Range Of Cooling Rates Research Articles

Thermodynamic calculation-assisted design of 500 MPa high performance steel by machine learning

In order to rationally design Q500 low-alloy wind power steel with stable microstructure and properties within a large cooling rate range, this study proposed a design system that utilizes thermodynamic database, machine learning (ML), and the multi-objective genetic optimization algorithm (MOGA). Thermodynamic calculation has been used to obtain a large amount of data on phase fractions and mechanical properties for various chemical composition at different cooling rates. By comparing the predictive capabilities of different ML models, the deep neural network (DNN) and MOGA were combined to design Q500 wind power steel. Subsequently, three groups of designed chemical compositions were appropriately selected for verification. The results indicate that the yield strength of the experimental steel was over 500 MPa, and the maximum error in microstructure and mechanical property prediction was 5.0%. The prediction results demonstrated the rationality of the composition design framework. Meanwhile, the method of combining the material database and ML proposed in this study can also be applied to the design of other low-alloy steels.

Effect of Mo on the Phase Transformation, Microstructure, and Hardness of Q500qENH Weathering Steel of Bridge Construction during Continuous Cooling

This article investigates the precipitation behavior of Mo element in the continuous cooling phase transformation process of weathering steel of bridge construction and its influence on microstructure and hardness. The results demonstrate that the addition of Mo leads to the narrowing of the phase transformation interval of proeutectoid ferrite and pearlite, the expansion of the bainite transformation zone, a delayed bainite phase transformation process, and the refinement of bainite microstructure. Moreover, theoretical calculations of bainite phase transformation kinetics reveal that the average activation energy for bainite transformation in Mo‐free and Mo‐containing steels is 94.8 and 134.0 kJ mol−1, respectively. The new grains of Mo‐free steel mainly grow in a 2D mode, while Mo containing steel mainly grows in a 1D mode, which confirms the retarding effect of Mo on bainite grain growth. Additionally, Mo significantly increases the number density and refines the size of precipitated phases in weathering steel of bridge construction. Within the cooling rate range of 0.1–30 °C s−1, the hardness of weathering steel of bridge construction is enhanced by Mo, with the magnitude of hardness increment decreasing as the cooling rate increases. This behavior is primarily governed by the combined influence of phase transformation strengthening and precipitation strengthening.

Crystallization of an undercooled Zn-based glass forming alloy

Zn-based metallic glasses (Zn40Mg11Ca31+xYb18-x, x = 0 - 4) are strong glass forming alloys with low glass transition temperatures. While the crystallization onset temperature is also low, the nucleation behavior has not been examined and analyzed quantitatively. For a Zn40Mg11Ca31Yb18 metallic glass alloy, the crystallization behavior was determined based on the application of the Flash differential scanning calorimetry (DSC) with ultrafast heating and cooling rates. The critical cooling rate range was measured for glass formation at 1.6(±0.2) × 104K/s. A temperature-time-transformation (TTT) diagram for the nucleation onset in an undercooled liquid was obtained by applying a series of isothermal exposures by fast heating from the glass state or fast cooling from the liquid state to the target temperatures. A heterogeneous nucleation analysis model was developed to interpret the experimental TTT diagram, which provided a further understanding of the nucleation in Zn-based metallic glasses.

Studies of the crystal structure of solid solutions (Sn2)1−x−y(GaAs)x(ZnSe)y, (GaAs)1−x(ZnSe)x grown from liquid phase

The possibility of growing (Sn2)1−x−y(GaAs)x(ZnSe)y, (GaAs)1−x(ZnSe)x solid solutions on a GaAs(100) substrate is shown at the temperature of the beginning of crystallization, respectively, TOC = 570 °C and TOC = 750 °C by the method of liquid-phase epitaxy from a limited tin solution-melt in the cooling rate range of 0.5–3 K/min. The content of the chemical composition and the perfection of the substrate-film boundary of epitaxial layers of solid solutions (Sn2)1−x−y(GaAs)x(ZnSe)y, (GaAs)1−x(ZnSe)x were studied using a scanning electron microscope (SEM).X-ray diffraction studies have shown that the resulting films are single-crystal with (100) orientation and have a sphalerite structure. Using a transmission electron microscope (TEM), we obtained HR TEM images of (GaAs)1−x(ZnSe)x poly- and single-crystal samples grown at a crystallization onset temperature TOC = 750 °C with forced cooling at a rate of 3 deg/min. and 1 deg/min. Some electrophysical and photoelectric properties of the samples have been studied.

