Multistage Heat Treatment Research Articles

Исследование влияния многостадийной термической обработки на особенности формирования микроструктуры криогенной конструкционной стали

Исследование влияния многостадийной термической обработки на особенности формирования микроструктуры криогенной конструкционной стали

Effect of heat treatment and cryogenics on hardness of ductile cast iron microstructure (FCD­50)

The results of researches of possibilities of increasing the mechanical properties of pig iron with ball-bearing graphite of brand FCD-50 by the combined heat treatment are described. The scheme of multistage heat treatment, the characteristic feature of which is the operation of cooling in liquid nitrogen, is proposed. The use of such a scheme allows for increased HRC hardness characteristics. The study of the microstructure of cast iron treated with this scheme has proved the efficiency of the cryogenic heat treatment stage and determined its rational regimes.To match or approximate the properties of steel, the mechanical properties of this ductile cast iron 50 (FCD-‎‎50) must be ‎enhanced by methods such as ACTDI (austemper cryogenic and temper Ductile Iron). In this procedure, the FCD-50 specimen is inserted into the electric kitchen heater at 350 °C ‎‎(room temperature), heated to 600 °C and held at this temperature for 45 minutes. Next, the ‎specimen is heated to 900 °C and held at this temperature for 1 hour ‎‎(temperature austenisation). Then, the specimen is placed in furnace 2 at 3,000 °C for 45 minutes ‎‎(austempering process). The specimen subsequently is cooled in ‎liquid nitrogen. In this study, additional specimens were obtained for hardness and ‎microstructure tests. The as-cast test yielded a ‎hardness value of 18.39 HRC. In the hardness test for the ACTDI process, 12 ‎hours of submersion yielded the highest hardness result of 24.25 HRC. ‎In terms of the changes in microstructure after soaking changes, the amount of perlite present in the specimen was greater than that of ferrite, and the arrangement of graphite ‎nodules was better than that of the as-cast material.‎

Improvement of small to large grain A15 ratio in Nb3Sn PIT wires by inverted multistage heat treatments

The next generation of superconducting accelerator magnets for the Large Hadron Collider at CERN will require large amounts of Nb3Sn superconducting wires and the Powder-In-Tube (PIT) process, which utilizes a NbSn2-rich powder core within tubes of Nb(7.5wt%Ta) contained in a stabilizing Cu matrix, is a potential candidate. However, the critical current density, Jc, is limited by the formation of a large grain (LG) A15 layer which does not contribute to transport current, but occupies 25-30% of the total A15 area. Thus it is important to understand how this layer forms, and if it can be minimized in favor of the beneficial small grain (SG) A15 morphology which carries the supercurrent. The ratio of SG/LG A15 is our metric here, where an increase signals improvement in the wires A15 morphology distribution. We have made a critical new observation that the initiation of the LG A15 formation can be controlled at a wide range of temperatures relative to the formation of the small grain (SG) A15. The LG A15 can be uniquely identified as a decomposition product of the Nb6Sn5(Cux), surrounded by a layer of rejected Cu, thus the LG A15 is not only of low pin density, but is not continuous grain to grain. We have found that in single stage reactions limited to 630 °C - 690 °C, the maximum SG A15 layer thickness prior to LG A15 formation is very sensitive to temperature, with a maximum around 670 °C. This result led to the design of four novel heat treatments which all included a short, high temperature stage early in the reaction, followed by a slow cooling to a more typical reaction temperature of 630 °C. We found that this heat treatment (HT) modification increased the SG A15 layer thickness while simultaneously suppressing LG A15 morphology, with no additional consumption of the diffusion barrier. In the best heat treatment the SG/LG A15 ratio improved by 30%. Unfortunately, Jc values suffered slightly, however further exploration of this high temperature reaction region is required to understand the limits to A15 formation in Nb3Sn PIT conductors.

