Number Of Heat Treatment Cycles Research Articles

Mechanical Properties of High Entropy Alloys Based on Rare Earth Elements with Yttrium and Scandium

Abstract—The high-entropy alloys GdTbDyHoSc and GdTbDyHoY having equiatomic composition are considered as promising materials for magnetic cold generators. The results of the alloys structure and chemical composition investigation are presented in this paper. The solidus and liquidus temperatures of the alloys under investigation were determined by the method of differential scanning calorimetry. Based on these data, an experimental mode of thermocyclic treatment was selected. There were no signs of alloys destruction after five cycles testing for heat resistance in the following regime: 15 min exposition at 1073 K (~0.6 of the melting temperature) and subsequent quenching in room temperature water. It was found that the applied heat treatment led to an increase in the hardness of the alloys by 2–3 times and a decrease in wear resistance by 4–40 times, depending on the composition of the alloys and the number of heat treatment cycles. A significant change in the properties of alloys is associated with the formation of oxides of the REM2O3 type not only on the surface of the alloys, but also in their volume, which is due to the high chemical activity of rare earth metals (REM). The presented data will be useful for the development of modes of thermal and thermomechanical processing of various alloys.

Fine tungsten precipitates in the matrix phase and their influence on the mechanical properties of a tungsten heavy alloy

ABSTRACT Post-sintering cyclic heat treatment of W–Ni–Co alloys results in the formation of an intermetallic phase which subsequently dissociates into fine tungsten precipitates in the matrix. The volume fraction of such precipitates in the matrix phase increases with the number of heat treatment cycles. In an effort to understand the influence of increasing fraction of precipitates on the mechanical properties of 92W–5Ni–3Co alloy, this study has been carried out with an increasing number (2, 4, 6 and 8) of heat treatment cycles. In addition, to understand and avoid the re-formation of the brittle intermetallic phase upon cooling, the cooling rate has also been varied by incorporating oil and water quenching in the processing. This study assumes significant importance considering the fact that W–Ni–Co alloys with fine precipitates in the matrix phase reportedly exhibit an excellent combination of tensile and impact properties in undeformed condition. With scope for further improvement in the properties by means of thermomechanical treatments, any refinement in the heat treatment parameters of these alloys will enhance the potential of this alloy system that is already being widely used in critical applications such as kinetic energy penetrators.

Ammonium nitrate with nanoporous structure: production methods, phase composition and morphological features of the surface

The article describes a new technology for the formation of nanoporous layers on the ammonium nitrate granule as a component of an industrial explosive. The main methods of porous ammonium nitrate (PAN) production are defined. The authors substantiate the effective implementation of the combined method of humidification and heat treatment. It is proposed to use a new granulator (vortex granulator) for the PAN obtaining method using humidification followed by heat treatment. The necessity to form a developed network of nanopores on the PAN granule surface is proved. A theoretical model describing the kinetics of humidifier’s film applying to the granule surface for subsequent drying and the nanoporous coating formation is presented. The phase composition of various samples of ammonium nitrate is studied. It is shown that all PAN samples do not undergo chemical transformations. The morphology of the PAN granule surface is studied to determine the porous structure nature. The studying results of the effect made by the type of humidifier, the heat treatment time and the number of heat treatment cycles on the PAN quality are demonstrated.

Challenging Ultra Grain Refinement of Ferrite in Low-C Steel Only by Heat Treatment

