Continuous Cooling Transformation Research Articles

Modeling of CCT diagrams for tool steels using different machine learning techniques

Continuous cooling transformation (CCT) diagram is an important basis to make an optimal heat treatment process of steels with a desired microstructure and properties. Therefore, it is of great practical importance to predict the CCT diagram rapidly and accurately, especially for costly and time-consuming material design by trial and error. In this study, machine learning approaches are provided to predict CCT diagrams of tool steels using relevant material descriptors including the chemical compositions, austenitizing temperature and cooling rate. Different machine learning techniques including the Multilayer Perceptron Regressor, k-Nearest Neighbours, Bagging and Random Forest are performed on experimental dataset for appropriate model selecting. Random forest is proved to be the best model to predict pearlite transition temperature and martensite transformation start temperature accurately. K-Nearest Neighbours and Bagging are suitable for predicting the start and end temperatures of bainite formation respectively. These optimal models are then used to predict the CCT diagrams of T8, 6CrW2Si, 4CrMoV, CrMn and Cr12W. Comparing with the calculation results by the commercial software JMatPro, these optimal models work much better for their distinguished prediction performance with high correlation coefficient and low error values, which is thus of great engineering significance.

Continuous Cooling Transformation Diagram, Microstructures, and Properties of the Simulated Coarse-Grain Heat-Affected Zone in a Low-Carbon Bainite E550 Steel

In order to provide important guidance for controlling and obtaining the optimal microstructures and mechanical properties of a welded joint, the continuous cooling transformation diagram of a new low-carbon Nb-microalloyed bainite E550 steel in a simulated coarse-grain heat-affected zone (CGHAZ) has been constructed by thermal dilatation method in this paper. The welding thermal simulation experiments were conducted on a Gleeble-3800 thermo-mechanical simulator. The corresponding microstructure was observed by a LEICA DM2700M. The Vickers hardness (HV) and the impact toughness at −40 °C were measured according to the ASTM E384 standard and the ASTM E2298 standard, respectively. The experimental results may indicate that the intermediate temperature phase transformation of the whole bainite can occur in a wide range of cooling rates of 2–20 °C/s. In the scope of cooling rates 2–20 °C/s, the microstructure of the heat-affected zone (HAZ) mainly consists of lath bainite and granular bainite. Moreover, the proportion of lath bainite increased and granular bainite decreased as the cooling rate increasing. There is a spot of lath martensite in the microstructure of HAZ when the cooling rate is above 20 °C/s. The Vickers hardness increases gradually with the increasing of the cooling rate, and the maximum hardness is 323 HV10. When the cooling time from 800 °C to 500 °C (t8/5) is 5–15 s, it presents excellent −40 °C impact toughness (273–286 J) of the CGHAZ beyond the base material (163 J).

Heat treatment behavior of alternator shafts steels

The present paper analyzes properties such as the hardness, the tensile and toughness behaviour and the microstructure for multiple bars of C40 and C45 steel used for shafts in alternators. The materials have been manufactured by rolling processes, followed by normalization treatments or forging processes followed by normalization treatments. Heat treatments are performed on steels to produce different microstructural phases that affect the mechanical properties. Microstructural changes during austenite decomposition depend on the transformation temperature region and cooling rate. Over the years, many experimental studies have been performed to reveal the microstructural changes during isothermal holding or continuous cooling of austenite for various kinds of steels. The isothermal transformation (IT) diagram, which is also called the time-temperature-transformation (TTT) diagram, and the continuous cooling transformation (CCT) diagram have been developed to graphically characterize the decomposition transformation as a function of time and temperature. However, when these diagrams are based on experimental measurements, several limitations are present. Each diagram is limited because the temperatures at which specific transformations occur can vary due to several factors, such as: the chemical composition of steel, the solid solution condition in the austenite prior to cooling and transformation, the presence of precipitates in austenite, the prior austenite grain size, and the applied stresses during the transformation.