Effect of sintering parameters (time and temperature) upon the fabrication process of organic binder-based metallic hollow sphere

ABSTRACTMetallic hollow spheres (MHSs) are developed to be used in structural applications in syntactic and metal foams. These foams are lightweight and energy-absorbing structures which also can be used for acoustic insulation. In this study, the fabrication process of MHSs with optimum mechanical properties has been investigated. To achieve this goal, polystyrene spheres were coated with iron powder and an organic binder. During the multi-stage heat treatment, the green spheres were sintered into MHSs. Sintering was done at various temperatures (1125, 1150, 1175 and 1200°C) at different durations (3:30, 4:30 and 5:30 h). The influence of the different sintering durations and temperatures on mechanical features, microstructure and density was studied as well. The obtained results indicate that samples that were sintered at the temperature of 1175°C for 4:30 h resulted in superior mechanical and physical properties.

Hierarchical Cu precipitation in lamellated steel after multistage heat treatment

The hierarchical distribution of Cu-rich precipitates (CRPs) and related partitioning and segregation behaviours of solute atoms were investigated in a 1.54 Cu-3.51 Ni (wt.%) low-carbon high-strength low-alloy (HSLA) steel after multistage heat treatment by using the combination of electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and atom probe tomography (APT). Intercritical tempering at 725 °C of as-quenched lathlike martensitic structure leads to the coprecipitation of CRPs at the periphery of a carbide precipitate which is possibly in its paraequilibrium state due to distinct solute segregation at the interface. The alloyed carbide and CRPs provide constituent elements for each other and make the coprecipitation thermodynamically favourable. Meanwhile, austenite reversion occurs to form fresh secondary martensite (FSM) zone where is rich in Cu and pertinent Ni and Mn atoms, which gives rise to a different distributional morphology of CRPs with large size and high density. In addition, conventional tempering at 500 °C leads to the formation of nanoscale Cu-rich clusters in α-Fe matrix. As a consequence, three populations of CRPs are hierarchically formed around carbide precipitate, at FSM zone and in α-Fe matrix. The formation of different precipitated features can be turned by controlling diffusion pathways of related solute atoms and further to tailor mechanical properties via proper multistage heat treatments.

Interactive formation of Cu-rich precipitate, reverted austenite, and alloyed carbide during partial austenite reversion treatment for high-strength low-alloy steel

Abstract

Effect of Multistage Heat Treatment on Microstructure and Mechanical Properties of High-Strength Low-Alloy Steel

The influence of Cu-rich precipitates (CRPs) and reverted austenite (RA) on the strength and impact toughness of a Cu-containing 3.5 wt pct Ni high-strength low-alloy (HSLA) steel after various heat treatments involving quenching (Q), lamellarization (L), and tempering (T) is studied using electron back-scatter diffraction, transmission electron microscopy, and atom probe tomography. The QT sample exhibits high strength but low impact toughness, whereas the QL samples mostly possess improved impact toughness but moderate strength, but the QLT samples again have degraded impact toughness due to additional tempering. The dispersion of nanoscale CRPs, which are formed during tempering, is responsible for the enhanced strength but simultaneously leads to the degraded impact toughness. The RA formed during lamellarization contributes to the improved impact toughness. Based on the present study, new heat treatment schedules are proposed to balance strength and impact toughness by optimizing the precipitation of CRPs and RA.

КОРРОЗИОННАЯ СТОЙКОСТЬ ЛИСТОВ СПЛАВА 1913 ПРИ МНОГОСТУПЕНЧАТЫХ РЕЖИМАХ ТЕРМИЧЕСКОЙ ОБРАБОТКИ И С ЗАЩИТНЫМИ НЕМЕТАЛЛИЧЕСКИМИ НЕОРГАНИЧЕСКИМИ ПОКРЫТИЯМИ

КОРРОЗИОННАЯ СТОЙКОСТЬ ЛИСТОВ СПЛАВА 1913 ПРИ МНОГОСТУПЕНЧАТЫХ РЕЖИМАХ ТЕРМИЧЕСКОЙ ОБРАБОТКИ И С ЗАЩИТНЫМИ НЕМЕТАЛЛИЧЕСКИМИ НЕОРГАНИЧЕСКИМИ ПОКРЫТИЯМИ

Data analysis and other considerations concerning the study of precipitation in Al-Mg-Si alloys by Atom Probe Tomography.