Relationship between Cluster-Arranged Nanoplate Formation and Mechanical Properties of Dilute Mg–Y–Zn Alloys Prepared by Combination of Low-Cooling-Rate Solidification and Extrusion Techniques

High-strength dilute Mg–Y–Zn alloys with cluster-arranged layer/nanoplate (CAL/CANaP) precipitates were developed via combined processes of low-cooling-rate solidification and extrusion techniques. The effects of CANaP morphology and deformation kink bands installation on the tensile properties of the extruded Mg–Y–Zn alloys were investigated. A slow-cooling solidification process with a cooling rate range of 0.1–0.01 K·s−1 produces a CAL-aggregated region in the α-Mg matrix. The CAL-aggregated region comprises long-period stacking ordered (LPSO) nanoplates with an intergrowth structure and the solo-CAL precipitates. The area fraction of the CAL-aggregated region increased with decreasing cooling rate. The microstructure of the extruded Mg99.2Y0.6Zn0.2 alloys prepared from low cooling rate-solidified ingots consisted of three characteristic regions: (i) dynamically recrystallized (DRXed) fine α-Mg grains, (ii) worked coarse α-Mg grains with a CAL-aggregated region, and (iii) worked blocky LPSO grains. The strength and ductility of the extruded Mg–Y–Zn alloys may be controlled by the volume fractions of the worked and DRXed grains, respectively. It is desirable to control the CANaP thickness and spacing to ∼1 µm and ∼0.8 µm or more, respectively, to promote DRX. Conversely, it is necessary to control the CANaP thickness and spacing to ∼1 µm and ∼0.8 µm or less, respectively, to form the worked grains in which kink bands are introduced.

Nonisothermal crystallization of poly(L‐lactic acid) promoted by polyols

AbstractThe inherent weak crystallization capacity has an impact on the processability and serviceability of poly(lactic acid) (PLA) and limits its application. D‐mannitol (DM) and xylitol (Xy), acting as a nucleation accelerant, were melt‐blended with poly(L‐lactic acid) (PLLA) and the nonisothermal crystallization behavior was investigated. At 1% dosage, the crystallization temperatures of polyol‐modified PLLAs are 10–15°C higher than those of neat PLLA in the cooling rate range of 5–20°C. The theoretical half‐time of crystallization (t1/2) of the Jeziorny model remains constant over the cooling rate range of 10–20°C min−1, in contrast to the greatly reduced t1/2 reported for heterogeneous nucleating agents. Xy is more effective than DM in promoting crystallization. It increases the crystallinity of PLLA to ~40% within the 1%–5% dosage; and the activation energy of nonisothermal crystallization calculated by the Friendman method follows the order of magnitude: neat PLLA ˃ PLLA/DM ˃ PLLA/Xy. Polyols promote the crystallization of PLLA by a mechanism different from heterogeneous nucleation, providing an efficient and biosafety approach to improve the performance of PLA.

Single-track study of A20X aluminum alloy fabricated by laser powder bed fusion: Modeling and experiments

Optimum process parameter window for recently developed metal powders used for laser powder bed fusion (LPBF) is strongly correlated with characteristics of each single track and single layer. In the present research, the influence of LPBF process parameters on geometrical and microstructural characteristics of A20X aluminum single tracks was studied theoretically and experimentally. Increasing the laser scan speed led to formation of non-homogenous, irregular single tracks. Moreover, enlarging the powder layer thickness to 120 µm induced balling phenomenon owing to a poor wetting of substrate by melt pool. Experimental measurements indicated that width and depth of melt pools decreased up to 53 % and 68 %, respectively by increasing the scan speed from 500 to 1700 mm/s and layer thickness from 40 to 120 µm. These findings were in agreement with predictions of an exponentially decaying heat input model used in this study. The model took into account thermo-physical properties of the alloy at different states. Bead height was observed to increase with the powder layer thickness, but remain unchanged with the scan speed. The process parameter window (energy density range) resulting in the conduction melting mode for the A20X alloy was found to be more than two times wider compared to conventional LPBF aluminum alloys. For all process parameters, a fine equiaxed grain structure was observed in the vertical section of the single tracks. Enhancing the scan speed and reducing the powder layer thickness resulted in a substantial grain refinement. This was well explained by the simulated cooling rates. In contrast to cast Al-Cu alloys, shape, size, and volume fraction of the second phase precipitates were determined to be independence of cooling rate over the investigated cooling rate range (1.4 × 106 to 7.5 × 106 °C/s).