It is well-known that grain refinement is one of the most effective ways to improve strength of metals without addition of alloying elements. In order to obtain bulky metals having ultrafine grained (UFG) microstructures with average grain sizes smaller than 1 μm, severe plastic deformation (SPD) processes have made a great success. However, there are still big barriers to realize UFG metallic materials, especially UFG steels, in large scale industries, since severe plastic deformation processes usually need special techniques and equipment, and large deformation forces are required for heavy plastic deformations. Cyclic heat treatments to repeat martensitic transformation and austenitization have been known as a simple way to fabricate fine-grained austenitic structures in steels. In the present study, we tried to make final ferrite microstructures ultrafine in a low-C steel by means of the cyclic heat treatment. Evolution of microstructures during the cyclic heat treatment was systematically investigated, putting stress on the change of grain sizes of austenite and ferrite. The austenite grain size decreased with increasing the number of heat treatment cycles, and the minimum average austenite grain size obtained was 11 μm. By having furnace-cooling from austenite states with various grain sizes, ferrite microstructures with different mean grain sizes were fabricated. We could successfully obtain a fine-grained ferrite structure with a mean grain size of 4.5 μm and nearly a random texture through the heat treatment without deformation. Microstructural features and mechanical properties of the obtained fine-grained ferritic structures were investigated by scanning electron microscope/electron back-scattering diffraction measurements and a tensile test at room temperature. The specimens with ferrite + pearlite microstructure with the smallest average ferrite grain size of 4.5 μm managed both high strength (yield strength of 375 MPa and tensile strength of 500 MPa) and large tensile ductility (uniform elongation of 20% and total elongation of 39%) in the simple 2Mn-0.1C steel.

Effect of Heat Treatment on the Mechanical Strength of Hollow Core Silicon Carbide Fibers

We have determined the tensile mechanical strength of Nicalon CG and Tyranno SAK hollow core silicon carbide fibers after repeated heat treatments at a temperature of 900°C. Analysis of the effect of the number of heat treatment cycles on strength characteristics of the two types of fiber has shown that the statistical strength and Weibull modulus of the Nicalon CG fiber decrease considerably in comparison with those of the Tyranno SAK fiber, which is due to the increase in the number and size of defects on the Nicalon CG fiber as the number of heat treatment cycles increases.

ТЕРМОЦИКЛИЧЕСКАЯ ОБРАБОТКА (ТЦО) МЕТАЛЛОВ – ПУТЬ К ПОЛУЧЕНИЕЮ ОПТИМАЛЬНЫХ СТРУКТУРЫ И СВОЙСТВ

The aim of this work is to study the basic principles of thermal cyclic processing (TCТ) of metals to obtain structures that determine the optimal complex of mechanical properties. The basic provisions of metal heating centers using periodically repeated heating and cooling cycles are given. The TCТ method, as a heat treatment method, is based on constant accumulation from cycle to cycle of positive changes in the structure of metals. Studies have shown that with rapid heating, the growth of austenitic grain occurs slowly and, therefore, heating to high temperatures (up to 10000C) does not lead to an intensive increase in grain. It has been established that grain size increases at a variable heating temperature 3 times slower than under isothermal conditions at the corresponding temperature. Provided that the growth rate of the new phase (austenite) is small and the nucleation rate of grains is significant, it turns out that by the end of the a®g transformation, a fine-grained structure is retained. Further heating or holding at a constant temperature leads to a rapid coarsening of austenite grains. If cooling (for example, in air) of rapidly heated steel is performed 10–150C higher than the temperature of the Ас1 point, then fine perlite grain is formed due to reverse recrystallization. With one thermal cycle, ferrite in subeutectoid steels almost does not undergo changes. But if several such heating and cooling are performed, then the entire ferrite-pearlite structure undergoes a change. It has been established that the higher the heating rate during heating and heating and the less overheating above Ас1, the finer the grain in carbon structural steel. However, this increases the need to increase the number of heat treatment cycles. The mechanism of structure formation explaining these phenomena and practical recommendations on the implementation of the process of the technical and economic process are presented. This approach makes it possible to form the optimal metal structure. At the same time, opportunities can be significantly expanded in terms of obtaining materials with desired properties and improving on this basis machines, structures, individual units and parts. All this puts TCТ in the category of promising areas in metalworking.

