Phase Transformations In Steels Research Articles

Industrial Steel Heat Treating: Numerical Simulation of Induction Heating and Aquaquenching Cooling with Mechanical Effects

This paper summarizes a mathematical model for the industrial heating and cooling processes of a steel workpiece corresponding to the steering rack of an automobile. The general purpose of the heat treatment process is to create the necessary hardness on critical parts of the workpiece. Hardening consists of heating the workpiece up to a threshold temperature followed by a rapid cooling such as aquaquenching. The high hardness is due to the steel phase transformation accompanying the rapid cooling resulting in non-equilibrium phases, one of which is the hard microconstituent of steel, namely martensite. The mathematical model describes both processes, heating and cooling. During the first one, heat is produced by Joule’s effect from a very high alternating current passing through the rack. This situation is governed by a set of coupled PDEs/ODEs involving the electric potential, the magnetic vector potential, the temperature, the austenite transformation, the stresses and the displacement field. Once the workpiece has reached the desired temperature, the current is switched off an the cooling stage starts by aquaquenching. In this case, the governing equations involve the temperature, the austenite and martensite phase fractions, the stresses and the displacement field. This mathematical model has been solved by the FEM and 2D numerical simulations are discussed along the paper.

Numerical Simulation of the Phase Transformations in Steels: A Case Study on Newly-Developed Multi-Phase Steels for Drop Forgings

Numerical Simulation of the Phase Transformations in Steels: A Case Study on Newly-Developed Multi-Phase Steels for Drop Forgings

A variational concept and its experimental calibration for temperature‐induced rate‐dependent phase transformations in steel

AbstractIn forming processes of metals, different physical phenomena are observable which may alter the material properties and behavior severely. These observations can be attributed to changes in the surface layer of the material. This research is a part of the transregional collaborative research center SFB/TRR 136 which deals with surface layer modifications in 42CrMo4 steels and aims to determine process independent correlations between material loads and material modifications. These correlations are called process signatures. The present work is concerned with investigating thermally induced solid‐solid phase transformations in steels. An example of a predominantly thermal process is the induction hardening process. In this process, an initially ferritic‐pearlitic structure is at first heated until reaching a temperature that lies above the temperature at which homogeneous austenite is formed. During this heating process, the steel undergoes diffusion‐driven phase transformations which are heating‐rate‐ and time‐dependent. After sufficiently long incubation, a homogeneous austenitic microstructure is obtained. Then, the specimen temperature is lowered at a very high cooling rate. This invokes high‐temperature rates on the microstructure and results in an austenitic‐martensitic phase transformation, see e.g. [1]. Based on the Principle of the Minimum of the Dissipation Potential (PDMP) as presented in [2], the austenitic‐martensitic transformation has been widely discussed in the context of shape memory alloys. We present a material model which is also able to show the ferritic‐austenitic transformation. The individual phase transitions are modeled using a rate‐ and transformation‐process‐dependent formulation of the dissipation potential. The presented material model is finally tested with numerical simulations.

Mean field model of phase transformations in steels during cooling, which predicts evolution of carbon concentration in the austenite

The objectives of the paper were twofold. The first was exploring possibility of fast and reliable modelling of phase transformations during cooling of steels, accounting for the evolution of the carbon concentration in the austenite. Existing discrete models require long computing times and their application to optimization of industrial processes is limited. Therefore, a model based on the modified JMAK equation was proposed. Control of the carbon concentration in the austenite during ferritic and bainitic transformations allowed to predict incomplete austenite transformation and occurrence of the retained austenite. Moreover, prediction of the onset of pearlitic transformation after the bainitic was possible. The model was validated by comparison the predictions with the results of physical simulations. Numerical simulations for various industrial processes were performed. Problem of the difference in the incubation time between isothermal and constant cooling rate tests was raised.