INVESTIGATION OF THE AUSTENITE DECAY KINETICS DURING CONTINUOUS COOLING OF STEEL WITH 0.84 % С, 0.44 % Si, 0.95 % Mn, 0.01 % B, 0.0006 % Са FOR RAILWAY RAILS OF THE NEW GENERATION

Purpose. Investigation of the supercooled austenite decay kinetics during continuous cooling of steel with 0.84 % C, 0.44 % Si, 0.95 % Mn, 0.01 % B, 0.0006 % Ca for railway rails of a new generation with increased operational properties to develop parameters of differential cooling. Method. The investigation of the austenite decay kinetics was carried out using differential thermal analysis. Samples were cooled at different cooling rates, temperature control was performed using chromel-aluminum thermocouples installed in the center of the sample. Microstructural investigations are performed using optical (AxioVert 200M mat) and electronic (SEM-106) microscopes. The interplate distance in pearlite is determined by the linear method, the diagonals are located perpendicular to the plate packages. Vickers hardness at a load of 10 kg. The number of structural components is recognized on microstructural images using the imageJ software complex. Results. The TKD analysis showed that at cooling rates of 0,06...5,96 °C/s, the structure of the investigated steel consists of pearlite. As the cooling rate increases, the morphology changes and the dispersion of pearlite increases: from medium pearlite to sorbitol-like. It should be noted that in the cooling rate range of 0,06...0,07°С/s, tertiary cementite is formed from ferrite. The diagram shows that at a cooling rate of 5.96 °С/s the hardness is 468 НВ (432 НВ), and the structure does not contain bainite. At a cooling rate of 1.47 °С/s, the hardness is 356 НV (345 НВ). This is slightly below the minimum allowable value (370 HV), but by interpolation it can be determined that the cooling rate must be at least 2.5 °C/s to achieve a hardness of at least 393 HV (370 HV). That is, when the rolling surface of the rail is cooled at a rate of 5.96 °C/s, the central volumes of the rail head at a rate of at least 2.5 °C/s, high-strength rails of the following categories can be manufactured: higher (DSTU (National Standard of Ukraine) 4344:2004) and R400HT (EN 13674-1:2011) from steel with 0.84 % С, 0.44 % Si, 0.95 % Mn, 0.01 % B, 0.0006 % Ca. Scientific novelty. Constructed thermokinetic diagram of steel austenite decay with 0.84 % С, 0.44 % Si, 0.95 % Mn, 0.01 % B, 0.0006 % Са. Practical value. It was established that with the use of experimental steel with 0.84 % C, 0.44 % Si, 0.95 % Mn, 0.01 % B, 0.0006 % Ca it is possible to achieve a hardness above 400NV without the formation of needle-like structures, therefore this is recommended chemical composition for the manufacture of high-strength rails of the R400NT category according to EN 13674-1-2011.

An investigation of crystallization kinetics of polyoxymethylene in processing conditions

AbstractIn this work, a very wide set of quiescent isothermal and non‐isothermal calorimetric experiments are carried out in order to analyze POM crystallization kinetics also under cooling rates comparable to those experienced by the polymer during processing. To investigate the effect of flow on the POM crystallization behavior, also some Linkam shearing tests are conducted. An enhancement of the POM crystallization process is observed under flow. A Kolmogoroff–Avrami–Evans (KAE) model for quiescent crystallization is proposed. It is based on nucleation (considering both a homogeneous and a heterogeneous process) and growth mechanisms. Model parameters of nucleation and growth are determined, and the overall model is able to describe the POM crystallization process in the whole cooling rate range adopted for the experiments. This allows to obtain a reliable description of the POM crystallization evolution during processing and provides important knowledge for managing the POM injection molding process and the final physical properties of POM products.