Hydrogen effect on 2.25Cr–1Mo–0.25V bainitic steel under aging heat treatment

In the present work, the mechanical properties and hydrogen interaction with microstructures obtained after thermal treatment of a 2.25Cr–1Mo–0.25V bainitic steel were investigated. The samples were heat-treated under two different conditions simulating shop-repairs and manufacturing procedures of reactors. The microstructures after each heat treatment showed MC, M2C, and M7C3 carbides, characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). The hydrogen diffusivity values after the first permeation for both conditions were 4.2 × 10−11 m2s−1 and 8.6 × 10−11 m2 s−1, and the solubility was 7.4 molHm−3 and 5.3 molHm−3, showing the influence trapping in the behavior of permeation curves. The TDS results indicate that a greater hydrogen trapping capability can be achieved with one of the studied conditions. Tensile testing of the hydrogenated and heat-treated samples, showed that the number of heat treatment cycles has a strong influence on the loss of ductility, which is mainly due to carbide growth and coalescence.

Simultaneous enhancement of ductility and strength in AISI 1080 steel through a typical cyclic heat treatment

In view of obtaining high ductility along with reasonable strength through accelerated lamellar disintegration process, a typical cyclic heat treatment consisting of a short duration (6min) holding at 775°C (above A1) followed by air cooling (at two different rates, viz. still air cooling and forced air cooling at an air flow rate of 8.7m3h−1) is executed on an annealed AISI 1080 steel up to 4 cycles. Substantial enhancement in the kinetics of lamellar disintegration is observed with durations of 44min and 2h 26min under cyclic forced air cooling and cyclic still air cooling, respectively. In case of cyclic still air cooling, under relatively slower cooling rate, the ‘lamellar thickening’ and ‘divorced eutectoid growth’ are augmented which tend to increase the size (greater than 1µm) of fragmented cementite particles (up to 3 cycles) and promote the generation of more spheroids in divorced eutectoid region (DER) with progress of cyclic heat treatment. Accordingly, as %DER increases with number of heat treatment cycles, hardness and strength properties gradually decrease up to 3 cycles and remain almost unaltered thereafter on execution of 4th cycle; while ductility increases gradually up to 4 cycles. On contrary, by virtue of the generation of more lamellar faults (cooling rate being faster), an intense ‘lamellar disintegration’ is observed in case of cyclic forced air cooling. This tends to reduce the size of fragmented cementite particles (to submicroscopic level) with progress of cyclic heat treatment. Furthermore, faster cooling (more degree of undercooling) promotes the generation of more plate shaped (non-spheroid) cementite particles with progress of cyclic heat treatment. Accordingly, under cyclic forced air cooling, strength properties and ductility gradually increase simultaneously with number of heat treatment cycles that leads to a substantially high ductility (%Elongation=34) along with reasonable strength (UTS=942MPa) after 4 cycles.

Effect of grain size on the superelastic response of a FeMnAlNi polycrystalline shape memory alloy

The effect of heat treatments on superelasticity was investigated in a FeMnAlNi polycrystalline alloy using incremental tensile strain tests at room temperature. Large grains of more than several millimeters can be obtained by a cyclic heat treatment process and the average grain size can be controlled by the number of heat treatment cycles. Improved superelastic response is observed in samples with large relative grain sizes, exhibiting increased elongation to fracture, lower critical stress for transformation, and higher reversibility.

Influence of the heat-treatment conditions on various types of multifilamentary Nb-46.5%Ti superconducting wires

Unlike the NbTi superconducting wires used for high critical current density, NbTi wires for Magnetic resonance imaging (MRI) magnets have larger and fewer NbTi monofilaments and different cross sections, which show different superconducting properties. This study investigated the effects of varying the temperature, number of heat-treatment cycles, and total strain over a wide range for multifilamentary Nb-46.5%Ti wires on a mass production scale for use in MRI magnets. The heat-treatment conditions were optimized for an NbTi superconducting wire and the critical current density and the n-value were measured as functions of the final strain at temperatures of 4.2 K and 7 T. We noticed that the superconducting properties increased with increasing final strain of the multifilamentary NbTi wire. The microstructure and the effects of the size and the distribution of α-Ti precipitates on the individual heat-treatment steps were observed by using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). We, consequently, found the heat-treatment conditions that provided the highest superconducting performance for the two types of NbTi wires used in this study, and the results of the study are expected to very helpful in establishing not only the heat-treatment conditions but also important manufacturing parameters, such as the total strain, even as the design of NbTi wires for used in MRI magnets as changing with industrial demand.

