Time Temperature Transformation Curves Research Articles

Effect of Cr on Isothermal Structure Transformation of Iron Oxide Scale

Herein, the effect of different contents of Cr on the isothermal structure transformation of FeO is systematically studied by a thermogravimetric analyzer, and it is found that the temperature range of eutectoid transformation of FeO in Fe–Cr alloy is 300–500 °C. Combined with the experimental data, the kinetic model of isothermal phase transformation of FeO is established based on Johnson–Mehl–Avrami–Kolmogorov equation, and the time–temperature–transformation curve of isothermal transformation of FeO is predicted, which follows the law of C curve, and the C curve shifts backward with the increase of Cr content. Therm‐calc thermodynamic software calculates the Fe–O phase diagrams of different Cr contents. Combined with the experimental results, the effect mechanism of Cr content on FeO eutectoid transformation is clarified. With the increase of Cr content, the stability of FeO decreases, and under the same isothermal temperature and time, high Cr content has greater undercooling, a higher driving force of phase transition, and the starting time of eutectoid transformation is earlier than that of lower Cr content.

Accelerated Isothermal Phase Transformation and Enhanced Mechanical Properties of Railway Wheel Steel: The Significant Role of Pre‐Existing Bainite

Exploring possible ways to accelerate bainite transformation kinetics and enhance mechanical properties, especially for large‐scale parts, attracted much attention both in industrial manufacturing and fundamental research. Herein, prior continuous cooling transformed bainite is introduced to promote isothermal transformation and improve strength toughness matching of bainite railway wheel steel. Phase transformation study by dilatometer indicates that the time–temperature–transformation curves of tested steel swing back at lower isothermal temperature. This acceleration effect is due to the formation of leaf‐shaped bainite at initial isothermal transformation stage. By tailoring the cooling rate, leaf‐shaped bainite can also be obtained during continuous cooling. It is confirmed that the formation of leaf‐shaped bainite during continuous cooling stage accelerates the subsequently isothermal bainite transformation. In situ microstructure evolution observation demonstrates that prior bainite/ austenite interface provides favorable nucleation site for initial isothermal bainite transformation. Moreover, pre‐existing bainite formed during continuous cooling stage can effectively divide the original austenite grain size. Concurrent enhancement of tensile strength and impact toughness in bainite wheel steel can be achieved by controlling the cooling rate.

Fracture Behavior and Mechanical Properties of a Thermomechanical TRIP Steel Under Simulated Heat Treatment Conditions

In this research, hypereutectoid Si–Mn TRIP steel was prepared via casting method followed by thermomechanical processing and different heat treatments. Subsequently, fracture behavior and mechanical properties of the prepared steel were investigated. The time–temperature–transformation curves of the specimens were predicted by a simulation software. Their mechanical properties were studied through tensile, hardness, and impact tests. The microstructure and fracture behavior of the specimens were evaluated using an optical microscope, scanning electron microscope and fractographic analysis. Results indicated that the heat-treated sample containing a multi-phase microstructure of bainite, martensite, pearlite and retained austenite, had the highest ultimate tensile strength around 1,300 MPa. The software predictions of the heat treatments were in good accordance with the microstructural results. The samples have undergone thermomechanical process and heat treatment including isothermal transformation showed brittle behavior with intergranular and cleavage fracture surfaces. In addition, the two-stage quenching cycles resulted in dual-phase microstructure composed of martensite embedded in retained austenite with a combination of brittle and ductile fracture behavior.

Isothermal transformation and precipitation behaviors of titanium microalloyed steels

The microstructure transformation and precipitation behavior of nano-carbides in Ti microalloyed steel during isothermal process were studied by a compression test on a Gleeble 3800 thermomechanical simulator and analyzed by optical microscopy, transmission electron microscopy and other methods. The results show that γ → α phase transformation and TiC precipitation take place in Ti microalloyed steel during the isothermal process, and time–temperature–transformation curve and precipitation–time–temperature (PTT) curve are all of “C”-type. During the isothermal process, the interphase precipitation of TiC mostly occurs at the period of the phase transformation, and the random precipitation of TiC mostly occurs on the ferrite after the phase transformation. The increment in yield strength at the initial stage of isothermal transformation mainly comes from phase transformation strengthening. With the increase in isothermal time, the precipitation hardening effect becomes more important for nucleation and growth of titanium carbides and eventually reaches the maximum value at the precipitation finished point of the PTT curve.

