % H₂O for up to 336 hours. Total pressure was 1 atm (1.02 × 10⁵ Pa). The gas velocity was between 0.05 and 10 cm/s while temperature was in the range 450–700°C. The samples are investigated by thermogravimetry, GI-XRD, SEM/EDX, GDOES, FIB and TEM/EDX. Oxidation is strongly affected by the vaporization of CrO₂(OH)₂ in H₂O/O₂ environment. The mechanism of vaporization of CrO₂(OH)₂ from a Cr₂O₃ surface is modelled by DFT calculations. In the absence of chromium vaporization the alloy forms a protective oxide consisting of a corundum-type solid solution (Fe_1–xCr_x)₂O₃. The vaporization of chromium tends to deplete the oxide in chromium. In some cases the oxide remains protective in spite of chromium depletion while in other cases there is a transition to breakaway oxidation. In the latter case a thick layered scale forms, consisting of an outer hematite part and an inner iron-chromium spinel. Oxidation behavior in an O₂+H₂O environment is to a large extent determined by the ability of the metallic substrate to supply the oxide with chromium by diffusion in order to compensate for the losses by vaporization. The corrosivity of the environment increases with the concentration of water vapor and oxygen, with the gas velocity and with temperature.     

The effect of compression stresses,stress level and stress order on fatigue crack growth of multiple site damage

Structural components are generally subjected to a wide stress spectrum over their lifetime. Service loads are accentuated at the areas of stress concentration, mainly at the connection of components. When there is a critical level of multiple site damage at connections, cracks link up to form a large crack which abruptly reduces the residual strength of the damaged structural member. Therefore, it is important to estimate the fatigue life before the cracks link up due to critical multiple site damage. In this study, the extended finite element method was applied to predict lifetime under constant amplitude cyclic loadings of fatigue tests on several multiple site damage specimens made of Al 2024‐T3. Then the multiple crack growths under service stress spectra are calculated to investigate the effects of compressive stress, stress orders and the effect of sequence cyclic loadings on stress levels by using Forman and NASGROW equations.     

Effect of the chromium content on the corrosion of nickel based alloys in primary water of pressurised nuclear reactors

Abstract

Stress corrosion cracking (SCC) is a damaging mode of alloys used in pressurised water reactors. Those damages led to the replacement of Alloy 600 (15% Cr) by alloy 690 (30% Cr) but the mechanism responsible for the SCC and the reason for the positive effect of chromium are not yet very well understood. In this paper, we studied the corrosion of synthetic alloys – with controlled chromium content varying from 5 to 30 wt%. Characterisation was done using SEM and TEM observations together with chemical analysis and mapping using EDX, EFTEM and SIMS. The outer oxide scale is composed of crystallites, beneath it the presence of a continuous chromium oxide is accompanied with a Cr depleted zone for alloys that contain more than 10% Cr. The penetration of oxygen over very large distances (several microns) on triple junctions is demonstrated, as well as the role of plastic deformation that modifies strongly the overall structure of the oxide scale.     

Ti-48Al-8Cr-2Ag纳米晶涂层对TiAlNb合金腐蚀行为的影响

研究了磁控溅射的Ti-48Al-8Cr-2Ag（原子数分数,％）纳米晶涂层对Ti-46．5Al-5Nb合金腐蚀行为的影响．结果表明,在600～700℃盐水蒸气的协同作用下,TiAlCrAg涂层使TiAlNb合金腐蚀速率降低．Nb是TiAlNb合金在700℃盐水蒸气协同作用下生成致密TiO2膜的主要原因．氯化物侵蚀α2相,使TiAlNb合金在盐和水蒸气综合作用下加速腐蚀,TiAlCrAg涂层中较少的α2相以及Laves相中较高的Cr含量是腐蚀速率降低的主要原因．     

The effect of stress ratio on fatigue crack growth rate in the absence of closure

A fractographic study¹ was performed on Al-alloy fatigue fracture surfaces produced by programmed load sequences. The load sequences included steps of constant amplitude cycles at three different stress ratios, each step is preceded by a small number of high amplitude cycles designed to avoid the influence of crack closure and to serve as fractographic markers. The experiments were conducted on different specimen geometries to produce conditions associated with a long crack under fully elastic conditions and a short crack in a notched coupon seeing high local post yield stress conditions. Crack sizes covered in the study ranged from 0.02 to 12 mm, and growth rates ranged from 2×10⁻⁷ to 4×10⁻⁵ mm cycle⁻¹. Fractographic evidence from the study suggests that the crack growth rate can vary by up to a factor of five with applied stress ratio change from 0.64 to 0.73. In the case of the long crack, the effect is less noticeable or totally absent. In the case of naturally initiating notch root cracks, the effect is more pronounced at higher stress level and lower crack growth rate.     

The effect of thermal history on the color of oxide layers in titanium 6242 alloy

A study was done to examine the effects of thermal history and surface pretreatment on the color of oxide layers in Titanium 6242 alloy. Coupons of the alloy underwent three different pretreatment options prior to thermal exposure to temperatures from 371 to 760 °C with periods from 6 min up to 100 h. After exposure it was found that there was not a simple correlation between color and thermal history. Also, it was not evident after exposure that the hardness could be traced to the thermal history. However, it was clear that color did rely on pretreatment options. The abrasive treatment resulted in shades of gold with a polished look while the milder treatments produced matte hues of blue. All coupons after exposure were found to have an oxide layer thickness of no more than 400 nm.     

The effect of grain refining and oxide inclusion on the fluidity of Al–4.5Cu–0.6Mn and A356 alloys

The effect of grain refinement and oxide inclusion on the fluidity of Al alloy was investigated with a test casting with eight thin flow channels. Pouring in air increased the amount of oxide in the A356 melt. The fluidity compared between normal A356 melt and contaminated melt. The amount of oxide was evaluated qualitatively by ultrasonic treatment. The flow length varied linearly with the pouring temperature. By adding Ti and Al–5Ti–B, fluidity increased. The grain size decreased by adding grain refiner. The fluidity depended on the degree of grain refining. It was noticed that pouring in air increased the amount of oxides in the melt by ultrasonic treatment. The fluidity of contaminated melt was decreased comparing to the normal one especially in lower temperature.     

The glow discharge plasma influence on the oxide layer and diffusion zone formation during process of thermal oxidation of titanium and its alloys

The influence of the glow discharge in argon and oxygen mixture on the effectiveness of the surface-hardening treatment of titanium and its alloys, is presented in this paper. The high solubility of oxygen in the titanium matrix is the base of the discussed technology. Interstitially placed oxygen causes significant hardness increment as well as wear resistance.The rectangular flat specimens of Ti6Al4V alloy underwent the process of plasma thermal oxidation (TO) in atmosphere Ar+O₂ which led to diffusion strengthening. The temperature of the process was 1223 K—lower than the higher temperature range of the recrystallization process. The experiments were carried out without plasma, with plasma presence during the whole process and the plasma presence only in diffusion stage. The substrates were examined with the use of scanning electron microscope. The chemical composition of formed oxides was determined. In addition to the X-ray analysis, the cross-section inspections as well as the hardness distribution in hardened layers assessment were also done.The main conclusion is that the plasma glow discharge presence during the whole process is not advantageous because the complex oxides that are difficult to dissolve in the matrix are formed. And although the surface hardness is the highest in this case, it is unfortunately connected with brittleness increase. The presence of plasma in diffusion stage is more preferable because it works “mechanically” on the oxides layer that is additionally thinner and causes the formation of thicker strengthened zone. However in both cases the use of plasma makes the process more effective than “typical” oxidation process.