Continuous Cooling Transformation Diagrams Research Articles

Development of a machine learning model for prediction of continuous cooling transformation diagrams in welding heat-affected zone

Development of a machine learning model for prediction of continuous cooling transformation diagrams in welding heat-affected zone

Microstructure estimation and validation of ER110S-G steel structures produced by wire and arc additive manufacturing

Wire and Arc Additive Manufacturing (WAAM) emerged as a manufacturing process for large scale structures with extensive form and design freedom. WAAM can be fully exploited once the relation between the transient thermal history and its relation to microstructure development and resultant mechanical properties is established. This relation can be further used for computational design tools such as Topology Optimization. This paper presents a model to predict the relation between the thermal history and solid-state phase transformations in a widely applicable High Strength Low Alloy steel ER110S-G. The transient thermal history of parts manufactured by WAAM is modelled using finite element analysis. The modelled thermal history is validated with thermocouple measurements. Our results show that a critical cooling cycle is responsible for the solid-state phase transformation in an AM part. The cooling rate of this particular cooling cycle is superimposed onto an experimentally constructed Continuous Cooling Transformation (CCT) diagram to determine the local solid-state phase fractions. The predicted phase fractions in three wall samples with different design and processing conditions of AM parts are used to predict the hardness. The predicted hardness is 10% higher than the measured hardness of AM samples. The effect of tempering is also considered in the model through JMAK equation. The results show that the tempering is caused in regions with high martensite content and it lowers the hardness by 8 − 10%. The micrographs of the AM parts show that the microstructural features are same for the AM parts with similar critical cooling rates.

Modelling of Phase Diagrams and Continuous Cooling Transformation Diagrams of Medium Manganese Steels

The aim of this manuscript was to study the influence of alloying elements on the phase transformation behavior in advanced high-strength multiphase steels. Continuous cooling transformation (CCT) and time–temperature–transformation (TTT) diagrams were calculated to analyze the stability of phases at variable time–temperature processing parameters. The analyzed materials were lean-alloyed transformation induced plasticity (TRIP) medium manganese steels. The simulations of the phase diagrams, the stability of the phases during simulated heat treatments, and the chemical composition evolution diagrams were made using Thermo-Calc and JMatPro material simulation softwares. The influence of alloying elements, i.e., Mn and C, were studied in detail. The computational and modelling results allowed the influence of alloying elements on equilibrium and non-equilibrium phase diagrams and microstructural and chemical composition evolutions to be studied. Good symmetry and correlation between computational softwares were achieved. The study allows for future optimization of the heat-treatment temperature and time conditions of modern medium-Mn automotive sheet steels.

鋳片浸漬冷却時の変態，3次元熱処理変形および応力発生の各挙動に及ぼす冷却条件およびγ粒サイズの影響

To optimize reverse transformation treatment, which is effective in preventing hot rolling cracking in the CC-HCR (Continuous Casting - Hot Charge Rolling) process, three-dimensional metallo-thermo-mechanical analyses were performed. The metallo-thermo-mechanics and a CCT (Continuous Cooling Transformation) diagram estimation method using JMatPro were used to obtain the required cooling time for reverse transformation in immersion cooling with water jet and to elucidate the effects of nonuniform cooling and γ grain size on thermal deformation and stress generation behaviors during cooling and the mechanisms of these events.

Application of a high-speed infrared thermographic camera to the study of HAZ softening in S960 welded joints