Proposition of an Empirical Functional Equation to Predict the Kinetics of Austenite to Ferrite Transformation in a Continuous Cooled IF-Ti-Stabilized Steel

The kinetics of phase transformations in isothermal processes is well described by the classic JMAK model. However, it is known that most industrial facilities employ continuous cooling processes and, for this condition, the JMAK model is adapted but, sometimes, without success. Considering the importance to predict critical temperatures, the present work proposes an alternative empirical functional equation to describe the kinetics of steel phase transformation under continuous cooling, being useful when the JMAK use is not successful. In this study, the continuous cooling austenite to ferrite transformation was experimentally characterized by dilatometry for an IF-Ti-stabilized steel. The proposed equation was compared with the classic adapted JMAK model regarding to evaluate its efficacy to fit the dilatometric experimental data. Using the constants obtained by the both model fittings as input parameters, a computational simulation was performed to determine the CCT diagram of the IF-Ti steel. The proposed functional equation was efficient to predict the critical temperatures, the kinetics of austenite decomposition and the CCT diagram of the studied steel. The results predicted by the proposed model greatly met the experimental data measured by dilatometry.

Mechanical Properties and Microstructural Aspects of Two High-Manganese Steels with TWIP/TRIP Effects: A Comparative Study

This article is focused on the mechanical behavior and its relationship with the microstructural changes observed in two high-manganese steels presenting twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP), namely Steel B and Steel C, respectively. Chemical compositions were similar in manganese, but carbon content of Steel B approximately doubles Steel C, which directly impacted on the stacking fault energy (SFE), microstructure and mechanical response of each alloy. Characterization of as-cast condition by optical microscope revealed a fully austenitic microstructure in Steel B and a mixed microstructure in Steel C consisting of austenite grains and thermal-induced (εt) martensite platelets. Same phases were observed after the thermo-mechanical treatment and tensile tests, corroborated by means of X-Ray Diffraction (XRD), which confirms no phase transformation in Steel B and TRIP effect in Steel C, due to the strain-induced γFCC→εHCP transformation that results in an increase in the ε-martensite volume fraction. Higher values of ultimate tensile strength, yield stress, ductility and impact toughness were obtained for Steel B. Significant microstructural changes were revealed in tensile specimens as a consequence of the operating hardening mechanisms. Scanning Electron Microscopy (SEM) observations on the tensile and impact test specimens showed differences in fracture micro-mechanisms.

Furnace for in situ and simultaneous studies of nano-precipitates and phase transformations in steels by SANS and neutron diffraction.

Interphase precipitation occurring during solid-state phase transformations in micro-alloyed steels is generally studied through transmission electron microscopy, atom probe tomography, and ex situ measurements of Small-Angle Neutron Scattering (SANS). The advantage of SANS over the other two characterization techniques is that SANS allows for the quantitative determination of size distribution, volume fraction, and number density of a statistically significant number of precipitates within the resulting matrix at room temperature. However, the performance of ex situ SANS measurements alone does not provide information regarding the probable correlation between interphase precipitation and phase transformations. This limitation makes it necessary to perform in situ and simultaneous studies on precipitation and phase transformations in order to gain an in-depth understanding of the nucleation and growth of precipitates in relation to the evolution of austenite decomposition at high temperatures. A furnace is, thus, designed and developed for such in situ studies in which SANS measurements can be simultaneously performed with neutron diffraction measurements during the application of high-temperature thermal treatments. The furnace is capable of carrying out thermal treatments involving fast heating and cooling as well as high operation temperatures (up to 1200 °C) for a long period of time with accurate temperature control in a protective atmosphere and in a magnetic field of up to 1.5 T. The characteristics of this furnace give the possibility of developing new research studies for better insight of the relationship between phase transformations and precipitation kinetics in steels and also in other types of materials containing nano-scale microstructural features.

ОБОСНОВАНИЕ КОРРЕКТИРУЮЩИХ ВОЗДЕЙСТВИЙ, КОМПЕНСИРУЮЩИХ ПОНИЖЕННОЕ СОДЕРЖАНИЕ УГЛЕРОДА В СТАЛИ ПРИ ПРОИЗВОДСТВЕ ХОЛОДНОКАТАНОГО ЛИСТОВОГО ПРОКАТА ДЛЯ ШТАМПОВКИ