Effect of Tungsten Addition on Continuous Cooling Transformation and Precipitation Behavior of a High Titanium Microalloyed Steel

Effects of tungsten addition on the continuous cooling transformation (CCT) characteristics and precipitation behavior of a high titanium microalloyed steel were investigated by dilatometry, optical microscopy, scanning and transmission electron microscopy, and hardness measurements. The results showed that the ranges of transformation products were moved to the right side of the CCT diagram when the 0.4% W was added. Accordingly, the following observations were made: (i) the ferrite phase transformation was shifted to the side of lower cooling rates and reduced temperatures; (ii) the bainite phase transformation region ran throughout the whole cooling rate range studied. Addition of W had a positive effect on the particle size refinement and number density increase of the precipitates. At the low cooling rates, in the range of <14 °C/s, W addition shifted the precipitation hardening peak to the low cooling rate side as the ferrite transformation induced stronger precipitation strengthening than the bainite one. Furthermore, the effect of W addition on phase transformation strengthening was obvious (increase in hardness: ~40Hv) at the high cooling rate range, over 14 °C/s.

Continuous Cooling Transformation Behavior of High Carbon Pearlitic Steel

The continuous cooling transformation behavior of high-carbon pearlitic steel was studied by employing optical microscopy, scanning electron microscopy, and the Vickers hardness test. The results show that the microstructure of the test steel is composed of proeutectoid cementite and lamellar pearlite in the cooling rate range of 0.05–2 °C/s and lamellar pearlite in the range of 2–5 °C/s. Further, martensite appears at 10 °C/s. With the increase in the cooling rate, the Vickers hardness of the test steel first decreases and then increases. In the industrial production of high-carbon pearlite steel, the formation of proeutectoid cementite at a low cooling rate needs to be avoided, and at the same time, the formation of martensite and other brittle-phase at a high cooling rate needs to be avoided.

Effect of cooling rate on the microstructure and mechanical properties of a high copper high nickel low carbon steel

A new composition steel was designed by adding 1wt.% Cu and 2wt.% Ni elements on the basis of low-carbon Ti-bearing microalloyed steel. The continuous cooling transition curve (CCT) is obtained by the phase transition curves, microstructure and hardness in the Gleeble3800 thermal simulation test machine. In order to further study the law of transformation and meet the needs of industrialized production, different cooling tests were carried out through industrialized trial production, and the corresponding organization and mechanical properties were analyzed. The results show that when the cooling rate increases, the microstructure transformation appears as granular bainite (GB) → granular bainite + lath bainite (GB+LB) → lath bainite (LB) → lath martensite (M). The mechanical properties increase with the increase of cooling rate, and the yield strength increases in a positive correlation trend. The results show that the steel can obtain good and stable comprehensive mechanical properties in a large cooling rate range without heat treatment. The best mechanical properties with a good combination of yield strength and impact performance are obtained under ultra-fast cooling conditions.

Continuous Cooling Transformation of Under-Cooled Austenite of SXQ500/550DZ35 Hydropower Steel

The expansion curves of the continuous cooling transformation of undercooled austenite of SXQ500/550DZ35 hydropower steel at different heating temperatures and cooling rates were measured by use of a DIL805A dilatometer. Combined with metallography and Vickers hardness measurement, the continuous cooling transformation diagrams (CCT) of the studied steel under two different states were determined. The results show that in the first group of tests, after the hot-rolled specimens were austenitized at 920 °C, when the cooling rate was below 1 °C·s−1, the microstructure was composed of ferrite (F), pearlite (P) and bainite (B). With the cooling rates between 1 °C·s−1 and 5 °C·s−1, the microstructure was mainly bainite, and martensite (M) formed as the cooling rate reached 5 °C·s−1. When the cooling rate was up to 10 °C·s−1, the microstructure was completely martensite and the hardness value increased significantly. In the second group of tests, after the hot-rolled specimens were quenched at 920 °C and then heated at an intercritical temperature of 830 °C, in comparison with the first group of tests, and except for additional undissolved ferrites in each cooling rate range, the other microstructure types were basically the same. Due to the existence of undissolved ferrite, the microstructures of the specimens heated at intercritical temperatures were much finer, and the toughness values at low temperatures were better.