Microstructure and Induced Defects of 6061 Al Alloy after Short Times Cyclic Semi-Solid Heat Treatment

Short times (5 min. /cycle) cyclic semi-solid heat treating process up to four cycles for 6061 aluminum alloy was studied. The microstructures of as-received and cyclic semi-solid heat treatment at the edge of samples at 630◦C for one to four cycles were investigated.Short time cyclic heat treatment up to two cycles improves the microstructure to be finer and more globular compared with asreceived one. Short time cyclic heat treatment significantly affects the microstructure at the edge of samples only due to short time heating.Optimum values of grain size and grain sphericity at the edge of samples for short cyclic heating time (5 min./cycle) of about 160 μm and 0.57 respectively. Increasing the number of heat treatment cycles above three increases both shrinkage porosity and micro-cracks formation.

Study of cyclic heat treatment on carbon steel

In this work, an annealed Ck85 carbon steel was subjected to cyclic heat treatment process that consisted of repeated short-duration (3.4 minutes) holding at 8000C (above Ac3 temperature) followed by forced air cooling. After 8 cycles (about a total 1 hour and 10 minutes duration of heating and cooling cycles), the microstructure mostly contained fine ferrite grains (grain size of 7 μm) and spheroidized cementite. This microstructure possesses an excellent combination of strength and ductility. The disintegration of lamellae through dissolution of cementite at preferred sites of lamellar faults during short-duration holding above Ac3 temperature, and the generation of defects (lamellar faults) during non-equilibrium forced air cooling were the main reasons of accelerated spheroidization. The strength property initially increased, mainly, due to the presence of finer micro constituents (ferrite and pearlite) and thereafter marginally decreased with the elimination of lamellar pearlite and appearance of cementite spheroids in the microstructure. Accordingly, the fractured surface initially exhibited the regions of wavy lamellar fracture (pearlite regions) along with dimples (ferrite regions). By increasing number of heat treatment cycles, the regions of dimples gradually consumed the entire fractured surface.

Effect of cyclic heat treatment on microstructure and mechanical properties of 50CrV4 steel

An annealed 50CrV4 steel was subjected to cyclic heat treatment process that consists of repeated short-duration (200 s) held at 840 °C (above Ac3 temperature of 790 °C) and short-duration (100 s) held at 700 °C (below Ac1 temperature of 710 °C). The spheroidization ratio of cementite and the average size of particles increase with increasing the cyclic number of heat treatment. After 5-cycle heat treatment, the spheroidization ratio of cementite is 100%, and the average size of the cementite particles is about 0.53 μm. After cyclic heat treatment, the hardness, ultimate tensile strength and yield strength of the experimental steel gradually decrease with increasing cyclic number of heat treatment. The elongation of the as-received specimens is about 7.4%, the elongation of the 1-cycle specimen is 14.3%, and the elongation of 5-cycle specimen reaches a peak value of 22.5%, thereafter marginally decreases to 18.3% after 6-cycle heat treatment. Accordingly, the fractured surface initially exhibits the regions of wavy lamellar fracture. By increasing the cyclic number of heat treatment cycles, the regions of dimples consume the entire fractured surface gradually. Some large dimples can be found in the fracture surface of the specimen subjected to six heat treatment cycles.

