Fatigue and creep fatigue crack growth behaviour of alloy 800 at 550°C

Fatigue and creep fatigue crack growth behaviour of alloy 800 at 550°C have been studied to analyse defect assessment in a steam generator. Different grades of alloy 800 have been investigated to reproduce the in service conditions. Fatigue crack growth (FCG) tests were conducted on CT20 and tubular specimens, then on welded tubes. Furthermore the influence of hold times on fatigue crack growth behaviour was studied.

The results obtained on material simulating the weld heat affected zone are in agreement with the tests conducted on welded tubes. Fatigue crack growth characteristics of aged and cold-worked aged material seem to be slightly improved in comparison with base material. Finally a hold time of one minute increases strongly the FCG threshold value determined in pure fatigue but has a negligible influence on crack growth rates.     

Creep fatigue behaviour of Type 321 stainless steel at 650°C

Abstract

Type 321 austenitic stainless steel has been used in the UK’s advanced gas cooled reactors for a wide variety of thin section components which are within the concrete pressure vessel. These components operate at typically 650°C and experience very low primary stresses. However, temperature cycling can give rise to a creep fatigue loading and the life assessment of these cycles is calculated using the R5 procedure. In order to provide materials property models and to validate creep fatigue damage predictions, the available uniaxial creep, fatigue and creep fatigue data for Type 321 have been collated and analysed. The analyses of these data have provided evolutionary models for the cyclic stress strain and the stress relaxation behaviour of Type 321 at 650°C. In addition, different methods for predicting creep fatigue damage have been compared and it has been found that the stress modified ductility exhaustion approach for calculating creep damage gave the most reliable predictions of failure in the uniaxial creep fatigue tests. Following this, validation of the new R5 methods for calculating creep and fatigue damage in weldments has been provided using the results of reversed bend fatigue and creep fatigue tests on Type 321 welded plates at 650°C in conjunction with the materials properties that were determined from the uniaxial test data.     

An experimental investigation of mixed-mode creep crack growth in Jethete M152 at 550°C

Creep crack growth tests have been made on Jethete M152 at 550°C under initially mixed-mode (i.e. K_I/K_II ≈ 1.6) and mode-II crack tip conditions using compact mixed-mode (CMM) specimens. The results of these tests have been compared with mode-I data obtained from compact tension (CT) tests, using a C* approach. The correlation between the mode-I, mode-II and mixed-mode data is reasonably good. However, the scatter band is greater than that obtained from the mode-I results only. The results indicate that the C* approach, which has been used successfully in mode-I situations, may also be useful for predicting creep crack growth in more general situations.     

High temperature oxidation behavior of ferritic steel in supercritical water at 550–700°C

Oxidation tests were conducted on ferritic steel T22 exposed to deaerated supercritical water at 550–700°C and 25 MPa. Oxide films formed on T22 had a double-layered structure with the outer layer consisting of iron oxide and the inner layer consisting of spinel oxide. Pores formed on the surface of samples initially but healed at longer exposure time and higher temperature. Cracks occurred along the grain boundaries in the oxide scale at 600–700°C for 200 and 400 h. The oxidation kinetics obeyed a near-parabolic law in all cases. The data of activation energy of T22 indicated that the likely oxidation rate-controlling step may be the outward diffusion of iron along the magnetite bulk.     

Wear behaviour at 600°C of surface engineered low-alloy steel containing TiC particles

The work aimed to develop surfaces that could resist wear at high temperatures, thus achieving a prolonged component life. Surface modification of a low-alloy steel by incorporating TiC particles has been undertaken by melting the surface using a tungsten inert gas torch. The dry sliding wear behaviour at 600°C of the original and modified surfaces was compared. Microscopic examination of both surfaces showed glazed layers across the wear tracks, with differing amounts of oxide and homogeneity. Extensive wear occurred on the steel surface, which showed deformation of the wear scar tracks and a steadily increased friction coefficient. The TiC addition reduced the wear loss, coinciding with a glazed layer 33% thinner than that on the low-alloy steel sample.     

