Melting during heat treatment of a sheet of a niobium alloy – Failure analysis

The niobium alloy Nb‐5.4 At% Hf‐2 At% Ti is an important structural material used for high temperature applications. It is commonly produced in the form of sheet; however, it is also available in other mill forms. The sheets are vacuum‐annealed to improve the formability prior to fabricating different components out of them through metal forming. Annealing in vacuum is adopted in view of the high proneness to oxidation of niobium‐ base materials. In the present instance vacuum annealing of a sheet of the niobium alloy was carried out at 1175 ^oC; a plate of the nickel‐base superalloy 600 was used as charge support. It was noticed that the vacuum annealing resulted in melting of the charge as well as the charge support material. The plate of superalloy 600 was then replaced by a sheet of commercially pure titanium and next trial of annealing heat treatment carried out; there was no problem of melting. Failure analysis was carried out. The analysis brings out how phase diagrams can be used to choose the right material for supporting the charge in order to prevent melting during heat treatment. The failure is very expensive, if the material to be heat treated is costly, as it happens to be in the present case.     

高性能连续碳纤维增强碳化硅复合材料的制备及其高温力学性能的研究

连续碳纤维增强碳化硅(Cf/SiC)复合材料以其耐高温、高强度、低密度等特性已成为新一代航空发动机高温部件的首选材料。采用聚合物浸渍裂解法(PIP)成功制备出适用于航空发动机高温部件的Cf/SiC复合材料,其密度为1.83g/cm3。在发动机典型工作温度1200℃条件下,通过本工艺制备Cf/SiC材料的弯曲强度高达712MPa,略高于材料的室温弯曲强度(641MPa)。这一现象可能由碳纤维在冷却过程中产生的残余应力引起。此外,在温度为1200℃、加载压力120MPa的条件下,材料的稳态高温蠕变速率为2.08×10-3%/h,基体开裂和界面滑动可能是材料宏观变形的主要原因。     

Evaluation of high temperature mechanical strength of Cr–Mo grade steel through small punch test technique

Remaining life assessment (RLA) of high temperature components calls for estimation current mechanical properties of the component material. For determination of mechanical properties of in-service components, large volumes of samples are required to be cut from components. The sample() removal warrants post-sampling repair by welding and mandatory post weld heat treatment. The time and the efforts involved in these exercises often limits the RLA to less reliable levels relying on the material’s original properties at the time of fabrication leading to over prediction.Research is being carried out for estimation of mechanical properties of materials by destructive testing of miniature samples under various modes. Small punch test (SPT) technology is one such technique which has generated significant interest of the researchers. However challenges exist to establish procedure of testing and development of field implementable characteristic co-relation between SPT parameters and material properties. Though reports are available on room temperature (RT) properties on variety of materials to a great extent, studies on estimation of high temperature properties through SPT are sparingly available. An attempt has been made to develop correlations between SPT parameters and high temperature mechanical properties of Cr–Mo grade material. This paper describes salient features of the test setup developed and used for conducting test at high temperature, experimental results on one of the Cr–Mo grade material followed by development of correlation equation for estimating tensile properties.     

High-temperature oxidation resistance improvement of titanium using laser surface alloying

Commercially pure titanium substrate was coated with aluminium by flame spraying or arc spraying techniques and was irradiated with a laser to form a laser-alloyed zone (LAZ). The microstructure of the LAZ was investigated. High-temperature oxidation tests were carried out in air at different high temperatures and the characteristics of the oxide layer were investigated using Auger electron spectroscopy. The aqueous corrosion tests were carried out in 3.5% NaCl solution at room temperature. The laser-processed material showed considerable improvement in the high-temperature oxidation-resistant property while the aqueous corrosion-resistant property of the LAZ was found to remain almost unaffected.     

Useful lifetime prediction of rubber component

Rubber material properties and lifetime evaluation are very important in design procedure to assure the safety and reliability of the rubber components. This paper discusses the failure mechanism and material tests were carried out to predict the useful lifetime of NBR and EPDM for compression motor, which is used in refrigerator component. The heat-aging process leads not only to mechanical properties change but also to chemical structure change so called degradation. In order to investigate the heat-aging effects on the material properties, the accelerated test were carried out. The stress–strain curves were plotted from the results of the tensile test for virgin and heat-aged rubber specimens. The rubber specimens were heat-aged in an oven at the temperature ranging from 70 °C to 100 °C for a period from 1 to 180 days. Compression set results changes as the threshold are used for assessment of the useful life and time to threshold value were plotted against reciprocal of absolute temperature to give the Arrhenius plot. By using the compression set test, several useful life prediction equations for rubber material were proposed.Significance: Useful lifetime estimation procedure employed in this study could be used approximately for the design of the rubber components at the early design stage.     