Towards an integrated experimental and computational framework for large-scale metal additive manufacturing

Using the Metal Big Area Additive Manufacturing (MBAAM) system, a thin steel wall was manufactured from a low carbon steel wire. The wall was then characterized comprehensively by high-throughput high-energy X-ray diffraction (HEXRD), electron backscatter diffraction (EBSD), and in-situ HEXRD tensile tests. With the predicted temperature histories from the finite element-based additive manufacturing process simulations, the correlations between processing parameters, microstructure, and properties were established. The correlation between the final microstructure with the predicted temperature history is well explained with the material's continuous cooling transformation (CCT) diagram calculated based on the composition of the low carbon steel wire. The final microstructure is dependent on the cooling rate during austenite to ferrite/bainite transformation during initial cooling and the subsequent reheating cycles. Fast cooling rate resulted in small ferrite grain size and fine bainite structure at the location closest to the base plate. Slower cooling rate at the side wall location and repeated reheating cycles to the ferrite-pearlite regions resulted in all allotriomorphic (equiaxed) ferrite with medium grain size with small amount of pearlite. With no reheating cycles, the top location has the slowest cooling rate and a large grained allotriomorphic ferrite and bainitic structures. The measured mechanical strength is then related to the microstructural feature size (grain or lath size) observed in those locations. A good correlation is found between the mechanical properties, microstructure features and the temperature history at various locations of the printed wall.

The Effects of Cooling Mode on the Properties of Ti–Nb Microalloyed High-strength Hot-rolled Steels

The industrial trials of two cooling modes, i e, water cooling in forepart + air cooling in later part (WAC) and air cooling in forepart + water cooling in later part (AWC), were carried out for a Ti-Nb microalloyed steel. The average cooling rates and coiling temperature were the same for two modes. The continuous cooling transformation (CCT) curve of the tested steel was drawn. The effects of the cooling mode on the microstructure, precipitates, and properties of the steels were investigated. Results show that the strength of the steel in the WAC mode is significantly larger than that in the AWC mode, mainly because the smaller the grain size, the more and finer the grain precipitates. Therefore, when the average cooling rate is constant, the fast cooling in the forepart is an effective method to increase the strength of steels. However, the increase in the strength is accompanied by the decrease in toughness, so that the toughness of the steel should be considered when changing the cooling mode.

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.

Phase transformation and microstructure evolution study in various alloy systems: An insight

Phase transformations occurring in various metals like steel, titanium and super alloysis integrated and presented in this paper. In steels, the microalloying with Nb and Nb-Mo decrease in transformation temperature indicates the acceleration of transformation kineticsin turn which affect the microstructure and continuous cooling transformation (CCT) diagram. Incase of titanium alloys, the thermohydrogen treatment (THT) uses hydrogen as temporary alloying element which facilitatethe alloys to be processed at lower stresses and/or temperatures. Thermomechanical processing sschedules are designed baseds ons the β transus temperature to produce a variety of novel microstructures. In the case of crystalline materials, plastic deformations assisted by dislocations and the concentration of dislocation increases with increasing plastic strainthus becoming a precursor of twinning.Stress-strain curves for different alloys suggested the microstructural changes account for deformation induced twinning and martensite transformation.

Quantification of microstructure to reveal the solidification path of an alloy

This paper reports on the development of Solidification Continuous Cooling Transformation diagrams that relate the solidification paths to the inherent microstructures of binary and ternary alloys. The methodology is based on the quantification of a solidified microstructure for its various phase fractions. This measured data is combined with well-established solidification models and phase diagrams to yield undercooling temperatures of individual phases. The thermal history and undercooling of different phases in the solidified alloy are estimated for a wide range of cooling rates (from 10-2 °Cs-1 to 105 °Cs-1). To describe the methodology, dedicated samples of Al-Cu, Al-Cu-Sc and Al-Si alloys were studied. With said alloys being solidified in a controlled manner, over a wide range of cooling rates and undercoolings, via Impulse Atomization, Electro-Magnetic Levitation, and Differential Scanning Calorimetry

Continuous Cooling Kinetics Modeling for a Medium–High Carbon Spring Steel

Continuous cooling transformation (CCT) diagrams of steel are the basis to determine the optimum heat treatment technology. Combined with metallography, the CCT diagram of medium–high carbon spring steel 60Si2CrA was obtained by dilatometric measurement at cooling rates from 0.5 to 50 K/s. It is found that ferrite, pearlite, and martensite were available in the experimental range. Taking into account of the incubation time and volume fraction evolution of the new phase, a new phase transformation kinetics model is established to estimate the fraction of ferrite and pearlite during continuous cooling process (CCP). Material constants in the model were determined and optimized by minimizing the errors between experimental and predicted data. The kinetics model was validated and proved its efficiency in predicting the new phase fraction of 60Si2CrA during continuous cooling and will probably constitute a useful tool for guiding the production process.