Development of high strength ductile hypereutectoid steel by cyclic heat treatment process

Abstract In this work a homogenizing annealed 1.24 wt% carbon steel was subjected to cyclic heat treatment process that consisted of repeated short-duration (6 min) holding at 894 °C (above Acm temperature) followed by forced air cooling. During short-duration holding at 894 °C, fragmentation (owing to partial dissolution) and thickening of cementite lamella occurred at the preferred high energy sites of lamellar faults. These undissolved fragmented cementite acted as pre-existing nuclei promoting the divorced eutectoidal reaction (DET) during forced air cooling that generated non lamellar region (NLR) in the microstructure apart from the presence of lamellar pearlite region (LPR) and cementite network (CN). The CN was also fragmented with repeated holding and cooling cycles. Besides, new lamellar fault sites were generated during forced air cooling to promote lamellar fragmentation. Accordingly, as the number of heat treatment cycles increases, more uniformly dispersed finer cementite particles (pre-existing nuclei) in austenite matrix was obtained at 894 °C. Therefore, the proportion of NLR increased and the proportions of LPR and CN decreased with the progress of cyclic heat treatment. After 8 cycles of heat treatment the microstructure mostly consisted of a combination of spheroids (0.55 ± 0.24 μm in size) and non-spheroids (mainly the plate shaped particles of 0.22 ± 0.10 μm in size) of cementite in 1:5 proportion by area% dispersed in ferrite matrix. The generation of cementite plates occurred through the motion of transformation disconnections. This provided an excellent combination of strength (UTS = 1086 MPa) and ductility (%elongation = 13) in this 1.24 wt% C steel. The property obtained had similarity to what was reported for a thermo-mechanically processed steel of much higher carbon content (1.8 wt%) where the microstructure consisted of solely the cementite spheroids of 0.65 μm average size dispersed in ferrite matrix.

Effect of cyclic heat treatment on microstructure and mechanical properties of 0.6 wt% carbon steel

In this work an annealed 0.6 wt% carbon steel was subjected to cyclic heat treatment process that consisted of repeated short-duration (6 min) holding at 810 °C (above Ac 3 temperature) followed by forced air cooling. After 8 cycles (about a total 1 h and 20 min duration of heating and cooling cycles), the microstructure mostly contained fine ferrite grains (grain size of 7 μm) and spheroidized cementite. This microstructure possessed an excellent combination of strength and ductility. The disintegration of lamellae through dissolution of cementite at preferred sites of lamellar faults during short-duration holding above Ac 3 temperature, and the generation of defects (lamellar faults) during non-equilibrium forced air cooling were the main reasons of accelerated spheroidization. The strength property initially increased mainly due to the presence of finer microconstituents (ferrite and pearlite) and thereafter marginally decreased with the elimination of lamellar pearlite and appearance of cementite spheroids in the microstructure. Accordingly, the fractured surface initially exhibited the regions of wavy lamellar fracture (pearlite regions) along with dimples (ferrite regions). With increasing number of heat treatment cycles the regions of dimples gradually consumed the entire fractured surface.

Heat Treatment Behavior of Tungsten Heavy Alloy

This paper focuses on the variations of static and dynamic properties of tungsten heavy alloy with heat treatment. The matrix phase of 93W-4.9Ni-2.1Fe (weight percent) has been penetrated into W/W grain boundaries during a cyclic heat treatment which consists of repeated isothermal holdings at 1150 °C and water quenching between them. By applying the cyclic heat treatment, the impact energy of tungsten heavy alloy is increased about three times from 57 to 170 J. When the tungsten heavy alloy is cyclically heat treated at 1150 °C and then re-sintered at 1485 °C, W/matrix interface is changed from round to undulated shape. The irregularity of the interface is increased with increasing the number of heat treatment cycles. From the measurement of the residual stress of W grains by X-ray diffraction, it is found that the irregularity of the interface is closely related with strain energy stemmed from the difference of thermal expansion coefficient between W particles and matrix phase. From dynamic ballistic test, it is found that the tungsten heavy alloy with undulated W grains forms many narrow fracture bands which are preferential for the self sharpening effect, thus, for the improvement of the penetration performance.

