Vibration failure in admission pipe of a steam turbine due to flow instability

High vibrations in admission piping of a steam turbine were analyzed. Vibration failure was detected after piping modification as part of upgrading a 300 MW power turbine plant searching for 10% power increment. However, after 1 year operation a vibration malfunction was detected in control valve and fittings of income piping with risk of cracking for maximum output. A study of computational fluid dynamics (CFD) revealed large steam flow instabilities produced by recirculation and high velocity exceeding a critical point. Measurements of natural frequency piping system with the turbine stall and subsequent measurements of frequency and vibration analysis during turbine operation indicated that recirculating flow plays a main role in the vibration problem by resonance. The paper discusses CFD results obtained with a proposed pipe configuration that reduces turbulence effects. Combined pressure slide and diameter increment in piping lead to reduced vibration turbine operation.     

Study on plastic deformation behavior of hot splitting spinning of TA15 titanium alloy

The plastic deformation behavior of hot splitting spinning of TA15 titanium alloy is a complex metal forming problem with multi-factor coupling interactive effects. In this paper, on condition of considering various thermal effects, a three-dimensional (3D) elastic–plastic coupled thermo-mechanical finite element (FE) model of hot splitting spinning of TA15 titanium alloy is established using the dynamic, temp-disp, explicit module of FE software ABAQUS. Based on the analysis of flow behaviors of TA15 titanium alloy, the mechanism and influence of materials plastic deformation behavior during the forming process are studied. The results show that, the flow stress of TA15 titanium alloy generally decreases with the increase of deformation temperature; at the same strain rate, the higher temperature is, the lower flow stress is. The temperature distributions along the circumferential direction of disk blank are even and the temperature of plastic deformation area is about 984 °C. The heat from plastic deformation and friction at local plastic deformation area is less than the dissipated heat, so the temperature just falls into approximately 945 °C. Radial spinning force as the driving force of plastic deformation increases gradually and reaches about 35 kN at the end. The maximum value of equivalent stress is presented in fillet part between disk blank and two mandrels. The distributions of equivalent plastic strain appear the large strain gradients and the obvious characteristics of inhomogeneous deformation. When friction factor on interfaces between disk blank and dies ranges from 0.4 to 0.6, the forming quality and precision are highest.     

Factors influencing arsenic and nitrate removal from drinking water in a continuous flow electrocoagulation (EC) process

An experimental study was conducted under continuous flow conditions to evaluate some of the factors influencing contaminant removal by electrocoagulation (EC). A bench-scale simulation of drinking water treatment was done by adding a filtration column after a rectangular EC reactor. Contaminant removal efficiency was determined for voltages ranging from 10 to 25 V and a comparative study was done with distilled water and tap water for two contaminants: nitrate and arsenic(V). Maximum removal efficiency was 84% for nitrate at 25 V and 75% for arsenic(V) at 20 V. No significant difference in contaminant removal was observed in tap water versus distilled water. Increase in initial As(V) concentration from 1 ppm to 2 ppm resulted in a 10% increase in removal efficiency. Turbidity in the EC reactor effluent was 52 NTU and had to be filtered to achieve acceptable levels of final turbidity (5 NTU) at steady-state. The flow regime in the continuous flow reactor was also evaluated in a tracer study to determine whether it is a plug flow reactor (PFR) or constantly stirred tank reactor (CSTR) and the results show that this reactor was close to an ideal CSTR, i.e., it was fairly well-mixed.     

Constitutive modeling of deformation in high temperature of a forging 6005A aluminum alloy

The 6005A aluminum alloy is one of the most widely used alloys in aeronautic and railway industries, yet its plastic deformation behavior under hot compression is still not fully understood. Isothermal compression tests of 6005A aluminum alloy were performed using a Gleeble-1500 device, up to a 70% height reduction of the sample at strain rates ranging from 0.01 s⁻¹ to 10 s⁻¹, and deformation temperatures ranging from 573 K to 773 K. Several modeling approaches, including flow stress–strain curves, a constitutive Arrhenius-type equation model, and processing maps were used to characterize the deformation behavior of the isothermal compression of 6005A aluminum alloy in this study. The related material constants (i.e. A, β and α) as well as the activation energy Q for 623–773 K and 573–623 K temperature regimes were determined. Two sets of constitutive exponent-type equations for the 6005A aluminum alloy were proposed. Furthermore, a change in deformation mechanism occurred when changing the temperature range from 623–773 K to 573–623 K.     