The relevance of the work. There are situations in the practice of metallurgical production when steel melting with a chemical composition intended to complete the determined order is reassigned to the production of rolled metal from another steel grade. With the formal accordance of chemical composition of melted steel to the requirements for a different steel grade, crucial control parameters of process that are traditionally used for a new grade of steel, may turn out to be inappropriate for structural and phase transformations in steel with real chemical composition. To prevent the re-lease of non-conforming products is necessary to develop and apply operational corrective actions. The purpose of the work is selection and rationale of operational corrective actions in the production of cold rolled sheet products of quality classes CQ, DQ and DDQ of mild quality steel to compensate for the reduced carbon content in meltings assigned to complete the respective orders. Microscopy methods were used to study the structure and phase composition of steel with a carbon content of 0,008 – 0,04 % in hot-rolled and cold-rolled states. Data on the mechanical properties of hot-rolled and cold-rolled steel were obtained by standardized tensile test methods. Based on the test results, re-liable multiple approximations of influence of control characteristics of the technology on properties of cold-rolled steel were constructed, which were subsequently used in optimization of thickness of semi finished rolled product for cold rolling as a compensating effect at the fluctuations of carbon content within the above limits. The novelty of the work lies in the identification and rationale of corrective actions for a technological system in which cold rolling is carried out on a continuous four-stand mill with roll feed. The results of the work consist in determining the temperature of the end of rolling and the increment of the thickness of the semi finished rolled product as operation-al compensating effects at assigning steel meltings with a low carbon content for rolling into a tech-nological system with a continuous four-stand mill with roll feed. The practical significance of the work is to prevent the yield of non-conforming products at using the proposed solution in the pro-duction of cold-rolled steel of quality classes CQ, DQ and DDQ 0,6–2,0 mm thick.

Coupled Temperature-Microstructure Model for Predicting Temperature Distribution and Phase Transformation in Steel for Arbitrary Cooling Curves

Abstract This study aims at developing a numerical model that can be employed for simulating the thermomechanical treatment to develop the advanced high strength steels. The developed numerical method is used to calculate the heat transfer coefficient of the quenching medium during the continuous cooling of the steel using the inverse heat transfer model for predefined cooling paths. Further, the phase transformation models are used to predict the final microstructure of the steel plate. The cooling rate, plate thickness, and rolling speed are varied to evaluate the temperature and microstructure distribution in the steel plate. It is found that on increasing the quenching time, the transformation fraction from austenite to ferrite and bainite phases increases and the corresponding martensite fraction decreases. The temperature variation in the plate is significant due to the change in plate thickness and rolling speed for a given quenching time. The present model will be useful for designing process parameters to obtain desired microstructures in third-generation advanced high strength steels.

Carbon heterogeneities in austenite during Quenching & Partitioning (Q&P) process revealed by in situ High Energy X-Ray Diffraction (HEXRD) experiments

Based on the evolution of the positions and intensities of the diffraction peaks, high energy X-ray diffraction (HEXRD) is recognized as the ultimate method to follow quantitatively in situ phase transformations in steels. However, the possible asymmetricity of diffraction peaks is seldom considered, and is known to bear information. A procedure for quantifying their skewness is proposed. In the case of a third generation high strength steel obtained by quench and partitioning (Q&P), the skewness is shown to be due to carbon heterogeneities at austenite/martensite interfaces developed at nanoscale, in agreement with prior post mortem atom probe tomography (APT) investigations.

Methodology for the emissivity estimation of radiation-type thermometers using the latent heat generated during the phase transformation of steels

This paper proposes a practical procedure for the emissivity estimation of radiation-type thermometers during the hot rolling process of steels. The temperature of a workpiece is typically measured by radiation-type thermometers during the hot rolling process of metals since such thermometers have several advantages to measure the temperature of moving objects in real time. However, a critical disadvantage of radiation-type thermometers is that infrared radiation strongly depends on the surface emissivity of an object, but determining emissivity is very challenging. Therefore, in this study, a new methodology for determining the emissivity of radiation-type thermometers was developed for the hot rolling process of steels based on the heat generation or absorption phenomena of phase transformations during this process. It was found that the starting temperature of phase transformation can be detected easily using radiation-type thermometers as well as thermocouple. We can thus identify the reference point for comparing the measured temperature by all the thermometers using the starting temperature of phase transformation during process. More interestingly, the transformation starting temperature of steel has a strong linear relationship with the cooling rate. Accordingly, it is possible to consider the transformation starting temperature as a material property that varies with cooling rate. The proposed technique was well applied to the steel wire rod cooling process and the optimal emissivity values of a pyrometer and thermocamera were identified efficiently, which is useful for many steel manufacturing applications. Finally, the more accurate temperature was achieved using the radiation-type thermometers by correcting the emissivity.