Crystallization Behavior of Liquid CaO-SiO2-FeO-MnO Slag in Relation to Its Reaction with Moisture

To help maintain the sustainability of the steel industry, we are developing a novel process to recover thermal energy (in the form of hydrogen) and valuable metal elements contained in steelmaking slags by reacting molten slags with moisture. The process is dependent on the structure and properties of the slag, of which the crystallization tendency is key, since surface phases affect the slag reactivity with the gas and enable selective formation of solid phases containing transition metals. In this paper, the precipitated phases of the molten synthetic CaO-SiO2-FeO-MnO slags after reacting with moisture were calculated by using thermodynamic package FactSage 7.0. Laboratory experiments were conducted to reveal the crystallization behavior of the targeted metal oxides in the slags with the aim of crystallization control. A hot stage-equipped confocal laser scanning microscope (CLSM) was used to in-situ observe the crystal precipitation on the surface of the liquid slag after reacting with moisture. Time temperature transformation (TTT) and continuous cooling transformation (CCT) diagrams were created from the precipitation behavior of crystals during cooling in the temperature range of 1873 K to 1173 K (1600 °C to 900 °C). The microstructures of the reacted slags were analyzed with a scanning electron microscope (SEM) equipped with an energy-dispersive spectrometer (EDS) and the phases present in the slag were characterized by X-ray diffraction (XRD). TTT curves of the reacted slags (with moisture) indicated that the nose temperature and critical time for nucleation located at 1473 K (1200 °C) and 89 seconds for the slag with basicity of 1.00. Further increasing the slag basicity to 1.25 and 1.50 increased the nose temperature to 1523 K and 1698 K (1250 °C and 1425 °C), respectively. CCT curves of the reacted slags (with moisture) indicated that the crystallization temperatures of precipitated phases increased with decreasing the cooling rate from 800 to 10 K/min, and the crystallization temperatures of primary phases increased with increasing slag basicity. Both magnetite (Fe3O4) and monoxide ((FeO)x(MnO)1−x) phases were detected by SEM-EDS and XRD in the reacted slags (with moisture). The amount of magnetite in the reacted slags increased from 25 to 32 pct to 36 pct and that of monoxide decreased from 5 to 2 pct to 1 pct with the slag basicity increasing from 1.00 to 1.25 to 1.50.

Dynamic Phase Transformation Behavior of a Nb-microalloyed Steel during Roughing Passes at Temperatures above the Ae3

A five-pass torsion simulation of the roughing passes applied during hot plate rolling was performed in the single-phase austenite region of a Nb-microalloyed steel under continuous cooling conditions. The deformation temperatures were approximately half-way between the Ae3 and the delta ferrite formation temperature (i.e., 250 °C above the Ae3) in which the free energy difference of austenite and ferrite is at maximum. The microstructures in-between passes were analyzed to characterize and quantify the occurrence of deformation-induced dynamic phase transformation. It was observed that about 7% of austenite transforms into ferrite right after the final pass. The results are consistent with the calculated critical strains and driving forces which indicate that dynamic transformation (DT) can take place at any temperature above the Ae3. This mechanism occurs even with the presence of high Nb in the material, which is known to retard and hinder the occurrence of DT by means of pinning and solute drag effects. The calculated cooling rate during quenching and the time–temperature–transformation curves of the present material further verified the existence of dynamically transformed ferrite.

Precipitation Behavior of σ Phase in Ultra-Supercritical Boiler Applied HR3C Heat-Resistant Steel

The precipitation kinetics of σ phase in commercial HR3C heat-resistant steel during aging at 650–800 °C was studied in the paper. Through morphology, composition and structural analyzing on the second phase in the HR3C steel, it was confirmed that the precipitations after aging were mainly NbCrN, M23C6 and FeCr type σ phase. The time-dependent mass change of the three precipitated phases showed that the linearly increased σ phase after aging at 750 °C–2000 h was transformed from NbCrN phase or M23C6 phase. According to the calculation on the volume fraction of electrolytically dyed σ phase, the time–temperature transformation (TTT) curve for σ phase at 1 vol% in two kinds of commercial HR3C steel (different in grain size) was obtained and analyzed. The nose of the TTT curve was located at around 750 °C for the two kinds of HR3C steel, and the larger grain size HR3C steel displayed a inhibit effect on the precipitation of σ phase. The impact energy of the HR3C steel after aging at 700 °C decreased obviously with the fracture mechanism changing from ductile fracture to intergranular brittle fracture, which was considered to be related to the density of σ-brittle phase after aging.