Atom Probe Tomography (APT) analysis and hardness measurements were used to characterize the early stages of precipitation in an Al–0.51 at%Mg–0.94 at%Si alloy as reported in the accompanying Acta Materialia paper [1]. The changes in microstructure were investigated after single-stage or multi-stage heat treatments including natural ageing at 298 K (NA), pre-ageing at 353 K (PA), and automotive paint-bake ageing conditions at 453 K (PB). This article provides Supporting information and a detailed report on the experimental conditions and the data analysis methods used for this investigation. Careful design of experimental conditions and analysis methods was carried out to obtain consistent and reliable results. Detailed data on clustering for prolonged NA and PA treatments have been reported.

Study of precipitation in Al–Mg–Si alloys by Atom Probe Tomography I. Microstructural changes as a function of ageing temperature

Atom Probe Tomography (APT) analysis and hardness measurements were used to characterise the early stages of precipitation in an Al–0.51at.%Mg–0.94at.%Si alloy. The changes in microstructure were investigated after single-stage or multi-stage heat treatments including natural ageing at 298K (NA), pre-ageing at 353K (PA), and automobile paint-bake ageing conditions at 453K (PB). A wide range of particle sizes and compositions was typically present after a given heat treatment. Changes in particle sizes and compositions were found to be continuous, without abrupt changes of chemistry or morphology during the evolution of the initial clusters into GP zones and fine-scale precipitates. No phases were observed which were intermediate between GP zones and β″. NA had a deleterious effect on the PB hardening response, which was caused by a decrease in the rate of formation of stable, elongated precipitates during the subsequent PB. This decrease appeared to be due mainly to the formation of clusters of low average Mg/Si ratio at 298K that did not act as effective precursors to the development of elongated precipitates upon PB. The presence of these clusters lowered the matrix solute concentrations, and suppressed nucleation of more efficient (higher Mg/Si ratio) precursors of the elongated phases.

Destructive Interactions between Pore Forming Agents and Matrix Phase during the Fabrication Process of Porous BiFeO3 Ceramics

Porous bismuth ferrite ceramics were synthesized by sacrificial pore former method. A mixture of BiFeO3 and 20 wt% of various pore formers including high density polyethylene, polyethylene glycol, polyvinyl alcohol, urea and graphite was intensively milled for 10 h in a planetary ball mill, uniaxially cold pressed and then subjected to the multi-stage heat treatment. The results revealed that urea and polyvinyl alcohol are appropriate candidates for maintaining the strength of the final porous structure. Density and porosity measurements showed that by employing 20 wt% of high density polyethylene and graphite, a porous sample with a maximum porosity of nearly 40% could be obtained. Mercury porosimetry results showed that using urea as a pore former gives porous bismuth ferrite with a mean pore diameter of 7 μm, uniform pore distribution as well as interconnected pores. Moreover, reactions between BiFeO3 matrix phase and thermal decomposition products of pore formers can lead to degradation of the BiFeO3 phase in the final porous samples. Analysis of X-ray diffraction patterns illustrated that in the samples processed with graphite, high density polyethylene and polyvinyl alcohol as pore former, BiFeO3 matrix phase partially or completely decomposed to intermediate phases of Bi2Fe4O9 and Bi25FeO40. Using of urea did not damage the matrix phase and porous BiFeO3 within the original perovskite structure could be prepared. Furthermore, thermodynamic investigation was carried out for prediction of possible interactions between matrix phase and pore former at elevated temperatures.

Influence of Discontinuous Precipitation on the Creep Behavior of a β-Solidified γ-TiAl Based Alloy

Abstract Owing to their excellent high temperature properties, intermetallic γ-TiAl based alloys are potentially suitable materials for applications in the aviation and automotive industry. In order to adjust a fine lamellar microstructure with optimum properties, the forging process is followed by a multi-stage heat treatment. During the final annealing process, a discontinuous precipitation reaction, a so-called cellular reaction, may occur which decomposes the fine lamellae in the α2/γ colonies. A stable microstructure is a crucial factor in the development of highly creep resistant alloys. In the scope of this work, various heat treatments were performed in order to adjust microstructures with a varying proportion of discontinuous precipitation. Furthermore, creep tests were performed at 800 °C and 150 MPa. Scanning electron microscopy in the back-scattered electron mode was used to characterize the microstructure's morphology. The respective phase fractions were determined by X-ray diffractometry. It could be shown that the creep resistance decreases with an increasing amount of discontinuous precipitation.