Analysis on hot deformation and following cooling technology of 25Cr2Ni4MoVA steel

The true stress–true strain curves of 25Cr2Ni4MoVA steel were obtained by uniaxial compression experiments at 850–1200 °C in the strain rate range of 0.001–10.0 s−1. And the dynamic continuous cooling transformation curves were obtained at the cooling rate range of 0.5–15.0 °C s−1 from the austenitization temperature of 1000 °C to the room temperature by pre-strain of 0.2 as well. The power dissipation map and the dynamic continuous cooling transformation diagram were constructed based on the data provided by these curves. Compared with the optical micrographs of the compressed samples, the full dynamic recrystallization region is located between 1000 and 1200 °C and at the strain rate range from 0.01 to 10.0 s−1 with the power dissipation efficiency not less than 0.33. In the full dynamic recrystallization region, the power dissipation efficiency increases and the dynamic recrystallization activation energy decreases with the temperature increasing. With the strain rate decreasing, the power dissipation efficiency increases firstly and then starts to decrease as the strain rate is less than 0.1 s−1, and dynamic recrystallization activation energy changes on the contrary. According to the dynamic continuous cooling transformation diagram, slow cooling is a better way for the hot-deformed piece with large size or complex shape to avoid cracking as the temperature of the piece is lower than 400 °C, and different cooling ways can be used for the hot-deformed piece with small size and simple shapes to obtain certain microstructure and meet good compressive properties.

Analytical modelling of PCM supercooling including recalescence for complete and partial heating/cooling cycles

Abstract Phase change material (PCM) experiencing supercooling and phase change hysteresis are widely reported in the literature. However, only few studies model analytically such PCM, and they mostly focus on either supercooling or phase change hysteresis, but rarely on both phenomena. Moreover, partial phase change cycles, with an incomplete melting or solidification, are rarely considered even if these processes occur frequently in latent heat thermal energy storage (LHTES) systems. The objective of this study is to model analytically the thermal behaviour of a PCM experiencing supercooling and phase change hysteresis, for complete and partial phase change cycles. The developed method, based on a heat source term, enables to model the recalescence process during the solidification. Currently rarely considered, the influence of the cooling rate on the supercooling degree and the recalescence process is evaluated with a phenomenological approach. For partial cycles, the different behavior between heating and cooling is modelled with the hysteresis model “curve scale” already validated in literature. The experimental validation is carried out on a PCM brick sample, monitoring both the heat flux and the PCM temperature, which enables to characterize a greater mass of PCM compared to direct scanning calorimeter (DSC) analysis. The selected PCM undergoing supercooling during solidification is PEG6000, a polymer suitable for domestic hot water (DHW) storage. Results show a good agreement between experimental and numerical results for complete heating and cooling cycles. The behavior laws used to model the solidification with the supercooling and recalescence processes are validated for the cooling rate range tested. The modelling is also satisfactory concerning the experiments on partial solidification for different temperature plateaus. However, the model fails to correctly represent the thermal behavior for a cooling process after a partial melting of the PCM. To conclude, the developed model enables to represent accurately the thermal behaviour of a PCM experiencing supercooling and phase change hysteresis for most of the phase change processes studied. Investigations on the thermal conditions influencing the crystalline structure and the effect on the phase change dynamic are suggested to improve supercooling modelling. The developed model needs to be validated for PCM having a higher supercooling degree, such as sugar alcohols or salt hydrates.

Microstructure and Mechanical Properties of P91 Steel during Heat Treatment: The Effect of the Cooling Speed during the Normalization Stage

The evolution of the microstructure and mechanical properties of P91 steel during heat treatments at different cooling speeds during the normalization stage was investigated. Results showed that normalized martensite with high hardness and strength was obtained over a wide range of cooling rates (higher than 200 °C/h) during the normalization stage of P91 steel. Within this cooling rate range, twin martensite gradually generated as the cooling rate decreased, which enhanced the brittleness of the normalized P91 steel. When the cooling rate was decreased to 25 °C/h, the P91 steel exhibited completely annealed microstructures consisting mainly of bulk α-Fe grains, with the formation of continuous M23C6 carbides at the grain boundaries. As a result, the annealed P91 steel had poor hardness and strength, as well as demonstrating increased brittleness. Notably, after tempering, the hardness and strength of normalized martensite were greatly weakened, while the toughness of tempered martensite greatly increased. After the tempering process, the continuous M23C6 GBs in the completely annealed P91 steel partly dissolved, which also remarkably enhanced the toughness.

