Continuous Cooling Transformation Diagrams Research Articles

Impact of niobium addition and non-metallic inclusions’ characteristics on the microstructure and mechanical properties of low-carbon CrNiMnMoB ultrahigh-strength steel: A comprehensive investigation

The effect of cooling rate and Nb addition on the phase transformation, microstructures, and mechanical properties of thermomechanically controlled hot-rolled low-carbon CrNiMnMoB ultrahigh-strength steel were studied. The aspects impeding Nb's benefits on hardenability, phase transformation, and mechanical properties were investigated. The hot rolling process was simulated, and a continuous cooling transformation diagram was constructed for the studied steels. Microstructures were analysed using laser scanning confocal microscopy and field emission scanning electron microscopy, with the latter also used for in-depth investigations of the non-metallic inclusions. Transmission electron microscopy was employed to investigate the precipitate characteristics. The concentrations of Nb in the solution and precipitates were quantified for the martensitic and granular bainitic structures. The effect of Nb in solution on phase transformation and strengthening is diminished, and the hardness is reduced by Nb precipitation at an early processing stage. Decreasing the cooling rate promotes the formation of granular bainitic structures rather than martensite. After being subjected to hot rolling followed by water quenching and air cooling, the microstructures are autotempered martensite and fully granular bainite, respectively. Various non-metallic inclusions, including Al2O3, MnS, and Al2O3–MnS, MnS·BN, Al2O3·BN and MnS·Al2O3·BN were observed. The number density and area fraction of non-metallic inclusions in the Nb-containing steel and the air-cooled samples are higher than those observed in the Nb-free and water-quenched samples. Adding 0.04 wt% of Nb slightly increased the strength, but the toughness properties dropped, mainly due to the higher number density and area fraction of non-metallic inclusions and primary large precipitates.

Modelling and numerical analysis for rotatory friction welding of U75V steel rails

This article presents a rotatory friction welding (RFW) method for U75V steel rail, aiming to mitigate challenges related to property discrepancies between the as-welded joints and the rail base metal (BM) and to narrow the heat-affected zone (HAZ) in conventional flash-butt welding (FBW) joints. A rotational intermediate plate is designed for rails with non-axisymmetric cross-sections, necessitating stationary during RFW. Advantages include achieving a relatively uniform welding heat input and maintaining the peak temperature of the contact interface near A1. To implement these concepts, a 2D finite element (FE) model for the RFW process of U75V rail steel rods was established and validated through experiments with identical process parameters. Microstructure predictions derived from continuous cooling transformation diagram confirm that ferrite microstructure is formed near A1 through rail steel RFW. Subsequently, a 3D FE model for intermediate plate RFW steel rails is developed to explore appropriate process parameter combinations. A suitable process parameters combination was identified, ensuring the peak temperature of the majority model contact interface does not exceed A1, resulting in a 76.7% reduction in HAZ (from ∼50 to 11.66 mm), and axial shortening of 8.10 mm, a significant decrease compared to the usual burn-off (30–40 mm) during FBW. These findings underscore the efficacy of this innovative welding solution and emphasize the significance of simulation technology in process optimization.

Quasi-equal strength and toughness matching rail steel joint obtained via friction welding below Ac1 temperature

U75V rail steel rods underwent continuous-drive friction welding (CDFW). Joint properties were evaluated through microstructural analysis and mechanical testing (tensile, impact, and microhardness). The thermo-mechanical behaviour of the CDFW process was simulated by the finite element method, while a computed continuous cooling transformation diagram was used to predict phase transformation, microstructures, and hardness variations. By reducing the spindle speed to 1000 r/min, the peak temperature in the weld centre zone (WCZ) remained below Ac1. Microstructures in WCZ exhibited a high density of geometrically necessary dislocations. The joints demonstrated comparable tensile strength, impact toughness, and microhardness to BM, with underlying mechanisms elucidated. This study showcases CDFW's potential in achieving rail steel joints with quasi-equal strength and toughness to the BM.

