High Temperature Failure Research Articles

High-temperature failure behaviour and mechanism of K-based additives in Li–Mg–N–H hydrogen storage systems

The high-temperature failure mechanism of K-based additives in a Mg(NH2)2–2LiH system was clarified for the first time.

Creep damage modelling of P92 pipe weld at 675°C

Creep data from ferritic steel P92 parent material, weld metal, cross-weld and heat affected zone specimens tested at 675°C are used to establish material behaviour models necessary for describing the creep behaviour of a P92 pipe with a circumferential weld. The method for determining the material properties for the heat affected zone of a P92 weld, in a single state variable creep damage constitutive model, using the data from cross-weld and impression creep tests, is described. The material properties for the parent material and weld metal regions have been already been published.Creep continuum damage finite element analyses are subsequently performed for a P92 pipe containing a circumferential weld, subjected to internal pressure and end loading, in order to assess the high temperature integrity and failure behaviour of a typical power plant component, from which the lifetime and potential failure modes can be predicted.

Long term out-of-pile thermocouple tests in conditions representative for nuclear gas-cooled high temperature reactors

During irradiation tests at high temperature failure of commercial Inconel 600 sheathed thermocouples is commonly encountered. As instrumentation, in particular thermocouples are considered safety-relevant both for irradiation tests and for commercial reactors, JRC and THERMOCOAX joined forces to solve this issue by performing out-of-pile tests with thermocouples mimicking the environment encountered by high temperature reactor (HTR) in-core instrumentation. The objective was to screen innovative sheathed thermocouples which would consecutively be tested under irradiation. Two such screening tests have been performed in high temperature environment (i.e. temperature in the range 1100–1150°C) with purposely contaminated helium atmosphere (mainly CH4, CO, CO2, O2 impurities) representative for high temperature reactor carburizing atmospheres. The first set of thermocouples embedded in graphite (mainly conventional N type thermocouples and thermocouples with innovative sheaths) was tested in a dedicated furnace at THERMOCOAX lab with helium flushing. The second out-of-pile test at JRC with a partly different set of thermocouples replicated the original test for comparison.Performance indicators such as thermal drift, insulation resistance and loop resistance were monitored. Through these long-term screening tests the effect of several parameters were investigated: niobium sleeves, bending, diameter, sheath composition as well as the chemical environment. SEM examinations were performed to analyze local damage (bending zone, sheath).The present paper describes the two tests, sums up data collected during these tests in terms of thermocouple behavior and describes further instrumentation testing work with fixed point mini cells for qualification under irradiation.

High temperature failure of natural gas feed burner pipe

The degradation of AISI 310 austenitic stainless steel pipe, which was used at high temperature in carbonaceous reducing atmosphere, was investigated in this work. In order to examine the causes of failure, various techniques including on-site investigation, optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction, metallography, and micro-hardness measurement were carried out. Scale separation and spallation were only observed at the internal surface of the pipe as a result of overheating service condition at the tip portion. The scale separation and spallation caused nozzle block and subsequent furnace shutdown. Growth of carbide precipitates and disintegration of alloys into dusts of coke and particles suggested carburization and metal dusting failure. It is recommended to monitor service temperature periodically at the tip portion. The service temperature should not exceed the designed value. Careful control of oxidizing/carburizing burning condition is strongly advised. Material selection is also discussed as an alternative means of failure prevention.

Microstructural evolution and high temperature failure of ferritic to austenitic dissimilar welds

Dissimilar metal welds between ferritic and austenitic alloys are used extensively in power plants. Premature failure of such welds can occur below the expected creep life of either base metal. This article reviews microstructural evolution in dissimilar welds and describes factors that contribute to premature failure. The microstructure in the as welded condition consists of a sharp chemical concentration gradient across the fusion line. Martensite forms within this gradient due to high hardenability and rapid cooling rates from welding. Upon aging, carbon diffuses down the chemical potential gradient from the ferritic steel toward the austenitic alloy. This can lead to formation of a soft carbon denuded zone in the ferritic steel, and nucleation and growth of carbides in the austenitic steel that produce high hardness. These differences in microstructure and hardness occur over distances of about 50–100 μm. Failure is attributed to the steep microstructural and mechanical property gradients, the large difference in coefficient of thermal expansion, formation of interfacial carbides that promote creep cavity formation, and preferential oxidation of the ferritic steel. Information is also provided on available creep rupture properties, remaining life estimation techniques, current best practices and research in progress.