Quench sensitivity of Al–Cu–Mg alloy thick plate

The quench sensitivity of Al–Cu–Mg alloy was investigated at different thicknesses of the thick plate. The quenching process was simulated via finite element analysis (FEA); time–temperature–property (TTP) curves and time–temperature–transformation (TTT) curves were obtained through hardness test and differential scanning calorimetry (DSC) test; and the microstructural observation was carried out by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Experimental results exhibit that the quench cooling rate decreases dramatically from the surface to the center of the plate, and the inhomogeneous quenching causes the difference in microstructure. With the decrease in quench cooling rate, constituent particles are coarsening gradually; the quantity of T-phase (Al20Cu2Mn3) increases and the S-phase (Al2CuMg) decreases. According to the precipitation kinetics analysis, the decrease in S-phase is caused by the increase in precipitate activation energy. So that the center of the plate shows the highest quenching sensitivity, which is consistent with the analysis of time–temperature–property curves and time–temperature–transformation curves.

Effect of Al2O3 Addition on Mineralogical Modification and Crystallization Kinetics of a High Basicity BOF Steel Slag

Basic oxygen furnace (BOF) steel slag is a main byproduct that is produced during the converter steelmaking process. The volume instability and fast crystallization of BOF slag limits its added-value application. This article aims to understand the effect of Al2O3 on the mineralogical modification and crystallization kinetics of a high basicity BOF steel slag. Continuous cooling transformation and time–temperature–transformation curves were constructed to determine the crystallization characteristics of BOF slag. The critical cooling rate to vitrify the slag was experimentally obtained. The crystallization sequence was clarified by integrating in situ and post-mortem observations with thermodynamic calculations. The results suggest that the addition of Al2O3 can effectively remove free lime, decrease the melting point, and improve the glass formation ability of the high basicity BOF slag. Undercooling the slag is enhanced by increasing the cooling rate and/or adding Al2O3. By steering the addition of Al2O3 and the cooling rate, BOF slag can be modified to obtain a more amorphous phase, presenting an enhanced potential to be a binder for added-value applications.

Quench Sensitivity of AA7N01 Alloy Used for High-Speed Train Body Structure

The quench sensitivity of AA7N01 aluminum alloy used for high-speed train body structure was investigated by end-quenching test, time–temperature-transformation curves simulation, transmission electron microscopy and scanning transmission electron microscopy. The hardness decreased with the increasing distance from the quenched end, and the depth of the age hardening layer was about 50 mm. The simulation results showed that the high quench sensitivity temperature was between 230°C and 300°C and the critical cooling rate should be larger than 2°C/s. The decrease of hardness along the increasing distance from the quenched end was attributed to the coarsening of the η equilibrium phase and the widening of the precipitate-free zone. The high-angle annular dark field results provided the direct evidence of the induced-precipitation effect of Al3Zr particles during the quenching process.

Prediction of ttt curves of cold working tool steels using support vector machine model

The cold working tool steels are of high carbon steels with metallic alloy additions which impart higher hardenability, abrasion resistance and less distortion in quenching. The microstructure changes occurring in tool steel during heat treatment is of very much importance as the final properties of the steel depends upon these changes occurred during the process. In order to obtain the desired performance the alloy constituents and its ratio plays a vital role as the steel transformation itself is complex in nature and depends very much upon the time and temperature. The proper treatment can deliver satisfactory results, at the same time process deviation can completely spoil the results. So knowing time temperature transformation (TTT) of phases is very critical which varies for each type depending upon its constituents and proportion range. To obtain adequate post heat treatment properties the percentage of retained austenite should be lower and metallic carbides obtained should be fine in nature.Support vector machine is a computational model which can learn from the observed data and use these to predict or solve using mathematical model. Back propagation feedback network will be created and trained for further solutions. The points on the TTT curve for the known transformations curves are used to plot the curves for different materials. These data will be trained to predict TTT curves for other steels having similar alloying constituents but with different proportion range.The proposed methodology can be used for prediction of TTT curves for cold working steels and can be used for prediction of phases for different heat treatment methods.

