Phase Transformations In Steels Research Articles

Digital holographic microscope for high spatial and temporal resolution in situ observation of dynamic phenomena of metals

A digital holographic microscope (DHM) was developed for the purpose of simultaneous observation and measurement of surface relief formation during deformation and phase transformation of steels. It was designed to accommodate the sample mounting component of a custom tensile tester. The Gabor wavelet transform method was used in the image reconstruction. To verify performance, the DHM was used to observe and measure height of slip bands formed during the monotonic loading 316L stainless steel. Data obtained has high temporal resolution (limited only by the camera’s frame rate capability) with lateral resolution approaching the diffraction limit. The results clearly show the evolving microstructures with height information available within each captured image.

On the role of interfacial coherency and carbon in niobium segregation at ferrite/austenite interface: An atomistic study

Co-segregation of carbon (C) and substitutional atoms at the interface can remarkably influence the kinetics of phase transformation in steels, while decoupling their contributions by experiments remains challenging. Here, we employ Molecular Statics and hybrid Molecular Dynamics/Monte Carlo simulation approaches to comparatively investigate the niobium (Nb) segregation behaviors at both the incoherent and semi-coherent ferrite/austenite interfaces in Fe-C-Nb systems. The calculations reveal that the atomic Virial stress perpendicular to the interface is crucial for the binding energy of Nb to the interface, and the C presence at the interface has both physical and chemical interactions with Nb. It is conventionally believed that C addition can promote Nb segregation due to their attractive interaction, while the current study indicates that C and Nb atoms tend to form clusters in ferrite, thereby weakening the Nb segregation. The present study provides a promising computational approach for investigating the elemental co-segregation in steels.

Comparison of Thermal Behaviors of Carbon and Stainless Steel Billets during the Heating Process.

The effect of thermal properties on the thermal behavior of a steel billet was investigated during the heating process to understand each effect and to provide process engineers with sufficient data to choose the optimal design conditions in reheating the furnace for hot rolling. Carbon steel and austenitic stainless steel (STS) were compared based on numerical simulations owing to the completely different thermal properties of these two steels: carbon steel having a phase transformation with a relatively high thermal conductivity and STS having no phase transformation with a relatively low thermal conductivity. The thermal conductivity affected the thermal behavior of the billet in the initial stage of heating owing to the high temperature difference between the surface of the billet and the gas in the furnace, i.e., the high Biot number. Accordingly, a non-firing zone and/or a preheating zone with a low gas temperature are necessary for high-alloyed steels including STS because the thermal conductivity of these steels is relatively low. The phase transformation of the carbon steels needs to occur in the primary heating zone, and this zone needs to have a relatively low gas temperature to reduce the temperature deviation or thermal stress in the billet. The heating pattern of the carbon steels and STSs in the reheating furnace should be designed differently considering the thermal conductivity and latent heat by the phase transformation of steels to obtain a high heating quality for the billet.

Microstructural engineering in carbon steel walls printed by directed energy deposition to enhance mechanical properties through heat-input control

The main object of the present study is focused on the systematic processing heat-input design based on directed energy deposition (DED) methodology, enrolling carbon steel phase transformation to control the mechanical properties. Three different 4-layer low-carbon alloy steel (ER70S-6) walls are fabricated to investigate this matter with varying heat-inputs. To justify heat-inputs, single layers by metal-inert gas (MIG) welding with different heat-inputs were carried out, and the results were used as input data to the 4-layers fabrication. In addition, to get a more homogenous structure, some walls' heat-inputs have descended from the first layer to the top layer. The microstructural studies show that reducing heat-input causes a higher solidification rate and the formation of finer columnar grains, which significantly affects the mechanical properties. Electron microscopy analysis reveals that a higher cooling rate in the wall with descending heat-input from bottom to top (496 J/mm to 382 J/mm), motivates formation of the acicular ferrite and bainite, which significantly enhances the mechanical properties. The exceptional tensile toughness of the wall, ∼28.86 GPa%, with an ultimate tensile strength (UTS) of ∼740 MPa, yield stress of ∼560 MPa, and an elongation to failure of ∼38 % depicted the impact of fine and multi-constituents' microstructure by heat-input control.

In-situ dynamic monitoring of phase transformation in steels using a multi-frequency electromagnetic sensor

An industrial-scale electromagnetic (EM) sensor is employed to non-destructively and dynamically monitor the microstructural phase transformation for mild steel samples with different thicknesses (3 mm, 6 mm, and 10 mm), high carbon steel (0.76 wt% C) and 2.25Cr–1Mo steel plates. The continuous cooling process of these samples in ambient air (cooling rate of 0.99–7.53 °C/s) on a run-out table is examined. To determine the phase fractions based on the EM sensor signal and measured temperature, a finite element (FE) model for the EM sensor is utilised. The obtained phase fractions are further validated by conducting dilatometry measurements independently, confirming the accuracy of the transformation behaviour and phase fraction determination derived from the EM sensor readings.