PECULIARITIES OF A DEFECT GENERATION DURING A HEAT TREATMENT OF LARGE DIAMETER DISLOCATION−FREE SILICON WAFERS WITH SPECIFIED DISTRIBUTION OF OXYGEN−CONTAINING GETTERING CENTRES

For this purpose the opportunities of perfect layer formation during RTA have been analyzed in dislocation−free silicon wafers. The RTA application is based on an opportunity of effective influence on a distribution of oxygen precipitate density on wafer thickness by means of control of vacancies and interstitial atoms distributions. However the decision of this important task is connected with an oc-currence of large local wafer stresses concentrated near fastening supports and a significant bend of large diameter Si wafers. Therefore in this project the mathematical modeling of three−dimension strain state and defect formation in large diameter Si wafer were investigated: the various ways of wafer fastening were analyzed and the opportunities of stresses reduction in Si wafers were determined. For the description of RTA defect formation the mathematical model taking into account of diffusion−recombination processes of vacancies and interstitial Si atoms, and also formation of vacancy clusters have been applied. On the basis of this model the time thermal RTA parameters were determinated: heating mode, hold time at the maximal temperature and cooling rate of wafer. They provide a formation of required perfect layer near wafer surface contained the corresponding vacancy concentration and cluster density on wafer thickness. The calculated results have been verified by authors of this project on test samples investigated by Light Microscopy (LM) and Transmission Electron Microscopy (TEM). Detailed LM and TEM researches of microdefect distributions and morphology have been carried out for the experimental Si wafers subjected to various RTA modes and multistage heat treatment in Belarusian plant «Integral».

Effect of heat treatment on the microstructure and properties of lithium disilicate glass-ceramics

The effect of heat treatment on microstructure and properties of SiO2–Li2O–K2O–Al2O3–ZrO2 glass-ceramics was investigated. The glasses were subjected to a multi-stage heat treatment. At the first stage at 650°C, Li2SiO3 was the main crystalline phase of all samples, which had a dimensional change with prolonging the heating time from 3h to 72h. After the second stage treatment at 830°C for 3h, all the Li2SiO3 crystals decomposed and Li2Si2O5 crystals formed with an opposite dimensional change from that of Li2SiO3. These Li2Si2O5 crystals were small, which could contribute to forming high translucency. The third stage at 550°C would release the internal strain and improve samples' bending strength, which could reach up to above 560MPa. The sample treated for 48h at the first stage had the smallest crystals and also obtained a highest real in-line transmission of 29% at 650nm with a thickness of 1.45mm and a bending strength above 581±25MPa.

Multi-Stage Heat Treatment of Second Generation TiAl Based Alloys

In this work the influence of the particular stages of a multi-stage heat treatment on microstructure and properties of the Ti-47Al-2W-0.5Si alloy was presented. The alloy was melted in a vacuum induction furnace at a special graphite crucible. The received alloy in form of cylindrical ingot with a diameter of 16 mm and a length of 120 mm was homogenized at 1400°C for 1 h with furnace cooling (Stage 1). The proposed further treatment consists of a cyclic heat treatment (Stage 2), under-annealing carried out in a two-phase α+γ area (Stage 3) and a short full annealing conducted in α-phase area (Stage 4). Selection of particular stage parameters including the upper cycle temperature, the soaking time at the upper cycle temperature, the cooling rate from the upper cycle temperature and number of cycles for the second stage and the temperature and time of annealing for the third and forth stage was presented. The purpose of the proposed treatment is microstructure refining. This refinement is associated with the massive transformation that occurs in the alloy after heating to a temperature of 1405°C, short soaking time and cooling in air. The effect of heat treatment parameters on the microstructure and hardness of the investigated alloy was characterized.

Multi-stage heat treatment of aluminum alloy AA7049

High strength 7xxx series aluminum alloys in the T6 temper are known to be highly susceptible to stress corrosion cracking (SCC). Retrogression and re-aging (RRA) is a heat treatment by which the resistance to SCC is enhanced without losing yield or tensile strength. The influence of multi-stage heat treatment on the properties and microstructure of AA7049 aluminum alloy was investigated. The microstructure of the alloy in various temper conditions was correlated to the electrical conductivity measurements, DSC analysis and TEM observation. The DSC analysis shows that RRA treatments lead to significant change in the microstructure and the resultant RRA precipitate microstructure is significantly different from both the T6 and T73 microstructures and is able to retain the strength of T6 temper while attaining the thermodynamic stability of the T73 temper.