Workability characteristics and mechanical behavior modeling of severely deformed pure titanium at high temperatures

In the present study, compression tests were performed at temperatures of 600–900 °C and at strain rates of 0.001–0.1 s⁻¹ to study the deformation and workability characteristics of commercially pure titanium after severe plastic deformation (SPD). It was found that the effects of temperature and strain rate are significant in dictating the steady state flow stress levels and the strain values corresponding to peak flow stress. The strain rate sensitivity (m) during hot compression of severely deformed Ti was shown to be strongly temperature dependent, where m increased with the increase in deformation temperature up to 800 °C. High temperature workability was analyzed based on the flow localization parameter (FLP). According to the FLP values, deformation at and below 700 °C is prone to flow localization. The flow behavior was predicted using Arrhenius type and dislocation density based models. The validities of the models were demonstrated with reasonable agreement in comparison to the experimental stress–strain responses.     

Plastic collapse of a stainless steel pressurized tube

The failure of a stainless steel tube which conducted oil at 300 °C has been analysed. The fracture surface of the broken tube was studied in the scanning electron microscope and the fracture mechanism found was the nucleation, growth and coalescence of voids. This mechanism is characteristic in materials plastically deformed before failure. Specimens for metallographic examination were cut from the damaged tube and from an intact tube to analyse both microstructures. No significant changes which could justify a microstructure’s embrittlement were detected. Hardness measurements were performed on the damaged and intact tubes. The broken tube was harder than the intact tube due to plastic deformation accumulated during failure. The pressure which is necessary to reach this hardening was analysed by the deformation theory of plasticity and it was found this pressure is close to that corresponding to the plastic instability. Consequently, the most plausible hypothesis of failure was due to an over-pressure which leads to the tube’s plastic collapse.     

An analysis of the deformation characteristics of a dual phase twinning-induced plasticity steel in warm working temperature regime

The deformation behavior of a dual phase twinning induced plasticity (TWIP) steel has been studied by means of continuous heating compression (CHC) testing technique. This has been performed in the range of room temperature to 300 °C (warm working regime) and the related experimental flow behavior has been compared with the theoretical ones. The derived deviations at 45 ± 5 °C, 100 ± 5 °C and 165 ± 5 °C have been properly addressed considering the related microstructural evolutions. The optical and scanning electron microscopy along with feritscope measurements have been carried out to explore the basis of any deviation. The results demonstrate the formation of ferrite at the austenite grain boundaries through deformation induced ferrite transformation mechanism. This effectively makes the structure softer at the initial stage of deformation (deviation i, 45 ± 5 °C). The initiation of twins within the austenite grains results in strengthening the structure and a small bump appears in the θ–ε curve (deviation ii, 100 ± 5 °C). In addition the responsible deformation mechanism of the steel is believed to change from mechanical twinning to dislocation slip at about 160 °C thereby a local decrease in the rate of work hardening occurs (deviation iii, 165 ± 5 °C).     

High frequency magnetic eigen excitations in a spin valve submitted to CPP DC current

We study the magnetization dynamics induced at low field by spin-transfer in a pillar-shaped spin valve. The spin valve is a square of 150 nm× 150 nm patterned from a film of IrMn 7 nm/CoFe, 2.4 nm/Ru, 0.8 nm/CoFe, 4.4 nm/Cu, 2.6 nm/CoFe, and 3.6 nm. The spin valve is studied in its anti-parallel state at 50 K. The high frequency voltage noise generated by the DC current flowing through the magneto-resistive device is used to identify the excitations induced by spin-transfer. Between an instability current of 1.72 mA and the switching current of 1.89 mA, we demonstrate the existence of pre-switch steady-state excitations, i.e. low amplitude precession. We study the frequency (10 GHz, red shift −1.46 GHz/mA) of this excitation, its line width (78–246 MHz), the power it carries (113 nW), and the current dependance thereof. We discuss those experimental findings using the formalism of Sun et al. and Valet et al., and show that the experimental behavior can be described by a macrospin approximation only at the very onset of the pre-switch excitations. The early saturation of the excitation power and the non-monotonic switching probability with the current are experimental indications that the pre-switch excitations are strongly non-uniform when approaching the switching current.     

Constitutive analysis of the hot deformation behavior of Fe–23Mn–2Al–0.2C twinning induced plasticity steel in consideration of strain

In this study, constitutive analysis has been carried out on Fe–23Mn–2Al–0.2C twinning induced plasticity (TWIP) steel. For this purpose, hot compression tests were conducted on a Gleeble-3500 thermo-mechanical simulator in the temperature range of 900–1150 °C and the strain rate range of 0.001–20 s⁻¹. The effects of deformation heating and friction on flow stress were analyzed and corrected. On the basis of Sellars–Tegart–Garofalo equation, the strain-dependent constitutive equations of the steel were derived. The results show that deformation heating has a significant influence on the flow stress at lower temperatures and higher strain rates, while the frictional effect is slight even at the highest strain level investigated. Comparison of the calculated flow stress with the experimental data suggests that the developed constitutive equations can adequately describe the relationships between the flow stress, strain rate, temperature and strain of the steel during hot deformation. This is supported by a high correlation coefficient (R = 0.996) and a low average absolute relative error (AARE = 3.31%) for the entire deformation condition range investigated.     