Fatigue crack growth of Fe81B13.5Si3.5C2 amorphous metal using magnetostrictive behaviour to generate ΔK

The fatigue crack growth behaviour of Fe₈₁B_13.5Si_3.5C₂ amorphous metal (Metglas 2605SC) was examined. An experimental apparatus was developed to generate the applied stress intensity range, _K using the magnetostrictive behaviour of the material. Cycles of elastic strain were accumulated at the natural frequency of the specimens (44 or 56 kHz) with the crack length being monitored optically at certain cycle increments. Through-the-thickness, centre-cracked panel specimens were tested in the as-cast and annealed conditions. Annealing temperatures were 250 and 450 °C. Several specimens annealed at 300 °C, while being subjected to a transverse magnetic field, were also tested. Examination of the fatigue crack growth rate data indicated no differences between the as-cast and the annealed only specimens. The specimens that were annealed while being subjected to the transverse magnetic field, however, exhibited much greater resistance to fatigue crack growth. Microscopy using a Kerr-effect magneto-optical microscope revealed that the magnetic domain boundaries within the material exerted a significant influence on the direction of the fatigue crack propagation and the overall crack growth rate behaviour.     

Thermal expansion of hydroxyapatite between − 100 °C and 50 °C

Hydroxy apatite (HAp) ceramic was synthesized using traditional sintering. Dilatometric and lattice thermal expansion properties of a HAp ceramic were evaluated at temperatures of ? 100–50 °C. In that temperature range, the dilatometric thermal expansion coefficient and the lattice thermal expansion coefficient of the HAp ceramic were, respectively, 10.6 × 10^? 6/°C and 9.9 × 10^? 6/°C. Furthermore, thermal expansion properties of a human tooth were measured. The thermal expansion coefficient of the horizontal direction perpendicular to the growing direction of a tooth was 15.5 × 10^? 6/°C; that of the vertical direction along with the direction of tooth growth was 18.9 × 10^? 6/°C at the temperature range described above.     

Tests on impact behaviour of micro-concrete-filled steel tubes at elevated temperatures up to 400°C

Concrete-filled tubular (CFT) columns are being more and more utilized in construction of tall buildings and bridges. The CFT column system, which has been proved to have excellent load carrying capacity and ductility, by static and simulated seismic loading tests, also has good dynamic impact behaviour. The impact resistance of small-size micro-concrete-filled steel tubes under axial impact load at elevated temperatures up to 400°C was experimentally studied by using a spilt Hopkinson pressure bar. The stress and strain time history curves of the tested specimens were recorded to analyze the impact behaviour of CFT at elevated temperatures. The failure patterns and the influence of temperature on the impact resistance of CFT are discussed. The test results show that CFT has an excellent impact resistant capacity at elevated temperatures and the dynamic behaviour of core concrete under high temperatures was discovered. A simplified calculation method to determine the impact resistant capacity of CFT at elevated temperatures is presented, which is validated by the tested results.     

Environmental effect on fatigue strength of stainless steel in PWR primary water – Role of crack growth acceleration in fatigue life reduction

The influence of the pressurized water reactor (PWR) water environment on fatigue life and fatigue crack growth rate was discussed. The fatigue lives of Type 316 stainless steel in the PWR water environment were investigated using cylindrical hollow specimens. The acceleration in the crack growth due to the environment was quantified by investigating spacing of striations and crack growth tests using compact tension specimens. The growth rates obtained could be represented by the strain intensity factor. It was shown that the fatigue lives estimated by crack growth prediction agreed with those obtained by the tests. Then, it was concluded that the reduction in the fatigue life due to the PWR water environment was brought about not by enhancement of crack initiation but by the acceleration of the crack growth.     