A Microstructural Sensitivity Study of 316H Austenitic Stainless Steel to Inter-Granular Creep Fracture

A preliminary sensitivity examination of the ductility exhaustion based creep damage prediction model, currently used in the R5 high temperature assessment procedure, showed that material property inputs had significant effects on damage prediction. In the present work, the link between the microstructural factors and the susceptibility to inter-granular high temperature creep failure is considered. The latter was judged to be associated with the low creep ductility. Here, the longitudinal section of a creep specimen and the fracture surface were examined. Auger electron spectroscopy was used to investigate the grain boundary composition in this specimen, which failed after a creep test of 1038h at 550°C under a triaxial stress state. The present results demonstrate that there is a possibility to correlate the susceptibility to high temperature inter-granular fracture from the low temperature fracture investigations. Finally, the susceptibility of the pre-treated 316H stainless steel to inter-granular high temperature failure and the contribution to the creep damage model are briefly discussed.

High Temperature Service Experience and Failure Analysis of some Heat Resistant Stainless Steels

Construction materials are exposed to aggressive high temperature environments and it is important for materials selection to identify the failure mechanism of corroded components. This paper describes work done at Outokumpu Avesta Research Centre, Sweden to diagnose the high temperature problems of two different stainless steel components. It describes how the severity of the corrosion was the result of interaction of a number of factors, and outlines the measures taken to alleviate the problems. The discussion highlights how heavy carburization could be a problem for the heat resistant grade 310S, used as construction material for tanks in charcoal production and how Cl-containing flue gases heavily corrode stainless steel.

Borehole indentor — A tool for assessing in-situ bulk ice strength and micromechanics

Abstract National Research Council (NRC) borehole indentor (BHI) is increasingly being used in the Arctic for measuring in-situ bulk strength of ice and its seasonal variation in conjunctions with investigations on climate change. Before NRC-BHI system was taken to the Arctic, its performance was evaluated on columnar-grained S1 ice in Dow's Lake, Ottawa. S1 ice with its huge and clear grains is ideal for microstructural analysis of indented ice and understanding the micromechanisms of failures (ductile and brittle) under three-dimensionally confined BHI conditions involving large volumes of ice. The upper yield (UY) strength was shown to exhibit power-law dependence on average stress rate to UY. This power-law is universally applicable to low-salinity (0.3 ppt) brackish-water S2 ice and various types of sea ice — first-year S3, granular and frazil, second-year S3 and multi-year ridge ice. The concept of shift function is introduced for quantifying the effect of temperature on BHI-UY strength. For 0.62 m thick S1 ice sheet at an average temperature of − 3.8 °C (or 0.986 T m ) the failure indentation is constant for strength of 15 to 30 MPa or stress of 1.5 × 10 − 3 E to 3.0 × 10 − 3 E — analogous to high-temperature creep failure strains under comparable normalized stresses in metal and complex alloys — thus establishing the same physics of deformation and failure processes. For BHI tests, the preferred independent and controllable variable, however, is the indentation rate. Both BHI-UY and BHI-flow strength (for 5 mm penetration, say) also obey power-law dependence on indentation rate and 0.1 mm s − 1 is suggested for standardization. While UY failures are characterized by parabolic zone of deformed, recrystallized and HIPed (hot isostatically pressed) ice, premature brittle fractures occurring at speeds at or higher than 0.2 mm s − 1 , are caused by the propagation of cracks nucleated at the ice-indentor contact surface. Two different measures of the degree of maturity in brittle fractures are proposed — one based on failure time and the other on UY stress.

Use of instrumented micro-indentation to study the mesoscopic elasto-plastic behavior of GH4145/SQ superalloy during high-temperature cyclic straining

In this paper the instrumented micro-indentation testing (IIT) was used to study the mesoscopic elasto-plastic behavior of nickel-base superalloy GH4145/SQ subjected to cyclic straining at an elevated-temperature. For this purpose, a series of experiments including high-temperature low-cycle fatigue tests, instrumented micro-indentation measurements and OM (TEM) examinations were carried out. The characteristic parameters of indentation (C, hmax, S, hr, We and WP) during high-temperature fatigue failure process were determined from the experimental indentation load–depth (P–h) curves. The fatigue mesoscopic elasto-plastic properties such as E, σy and n were estimated using Dao et al.’s analysis algorithm [19], combined with the indentation characteristic parameters, and their distribution patterns were verified in a statistical sense. Microstructural examinations using both OM and TEM were performed to provide the micromechanisms for the fatigue mesoscopic elasto-plastic behavior of the superalloy during high-temperature low-cycle fatigue.