Effect of particle size on densification of pure magnesium during spark plasma sintering

The effect of particles size ranges (<38 μm, 75–150 μm, 270–550 μm) of atomized magnesium powders on densification mechanisms during spark plasma sintering (SPS) process was investigated. The intrinsic driving force, local pressure and current of Mg powders with different particle sizes were analyzed by theoretical calculation. The results obviously indicate that the densification of pure magnesium can be improved by the reduction of particle size, suggesting the intrinsic driving force, local pressure and current intensity are enhanced significantly by a decrease in the particle size at the same sintering conditions, which can promote shrinkage of pores, formation of the sintering neck and mass transportation in the SPS process. Not only that, rapid densification is also interpreted in term of mechanical movement of particles, Joule heating effect and plastic deformation. However, the mechanical movement of the large particles is higher than that of small particles due to high punch displacement, and plastic deformation, detected by scanning electron microscopy, plays a main role in densification for large particles in the case during the sintering. Joule heating effect is the key factor for densification of small Mg particles, and high densification degree can be obtained by sintering small particles.     

A novel severe plastic deformation method for fabricating ultrafine grained pure copper

A novel severe plastic deformation (SPD) method entitled elliptical cross-section spiral equal-channel extrusion (ECSEE) was proposed to fabricate ultrafine grained (UFG) pure copper. The principle of ECSEE process was adopted to accumulate shear stress within the workpiece without any cross-section area change. In order to primarily demonstrate the deformation characteristic and refinement ability of ECSEE method, the simulated and experimental investigations were both done. In the case of simulation, the ECSEE-ed workpiece containing scribed grids was analyzed for the flow net change. Simulated results indicated the trend of effective strain distribution decreased from the circumferential area to the central area on the cross-section of ECSEE-ed workpiece. In experimental investigations of a single-pass of ECSEE, a significant grain refinement from 10–50 μm to 1–10 μm was mainly in the circumferential area of the cross-section for processed workpieces. During the ECSEE deformation, shear strain as an essential role conduced the grain refinement. Besides, a significant increase of hardness, from ∼40 Hv to ∼85 Hv, was examined. The distribution characteristic of refinement and hardness were both consistent with that of simulated effective strain.     

An investigation into the mechanical behavior of a new transformation-twinning induced plasticity steel

In recent years, the transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) steels have been the focus of great attention thanks to their excellent tensile strength-ductility combination. Accordingly the mechanical behavior of an advanced microalloyed TRIP–TWIP steel, the compression tests were conducted from 25 to 1000 °C. This experimental steel shows a high compressive strength of 1280 MPa with the yield strength of 385 MPa as well as an outstanding strain hardening rate of about 14,000 MPa at the 25 °C. In addition the results indicate that the plastic deformation in the range of 25–150 °C is controlled by both the strain-induced martensite formation and mechanical twinning. However the mechanical twinning has been speculated as the only deformation mechanism in the temperature range of 150–1000 °C. This as well has led to an outstanding grain refinement via grain partitioning. The occurrence of mechanical twinning at such high temperatures is a novel observation in this grade of TRIP–TWIP high manganese steels.     

Hot tensile deformation behaviors and constitutive model of 42CrMo steel

The hot tensile deformation behaviors of 42CrMo steel are studied by uniaxial tensile tests with the temperature range of 850–1100 °C and strain rate range of 0.1–0.0001 s⁻¹. The effects of hot forming process parameters (strain rate and deformation temperature) on the elongation to fracture, strain rate sensitivity and fracture characteristics are analyzed. The constitutive equation is established to predict the peak stress under elevated temperatures. It is found that the flow stress firstly increases to a peak value and then decreases, showing a dynamic flow softening. This is mainly attributed to the dynamic recrystallization and material damage during the hot tensile deformation. The deformation temperature corresponding to the maximum elongation to fracture increases with the increase of strain rate within the studied strain rate range. Under the strain rate range of 0.1–0.001 s⁻¹, the localized necking causes the final fracture of specimens. While when the strain rate is 0.0001 s⁻¹, the gage segment of specimens maintains the uniform macroscopic deformation. The damage degree induced by cavities becomes more and more serious with the increase of the deformation temperature. Additionally, the peak stresses predicted by the proposed model well agree with the measured results.     