Effect of Mo and Mo+Nb Additions on the Phase Transformation and Microstructure of a Developed Low-Carbon CrNiMnB Ultrahigh-Strength Steels with a Preceding Hot Deformation

The influence of molybdenum, and molybdenum with niobium addition on the phase transformation behaviour of a developed low-carbon CrNiMnB ultrahigh-strength steels, was investigated. Gleeble 3800 thermomechanical simulator was employed to simulate the hot-rolling process and to get the dilatation curves. After austenitization at 1250 °C for the complete dissolution of carbides, specimens received 0.6 total strain (i.e., 0.2 at 1100 °C and 2 x 0.2 at 900 °C) followed by cooling at various cooling rates (CRs) in the range of 2-60 °C/s. The final microstructures were investigated using laser scanning confocal microscopy, field emission scanning electron microscopy, and hardness measurements. Then the continuous cooling transformation diagrams were constructed based on the dilatation curves, microstructure, and hardness values. The electrolytic extraction method was used to assess the elements' distribution and the composition of the forming precipitates. The addition of Mo increased the hardenability, decreased the transformation temperatures, and promoted the formation of low-temperature transformation products i.e., martensite and bainite ferrite, at different CRs and inhibit the formation of polygonal ferrite. The formation of coarse precipitates neglected the effect of Mo+Nb addition, decreased the hardenability and expanded the region of BF formation to high CRs. The variation in the hardness with microstructural changes was discussed.

Proposal and Assessment of a Multiple Cycle-Continuous Cooling Transformation (MC-CCT) Diagram for Wire Arc Additive Manufacturing of Thin Walls

Continuous cooling transformation (CCT) diagrams of base metals are common in welding. They can be built using physical or numerical simulations, each with advantages and limitations. However, those are not usual for weld metal, considering its variable composition due to the dilution of the weld into the base metal. Wire Arc Additive Manufacturing (WAAM) is a distinctive case in which the interest in materials comparable with weld composition raises attention to estimating their mechanical properties. Notwithstanding, this concept is still not used in WAAM. Therefore, the aim of this work was to address a methodology to raise MC-CCT (Multiple Cycle Continuous Cooling Transformation) diagrams for WAAM by combining physical and numerical simulations. A high-strength low-alloy steel (HSLA) feedstock (a combination of a wire and a shielding gas) was used as a case study. To keep CCT as representative as possible, the typical multiple thermal cycles for additive manufacturing thin walls were determined and replicated in physical simulations (Gleeble dilatometry). The start and end transformations were determined by the differential linear variation approach for each thermal cycle. Microstructure analyses and hardness were used to characterise the product after the multiple cycles. The same CCT diagram was raised by a commercial numerical simulation package to determine the shape of the transformation curves. A range of austenitic grain sizes was scanned for the curve position matching the experimental results. Combining the experimental data and numerically simulated curves made estimating the final CCT diagram possible.

Continuous Cooling Transformation Diagram of Case Hardening Steel by Instrumented Jominy Test

Abstract Determining the phase transition data of technological materials used for example to design heat treatment processes is generally a complex and labor-intensive process if high data fidelity is required. This work presents a method based on the standard Jominy end quench test with additional instrumentation that can generate many temperature histories required for the generation of continuous cooling diagrams at the same time, greatly simplifying the process. This method is applied to a simple 16MnCr5 case hardening steel to show the practical execution. The resulting diagram and microstructure is consistent with more complex test methods, showing good performance of the much simpler test method.

Effect of TiO<sub>2</sub> Addition and Cooling Rate on Crystallization Behavior of Separated Slag Containing Low-grade RE

With the continued exploitation and utilization of high-grade rare earth ores, it is increasingly important to extract rare earths from separated slag containing low-grade rare earth. The X-ray powder diffraction, scanning electron microscopy, electron probe micro-analyzer and confocal laser scanning microscopy were used to explore the influence of TiO2 and cooling rate on the crystallization of CaO-SiO2-TiO2-10 wt% P2O5-8 wt% Nb2O5-5 wt% CeO2-5 wt% CaF2 slag system. In this study, the britholite was precipitated selectively as the Ce-enriched phase. When TiO2 was added at less than 12 wt%, the britholite was promoted to crystallize meanwhile the Ca2Nb2O7 was suppressed. However, CaTiSiO5 inhibited the growth of britholite when the TiO2 content exceeded 15 wt%. The non-isothermal crystallization kinetics had also been investigated for the TiO2 content and cooling rate varied from 0-18 wt% and 10-40°C/min, respectively. The continuous cooling transformation diagram and the relative crystallinity of the primary crystals were also constructed. Based on the observation and measurement of crystallization process, the modified Avrami model was applied to determine the crystallization mode of britholite with 9 wt% TiO2 addition. It was constant nucleation rate and one-dimensional growth with diffusion controlled. Considering the nucleation and growth of crystals, 20–30°C/min was preferred to be the reasonable parameter during cooling stage.