Cold sintering of magnetic BaFe12O19 and other ferrites at 300 °C

Densifying ZnFe₂O₄ and BaFe₁₂O₁₉ at 300 °C to values greater than 90% of theoretical is demonstrated via hydroflux-assisted densification (HAD), a derivative of the cold sintering process employing non-aqueous, flux-based mass transport phases to facilitate particle consolidation. Previous attempts to cold sinter these materials with aqueous-based mass transport phases were not as successful with final densities?<?90%. Attempts to densify NiFe₂O₄ and (Ni_0.5Zn_0.5)Fe₂O₄ only achieved densities around 80%, indicating an alternative transport phase may be needed to achieve high densities in Ni-containing materials. Magnetic hysteresis measurements of the low-temperature densified BaFe₁₂O₁₉ samples produced magnetic saturation values as high as 93 emu/g and coercive fields as high as 1789 Oe, which are comparable to values reported in the literature for this material produced via other processing techniques. Additional techniques are suggested to further optimize the magnetic properties of BaFe₁₂O₁₉ densified following the HAD approach.

     

The isothermal section of the Al-Si-B ternary system at 700 °C and its applications

The isothermal section of the Al−Si−B ternary system at 700 °C has been calculated using FactSage software and experimentally determined using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and x-ray powder diffraction(XRD). The calculated results show that there are six three-phase regions in the isothermal section: Liquid-Al+AlB₂+(Si), AlB₂+AlB₁₂+(Si), (Si)+AlB₁₂+SiB₃, AlB₁₂+SiB₃+SiB₆, AlB₁₂+SiB₆+SiB₁₄, AlB₁₂+SiB₁₄+(B). Four three-phase regions of Liquid-Al+AlB₂+(Si), AlB₂+AlB₁₂+(Si), (Si)+AlB₁₂+SiB₃ and AlB₁₂+SiB_n+(B) were confirmed experimentally, which are consistent with calculated results. The small AlB₂ particles in the Al-3B alloy have good refining effect on the primary α-Al phase in the Al-10Si alloy, which greatly refines the microstructure and improves the mechanical properties of the Al-10Si alloy.     

Effect of water vapour on growth and adherence of chromia scales formed on Cr in high and low pO2-environments at 1000 and 1050°C

Abstract

The oxidation behaviour of pure Cr at 1000 and 1050°C was studied in Ar–O₂ and Ar–H₂–H₂O mixtures. It was found that in the low-pO₂ gases the oxide scales exhibited higher growth rates than in the high-pO₂ gases. The scales formed in the low-pO₂ gases showed substantially better adherence during cooling, than scales formed in the high-pO₂ gases. These differences in growth rate and adherence can be correlated with differences in size and location of the in-scale voids formed during the isothermal exposure. Exposures in Ar-O₂-H₂O mixtures revealed that the differences in scale growth rates as well as in scale void formation and growth are not primarily related to differences in the oxygen partial pressure of the atmosphere but to the presence of water vapour in the test gas. At sufficiently high H₂O/O₂-ratios, water vapour promotes oxide formation at the scale/metal interface thereby suppressing excessive growth of existing voids, and also as a consequence improved scale adherence. Whether the enhancement of inward scale growth is related to transport of H₂O- or H₂-molecules or due to OH^? ions, cannot be derived with certainty from the present results.     

Evolution of M23C6 phase in HR3C steel aged at 650 °C

The evolution of the morphology, composition, and particle size of the M₂₃C₆ phase in HR3C steel aged at 650 °C was analysed, by means of metallographic microscopy, scanning electron microscopy, and transmission electron microscopy. The results showed that the Cr/Fe ratio in the M₂₃C₆ phase, in the form of irregular square and long strip, increased with the ageing time. Fe was gradually replaced by Cr in the M₂₃C₆ lattice. The M₂₃C₆ phase gradually evolved and then became stable after ageing for 2000 h.     