Hot deformation behavior of an austenitic Fe–20Mn–3Si–3Al transformation induced plasticity steel

Hot deformation behavior of an austenitic Fe–20Mn–3Si–3Al transformation induced plasticity (TRIP) steel was investigated by hot compression tests on Gleeble 3500D thermo-mechanical simulator in the temperature ranges of 900–1100 °C and the strain rate ranges of 0.01–10 s⁻¹. The results show that the flow stress is sensitively dependent on deformation temperature and strain rate, and the flow stress increases with strain rate and decreases with deformation temperature. The peak stress during hot deformation can be predicted by the Zener–Hollomon (Z) parameter in the hyperbolic sine equation with the hot deformation activation energy Q of 387.84 kJ/mol. The dynamic recrystallization (DRX) is the most important softening mechanism for the experimental steel during hot compression. Furthermore, DRX procedure is strongly affected by Z parameter, and decreasing of Z value lead to more adequate proceeding of DRX.     

Microstructure and mechanical properties of duplex stainless steel subjected to hydrostatic extrusion

The nanostructure and mechanical properties of ferritic-austenitic duplex stainless steel subjected to hydrostatic extrusion were examined. The refinement of the structure in the initial state and in the two deformation states (ε = 1.4 and ε = 3.8) was observed in an optical microscope (OM) and a transmission electron microscope (TEM). The results indicate that the structure evolved from microcrystalline with a grain size of about 4 μm to nanocrystalline with a grain size of about 150 nm in ferrite and 70 nm in austenite. The material was characterized mechanically by tensile tests performed in the two deformation states. The ultimate strength appeared to increase significantly compared to that in the initial deformation stages, which can be attributed to the grain refinement and plastic deformation. The heterogeneity observed in microregions results from the dual-phase structure of the steel. The results indicate that hydrostatic extrusion is a highly potential technology suitable for improving the properties of duplex steels.     

Design and performance of a 4He-evaporator at <1.0 K

A helium evaporator for obtaining 1 K temperature has been built and tested in laboratory. This will function primarily as the precooling stage for the circulating helium isotopic gas mixture. This works on evaporative cooling by way of pumping out the vapour from the top of the pot. A precision needle valve is used initially to fill up the pot and subsequently a permanent flow impedance maintains the helium flow from the bath into the pot to replenish the evaporative loss of helium. Considering the cooling power of 10 mW @1.0 K, a 99.0 cm³ helium evaporator was designed, fabricated from OFE copper and tested in the laboratory. A pumping station comprising of a roots pump backed by a dry pump was used for evacuation. The calibrated RuO thermometer and kapton film heater were used for measuring the temperature and cooling power of the system respectively. The continuously filled 1 K bath is tested in the laboratory and found to offer a temperature less than 1.0 K by withdrawing vapour from the evaporator. In order to minimize the heat load and to prevent film creep across the pumping tube, size optimization of the pumping line and pump-out port has been performed. The results of test run along with relevant analysis, mechanical fabrication of flow impedance are presented here.     

Aluminum matrix composites reinforced with multi-walled boron nitride nanotubes fabricated by a high-pressure torsion technique

Aluminum matrix composites loaded with various fractions of multi-walled, well-structured boron nitride nanotubes (BNNTs), up to 5 wt.% fractions, were fabricated using powder constituents by means of a high pressure torsion technique (HPT) at room temperature under 5 GPa pressurization. Transient ultrathin amorphous-like layers, with a thickness of 2–5 nm, composed of Al(BNO) phases, which formed under severe plastic deformation and developed under further heat treatments of the composites at 350 °C and 450 °C, were detected at the interfacial regions between Al grains and tightly embracing them BN layers. Room temperature hardness and tensile tests on fabricated composites before and after heat treatments were conducted. The highest value of room temperature tensile strength was obtained on Al-5 wt.% BNNT samples annealed at 450 °C, that reached up to ~ 420 MPa, thus exhibiting more than a doubled increase in strength compared to HPT-fabricated pure Al samples under identical compacting conditions.     

Anisotropic and temperature effects on mechanical properties of copper nanowires under tensile loading

Atomistic simulations are used to investigate the mechanical properties of copper nanowires (NWs) along 〈1 0 0〉, 〈1 1 0〉 and 〈1 1 1〉 crystallographic orientations under tensile loading at different temperatures. The inter-atomic interactions are represented by employing embedded-atom potential. To identify the defects evolution and deformation mechanism, a centrosymmetry parameter is defined and implemented in the self-developed program. The simulations show that Cu NWs in different crystallographic orientations behave differently in elongation deformations. The stress–strain responses are followed by a particular discussion on yield mechanism of NWs from the standpoint of dislocation moving. Generally, the study on the incipient plastic deformation will be helpful to further understanding of the mechanical properties of nanomaterials. In addition, the Young’s modulus decreased linearly with the increase of temperature. The crystal structure is less stable at elevated temperatures.