Effects of cooling rate on microstructure, mechanical properties, and residual stress of Fe-2.1B (wt%) alloy

The continuous cooling transformation (CCT) curve of an Fe-2.1B (wt%) alloy is obtained using a Gleeble 1500D thermomechanical simulator. The microstructure, mechanical properties, and residual stress of alloy specimens with various cooling rates are examined. The results reveal that the cooling rate has a great influence on the matrix microstructure of the Fe-2.1B (wt%) alloy. Pearlite is formed at the cooling rate of 0.1 K/s, pearlite and martensite are formed in the cooling rate range of 0.2–0.5 K/s, and only martensite remains in the matrix when the cooling rate exceeds 0.5 K/s. In addition, as the cooling rate increases, the dislocation density in the matrix increases, and this, in turn, leads to an increase in the volume fraction of the M23(B, C)6 phase. The precipitation of M23(B, C)6 causes the decrease in the (B + C) contents of the matrix, which, in turn, reduces the microhardness of the matrix to some extent. Meanwhile, the large residual compressive stress of the alloy increases with increasing cooling rate. The maximum residual compressive stresses induced by the cooling rates of 0.5 and 30 K/s are approximately 18% and 36%, respectively, higher than that induced by the cooling rate of 0.1 K/s. Moreover, when the cooling rate increases from 0.1 K/s to 0.5 K/s, the macrohardness and bending stress increase significantly owing to an increase in Vm/Vpr (where Vm represents volume fraction of martensite, Vpr is the volume fractions of pearlite and austenite). When the cooling rate exceeds 0.5 K/s, the macrohardness and bending stress decrease gradually because of the decrease in the (B + C) contents of the matrix and an increase in the residual stress. The critical cooling rate (0.5 K/s) may be the optimal cooling rate of Fe-B alloys.

Cracking and segregation in high-alloy steel 0.4C1.5Mn2Cr0.35Mo1.5Ni produced by thick continuous casting

Based on our innovative application of using thick continuous casting slab 0.4C1.5Mn2Cr0.35Mo1.5Ni (high alloy) for the production of high-quality mould steel, the present study investigated the high cracking susceptibility of high-alloy steel and segregation in continuous casting slab. The thermal expansion and the continuous cooling transformation (CCT) curve measurement, together with a high temperature in situ observation, confirmed the martensite phase transition happening at approximately 583 K that would result in an increase in the hardenability and cracking susceptibility. The cracking susceptibility zone was determined by high-temperature mechanical properties measurement. The high-alloy mould steel has no II brittle zone, and III brittle zone is 973–1148 K. As a conclusion, the straightening temperature should be above 1148 K to avoid the cracking during the continuous casting. Moreover, the elemental segregation of carbon, sulfur, chromium, and molybdenum along the cracking was examined by electron probe microanalysis (EPMA) quantitative analysis that might be another reason for the steel crack formation. It shows that Martensite phase transition happened at approximately 583 K that would result in an increase in the hardenability and cracking susceptibility.

Retained austenite phase detected by Mössbauer spectroscopy in ASTM A335 P91 steel submitted to continuous cooling cycles

Samples of ASTM A335 P91 steel submitted to continuous cooling at different rates were analyzed in the form of foils and powders by means of Mössbauer spectroscopy. The Continuous Cooling Transformation (CCT) diagram of steel ASTM A335 P91 displays two basic microstructural domains at low temperatures – ferritic and martensitic – whose limits depend on the austenite holding temperature, the precise chemical composition and the cooling conditions from the austenite mother phase. Under certain conditions, the martensitic transformation may not be completed, leading to a final microstructure with a non-negligible percentage of the austenite phase retained in a metastable state. This retained austenite could be detrimental for the mechanical properties of the steel.Mössbauer analysis suggested that powdering process promotes the retained austenite transformation to martensite; in particular, in the present case, all the austenite transformed into martensite during powdering. Foil samples instead displayed retained austenite whose relative fraction was determined as a function of the cooling rate. At the same time, the carbon content of retained austenite was estimated for the faster cooled samples; a preliminary explanation for the observed trends is given.