Effect of Cr2O3 Content on the Viscosity and Crystallization Properties of Continuous Casting Mold Slag

Herein, the effect of Cr2O3 on the crystallization and flow behavior of CaO–SiO2–Al2O3–Na2O–CaF2 continuous casting mold slags is investigated. The viscosity in the slag at 1200–1500 °C is measured by the rotating cylinder method and a quantitative relationship between the viscosity and the Raman structure Q3/Q2 is constructed. The time–temperature–transformation and continuous cooling transformation diagrams for various slags are obtained using a single hot‐thermocouple technique. The results show that with the crystallization temperature decreases, it can be observed that the crystal growth mode gradually changes from 3D spherical and 2D massive to 1D needle‐like. The study finds the transition from network modifier to network forming agent in the slag for Cr2O3 content from 0 to 3 wt%. The Cr2O3 added to 1–2 wt% decreases the viscosity and promotes crystallization. The viscosity increases and the crystallization is inhibited when the content of Cr2O3 increases to 3 wt%. Meanwhile, when Cr2O3 is added to 3 wt%, the Cr2O3 crystalline phase precipitates except for the main crystalline phase 2CaO·SiO2.

Microstructural Changes and Determination of a Continuous Cooling Transformation (CCT) Diagram Using Dilatometric Analysis of M398 High-Alloy Tool Steel Produced by Microclean Powder Metallurgy.

The paper deals with the dilatometric study of high-alloy martensitic tool steel with the designation M398 (BÖHLER), which is produced by the powder metallurgy process. These materials are used to produce screws for injection molding machines in the plastic industry. Increasing the life cycle of these screws leads to significant economic savings. This contribution focuses on creating the CCT diagram of the investigated powder steel in the range of cooling rates from 100 to 0.01 °C/s. JMatPro® API v7.0 simulation software was used to compare the experimentally measured CCT diagram. The measured dilatation curves were confronted with a microstructural analysis, which was evaluated using a scanning electron microscope (SEM). The M398 material contains a large number of carbide particles that occur in the form of M7C3 and MC and are based on Cr and V. EDS analysis was used to evaluate the distribution of selected chemical elements. A comparison of the surface hardness of all samples in relation to the given cooling rates was also carried out. Subsequently, the nanoindentation properties of the formed individual phases as well as the carbides, where the nanohardness and reduced modulus of elasticity (carbides and matrix) were evaluated.

Improvements in welding continuous cooling transformation diagram of high-strength low-alloy steel weld

The welding continuous cooling transformation (CCT) diagram concerning microstructure evolution in a fine zone of the weld joint provides essential data for microstructure control in practice. In this study, a welding CCT diagram was improved to be elaborate by using the expanded lever rule. Processes for obtaining pieces of phase information are explained in detail. Improvements are: (i) phase areas are clearly presented, (ii) phase volume percentages are given, and (iii) overlapped phase areas are revealed. To meet high-strength requirement, a heat input of 21 kJ cm−1 was referenced from the welding CCT diagram, for achieving acicular ferrite and lath-like martensite. Transformed phase volume fractions are estimated and high strength and good toughness are obtained due to the elaborate phase information.

Investigation on residual stress of EQ56 high strength steel butt weld

In the welding process of high-strength steel (HSS), the intense temperature change in joints induces the formation of welding residual stress (WRS), which increases the fatigue crack growth rate. Developing an effective finite element (FE) model for accurately predicting the WRS distribution is critical to improve the quality of welded structures. Experimental tests and numerical simulations were carried out to investigate the WRS distribution of EQ56 HSS. The thermal expansion curves at various heating and cooling rates were measured through thermal simulation experiments for achieving the continuous cooling transformation (CCT) diagram of EQ56 HSS. Based on the CCT diagram, the thermo-metallurgical-mechanical (TMM) FE model was developed to consider the influence of solid-state phase transformation (SSPT) on distribution and magnitude of WRS for welded EQ56 HSS joints, as well as material hardening law. Comparison of predicted WRS distribution and experimental data shows that plastic hardening laws of materials significantly affect the precision of WRS distribution, and combined hardening law gives most reliable prediction. The FE model considering SSPT behavior provides more accurate estimation of WRS distribution than those without considering SSPT behavior.

A Parameterized Leblond-Devaux Equation for Predicting Phase Evolution during Welding E36 and E36Nb Marine Steels.

High heat input welding can improve welding efficiency, but the impact toughness of the heat-affected zone (HAZ) deteriorates significantly. Thermal evolution in HAZ during welding is the key factor affecting welded joints' microstructures and mechanical properties. In this study, the Leblond-Devaux equation for predicting phase evolution during the welding of marine steels was parameterized. In experiments, E36 and E36Nb samples were cooled down at different rates from 0.5 to 75 °C/s; the obtained thermal and phase evolution data were used to construct continuous cooling transformation diagrams, which were used to derive the temperature-dependent parameters in the Leblond-Devaux equation. The equation was then used to predict phase evolution during the welding of E36 and E36Nb; the quantitative experimental phase fractions of the coarse grain zone were compared with simulated results to verify the prediction results, which are in good agreement. When heat input is 100 kJ/cm, phases in the HAZ of E36Nb are primarily granular bainite, whereas for E36, the phases are mainly bainite with acicular ferrite. When heat input increases to 250 kJ/cm, ferrite and pearlite form in both steels. The predictions agree with experimental observations.