Effect of Different Heat Treatments on Microstructure and Mechanical Properties of the Martensitic Gx12CrMoVNbN9-1 Cast Steel / Wpływ Parametrów Obróbki Cieplnej Na Mikrostrukture I Własciwosci Mechaniczne Martenzytycznego Staliwa Gx12CrMoVNbn9-1

The paper presents a research on the influence of multistage heat treatment with the assumed parameters of temperature and time on the microstructure and mechanical properties of high-chromium martensitic GX12CrMoVNbN9-1 (GP91) cast steel. In the as-cast state GP91 cast steel was characterized by a microstructure of lath martensite with numerous precipitations of carbides of the M23C6, M3C and NbC type, with its properties higher than the required minimum. Hardening of the examined cast steel contributes to obtaining a microstructure of partly auto-tempered martensite of very high strength properties and impact strength KV on the level of 9-15 J. Quenching and tempering with subsequent stress relief annealing of GP91 cast steel contributed to obtaining the microstructure of high-tempered lath martensite with numerous precipitations of the M23C6 and MX type of diverse size. The microstructure of GP91 cast steel received after heat treatment was characterized by strength properties (yield strength, tensile strength) higher than the required minimum and a very high impact energy KV. It has been proved that GP91 cast steel subject to heat treatment No. 2 as a result of two-time heating above the Ac3 temperature is characterized by the highest impact energy.

Mechanical properties of GX12CrMoVNbN91 (GP91) cast steel after different heat treatments

The paper presents the results of research on the influence of multistage heat treatment of different types on the microstructure and properties of high-chromium martensitic GX12CrMoVNbN9-1 (GP91) steel. The materials under investigation were samples taken from a test coupon. The heat treatment of GP91 cast steel was performed for the temperature and time parameters typical of the treatment of multi-ton steel casts. The results of our research proved that, in the as-cast state, GP91 cast steel is characterized by a coarse-grain microstructure of lath martensite with precipitations of carbides, such as M23C6 , M3C , and NbC. The cast-steel microstructure in the as-cast state ensured the required standard mechanical properties. The heat treatment of GP91 cast steel contributed to getting a fine-grain microstructure of high-tempered lath martensite with numerous precipitations of M23C6 and MX carbides of diverse sizes. The structure of GP91 cast steel received as a result of heat treatment makes it possible to obtain high plastic properties, mainly impact strength, by maintaining its strength properties (yield strength) on the required minimum level.

Grain Refinement of Ti-48Al-2Cr-2Nb Alloy by Heat Treatment Method

In this paper, the possibility of refining grain of Ti-48Al-2Cr-2Nb alloy in the processes of multi-stage heat treatment consisted of initial heat treatment, cyclic heat treatment and under-annealing was evaluated. Microstructural changes that take place during the particular heat treatment procedures were also described. It was demonstrated that due to the application of combined cyclic heat treatment and under-annealing almost 24-fold grain refinement in relation to the state after homogenising could be obtained. Probable mechanisms of grain refinement in the proposed heat treatment processes were also presented and influence of individual procedures of the proposed treatment on selected properties of the investigated alloy was described

Internal-getter formation in nitrogen-doped dislocation-free silicon wafers

An experimental study is reported concerning the formation of defects in nitrogen-doped dislocation-free silicon wafers under a multistage heat treatment to produce an internal getter, the first stage being rapid thermal annealing (RTA) under different conditions. The experiments are conducted on p-Si(100) wafers of diameter 200 mm with an oxygen content of (6−7) × 1017 cm−3 and a doping level of 1.6 × 1014 cm−3, the resistivity being 10–12 ω cm. The processed wafers are examined by optical microscopy and transmission electron microscopy. With normal conditions of RTA (argon, 1250°C, 20 s), the process is found to be incapable of creating a defect-free subsurface layer of adequate thickness, though it is able to provide the desired system of defects in the bulk. The aim is achieved by changing to sequential RTA in oxygen and argon as the first stage. The reasons for the results are presented.