Hot strip rolling of low-carbon advanced high-strength (AHS) sheet steels is difficult because their products require specific chemical compositions to achieve excellent mechanical properties. The formation of a heat-affected zone (HAZ) during welding is therefore a challenge in terms of maintaining high strength values. This fact complicates the formation of the softening area when the hardness in the HAZ rapidly decreases. According to the continuous cooling transformation diagram (CCT), the chemical composition of the base metal is directly related to the fine-grained martensitic microstructure arising from very high cooling rates and microalloying up to 0.2 wt% by Nb, V, and Ti. The gas metal arc welding (GMAW) was used to perform the study, what presents a challenge for thin sheets because of very narrow HAZ formation, suscepible to HAZ softening phenomenon. The width of the HAZ was about 6 mm and determined by applying an infrared thermographic camera to measure the real-time temperature development. The significant reduction in microhardness, known as the softening phenomenon, occurred in the intercritical heat-affected zone or at the boundary between intercritical and subcritical HAZ, according to real-time diagnostics of the temperature development and the microhardness results. The amount of heat produced during welding plays an important role in this context and is directly connected to the heat input of the welding process. The softening area can be reduced and its impact on the mechanical properties can be lowered by appropriately modifying the welding conditions, e.g., optimising the heat input. The purpose of this paper is to point out the change in temperature during the welding process.

Selective precipitation and non-isothermal crystallization kinetics of britholite from low grade REE-bearing slag

In order to improve the recovery rate of rare earth, the effect of P2O5 and cooling rate on the selective precipitation of CaO–SiO2–TiO2–P2O5–Nb2O5–CeO2–CaF2 slag system was explored using X-ray powder diffraction, a scanning electron microscope, an electron probe micro-analyzer and a confocal laser scanning microscope. The apatite (Ca2.02Ce7.98Si6.00O26.00) is tiny and difficult to be separated from the slag without P2O5. When the addition of P2O5 is 6 wt%–10 wt%, the cerium and phosphorus are found to be enriched in britholite (Ca5–xCex[(SiO4)x(PO4)3–x]F) phase while x is negatively correlated with P2O5 content in the slag. The influence of P2O5 and cooling rate on the non-isothermal crystallization kinetics was also investigated. For the cooling rate between 5 and 40 °C/min, continuous cooling transformation diagram of britholite was constructed when the P2O5 content is between 6 wt%–10 wt%. The modified Avrami model was applied to determine the crystallization mode of slag with 10 wt% P2O5. It is shown that the crystallization mode is diffusion-controlled with constant nucleation rate and one-dimensional growth. In consideration of quantity and volume of crystals, the reasonable cooling rate is between 10 and 30 °C/min.

Recycling‐Induced Copper Contamination of a 42CrMo4 Quench and Tempering Steel: Alterations in Transformation Behavior and Mechanical Properties

The transformation behavior and the mechanical properties of three laboratory produced steels are presented. To investigate the influence of copper contamination, the reference alloy 42CrMo4 is modified and alloyed with 0.5 and 1.0 wt% copper, respectively. The transformation behavior is investigated through dilatometry that yields continuous cooling transformation diagrams, while the mechanical properties are investigated by hardness measurements as well as Charpy‐V‐notch impact and tensile tests. Accompanying microstructure investigations by means of light optical, scanning electron, and transmissions electron microscopy and thermodynamic simulations (CALPHAD) are performed. It is demonstrated that copper contamination that will be unavoidable when increasing the overall steel recycling fraction may contain the potential for increasing the hardness as well the strength of standard high strength steels, although it is considered to be very detrimental during steel production. Together with reduced environmental footprints and production costs due to the utilization of lower grade scrap with more Cu‐contamination, the development of alternative processing routes for Cu‐contamination‐tolerant high strength steels is pursued.

Intercritical continuous cooling transformation diagram for the manufacture of low-alloyed low-carbon multiphase steels

This paper deals with the construction of an experimental intercritical continuous cooling transformation (CCT) diagram for the manufacture of low-alloyed low-carbon multiphase steels. An in-situ dilatometric study was conducted both to determine the critical transformation temperatures and to follow the evolution of austenite on continuous heating. A quantitative analysis of the transformed austenite was conducted from the dilation curves using the lever rule method to further investigate the intercritical austenite decomposition for different cooling rates. Time and temperatures required for phase transformations were determined from the first derivative of the dilation curves, obtained as a function of cooling rate. Critical points were then plotted to construct the corresponding CCT diagram. Results show that the experimental CCT diagram can be used to design thermal cycles for the development of multiphase steels.