Nucleation kinetics in Al-Sm metallic glasses

The isothermal nucleation kinetics in Al-Sm metallic glasses with low Sm concentrations (xSm) was studied using molecular dynamics simulations in order to calculate time–temperature–transformation curves. The average delay time of Al nanocrystal nucleation was found to increase exponentially with xSm, whereas the estimated critical cooling rate necessary to avoid crystallization decreases exponentially with xSm. Sm solutes were found to suppress Al nucleation by increasing the attachment barrier and therefore by reducing the attachment frequency. The analysis shows that the attachment of Al to the evolving nucleus has the same characteristics as Al diffusion within the amorphous matrix and they both take place heterogeneously via collective movement of a group of Al atoms.

Formation of G-phase in 20Cr32Ni1Nb Stainless Steel and its Effect on Mechanical Properties

A series of tensile tests, Charpy impact tests, optical microscopy observations, and field emission-scanning electron microscopy examinations, were carried out to investigate the mechanical properties and microstructural evolution of 20Cr32Ni1Nb steel. Experimental results indicate that the as-cast microstructure of the steel typically consists of a supersaturated solid solution of austenite matrix with a network of interdendritic primary carbides (NbC and M 23C6). In the ex-service samples, large amounts of secondary carbides precipitate within austenite matrix. Besides the growth and coarsening of NbC and M 23C6 carbides during service condition, the Ni-Nb silicides known as G-phase (Ni16Nb6Si7) are formed at the interdendritic boundaries. The microstructural evolution results in the degradation of the mechanical properties of the ex-service steel. In addition, the precipitate rate of G-phase, depending in part on Si content, varies greatly for the 20Cr32Ni1Nb steel, which plays a key role in the long-term microstructural stability of the steel. Based on the X-ray diffraction data, time–temperature–transformation curve for the steel is obtained from the aged specimens.

Crystallization, high temperature deformation behavior and solid-to-solid formability of a Ti-based bulk metallic glass within supercooled liquid region

Empirical information of crystallization was first deduced using differential scanning calorimeter for a monolithic Ti44.4Zr6.6(Cu0.8Ni0.15Sn0.05)49 bulk metallic glass with 2 mm in diameter, characterized by a high activation energy and short incubation time for crystallization. Through the understanding of the compressive deformation modes as a function of temperature and strain rate within supercooled liquid region, processing map was constructed based on dynamic materials model. By overlapping time-temperature transformation curve onto processing map, macroscopic formability was found to have a good correspondence with the region of iso-efficiency contours having power dissipation efficiency above 0.4 without in-situ crystallization.

Formation, Time–Temperature–Transformation curves and magnetic properties of FeCoNbSiBP metallic glasses

Starting from the Fe52Co23Nb4Si5B16 alloy having good soft magnetic properties and tendency to amorphization, the addition of P, a glass former in Fe-based alloys, is explored up to 4at.%.The alloys were rapidly solidified by means of planar flow casting and have been characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC) and high temperature differential thermal analysis (HT DSC). The crystallization mechanism has been studied as a function of temperature: it involves primary formation of a bcc phase at low P content and becomes eutectic at 4% P. A remarkable achievement are Time–Temperature–Transformation (TTT) diagrams for primary crystallization obtained by DSC. Magnetisation measurements as a function of temperature in parallel with thermal analysis revealed details of the crystallization sequence. The magnetic softness of alloys has been shown with hysteresis cycles.The use of P appears favourable to amorphization, thermal and magnetic properties up to 2at.%.