Ageing and cracking of simulated heat affected zones in 321 stainless steel

The heat affected zone (HAZ) of stabilised austenitic stainless steel welds may exhibit a serious form of intercrystalline cracking during service at high temperature. This type of embrittlement, well known as stress relief cracking, is related to thermal ageing: a fine and abundant intragranular Ti(C,N) precipitation appears near the fusion line during service at high temperature and modifies the mechanical behaviour of the HAZ. To analyse this embrittlement micro mechanism and to assess the lifetime of real components, different HAZ were simulated by various solution heat treatments, cold rolling and ageing conditions. The mechanical behaviour of these resulting materials was investigated using creep and tensile tests on smooth bars. Then, creep tests were carried out and simulated on notched bars by finite element calculations. A damage model was identified from intergranular damage measurements made on notched specimens and compared with calculated mechanical fields. Further tests on fatigue precracked specimens showed that crack propagation occurred under stress relaxation conditions in simulated HAZ material.     

80% Ni-20% Cr Powdered Metal Heating Element

80% Ni and 20% Cr powder mixture was admixed with some minor components. These powder mixtures were cold isostatically pressed, and sintered at 1200^°C or 1300^°C in various atmospheres for one hour. Compacts sintered in neutral or reducing atmospheres could be rolled to high density, while those sintered in air were too brittle to be rolled. The microslructure of the rolled sample was homogeneous solid solution. Fusion tests was carried out on the commercial Nichrome wire and showed that this material began to sag at 1200^°Cand sagging grew and broke at 1400^°, while the rolled sample of the present work, when heated, kept straight, untii it broke at 1400^°C. The microstructure of the sample unable to be rolled composed of some pure nickel islands embedded in a matrix of oxide. This sample, when heated, kept straight, until it broke at a temperature of higher than 1700^°C.     

The effect of temperature on hydrogen induced fracture in UNI 40NiCrM07 steel

The influence of the temperature on hydrogen embrittlement in UNI 40NiCrMo7 steel was evaluated by means of delayed failure tests. The tests were carried out in the temperature interval 2°–60°C using samples which had undergone two different heat treatments. Hydrogen was introduced into the samples in intensiostatic conditions (d.d.c. 10mA/cm²) by means of cathodic charge in a 0.1N sulphuric acid solution thermostatically maintained at the test temperatures. The results obtained showed a noticeable influence of the temperature on threshold load values and crack incubation times in the tests carried out on quenched material, whereas this influence was found to be negligible in the tests performed on tempered material. This different behaviour was attributed to the effect of the temperature on movement kinetics in the dislocations, responsible for the accumulation and transport of hydrogen into the zones of maximum stress, and on hydrogen diffusion in the metallic lattice.     

Failure mode analysis of two crankshafts of a single cylinder diesel engine

This paper reports an investigation carried out on two damaged crankshafts of single cylinder diesel engines used in agricultural services for several purposes. Recurrent damages of these crankshafts type have happened after approximately 100 h in service. The root cause never was imputed to the manufacturer. The fatigue design and an accurate prediction of fatigue life are of primordial importance to insure the safety of these components and its reliability. This study firstly presents a short review on fatigue power shafts for supporting the failure mode analysis, which can lead to determine the root cause of failure. The material of these damaged crankshafts has the same chemical composition to others found where the same type of fracture occurred at least ten years ago. A finite element analysis was also carried out in order to find the critical zones where high stress concentrations are present. Results showed a clear failure by fatigue under low stress and high cyclic fatigue on crankpins.     

High‐Temperature Corrosion of NiTi‐Shape‐Memory Alloys

Semi‐finished products and components made of NiTi‐shape‐memory alloys (NiTi‐SMA) are often subjected to heat treatment after their fabrication. During this heat treatment, oxide layers begin to form which contain a high amount of titanium. In this investigation special attention was drawn to the selective oxidation of Ti because a TiO_X‐layer can represent a Ni‐barrier and may therefore be of special use for medical applications. A comparison of the following three samples was carried out: A sample oxidised at room temperature, another that was heat‐treated in ambient air (600 °C/1min) and a third sample that was subjected to a heat treatment (600 °C/1min) in an atmosphere that oxidises titanium but reduces NiO in order to achieve a selective oxidation of the titanium. The analysis of the oxide layers was carried out by means of x‐ray photoelectron spectroscopy (XPS). It was shown that the ratio of titanium to nickel in the oxide layer can be substantially increased when performing the annealing treatments in a partial reducing atmosphere. Furthermore, a thermo‐gravimetric investigation of the material was carried out at 600 °C in dry air in order to estimate the growth of the oxide layers.     