The influence of surface steps on the optical and electronic anisotropy of Ag(110)

The Ag(110) surface was studied at various stages of annealing and ion bombardment cycles by means of reflection-anisotropy spectroscopy (RAS) and scanning tunneling microscopy (STM). At low fluence and a limited number of heat treatment cycles, a positive RAS signal at 3.8 eV, followed by a negative peak at 3.9 eV, was recorded. STM showed that the room-temperature surface corresponding to this curve profile had steps whose edges were parallel with the in-plane [110] direction. At longer sputtering times, with several cycles of annealing, we obtained a statistically isotropic distribution of steps and terraces. Ag(110) surfaces of the latter kind result in RAS curves where the positive low-energy component at 3.8 eV is absent. We discuss the spectra in terms of local-field calculations where the screened dipole–dipole interaction coefficients are modified by surface steps. If the step edges are isotropically distributed instead of parallel with the [110] direction, the strength of the low-energy part of the RAS curve is reduced. However, the calculated reduction in strength is not enough to account for the experimental results. Step-induced coupling to surface plasmons is an additional mechanism, which gives a much stronger reduction in strength. The influence of both effects on the RAS curve increases with decreasing correlation length. The plasmon-based mechanism takes place already at lengths of the order of 10 2 nm, whereas the cut-off in the dipole–dipole interaction needs correlation lengths that are almost one magnitude lower to be important.

Undulation of W/matrix interface by resintering of cyclically heat-treated W-Ni-Fe heavy alloys

When liquid-phase-sintered W-Ni-Fe alloys were cyclically heat treated at 1100 °C and resintered at 1485 °C, undulation of W/matrix interface resulted. The irregularity of the interface increased with the number of heat-treatment cycles. The residual thermal stress of W grains measured by X-ray diffraction increased with the number of heat-treatment cycles and exceeded the yield stress of W single crystal in certain crystallographic directions. A calculation by the finite element method (FEM) also showed nonuniform distribution of thermal stress on W grains. Local yielding of W grains is believed to occur during the cyclic heat treatment. The observed undulation of W/matrix interface appears therefore to result from preferential dissolution of material from regions with higher strain energy and precipitation of material at regions with lower strain energy at the resintering temperature. The undulation disappeared with the grain growth during prolonged resintering.

Udulation of W/matrix interface by resintering of cyclically heat-treated W [sbnd]Ni [sbnd]Fe heavy alloys

When liquid-phase sintered W Ni Fe alloys were cyclically heat-treated at 1100 °C adn resintered at 1485 °C, undulation of W/matrix interface resulted. The irregularity of the interface increased with the number of heat-treatment cycles. The residual thermal stress of W grains measured by X-ray diffraction increased with the number of heat-treatment cycles and exceeded the yield stress of W single crystal in certain crystallographic directions. A calculation by the finite element method also showed non-uniform distribution of thermal stress on W grains. Local yielding of W grains is believed to occur during the cyclic heat treatment. The observed undulation of W/matrix interface appears therefore to result from preferential dissolution of material from regions with higher strain energy and precipitation of material at regions with lower strain energy at the resintering temperature. The undulation disappeared with the grain growth during prolonged resintering.

Some aspects of the thermal cycling resistance of sintered materials

1. Thermal cycling resistance is not merely a material property, but depends also on the size and shape of parts made from a given material; its value for parts with equal dimensions in different directions is greater than in the case of a substantial disparity between these dimensions. 2. The thermal cycling resistance of parts of equal shape increases with decreasing size of the parts and with increasing ductility (decreasing brittleness) of the material. 3. The thermal cycling resistance of alloys of titanium carbide with nickel increases with increasing nickel content and sintering temperature. 4. A regularity was found in the change of electrical resistivity during cyclic heat treatment. The increase of electrical resistivity,Δρ=ρ−ρ 0 ,is proportional to the number of heat treatment cycles.