Inverse Analysis of Forming Processes Based on FORGE Environment

In the field of materials forming processes, the use of simulation coupled with optimization is a powerful numerical tool to support design in industry and research. The finite element software Forge®, a reference in the field of the two-dimensional and three-dimensional simulation of forging processes, has been coupled to an automatic optimization engine. The optimization method is based on meta-model assisted evolutionary algorithm. It allows solving complex optimization problems quickly. This paper is dedicated to a specific application of optimization, inverse analysis. In a first stage, a range of reverse analysis applications are considered such as material rheological and tribological characterization, identification of heat transfer coefficients and, finally, the estimation of Time Temperature Transformation curves based on existing Continuous Cooling Transformation diagrams for steel quenching simulation. In a second part, a novel inverse analysis application is presented in the field of cold sheet forming, the identification of the material anisotropic constitutive parameters that allow matching with the final shape of the component after stamping. The advanced numerical methods used in this kind of complex simulations are described along with the obtained optimization results. This article shows that automatic optimization coupled with Forge® can solve many inverse analysis problems and is a valuable tool for supporting development and design of metals forming processes.

Multi-Stage Control of Waste Heat Recovery from High Temperature Slags Based on Time Temperature Transformation Curves

This paper presents a significant method and a basic idea of waste heat recovery from high temperature slags based on Time Temperature Transformation (TTT) curves. Three samples with a fixed CaO/SiO2 ratio of 1.05 and different levels of Al2O3 were designed and isothermal experiments were performed using a Single Hot Thermocouple Technique (SHTT). The TTT curves established through SHTT experiments described well the variation of slag properties during isothermal processes. In this study, we propose a multi-stage control method for waste heat recovery from high temperature slags, in which the whole temperature range from 1500 °C to 25 °C was divided into three regions, i.e., Liquid region, Crystallization region and Solid region, based on the TTT curves. Accordingly, we put forward an industrial prototype plant for the purpose of waste heat recovery and the potential of waste heat recovery was then calculated. The multi-stage control method provided not only a significant prototype, but also a basic idea to simultaneously extract high quality waste heat and obtain glassy phases on high temperature slags, which may fill the gap between slag properties and practical waste heat recovery processes.

Virtual front tracking cellular automaton modeling of isothermal β to α phase transformation with crystallography preferred orientation of TA15 alloy

A virtual front tracking modified cellular automaton (CA) method is applied to simulate solid phase transformation with a specific crystallographic preferred orientation in a TA15 alloy, to eliminate the dependence of the traditional CA method on space meshing. Simulation results demonstrate the capabilities of the new model in growth anisotropy modeling at large space and time scales, as well as quantitative analysis and description of the precipitated new phase morphology relates to the solute diffusion space by comparison with the diagonal modeling method and rotation of cell sites technique. The isothermal phase transformation kinetics is analysed, which exhibits the desired agreement with the prediction result of the Johnson–Mehl–Avrami analytical equation, thereby a time–temperature-transformation curve is predicted. The crystallography characteristics are consistent with electron backscattered diffraction analysis data. Using the established model, microstructure evolution during the isothermal heat treatment of the TA15 alloy is simulated so that the microstructure heredity with thermal cycling is vividly reflected.

Development of crystalline phases in sintered glass-ceramics from residual E-glass fibres

The crystalline phase evolution during firing of glass-ceramic materials from residual E-glass fibres was investigated as a function of temperature and time. The thermal stability and the mechanism of crystallisation were studied by differential scanning calorimetry (DSC). The mineralogical and microstructural characterisation of the sintered glass-ceramics was carried out by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM), respectively. The crystallisation behaviour was depicted by the TTT (Time–Temperature–Transformation) curve. The activation energies for crystallisation were calculated by the Friedman differential isoconversional, ASTM and Kissinger methods. The results show that devitrification of the glass leads to a series of glass-ceramic materials composed of wollastonite and plagioclase s.s. Their microstructure is composed of a dense network of crystals, which is responsible for the high mechanical properties exhibited by these materials.

XRD study of the kinetics of β ↔ α transformations in tin

The transformation kinetics of βSn (white tin) to and from αSn (grey tin) are studied by synchrotron X-ray diffraction of seeded powder samples of 99.99% Sn. An analysis of thermal expansion behaviour revealed that the volume change of transformation increases as the temperature decreases. The βSn → αSn transformation was well described by Johnson–Mehl–Avrami kinetics with an Avrami exponent of 3, which was confirmed to be due to three-dimesional growth from pre-existing nuclei by a microstructural study. The αSn → βSn transformation exhibited a decreasing Avrami exponent from ∼4 to ∼1 during the transformation. Time–Temperature–Transformation curves are plotted from the isothermal transformation data and are compared with past work.