Accounting for random character of recrystallisation and uncertainty of process parameters in the modelling of phase transformations in steels

ABSTRACT The problem of multiphase microstructure heterogeneity was considered in this paper. The stochastic model of multi-step hot deformation was applied to calculate histograms of the dislocation density and the grain size at the beginning of phase transformations. These histograms were used as input data for simulation of phase transformations using conventional deterministic modelling. Heterogeneity of the phase composition was calculated for various constant cooling rates, as well as for the industrial laminar cooling process. In the latter case, uncertainty of the boundary conditions was additionally accounted for. The results showing an influence of the heterogeneity of the dislocation density and the grain size on the microstructure heterogeneity in the final product can be considered reliable, because they are based on the material models, which were identified and verified experimentally in earlier publications. The effect of the uncertainty of the boundary conditions is presented in a qualitative manner only, and may be the subject of future research.

Phase-field Modeling and Simulation of Solid-state Phase Transformations in Steels

The phase-field method is used as a powerful and versatile computational method to simulate the microstructural evolution taking place during solid-state phase transformations in iron and steel. This review presents the basic theory of the phase-field method and reviews recent advances in the phase-field modeling and simulation of solid-state phase transformations in iron and steel, with particular attention being paid to the modeling of the austenite-to-ferrite, pearlitic, bainitic, and martensitic transformations. This review elucidates that the phase-field method is a promising computational approach to investigate the microstructural evolutions (e.g., interface migration, solute diffusion, and stress/strain evolutions) that take place during the phase transformations. It also indicates that further improvements are required to enhance the predictive accuracy of the phase-field models developed to date. Finally, this review discusses the critical challenges and perspectives for the further improvement of the phase-field modeling of solid-state phase transformations in steel, i.e., the modeling of heterogeneous nucleation, the abnormal effect of the diffusion interface, and material parameter identification.

In-Situ X-ray Diffraction Analysis of Metastable Austenite Containing Steels Under Mechanical Loading at a Wide Strain Rate Range

This paper presents and discusses the methodology and technical aspects of mechanical tests carried out at a wide strain rate range with simultaneous synchrotron X-ray diffraction measurements. The motivation for the study was to develop capabilities for in-situ characterization of the loading rate dependency of mechanically induced phase transformations in steels containing metastable austenite. The experiments were carried out at the DanMAX beamline of the MAX IV Laboratory, into which a custom-made tensile loading device was incorporated. The test setup was supplemented with in-situ optical imaging of the specimen, which allowed digital image correlation-based deformation analysis. All the measurement channels were synchronized to a common time basis with trigger signals between the devices as well as post-test fine tuning based on diffraction ring shape analysis. This facilitated precise correlation between the mechanical and diffraction data at strain rates up to 1 s−1 corresponding to test duration of less than one second. Diffraction data were collected at an acquisition rate of 250 Hz, which provided excellent temporal resolution. The feasibility of the methodology is demonstrated by providing novel data on the kinetics of the martensitic phase transformation in EN 1.4318-alloy following a rapid increase in strain rate (a so-called jump test).

Transition to body-centered cubic structure in Au thin films under electron-beam irradiation

Gold (Au) is well known to transition to a body-centered cubic (bcc) structure at high temperature and pressure. However, this study shows that such conditions may not be necessary. In this study, Au thin film specimens for transmission electron microscopy (TEM) undergo a transition from face-centered cubic (fcc) to bcc structure under electron-beam irradiation in TEM. Some regions in the Au thin film specimens are selectively thinned during electron-beam irradiation. The thin regions are considered to be subjected to the localization of energy deposition by electronic excitations by incident electrons due to their finite size, which causes the regions to be heated. Of the excitations, Auger decays are a possible source of Au heating. The temperature rise due to Auger decays is successfully simulated to be accompanied by stress development in the thin regions. Sometimes the thin regions transition from face-centered cubic (fcc)-to-bcc phases. The fcc matrix and the bcc phase have orientation relationships similar to those found in the fcc-to-bcc phase transformation in steels, which indicate that the transition to the bcc structure occurs through a displacive mechanism. It is suggested that the phase transition is facilitated for thin film morphology and that crystal defects caused by electron-beam irradiation and stress development stabilize the more open bcc structure.

Accounting for the random character of nucleation in the modelling of phase transformations in steels

Accounting for the random character of nucleation in the modelling of phase transformations in steels

The model of austenite structure state taking into account fluctuations of magnetic nature

The results of magnetic state simulation of carbon steel austenite are presented in this work. Quantitative parameters of the areas of short-range magnetic order, which are presented in paramagnetic austenite matrix and are observed as potential places of α-phase nucleation, are examined. This austenite heterogeneity in steels and clean iron is confirmed by the experimental data; however, detailed quantitative study meets technical complication of such experiments. It is suggested to use the Ising model for realization of the calculation experiment. The obtained spin configurations were analyzed for determination of geometric parameters of magnetic inhomogeneities and period of their sustainable existence. Calculations were conducted within the temperature range of austenite phase existence in steel, as well as without and with taking into account presence of external magnetic field, which strengthens existing austenite heterogeneity and has the effect on conduction of phase transformations in steel. The results of simulation confirm presence of the areas of short-range magnetic order in austenite, their size is close to the existing theoretical assessments and experimental data. It is shown that temperature rise leads to decrease of dimensions, number and life period of clusters, but these parameters are still important at the temperature of martensite transformation in steel. Overlapping of external magnetic field creates the conditions for existence of large-size ferromagnetic clusters also at more high temperatures, as well as increases time of their sustainable existence. The external magnetic field provides the conditions for more intensive and multiplicative nucleation of ferromagnetic phase, similar to short-range magnetic order fluctuations in location of atoms during liquid metal cooling. 