Development and Application of Cast Steel Numerical Simulation System for Heat Treatment

A numerical simulation system for heat treatment of cast steel was developed using C++ environment, which has been integrated into the InteCAST platform as a new feature module. The system mainly includes heat treatment process inquiry module, initialization and property parameter setting module, automatic import and display module of time-temperature-transformation (TTT) and continuous cooling transformation (CCT) curves, heat treatment process design module, organization, performance and stress–strain prediction module. This paper introduces the mathematical model and numerical solution algorithm in the process of numerical simulation of the heat treatment organization, performance and stress–strain as well as the interface operation process and application examples of the system.

Effect of Bubbles on Crystallization Behavior of CaO–SiO2 Based Slags

Surface longitudinal cracks are a serious problem and particularly prevalent in the casting of peritectic steel (carbon content between 0.10%C and 0.18%C, non-alloyed). It is usually alleviated by controlling the horizontal heat transfer from the steel shell to the mold through increasing the crystallization performance of slags. In the actual continuous casting process, a large number of bubbles are formed in the molten slags, and the crystallization properties of the mold fluxes are affected by bubbles. Therefore, an investigation of the influence of bubbles on the crystallization performance of mold fluxes was carried out by applying the hot thermocouple technique and it is hoped that surface longitudinal cracks can be solved in this way in the peritectic steel casting process. The continuous cooling transformation (CCT) diagrams and time–temperature transformation (TTT) diagrams were constructed for an analysis of the crystallization kinetics. The results showed that the crystallization ability of mold fluxes was enhanced by adding bubbles through shortening the incubation time of crystallization, increasing the critical cooling rate, and decreasing the activation energy of crystallization. As a result, the crystalline fraction, slag film thickness, and surface roughness of the slag films were improved, but the crystalline phase was not affected by bubbles. With an increase of the bubble content remaining in the molten slag, the growth mechanism of the cuspidine crystal phase changed from a low dimension to a high dimension, and the content of the molten slag’s structure unit (Q1) needed for cuspidine in the molten slag was markedly increased.

Study on Continuous Cooling Transformation and Microstructure of 500 Mpa Grade Steel for Railway Freight Car Body

Complete the composition design and laboratory smelting of high strength weathering steel for railway freight car body of grade 500MPa. The actual continuous cooling transformation curve (CCT curve and cold speed of 0.5-50 °C/s) was measured through the Gleeble3500 thermal simulation test machine with expansion method and alloy phase method, observing the microstructures by optical microscope and SEM. The hardness of the samples under different cooling rates were measured by Vivtorinox hardness tester. Study on the effect of cooling rates on the microstructures and hardness of the steels.

Effect of Process Parameters on Microstructure and Properties of 1500 MPa Grade Hot Formed Steel without Boron but Containing Niobium

In this paper, a new type of automotive 1500 MPa grade hot-formed steel without boron but containing niobium was subjected to thermoforming experiments. The phase transition point and Continuous Cooling Transformation (CCT) curve of the hot-formed steel were measured by thermal dilatometer, and then the best austenitizing parameters was determined. The microstructure of the cold-rolled sheet and the hot-formed steel sheet were observed by electron microscopy. The microstructure of the steel sheet after hot forming was studied by X-ray diffraction (XRD) method to determine whether the microstructure after hot forming had residual austenite. The influence of residence conditions on its mechanical properties was studied. The experimental results has shown that the microstructure of the original cold-rolled sheet is mainly composed of ferrite and pearlite. After thermoforming, the basic microstructure are martensite and a small amount of ferrite; When the hot forming parameters is that 900 °C of the heating temperature, 3 min of the holding time, 8 s of the residence time, quenching temperature is the room temperature, the new 1500 MPa grade hot formed steel has the best mechanical properties that the tensile strength is 1519 MPa, the yield strength is 1060 MPa, the yield ratio is 0.73, and the elongation reaches 10.52%. The result shows that the new 1500 MPa grade hot formed steel could obtain excellent mechanical properties through a reasonable process under the premise of ensuring hardenability.

Development of Niobium Microalloyed Steel for Railway Wheel with Pearlitic Bainitic Microstructure

Heavy haul transportation (load over 30 tons/axle), as well as the axle load, has been more and more used in Brazil and worldwide. The stress generated in the wheel-rail contact with loads up to 30 tons/axle is around 760 MPa, which causes premature wear and cracks of conventional wheels (AAR (Association of American Railroads) class C). Microalloyed wheels are fundamental on heavy haul transport, whose main function is to combine high hardness, ductility, and yield strength of the material in order to prevent shelling. The main purpose of this research is to develop a new microalloyed wheel steel with niobium addition that meets all the requirements of the AAR class D material with mixed microstructure composed of pearlite and bainite. The 0.71C/0.020Nb steel developed in this study (Nb material) for railroad achieved the standards required for AAR class D in all mechanical properties, with fracture toughness higher than the usual vanadium microalloyed steel used in comparison. The Continuous Cooling Transformation (CCT) diagram showed the presence of bainite even at very low cooling rate, in the range between 0.3 - 2 °C/s. These cooling rates to form bainite are much lower than those observed in other steels with similar composition.