Finite Element Modelling and Experimental Testing of Thermomechanical Behaviour and Failure of Titanium Superplastic Forming Dies

This paper describes high temperature cyclic and creep relaxation testing and modelling of a high nickel-chromium material (XN40F) for application to life prediction of superplastic forming (SPF) tools. An experimental test programme to (i) characterise the high temperature cyclic elastic-plastic-creep behaviour of the material over a range of temperatures between 20oC and 900oC, including cyclic controlled strain-range tests at different strain-rates and creep relaxation tests, and (ii) identify the material constants relevant to thermo-mechanical fatigue (TMF) life prediction, is described. The objective of the material testing is the development of high temperature material and failure models for cyclic analyses and life prediction of SPF and diffusion bonding (DB) dies for titanium aerospace components.

Densification and High Temperature Deformation in Oxide Ceramics

Bulk ceramics are usually obtained from sintering of powders, involving both densification and growth. The kinetics of grain growth are examined with a view to producing bulk nanoceramics. The high temperature deformation and failure of oxide ceramics are also examined with reference to diffusion processes.

Some Fundamental Aspects of Redesign and Remanufacture of High Temperature Components

The impact of remanufacturing on the conservation of energy and resources has been well recognized during the last decade. When the relevant technologies are applied for high temperature components in power and process industries, a redesign of the component life should be required due to the time-dependent feature of high temperature failure. In order to provide some fundamentals for redesign and remanufacture of high temperature components, mechanical behavior of a two-bar structure with one bar being remanufactured is analyzed. An optimal repairing time is given. From the viewpoint of creep damage, various high temperature structures are analyzed by using damage coupled finite element method. Suggestions for life extension remanufacture are proposed for typical high temperature components.

2426 一方向凝固超合金の結晶成長軸と直交する高温疲労荷重下における破断面の形成におよぼす不均質組織の影響(S13-3 高温材料・構造の強度と破壊(3),S13 高温材料・構造の強度と破壊)

2426 一方向凝固超合金の結晶成長軸と直交する高温疲労荷重下における破断面の形成におよぼす不均質組織の影響(S13-3 高温材料・構造の強度と破壊(3),S13 高温材料・構造の強度と破壊)

High-temperature failure of GaN LEDs related with passivation

This paper analyses the thermally-activated failure mechanisms of GaN LED test-structures related with the presence of a hydrogen rich SiN passivation layer, by comparing the electrical and optical behaviour of samples with and without passivation during thermal stress. The analysis was carried out by means of electroluminescence , cathodoluminescence , emission microscopy and current–voltage measurements. Thermal treatment induced degradation only on the samples with passivation: identified degradation modes were an efficiency decrease exponential in time, emission crowding, and a forward voltage increase. On the other side, thermal treatment did not change the behaviour of the LEDs without passivation. An interpretation for the degradation of the passivated samples is the following: as a consequence of passivation deposition, a considerable amount of hydrogen is incorporated in the passivation layer. Heating at 250 ∘ C allows this hydrogen to interact with the LED surface, thus worsening the transport properties of p -GaN and of the p -ohmic contact, and then the current and emission distribution, inducing the observed degradation and emission crowding. The activation energy of the degradation process was found to be equal to 1.3 eV. Comparison between spectral electroluminescence and cathodoluminescence measurements shows how the mechanism mentioned above is not the only ageing cause and the thermal worsening of QW confinement and/or the creation of nonradiative centers possibly contribute to the LED damage.

Micro-Area Analysis of the Corrosion Products Formed on Alloy 825 Exposed to Simulated Waste Incineration Boiler Conditions

An analytical study of high-temperature corrosive failure in a waste incineration plant was conducted using a new type EPMA equipped with a Schottky type field emission gun. For sample preparation, a cross-section polisher that uses an argon ion beam was applied. Both the element distribution through the alloy substrate to the scale and the chemical states of sub-micron sized corrosion products were revealed, leading to inferences about the corrosion mechanism.