Microstructure and Mechanical Properties of Martensitic High-Strength Engineering Steel

A promising martensitic steel with good hardenability is studied. In the cooling rate range 0.1–30°C/sec and the only transformation recorded by a dilatometer starts at an Ms temperature of 355 ± 10°C. Microstructure and mechanical properties of the steel studied are analyzed after various heat treatment regimes: cooling from the austenitizing temperature at various rates (from 0.02 to 20°C/sec), austempering at 280–380°C, and oil quenching and tempering at 200–600°C. It is shown that a 1000-fold change in the cooling rate has a weak effect on steel strength. Bainitic transformation in the steel in question occurs in the martensitic transformation temperature range, which leads to a reduction in impact strength compared with a martensitic microstructure both after slow cooling and isothermal hardening.

Boride formation behaviour and their effect on tensile ductility in cast TiAl-based alloys

Boron has been used to refine the microstructures in TiAl castings, such as low-pressure turbine (LPT) blades, to improve mechanical properties. However, boride precipitates with undesirable morphologies could reduce ductility and even entirely remove the benefits of grain refinement. Boride size and morphology in variant alloys based on Ti45Al2Mn2Nb1B has found to be closely related to alloying element species and solidification conditions, leading to distinctly different boride formation behaviour. It has been shown that Hf promotes the formation of thin curvy boride flakes whilst Ta promotes the formation of thick straight boride ribbons in the studied cooling rate range. The boride crystal structure changes from TiB with the B27 structure for coarse straight boride to TiB with Bf structure for the curvy boride. Curvy borides have the strongest effect in reducing ductility, regardless of alloy composition.

Atom-probe tomographic and dilatometric studies of phase-transformations after inter-critical annealing of a low-carbon dual-phase steel

Phase-transformations and microstructural evolution during and after inter-critical annealing of a low-alloy, dual-phase steel are studied utilizing dilatometric measurements, and microstructural characterization methods, including scanning electron microscopy, synchrotron X-ray diffraction, and atom-probe tomography. Dilatometric measurements reveal that the sample contains ~72% austenite, after annealing for 5-min at 871 °C, which agrees with the water-quenched microstructure, and ~90% after further annealing to 1 h, which agrees with thermodynamic predictions. A continuous-cooling transformation (CCT) diagram is constructed, within the cooling-rate range between 1 °C/s and 70 °C/s. The incubation time for the ferrite-transformation is <1 s, after inter-critical annealing, and the secondary-phases are mainly pearlite for cooling-rates of <5 °C/s, and martensite for cooling-rates >10 °C/s, which is confirmed utilizing scanning electron-microscope observations. The microhardness increases as the cooling rate increases, and an abrupt slope-change is explained by the major secondary-phase changing from martensite to pearlite between 5 °C/s and 10 °C/s. Atom-probe tomography demonstrates the coexistence of martensite and pearlite as secondary-phases for 30 °C/s and a switch in austenite-ferrite transformation mechanisms from para-equilibrium (70 °C/s) to partitioning local-equilibrium (30 °C/s). A microstructural map is constructed to reveal the austenite decomposition products in the cooling-rate range between 1 °C/s and 70 °C/s. Based on composition measurements of the matrix, utilizing APT, the effects of cooling-rates on solid-solution-strengthening are estimated, and the results demonstrate that solid-solution strengthening, especially carbon-redistribution, accounts for the majority of the strength variations due to different cooling-rates.

Influence of Al deoxidation on the formation of acicular ferrite in steel containing La

AbstractThe formation of acicular ferrite (AF) in steel containing Lanthanum (La) with different contents of Aluminum (Al) has been investigated. It has been found that the amount of AF in La treated steel killed by Al is less than the one with little content of Al. The reasons are as follows: first, Al can change the type of La-containing inclusions and may reduce the effectiveness of La inclusion to induce the nucleation of AF. second, Al can make the cooling rate range of AF nucleation narrower in La treated steel. What’s more, Al can increase the decomposition temperature of austenite, and all these make AF formation difficult. In this study, the content of Al should be controlled to less than 0.018 wt% to obtain a more effective inclusion to induce the formation of AF.