A novel analytical approach to ascertain hardenability of plain carbon steel

ABSTRACT An analytical model is conceived to correlate thermal processing parameters, phase transformation parameters, microstructural features and the resultant hardening response of steel. Three equations evolved thereby readily resolve the issues of direct graphical determination of hardenability parameters, variation of unhardened core region with the diameter of steel and quantitative prediction of the microstructure. The model is successfully validated with existing experimental database of hardenability parameters following chemical composition-cum-grain size method as well as in view of own experimentation carried out on available steel. This elucidates a new conduit of understanding thermal processing-based solid state phase transformation in steel.

Finite Element Simulation of the Effect of Phase Transformation on Residual Stress in a Thick Section T-Joint

It has long been known that residual stresses can profoundly affect the integrity of engineering components. Evidence has recently emerged to confirm that solid-state phase transformations in steels can significantly influence the as-welded residual stresses. A two-dimensional thermo-mechanical, generalized plane strain finite element model was created to simulate the effect of phase transformation on residual stress during the multi-pass weld process in a thick section T-joint. The effect of phase transformation and martensite start temperatures was investigated. The results showed that phase transformation generated compressive residual stress underneath the last bead to be deposited for this multi-pass weld model. However, some constraints around the bead were essential to provide that stress. Tensile residual stress was generated in the bulk of the weld area when phase transformation was considered. Therefore, phase transformation may be helpful for single pass and other groove welds but may be unhelpful in the case of the T-joint examined here. The effect of the martensite start temperature is small compared with the main difference between having a phase transformation and not having one.

Optical Detection of Phase Transformations in Steels: An Innovative Method for Time-Efficient Material Characterization During Tailored Thermo-mechanical Processing of a Press Hardening Steel

With an innovative optical characterization method, using high-temperature digital image correlation in combination with thermal imaging, the local change in strain and change in temperature could be determined during thermo-mechanical treatment of flat steel specimens. With data obtained by this optical method, the transformation kinetics for every area of interest along the whole measuring length of a flat specimen could be analyzed by the generation of dilatation curves. The benefit of this innovative optical characterization method compared to a dilatometer test is that the experimental effort for the design of a tailored component could be strongly reduced to the investigation of only a few tailored thermo-mechanical processed specimens. Due to the implementation of a strain and/or temperature gradient within the flat specimen, less metallographic samples are prepared for hardness analysis and analysis of the microstructural composition by scanning electron microscopy to investigate the influence of different process parameters. Compared to performed dilatometer tests in this study, the optical method obtained comparable results for the transformation start and end temperatures. For the final design of a part with tailored properties, the optical method is suitable for a time-efficient material characterization.Graphical

Variational modeling of temperature induced and cooling-rate dependent phase transformations in polycrystalline steel

Phase transformations in steel have a high impact on the material properties. The material is modified by a changed stiffness as well as residual stresses which are a result of transformation strains and plastic deformations during the phase transition. In the current work, we propose a variational material model which is able to display the phase transformations which occur during thermal processes like induction hardening and grinding. Depending on the cooling rate, the material model is able to show the transformation between austenite and martensite, austenite and ferrite as well as a mixture of both. The polycrystalline structure of the steel is taken into account by use of different oriented grains which are considered by a combined Voigt–Reuss bound for the energy of the phase and grain mixture. The material model is derived using the principle of the minimum of the dissipation potential for non-isothermal problems. Compared to classical approaches in material science, the model is thermomechanically coupled. Thus, the phase transformation is also influenced by the stress state and individual martensitic variants are favored during the transformation process, which is an important property for applications to process chains. The model’s ability to show the complex material behavior is proven by calculations on the material point level as well as by finite element simulations.

Application of phase-field modeling in solid-state phase transformation of steels

Application of phase-field modeling in solid-state phase transformation of steels

Numerical Modeling of Phase Transformations in Dual-Phase Steels Using Level Set and SSRVE Approaches.

Development of a reliable model of phase transformations in steels presents significant challenges, not only metallurgical but also connected to numerical solutions and implementation. The model proposed in this paper is dedicated to austenitic transformation during heating and ferritic transformation during cooling. The goal was to find a solution which allows for the decreasing of computing time without noticeable decreasing the accuracy and reliability of the model. Proceedings to achieve this goal were twofold. Statistically Similar Representative Volume Element was used as a representation of the microstructure. It allowed for the reducing of the complexity of the computational domain. For the purpose of the model, carbon diffusion was assumed to be the main driving force for both transformations. A coupled finite element–level set method was used to describe growth of a new phase. The model was verified and validated by comparing the results with the experimental data. Numerical tests of the model were performed for the industrial intercritical annealing process.

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.