Dopant effect on high-temperature plastic flow behavior and grain boundary chemistry in oxide ceramics

The grain boundaries in high-purity oxide ceramics, such as Al 2 O 3 and TZP, are often free from glass phase, and the high-temperature plastic flow or grain boundary failure is sensitive to small levels of doping by various cations. For example, the high-temperature creep strain rate in fine-grained, polycrystalline Al 2 O 3 is highly retarded by 0.1 mol% Lu 3 + or Zr 4 + -doping. The elongation to failure in superplastic TZP is improved by 0.2 - 3 mol% Ge 4 + -doping. Such a dopant effect is attributed to changes of the grain boundary diffusion due to the segregation of dopant cation along the grain boundaries. Quantitative analysis on the atomic structure and chemical bonding state along the grain boundaries by high-resolution electron microscopy, energy-dispersive X-ray spectroscopy, electron energy-loss spectroscopy and molecular orbital calculations will provide theoretical guiding principles to design high-performance oxide ceramics in the near future.

Toughness and strength characteristics of Nb-W-Si ternary alloys prepared by Arc melting

This article describes the room-temperature and high-temperature mechanical properties and failure modes of series Nb-W-Si alloys—Nb-10W, Nb-10Si, Nb-10Si-5W, Nb-10W-5Si, and Nb-10W-10Si—prepared by arc melting. For the Nb-10W alloy, the microstructure was a monolithic Nb solid solution (Nbss) with a grain size up to a few hundred microns, while the other four alloys consisted of primary Nbss and a eutectic of Nbss/Nb5Si3 (5-3 silicide) as a result of replacing Nb with Si. Among all alloys, the Nb-10W showed the highest fracture toughness of about 15.3 MPa√m1/2 and the lowest 0.2 pct yield compressive strength of 90 MPa at 1670 K. Conversely, the Nb-10Si-10W had the highest 0.2 pct yield strength of about 330 MPa at 1670 K and the lowest fracture toughness of 8.2 MPa√m1/2. It is suggested that toughness is supplied by the metallic Nbss phase, while high-temperature strength is mainly provided by the brittle silicide phase. For the Nb-10W alloy with the monolithic Nbss, intergranular cleavagelike crack propagation is the fracture mode at room temperature, and dislocation movement within the grains and grain-boundary sliding are the dominant modes of high-temperature failure. With two-phase Nbss/Nb5Si3 microstructures, the compressive damage of all four alloys at high temperature was dominated by debonding of the interfaces between the Nbss and the silicide; however, the fracture mode at room temperature is transgranular, controlled by the primary Nbss cleavage.

Some selected topics in creep cavitation

The nucleation and growth of intergranular cavitation is one of the major contributors to high-temperature failure. We focus here on three developments in this area. The first is the effect of combined creep deformation and diffusive mass transport upon void growth. Second, we discuss environmental influences upon cavity growth, principally in the form of gas-producing internal reactions. Finally, we outline some of the experimental observations concerning randomness in creep cavitation and modeling efforts intended to capture some of these random effects.

Structural analysis of electrosleeved tubes under severe accident transients

A flow stress model was developed for predicting failure of electrosleeved PWR steam generator tubing under severe accident transients. The electrosleeve, which is nanocrystalline pure nickel, loses its strength at temperatures greater than 400°C during severe accidents because of grain growth. A grain growth model and the Hall–Petch relationship were used to calculate the loss of flow stress as a function of time and temperature during the accident. Available tensile test data, as well as high-temperature failure tests, on notched electrosleeved tube specimens were used to derive the basic parameters of the failure model. The model was used to predict the failure temperatures of electrosleeved tubes with throughwall and part-throughwall axial cracks in the parent tube during a postulated severe accident transient.

Oxidation of Inconel 718 in Air at High Temperatures

Experiments were conducted with Inconel 718 at high temperatures to evaluate the rate of oxidation of the material over as wide a temperature range as possible, as well as to determine the high-temperature failure limit of the material. Samples of Inconel 718 were inserted into preheated furnaces at temperatures ranging from 973 to 1620 K and oxidized in air for varying periods of time. After being oxidized in air at a constant temperature for the prescribed time and then being allowed to cool, the samples were reweighed to determine their mass gain due to the uptake of oxygen. From these mass-gain measurements, it was possible to identify three regimes of oxidation for Inconel 718: a low-temperature regime in which the samples behaved as if passivated after an initial period of transient oxidation, an intermediate-temperature regime in which the rate of oxidation was limited by diffusion and exhibited a constant parabolic rate dependence, and a high-temperature regime in which material deformation and damage accompanied an accelerated oxidation rate above the parabolic regime.