Material extrusion thermal model mapped across polyetheretherketone isothermal and continuous cooling transformation charts

This study provides an insight into the polyetheretherketone (PEEK) crystallinity progression throughout the material extrusion (MEX) additive manufacturing process as a function of time and temperature, comparing it with the isothermal and continuous cooling transformation charts created over a wide range of isothermal crystallisation temperatures and constant cooling rates. The isothermal and non-isothermal crystallisation kinetics were explored using Differential Scanning Calorimetry (DSC) and Fast Scanning Calorimetry (FSC). The half-time, onset and ending of crystallisation were obtained for isothermal crystallisation temperatures between 150 °C and 330 °C, while the crystallisation under constant cooling was obtained using rates between − 0.5 K s −1 and − 45 K s −1 . The results were used to draw the Continuous Cooling Transformation (CCT) and the Time-Temperature Transformation (TTT) diagrams and calculate the Avrami numbers using the parallel Avrami model. These results were then compared to the degree of crystallinity as a function of time and temperature for the MEX process. To evaluate the crystallisation within the MEX process a 1D transient transfer heat model was used to obtain the printing thermal profile, which was replicated using the FSC technique. The results showed that for the MEX printing process, the crystallisation usually is a product of a combination of rapid cooling and heating processes followed by periods of greater thermal stability which, depending on the nature of the process, can approach a quasi-isothermal crystallisation process. By superimposing the process thermal profile on the TTT and CCT diagrams and comparing the crystallinity values measured from each point in the thermal profile, it was possible to understand the crystallinity evolution and the remelting of the build surface promoted by the subsequent printed layers. • Temperature fluctuations in the MEX process are a combination of ultra-fast dynamic cooling steps with heating and remelting. • The FSC and standard DSC allowed the replication of the modelled thermal profile. • The crystallinity varies as a function of local temperature and the fluctuations caused by the multi-layer printing process. • Higher chamber temperature increased the crystalline phase remelting, resulting in greater mobility of polymeric chains. • Shorter return times improved overall crystallinity levels due to more frequently heat input in the building surface.

Prediction of continuous cooling transformation diagram for weld heat affected zone by machine learning

Prediction of continuous cooling transformation diagram for weld heat affected zone by machine learning

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.

Investigation of microstructural and mechanical properties of hot forged 31Mn4 dual phase steel with computer-aided simulations

ABSTRACT In this study, it is aimed to realise the hot forging process design and prototype production of the steel yielding support connection clamp, the critical safety element of underground mining ground support systems, by using 31Mn4 dual phase steel. In this context, at the end of the forging and deburring processes, air cooling was designed, simulated and cooling rates were obtained. The cooling curve was integrated into the continuous cooling transformation diagram and phase formation was predicted. The distribution of pearlitic, ferritic, bainitic structures and formation rates, hardness distributions, strength data were also obtained. The hardness and tensile strength calculations, microstructural examinations and SEM analysis were carried out for validation. It has been determined that pearlitic-ferritic microstructures are formed in regions where the cooling rate is slow. In thinner sections where the cooling rate is around 1.2°C/sec, ferritic structures become smaller and bainite phase was observed. As a result, the average tensile strength in 1st and 2nd regions were recorded as 684.5, 690 MPa in simulation, while these values were recorded as 688.45 and 694.15 MPa in tensile tests. It has been determined that this result corresponds to the 99.4% accuracy rate of the prototype obtained by simulation-supported and real productions.

Determination of CCT Diagram by Dilatometry Analysis of High-Strength Low-Alloy S960MC Steel.