高温后半灌浆套筒抗拉性能试验研究

对灌浆料标准试块和铸铁半灌浆套筒连接件进行了常温、200℃、300℃、400℃、600℃的抗压强度试验和单向拉伸试验。研究了温度对灌浆料性能的影响,以及锚固长度、保护层厚度对高温后半灌浆套筒钢筋连接试件抗拉性能的影响,分析了高温后半灌浆套筒连接破坏模式的转变。结果表明:灌浆料抗压强度随温度的升高而降低,在经历600℃高温后,灌浆料抗压强度损失达73%;套筒试件经历高温后由钢筋屈服后断裂破坏逐渐变为钢筋屈服拔出破坏,最终变为钢筋屈服前拔出破坏;无保护层试件抗拉性能在各个温度下均低于有保护层试件。基于试验数据给出了灌浆料抗压强度随温度变化的计算公式及半灌浆套筒极限拉力随温度和锚固长度变化的计算公式,为相关设计及工程施工提供参考。     

Failure Analysis of Wear Block of Crush Deck Used in Sinter Plant

Failure analysis of wear block used in the crush deck of a Sinter Plant was carried out. Crush deck acts as a platform to transfer the hot sinter cake from sinter machine to crushing unit. Hot sinter cake with a temperature around 800 °C falls on the crush deck from the discharging end of rotating sinter machine. This investigation consists of visual observation, chemical analysis, characterization of microstructures, energy dispersive spectroscopy, and measurement of hardness. Analysis revealed that this is an iron-based material with Cr–Ni alloying with higher hardness and lower toughness, which was due to improper microstructure. Microstructure revealed a large number of primary carbides along with needle-shaped carbides, which drastically reduce the toughness of the material. Application point of view the wear block should have optimum combination of toughness and wear resistance at elevated temperature; however, analyses showed the actual wear block had with higher wear resistance and lower toughness. This was reflected in the undesirable microstructure observed during metallographic study. It was thus concluded that the component failed before a proper life cycle was achieved due to poor material properties. Based on the failure analysis suitable heat treatment process was recommended. Metallurgical study carried out on the suggested material, showed significant improvements in material property. Trial of recommended wear block was taken at the plant level and life of the recommended material improved twice than that of previous one.     

Effect of notch root radius on tensile behaviour of 316L(N) stainless steel

Abstract

Type 316L(N) stainless steel (SS) is used as the major structural material for high temperature components of sodium cooled fast reactors. The influence of notch root radius on the tensile behaviour of 316L(N) SS under multi-axial stress state was investigated. Double U-notches with five different kinds of notch geometry were incorporated symmetrically into the tensile testing specimens by changing the notch root radius while keeping the gross diameter, net diameter and notch depth as the same for all the notches. The notch root radius was varied as 0·25, 0·5, 1·25, 2·5 and 5 mm. Tensile tests were carried out on the notched specimens at room temperature (298 K) and at 923 K at a constant strain rate of 3×10^?3 s^?1. The tensile strength and yield strength of notched specimen of 316L(N) SS increased with decrease in notch radius at both the temperatures and the notch severity was less pronounced at high temperature. The fractured notch surface was analysed using scanning electron microscope and unfractured notch was sliced along the axis and observed under optical microscope. Finite element analysis was performed on the models of notched specimens with various notch root radii. These results showed that Von Mises equivalent stress which was derived from triaxial stresses decreased with decrease in notch radius. The shift of location of peak values of maximum principal stress and hydrostatic stress towards the axis of the specimen, leading to formation of cracks, occurred at a lower nominal stress when the notch radius was increased.     

Prediction of fatigue lifetime under multiaxial cyclic loading using finite element analysis

Life prediction for GH4169 superalloy thin tubular and notched specimens were investigated under proportional and nonproportional loading with elastic–plastic finite element analysis (FEA). A strain-controlled tension–torsion loading was carried out by applying the axial and circular displacements on one end of the specimen in the cylindrical coordinate system. Uniaxial cyclic stress–strain data at high temperature were used to describe the multi-linear kinematic hardening of the material. The comparison between FEA and experimental results for thin tubular specimen showed that the built model of FE is reliable. A fatigue damage parameter was proposed to predict the fatigue crack initiation life for notched specimen. The results showed that a good agreement was achieved with experimental data.     

Dynamic mechanical response of polyetheretherketone (PEEK) exposed to cyclic loads in the high stress tensile regime

Tensile fatigue tests of PEEK at high load levels were carried out and analyzed regarding the dynamic mechanic behavior of the material during the tests. The experiments were conducted in the high stress tensile regime of PEEK. After a short period of unsteady temperature increase distinct material changes were observed during the tests at medium and high load levels. The storage modulus increased continuously while the loss modulus, loss factor (tan δ) and the dissipation energy rate decreased continuously. It was concluded that the accumulation of residual stresses because of partially irreversible deformation causes this effect.     