The effects of operational thermal cycling on mechanical and magnetic properties of structural steels

This article shows the experimental results of the operational thermal cycling modeling of structural carbon steels with temperature range from the point of phase transition of type II cementite to the beginning of phase transformations of steel. In this study the influence of temperature that the number of thermal cycles and the histogram of temperature loading on the magnetic and mechanical properties of the steels were investigated. The possibilities of controlling the residual life of the elements, which are subjected to thermal cycling on changing the magnetic properties of metal, were determined. The functional dependence of the coercive force and hardness on the number of thermal cycles at a given temperature is obtained.

A Phenomenological Mechanical Material Model for Precipitation Hardening Aluminium Alloys

Age hardening aluminium alloys obtain their strength by forming precipitates. This precipitation-hardened state is often the initial condition for short-term heat treatments, like welding processes or local laser heat treatment to produce tailored heat-treated profiles (THTP). During these heat treatments, the strength-increasing precipitates are dissolved depending on the maximum temperature and the material is softened in these areas. Depending on the temperature path, the mechanical properties differ between heating and cooling at the same temperature. To model this behavior, a phenomenological material model was developed based on the dissolution characteristics and experimental flow curves were developed depending on the current temperature and the maximum temperature. The dissolution characteristics were analyzed by calorimetry. The mechanical properties at different temperatures and peak temperatures were recorded by thermomechanical analysis. The usual phase transformation equations in the Finite Element Method (FEM) code, which were developed for phase transformation in steels, were used to develop a phenomenological model for the mechanical properties as a function of the relevant heat treatment parameters. This material model was implemented for aluminium alloy 6060 T4 in the finite element software LS-DYNA (Livermore Software Technology Corporation).

Инновационная технология термической обработки боковой рамы тележки грузового вагона, обеспечивающая повышение ее усталостной прочности

Basic regulations of the thermal strengthening technology of a bogie side frame in a freight car made of mild steel based on the procedure developed for the parameter definition of thermal treatment on the results of the computer simulation of kinetics of structural and phase transformations in steel are shown. The results of the computer simulation of structure formation in 20 GL steel containing 0.42 and 0.53% of silicon at the realization of different cooling intensity are shown. The computer simulation is carried out on the basis of the developed procedure formed on the basis of the austenite decomposition diagram taking into account different cooling rates. The experimental assessment of a bogie side frame state after realization of the technology developed is carried out.

A cellular automaton model integrated with CALPHAD-based thermodynamic calculations for ferrite-austenite phase transformations in multicomponent alloys

A cellular automaton (CA) model, which is directly coupled with thermodynamic calculations based on the CALPHAD method, is developed for the simulation of ferrite (α)-austenite (γ) phase transformations involving the partitioning and long-range diffusion of both interstitial and substitutional elements in multicomponent alloys. A data management scheme is proposed, which renders the efficient coupling between the kinetic simulation using the CA approach and thermodynamic calculations. The present CA model is applied to simulate the cyclic phase transformations between the α- and γ-phases for a quaternary Fe-0.02C-0.2Mn-0.1Si (wt%) alloy in both one- and two-dimensions. The simulated curves of α-volume fraction varying with temperature clearly illustrate the different α-γ transformation stages. The transformation kinetics and concentration profiles of Mn and Si obtained by the 1-D CA simulation agree well with DICTRA predictions. The 2-D simulation of a single circular grain is found to have shorter stagnant stages and faster transformation kinetics than that of the 1-D planar case. This can be attributed to the different growth geometries of 1-D planar and 2-D circular, and the effect of substitutional element Mn on the migrating interface. The non-uniform distribution of C and residual spikes of Mn and Si can be clearly delineated in the 2-D multi-grain simulation. It is shown that the data management scheme proposed in the present work significantly reduces the simulation time for the required thermodynamic data by two orders of magnitude approximately, which makes it feasible to perform valid simulations of microstructural evolution and solute distributions during α-γ phase transformations in steels with more than three components.