High-strength steels are used more than general structural steel due to their combination of properties such as high strength, good toughness and weldability. They are mainly used in the manufacture of heavy vehicles for the mining industry, cranes, transportation, etc. However, welding these grades of steel brings new challenges. Also, a simulation for welding high-strength steel is required more often. To insert a material database into the simulation program, it is necessary to conduct investigations using CCT (Continuous Cooling Transformation) diagrams, welded joints research, and more. To investigate the behavior of S960MC steel during heating and cooling, we used dilatometry analysis supported by EBSD (Electron Backscatter Diffraction) analysis. A CCT diagram was constructed. The transformation temperatures of Ac1 and Ac3 increase with increasing heating rate. The Ac1 temperature increased by 54 °C and the Ac3 temperatures by 24 °C as the heating rate increased from 0.1 °C/s to 250 °C/s. The austenite decomposition temperatures have a decreasing trend in the cooling phase with increasing cooling rate. As the cooling rate changes from 0.03 °C/s to 100 °C/s, the initial transformation temperature drops from 813 °C to 465 °C. An increase in the cooling rate means a higher proportion of bainite and martensite. At the same time, the hardness increases from 119 HV10 to 362 HV10.

PROCESSES OF WELDED JOINT STRUCTURE FORMATION DURING ELECTRIC RESISTANCE WELDING OF RAILWAY RAILS

During operation of a continuous welded rail track, most of the fractures occur in welded joints. Welded joints failures take place due to various reasons including the presence of the hardened areas in the welded joint and increased local wear of the rolling surface. The paper presents the dilatometry results and the obtained continuous cooling transformation diagram of the supercooled austenite decomposition for R350LHT rail steel, which make it possible to define the critical cooling rate. It was determined that during welding in hard modes the welded joint cooling rate is lower than the critical one which leads to the martensite formation in the welded seam. The positions of critical points and phase equilibrium areas for the given rail steel were also modeled. It is proposed to use a new method for welding railroad rails based on the knowledge of dependences of the structural components dispersity and, first of all, pearlite and carbide particles formed in the process of obtaining welded joints of rails on the composition of steel and the conditions of their cooling.

Algorithm for Determining Time Series of Phase Transformations in the Solid State Using Long-Short-Term Memory Neural Network.

In the numerical analysis of manufacturing processes of metal parts, many material properties depending on, for example, the temperature or stress state, must be taken into account. Often these data are dependent on the temperature changes over time. Strongly non-linear material property relationships are usually represented using diagrams. In numerical calculations, these diagrams are analyzed in order to take into account the coupling between the properties. An example of these types of material properties is the dependence of the kinetics of phase transformations in the solid state on the rate and history of temperature change. In literature, these data are visualized Continuous Heating Transformation (CHT) and Continuous Cooling Transformation (CCT) diagrams. Therefore, it can be concluded that time series analysis is important in numerical modeling. This analysis can also be performed using neural networks. This work presents a new approach to storing and analyzing the data contained in the discussed CCT diagrams. The application of Long-Short-Term Memory (LSTM) neural networks and their architecture to determine the correct values of phase fractions depending on the history of temperature change was analyzed. Moreover, an area of research was elements that determine what type of information should be stored by LSTM network coefficients, e.g., whether the network should store information about changes of single phase transformations, or whether it would be better to extract data from differences between several networks with similar architecture. The purpose of the studied network is strongly different from typical applications of artificial neural networks. The main goal of the network was to store information (even by overfitting the network) rather than some form of generalization that allows computation for unknown cases. Therefore, the authors primarily investigated in the ability of the layer-based LSTM network to store nonlinear time series data. The analyses presented in this paper are an extension of the issues presented in the paper entitled “Model of the Austenite Decomposition during Cooling of the Medium Carbon Steel Using LSTM Recurrent Neural Network”.

Mo addition of high-strength seismic rebars subjected to Tempcore processes for tensile strength enhancement

High-strength seismic reinforcing bars (rebars) reduce seismic damages by achieving low yield ratios. The effect of adding Mo to high-strength Tempcore rebars for achieving low yield ratios was investigated. The continuous cooling transformation diagram shifted with Mo addition to achieve higher hardenability, thereby producing a low-temperature transformed phase, inside the rebar. The work-hardening exponent increased with increasing Mo content, which was due to the fine grain size of the low-temperature transformed phase and its high dislocation density. The addition of 0.11 wt% Mo provided a feasible solute drag effect for constructional steels and achieved high strength and excellent seismic resistance.