Decomposition of γ-Fe in 0.4C–1.8Si-2.8Mn-0.5Al steel during a continuous cooling process: A comparative study using in-situ HT-LSCM, DSC and dilatometry

Continuous cooling transformation (CCT) diagrams represent roadmaps for producing all heat-treatable steels. CCT curves provide valuable information on the solid-state phase transformation sequence, depending on the defined cooling strategies, the alloying concept of the steel and previous processing steps. The experimental characterization of CCT diagrams is usually done on a laboratory scale applying thermal analysis of dilatometry. In current research studies, however, also other in-situ methods such as high-temperature laser scanning confocal microscopy (HT-LSCM) or differential scanning calorimetry (DSC) are frequently used to investigate phase transformations during thermal cycling. In the present study, HT-LSCM observations and DSC analysis are critically compared with dilatometry results by investigating the CCT diagram of a 0.4%C-1.8%Si-2.8%Mn-0.5%Al (in mass pct.) advanced steel grade. Furthermore, classical examinations by optical microscopy and hardness measurements were performed to support the analysis. In general, very good consistencies between all experimental techniques were identified in determining the transformation start temperature for pearlite, bainite and martensite. The optical microscopy confirmed the observed phase transformations and the results correlated with the measured hardness response. Based on the results, coupling of HT-LSCM and DSC is considered as a valuable novel approach to plot CCT diagrams in future research.

Effect of CaO/Al2O3 Ratio on Physical Properties of Lime-Alumina-Based Mould Powders

High-aluminium steels contain a significant amount of aluminium. The reaction between Al in the liquid steel and SiO2 in lime-silica-based mould powders during the continuous casting process of high Al steel causes chemical compositional changes in the mould powders, subsequently affecting the surface quality of slabs. In order to solve the aforementioned problem, lime-alumina-based mould powders have been developed, which can lead to an increase in the surface quality of cast slabs by inhibiting steel/slag interaction. However, the mould slag tends to crystallise easily, which leads to a deterioration of the mould lubrication. In view of this aspect, it is important to develop and optimize lime-alumina-based mould powders to meet the requirements of continuous casting of high-aluminium steels. In this study, the changes in crystallinity, viscosity and melting temperature of lime-alumina-based mould powders with the effects of increasing the CaO/Al2O3 ratio have been observed through STA (Simultaneous Thermal Analysis), HSM (Hot Stage Microscopy), XRD (X-ray Diffraction), IPT (Inclined Plate Test) and rotational viscometer. The crystallisation behaviour of these mould powders was evaluated by generating CCT (continuous cooling transformation) diagrams. Additionally, the changes in steel chemistry have also been analysed using XRF (X-ray fluorescence) and ICP (Inductively Coupled Plasma Mass Spectrometer). The results of these analyses demonstrated that crystallinity of lime-alumina-based mould powder is increased while the initial crystallisation temperature and viscosity are decreased by CaO/Al2O3 additions. However, the degree of steel/slag interaction decreases with an increase in Al2O3 content.

Prediction of continuous cooling transformation diagrams in steels using light gradient boosting and rule-based optimization

ABSTRACT Continuous cooling transformation (CCT) diagram is an indispensable tool for new product development in the steel industry and it is traditionally plotted via extensive thermomechanical studies. To reduce the cycle time of product development, a machine-learning model to predict the CCT of a new grade (chemistry and processing parameters) has been developed in this work. The idea is to have a holistic predictive model suitable for integrated steel manufacturing plant, instead of having many models for different class of material compositions. A wide range of data set is prepared to this effect and explored with multiple regression-based machine-learning models. Light Gradient Boosting Machine (LGBM) demonstrates exceptional results amongst the rest, and it is topped off with a metallurgical constraint/correction-factor to obtain the final model. The model is validated against test data as well as against some well-established metallurgical correlations and is found to perform fairly well.

In-situ characterization of silicate slag crystallization by electrical impedance spectroscopy: Construction of continuous cooling transformation diagram

In-situ characterization of silicate slag crystallization by electrical impedance spectroscopy: Construction of continuous cooling transformation diagram

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.