REVISITING THE NATURE OF SITES OF MARTENSITE NUCLEATION DURING STEEL HARDENING

Presence of microvolumes most prepared for the martensite emergence in austenite is discussed. Aming many works dealing with martensitic transformations, rare works are devoted to the location of martensite origin. This aspect of transformation is important, since it allows us to obtain new knowledge about scenarios for γ → α transformation development during quenching of steel. The martensite embryos are submicron austenite volumes that are most prepared for phase transition and are characterized by increased energy. Experimental results were obtained by the methods of high-temperature metallography. Steel structure observed as a result of vacuum etching was studied, as well as the surface relief caused by shear during the martensitic transformation. The resulting structural patterns made it possible to observe most of the possible places for martensite emergence: nonmetallic inclusions, twins, high-angle and small-angle grain boundaries, previously formed martensite crystals, dislocations and elements of the disclination structure. It is shown that a high dislocation density is observed in the twin area, which facilitates nucleation of martensite as a result of disappearance of part of elastic energy of the dislocation when atoms inside the embryo are rearranged. When nucleation occurs on the grain boundaries, energy is released, which is used to construct a new interphase boundary and to compensate emerging elastic energy. The relative energy of the boundaries of different types was estimated by the method of multi-beam interferometry. The depth of the grooves that were formed on the surface by thermal etching was measured. Elements of disclination structure resulting from inhomogeneous deformation were observed, which are also sites of germinal centers formation. It is noted that nanoareas with ferromagnetic order, which are present in paramagnetic austenite, may not be observed with the help of the technique used in this work. However, magnetism plays a decisive role in realization of one or another scenario of the development of phase transformation in steels. Obtaining data on the interaction of ferromagnetic areas in austenite with each other, with crystal lattice defects, the magnetic field, and data on their lifetime, number and size is an important task for future research.

Em Sensor Array System and Performance Evaluation for In-line Measurement of Phase Transformation in Steel

In a hot strip mill (HSM), the evolution of phase transformation in steel during the dynamic cooling process on a runout table has a significant effect on the microstructure and mechanical properties of hot-rolled materials and further processing in the subsequent processing steps. An electromagnetic (EM) sensor array system, EMSpec® (ElectroMagnetic Spectroscopy), has been developed for in-line measurement of steel phase transformation. The first industrialised system has been installed on the run-out table (ROT) of HSM #2 at Tata Steel in the Netherlands for industrial trials. The EMSpec system consists of multiple sensor nodes located at different positions on the run-out table. Each sensor node measures the impedance spectrum, from which the amount of transformed phase fraction is determined based on a measurement model. All of the sensor nodes are calibrated for the delivery of proper sensor signals, such that progressively increasing phase transformation of the steel strip travelling from one node to the next can be correctly measured. Besides the sensing principle and system calibration, this paper presents in-line measurement results, which are interpreted and compared with phase transformation predictions from a physical thermodynamic and kinetic phase transformation model.

Microstructure and heat treatment effect on transformation strain in steels: part 2 modelling

ABSTRACTAnisotropic dilatation during phase transformation in steels can be caused by banded microstructure. As well as the directionality in mechanical properties, the anisotropic transformation strain can result in distortion and dimension change of the products. Hence, mechanical assessment of this phenomenon is of great importance. In this paper, a numerical approach using Crystal Plasticity Fast Fourier Transform (CPFFT) has been developed and used, reproducing the anisotropic transformation stain during heating (experimental results are reported in Part 1). It was confirmed by the numerical simulation that the banded microstructure in microstructure can be one of the most important reasons for the anisotropy of transformation strain.

An original unified approach for the description of phase transformations in steel during cooling: first application to binary Fe–C

Exploiting Landau’s theory of phase transformations, defining an original order parameter and using the phenomenological transformation temperatures, it is reported that it is possible to describe in a global approach the conditions for the formation of each constituent (ferrite, bainite, martensite) from austenite during cooling in steel. It allowed to propose a new rigorous classification of the different thermodynamic conditions controlling each phase transformation. In a second step, the approach predicts naturally the effect of cooling rate on the bainite start temperature. Finally, perspectives are assessed to extend the approach in order to take into account the effect of an external field such as applied stress.

Phase transformations in steels under controlled quenching in magnetic fluid

Phase transformations in steels under controlled quenching in magnetic fluid