Fatigue-creep interaction performance of Incoloy 825 nickel-based superalloy at 650 °C

The fatigue-creep interaction performance of Incoloy 825 nickel-based superalloy at 650 °C was investigated through introducing the tensile, compressive, and tensile-compressive strain hold time at the controlled total strain amplitude Δϵ_t = 0.3 %∼0.7 %. The results show that the Incoloy 825 nickel-based superalloy exhibits the cyclic hardening behavior, the cyclic hardening behavior followed by cyclic softening behavior and the cyclic hardening behavior followed by cyclic stability during the cyclic deformation with tensile strain hold time, while the alloy exhibits the cyclic hardening behavior and the cyclic hardening behavior followed by the cyclic stability during the cyclic deformation with compressive and tensile-compressive strain hold time. The relationship between both plastic and elastic strain amplitudes with reversals to failure for the alloy shows a single slope linear behavior, which can be described by the Coffin-Manson and Basquin equations, respectively. The deformation mechanism of the alloy under three loading condition of fatigue-creep interaction is mainly the planar slip. In addition, under three loading condition of fatigue-creep interaction, the cracks initiate and propagate in the transgranular mode.     

Effect of grain size on the irradiation response of grade 91 steel subjected to Fe ion irradiation at 300 °C

Journal of Materials Science - Irradiation using Fe ion at 300&nbsp;°C up to 100 dpa was carried out on three variants of Grade 91 (G91) steel samples with different grain size ranges:...     

Microstructural characterization of nitrided beta Ti–Mo alloys at 1400 °C

In this work, the two Ti₉₂Mo₈ and Ti₈₄Mo₁₆ alloy compositions were gas nitrided at 1400 °C. The microstructure and the chemical composition of the gas nitrided surfaces were investigated by scanning electron microscopy and by electron probe microanalysis. Two internal needle-like nitride precipitates, α-(Ti,N) and δ-TiN_0.3, were observed. Their crystallographic orientation relationships in the β matrix were determined by electron backscattering diffraction.     

Accelerated transformation of brushite to octacalcium phosphate in new biomineralization media between 36.5 °C and 80 °C

This study investigated the hydrothermal transformation of brushite (dicalcium phosphate dihydrate, DCPD, CaHPO₄·2H₂O) into octacalcium phosphate (OCP, Ca₈(HPO₄)₂(PO₄)₄·5H₂O) in seven different newly developed biomineralization media, all inspired from the commercial DMEM solutions, over the temperature range of 36.5 °C to 90 °C with aging times varying between 1 h and 6 days. DCPD powders used in this study were synthesized in our laboratory by using a wet-chemical technique. DCPD was found to transform into OCP in the Ca²⁺, Mg²⁺, Na⁺, K⁺, HCO₃^?, Cl^? and H₂PO₄^? containing aqueous biomineralization media in less than 72 h at 36.5 °C, without stirring. The same medium was able to convert DCPD into OCP in about 2 h at 75–80 °C, again without a need for stirring. Samples were characterized by using powder X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM).     

Hot corrosion behavior of porous nickel-based alloys containing molybdenum in the presence of NaCl at 750 °C

The hot corrosion of porous Ni-23Cr-xMo (0%, 4.5%, 9.0%, 13.5%, mass fraction) alloys tested at 750 °C under cyclic procedure was investigated in order to elucidate the effect of Mo addition on hot corrosion in the presence of NaCl. The hot corrosion experiments were performed at 750 °C in air with 4 mg cm^− 2 NaCl deposit. The performance of the alloys was evaluated by the results of weight change kinetics. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS) were used to characterize the corrosion products. The results indicate that NaCl accelerated the oxidation of the alloys by chloridized elements Mo and Cr. Among the porous Ni-23Cr-xMo alloys, Ni-23Cr-9Mo alloy exhibited the best hot corrosion resistance due to the formation of NiO-NiCr₂O₄-Cr₂O₃ oxide scales. Furthermore, these oxide scales were confirmed more effective to protect the alloys after adding of Mo.