Microstructural adjustment of hot-rolled Ti–6Al–4V based on a CCT diagram

The effects of post-processing heat treatments on hot-rolled Ti–6Al–4V with extra low interstitial concentration were investigated. The 2 mm sheets were rolled in the single β phase field region and subsequently cooled in air. A continuous cooling transformation (CCT) diagram was generated, revealing the strong influence of the cooling rate on both microstructure and hardness. Cooling rates between 0.01 and 200°C s−1 were selected. While rapid cooling resulted in a martensitic transformation β → α′, a decrease in the cooling rate led to a progressive spheroidization of the lamellar (α + β) microstructure. With a focus on the established CCT continuous cooling transformation diagram, this study contains insight on the microstructure evolution of the Ti–6Al–4V ELI alloy during cooling from the single β phase field region.

Extended Continuous Cooling Transformation (CCT) Diagrams Determination for Additive Manufacturing Deposited Steels.

Continuous cooling transformation (CCT) diagrams are widely used when heat treating steels and represent which type of phase will occur in a material as it is cooled at different cooling rates. CCT diagrams are constructed on the basis of dilatometry measurements on relatively small testing samples (cylindrical shape with diameter of 4mm and length of 11 mm in this study). The main aim of this work was to demonstrate the possibility of evaluating the tensile test properties using mini-tensile tests from miniature volumes (1.4 × 10−7 m3 for one sample) subsequent to determination of the CCT diagram and to extend a standard CCT diagram with information about strength, ductility and the estimated value of the work-hardening coefficient. Mini-tensile tests (MTT) were recently developed due to the low availability of experimental material and have already been successfully used for local mechanical property characterization of metals. CCT diagrams were constructed for 42CrMo4 steel prepared by the laser-directed energy deposition (L-DED) process, for commercially available 42CrMo4 steel conventionally manufactured (for comparison of traditional processing and AM preparation) and for H13 tool steel deposited by the selective laser melting (SLM) process.

A hybrid machine learning model for predicting continuous cooling transformation diagrams in welding heat-affected zone of low alloy steels

Continuous cooling transformation diagrams in synthetic weld heat-affected zone (SH-CCT diagrams) show the phase transition temperature and hardness at different cooling rates, which is an important basis for formulating the welding process or predicting the performance of welding heat-affected zone. However, the experimental determination of SH-CCT diagrams is a time-consuming and costly process, which does not conform to the development trend of new materials. In addition, the prediction of SH-CCT diagrams using metallurgical models remains a challenge due to the complexity of alloying elements and welding processes. So, in this study, a hybrid machine learning model consisting of multilayer perceptron classifier, k-Nearest Neighbors and random forest is established to predict the phase transformation temperature and hardness of low alloy steel using chemical composition and cooling rate. Then the SH-CCT diagrams of 6 kinds of steels are calculated by the hybrid machine learning model. The results show that the accuracy of the classification model is up to 100%, the predicted values of the regression models are in good agreement with the experimental results, with high correlation coefficient and low error value. Moreover, the mathematical expressions of hardness in welding heat-affected zone of low alloy steel are calculated by symbolic regression, which can quantitatively express the relationship between alloy composition, cooling time and hardness. This study demonstrates the great potential of the material informatics in the field of welding technology.

Thermodynamic and Kinetic Calculation of High Strength Aluminum-Lithium Alloy

High strength Al-Li alloy is the research focus of aluminum alloy. In the present paper, high strength Al-Li alloy 2A97 composed of Al-Li-Cu-Zn-Mg-Mn-Zr system is studied by thermodynamic and kinetic calculation. The equilibrium phase diagram and the metastable phases of 2A97 were calculated by thermodynamic method. The solidification phase diagram of 2A97 was obtained using the Scheil–Gulliver model. The continuous cooling transformation diagram and isothermal aging curves were calculated using the kinetic method. In addition, the 2A97 microstructures of cast, homogenized, hot-rolled, and solid solution were observed by a scanning electron microscope. The results can be used for the process optimization and microstructure control of 2A97. In particular, the research results of this work can be used for the determination of the homogenization temperature and solid-solution temperature.