Prediction of microstructure composition in steel plate heated using high power Yb:YAG laser radiation

This work concerns numerical modelling and computer simulations of temperature field and phase transformations during Yb:YAG laser heating of sheets made of S355 steel. The distribution of laser power emitted by Trumpf laser head D70 is used in the analysis. The heat source is modelled on the basis of interpolation algorithms using geostatistical kriging method. Coupled heat transfer and fluid flow in the fusion zone are described respectively by transient heat transfer equation with convective term and Navier-Stokes equation. The kinetics of phase transformations and volumetric fractions of arising phases are obtained on the basis of Johnson-Mehl-Avrami (JMA) and Koistinen-Marburger (KM) models. Continuous Heating Transformation (CHT) diagram is used for heating process and Continuous Cooling Transformation (CCT) diagram is used for heated steel with the decomposition of final volume fractions of phases transformed form austenite dependant on cooling rates.

Effect of simulated thermomechanical processing on transformation behavior and microstructure of 82B steel

The effects of loop-laying temperature and austenite deformation on the phase transformation behavior during continuous cooling, microstructure, and pearlite interlaminar spacing in 82B steels were investigated. Static and dynamic continuous cooling transformation (CCT) diagrams were measured with a Gleeble-3500 thermal simulator, and the mechanisms governing changes in the initial temperature, initial time, and duration of the phase transformation zone were also analyzed and discussed. The results show that CCT diagram shifted to the bottom right, the initial temperature of the phase transition decreased, the initial time of the phase transition increased, the duration of the phase transition increased, and the lamellar spacing of pearlite was finer as the loop-laying temperature increased. The initial phase transition time decreased, and the phase transition duration first reduced, then increased, and finally decreased in the static condition and in the dynamic condition at 850 °C as the cooling rate increased. Meanwhile, the phase transition duration continuously decreased in the dynamic condition at 900 °C. At a given loop-laying temperature, the lamellar spacing in pearlite was finer due to austenite deformation compared with the undeformed case. Compared with the results shown in the dynamic CCT diagram, the corresponding phase diagrams of the static CCT diagram slightly shifted to the bottom right. Moreover, there was a clear linear relationship between the reciprocal of the lamellar spacing in pearlite and the average undercooling degree in the phase transformation zone.

Continuous Cooling Phase Transformation Rule of 20CrMnTi Low-Carbon Alloy Steel

Gear steel is a ferritic steel. In the rolling process, the ideal structure is ferrite + pearlite, and bainite or martensite is not expected. However, due to the high alloy content, the hardenability is good, and the bainite or martensite structure is very likely to be generated upon cooling after rolling. In this paper, phase transformation rules during continuous cooling of 20CrMnTi with and without deformation were studied to guide the avoidance of the appearance of bainite or martensite in steel. A combined method of dilatometry and metallography was adopted in the experiments, and the dilatometer DIL805A and thermo-simulation Gleeble3500 were used. Both dynamic and static continuous cooling transformation (CCT) diagrams were drawn by using the software Origin. The causes of those changes in starting temperature, finishing temperature, starting time and transformation duration in ferrite-pearlite phase transformation were analyzed, and the change in Vickers hardness of samples with different cooling rate was discussed. The results indicate that with different cooling rate, there are three phase transformation zones: ferrite-pearlite, bainite and martensite. Deformation of austenite accelerates the occurrence of transformation obviously and moves CCT curve to left and up direction. When the cooling rate is lower than 1 °C/s, the phases in samples are mainly ferrite and pearlite, which is the ideal microstructure of experimental steel. As the cooling rate increases, starting temperature of ferrite transformation in steel decreases, starting time reduces, transformation duration gradually decreases, and the Vickers hardness of samples increases. Under the cooling rate of 0.5 °C/s, ferrite transformation in deformed sample starts at 751.67 °C, ferrite-pearlite phase transformation lasts 167.9 s, and Vickers hardness of sample is 183.4 HV.