Dielectric and impedance characteristics of Ba(Bi0.5Nb0.5)O3 ceramics

A lead free polycrystalline material Ba(Bi_0.5Nb_0.5)O₃ was prepared using a high-temperature mixed oxide technique using high purity ingredients. The formation of the material in monoclinic crystal structure was confirmed by an X-ray structural analysis at room temperature. The nature and texture of microstructure by scanning electron microscopy show that the compound has well defined grains uniformly distributed throughout the surface of the sample. Detailed studies of dielectric and impedance properties of the material, carried out in the frequency range of (1 kHz–1 MHz) at different temperatures (30 °C to 475 °C), have shown many interesting properties. Dielectric study showed an existence of diffuse phase transition around 317 °C. The temperature dependence of impedance parameters (impedance, modulus etc.) of the material exhibits a strong correlation of its micro-structure (i.e., bulk, grain boundary, etc.) with the electrical parameters. An existence of negative temperature coefficient of resistance (NTCR) type behavior in the material similar to that of semiconductors was also observed. The complex electric modulus analysis indicates the existence of hopping conduction mechanism in the system with non-exponential type of conductivity relaxation. The nature of variation of dc conductivity with temperature confirms the Arrhenius behavior of the material. The ac conductivity spectra show a typical signature of an ionic conducting system, and are found to obey Jonscher’s universal power law. The temperature dependent pre-exponential factor (A) shows peak and frequency exponent (n) possesses a minimum at transition temperature.     

Some effects of trace elements in reducing creep performance of high–strength superalloys

Abstract

In the high–strength superalloys used for ‘hot-end’ components in gas turbine engines, small amounts of elements with low melting points can have a seriously detrimental effect on high–temperature performance. Some of the more recent work carried out to define harmful levels of trace element content and to develop an understanding of the mechanisms involved is briefly reviewed in this paper. The emphasis is on results available for cast alloys; the problem of gaseous impurities in recycled material is also considered. The implications for the specification and control of trace element content are briefly discussed.

MST/161     

Diffusion bonding of ferritic oxide dispersion strengthened alloys to austenitic superalloys

The superior high temperature mechanical strength and oxidation resistance of ferritic oxide dispersion strengthened (ODS) tubular alloys are compromised by the difficulties encountered in joining. Conventional fusion welding techniques generate a weld fusion zone which is devoid of the mechanical strength exhibited by the base material. Therefore, more sophisticated solid state joining techniques, such as diffusion bonding, must be employed when joining ODS materials. This paper describes a series of solid state diffusion bonding experiments carried out between two tubular ferritic ODS alloys and two high temperature austenitic alloys. Careful control of bonding conditions produced pressure retaining joints between one of the tubular ODS alloys and both austenitic alloys. The successful joint design was incorporated into the manufacture of a tubular creep component, which enabled a series of internally pressurized creep tests to be carried out. The microstructure developed at the bond interface of each of the four separate material couples is described and the high temperature performance of the pressure retaining joints is discussed.     

Cracking of AFNOR 7020 aluminium alloy component: A metallurgical investigation

Aluminium alloy of Al–4.5Zn–1.5Mg system, belonging to age hardenable, high strength category is being used for the fabrication of various components. The Al alloy component was part of a conduit line for filling the liquid chemical to its storage tank. The component consisted of an extruded tube, TIG welded to a bulb, which was fabricated from Al alloy forgings. A crack was observed on the conduit tube at the region where tube was welded to the bulb. Detailed investigations were carried out on the cracked component, which revealed insufficient working of the ingot during forging, resulting in remnant cast structure along with a heavy network of low melting point compound throughout the material. The cracks, which were initiated under the stresses induced during thermal treatment, machining and assembly, were found propagating through the eutectic network.

This paper highlights the investigations carried out on the failed components.     

The effect of random grain distributions on fatigue crack initiation in a notched coarse grained superalloy specimen

Coarse grained superalloys are of large interest in high temperature applications, and can be found in e.g. gas turbine components, where great care must be given with respect to high temperature fatigue. Due to the large grain size, the material behaviour at e.g. sharp notches cannot be considered homogeneous. As a consequence, the fatigue behaviour is likely to expose a large variation. In order to numerically investigate this variation, a Monte Carlo analysis has been carried out by 100 FE-simulations of notched specimens, where placements and orientations of the grains were randomised. Furthermore, each grain was modelled as a unique single-crystal, displaying both anisotropic elastic and plastic behaviour and tension/compression asymmetry. The effect of randomness was investigated by the obtained dispersion in fatigue crack initiation life. It was concluded that the fatigue life behaviour of coarse grained nickel-base superalloys may show a considerable variation, which cannot be captured by one single deterministic analysis based on data for a homogenised material. Furthermore, the dispersion is of such a magnitude that it needs to be taken into account in industrial applications where highly stressed coarse grained materials are used.