Transformation Diagrams Research Articles

Modelling the continuous cooling transformation diagram of engineering steels using neural networks

Abstract The neural network model of Van der Wolk et al. [1] describes the effect of composition on the phase regions of the continuous cooling transformation (CCT) diagram, yet does not consider the fractions of microstructural components and the hardness data that are often quoted in CCT diagrams. In the present paper, the construction of two more neural network models, one for the fractions of ferrite, pearlite, bainite and martensite in the microstructure, and one for the hardness after cooling, using the data of 338 and 412 diagrams, respectively. The accuracy of each model was found to be similar to the expected experimental error; moreover, the models were found to be mutually consistent, although they have been constructed independently. Furthermore, the trends in these properties for alloying elements can be quantified with the models, and are largely in line with metallurgical expectations.

Modelling the continuous cooling transformation diagram of engineering steels using neural networks

Abstract A neural network model for the calculation of the phase regions of the continuous cooling transformation (CCT) diagram of engineering steels has been developed. The model is based on experimental CCT diagrams of 459 low-alloy steels, and calculates the CCT diagram as a function of composition and austenitisation temperature. In considering the composition, 9 alloying elements are taken into account. The model reproduces the original diagrams rather accurately, with deviations that are not larger than the average experimental inaccuracy of the experimental diagrams. Therefore, it can be considered an adequate alternative to the experimental determination of the CCT diagram of a certain steel within the composition range used. The effects of alloying elements can be quantified, either individually or in combination, with the model. Nonlinear composition dependencies are observed.

Isothermal Transformation Diagrams of Austenite in Maraging Steel 08H15N5D2T at Overcooling

The diagrams of isothermal transformation based on kinetic curves R = (τ) for retained austenite in high-strength alloy low-carbon at overcooling have been built. It is shown that the temperature of quenching influences the stability at overcooling and resistance to isothermal transformation of austenite at sub-zero temperatures.

In situ monitoring of phase transformation kinetics under rapid heating of Ti–6Al–4V titanium alloy

The phase transformation kinetics of Ti–6Al–4V alloy was studied during rapid heating using in-situ high energy synchrotron diffraction and in-situ electrical resistivity measurement for heating rates varying from 0.03 to 200 °C.s−1. The application of Rietveld refinement to high energy synchrotron data tracked the mass fraction, mean lattice parameters and Full Width at Half Maximum (FWHM). For increasing heating rates, results evidenced a shift in the dissolution of α to β phase temperature range from 600 °C for 0.25C.s−1 to 960 °C for 200 °C.s−1. A Continuous Heating Transformation diagram (CHT) was constructed demonstrating the effect of high heating rate. The result also demonstrated a much narrower temperature transformation range at high temperature. FWHM showed a time-dependent decrease of the stress level and/or chemical heterogeneity in the β phase before the start of the α phase dissolution. The mean lattice parameter evolution of β phase suggested that the α phase dissolution kinetics was diffusion controlled despite a heating rate reached of 200 °C.s−1, leading to chemical heterogeneities in the β phase.

Influence of thermomechanical processing on the microstructural and mechanical properties of steel 22MnB5

In order to reduce CO2 emissions in the transport sector, the approach of load-adapted components is increasingly being pursued. For the design of such components, it is crucial to determine their resulting microstructure and mechanical properties. For this purpose, continuous cooling transformation diagrams and deformation continuous cooling transformation diagrams are utilized, however, their curves are strongly influenced by the chemical composition, the initial state and especially the process parameters.In this study, the influence of the process parameters on the transformation kinetics is systematically investigated using an innovative characterization method. The experimental setup allowed a near-process analysis of the transformation kinetics, resulting microstructure and mechanical properties for a specific process route with a reduced number of specimens. A systematic investigation of the effects of different process parameters on the microstructural and mechanical properties made it possible to reveal interactions and independencies between the process parameters in order to design a partial heating or differential cooling process. Furthermore, the implementation of two different cooling conditions, representative of differential cooling in the die relief method with tool-contact and non-contact areas, showed that the soaking duration has a significant influence on the microstructure in the non-contact tool area.

Methodology of scientific selection of accumulation modes and preliminary heat treatment of forgings made of medium carbon medium alloy steels

This article considers issues related to the forgings from medium-carbon medium-alloyed steel grades. It is necessary to take into account the processes of phase and structural transformations when assigning modes of accumulation of ingots after forging and preliminary heat treatment (PHT). The main provisions of the appointment of PHT modes were analyzed. The paper formulates their limited application when using diagrams of isothermal transformation of austenite, built according to standard techniques.

Влияние холодной деформации и термической обработки на структуру, текстуру и свойства титанового псевдо-β-сплава

The effect of cold plastic deformation and heat treatment on the phase composition, structure, texture, and mechanical properties of sheets of pseudo-β-titanium alloy VT35 has been studied. It has been established that after cold rolling of sheet semi-finished products, a multicomponent texture (111) + (112) + (100) is formed, which during subsequent heat treatment is transformed into a similar texture but with a lower degree of texturing. A generalized diagram of phase and structural transformations with temperature-time conditions for the onset of polygonization, recrystallization, and decay of the β-phase was constructed. It is shown that, subject to the sequence and kinetics of the processes occurring during heat treatment, it is possible to form a structure of various types in the alloy, which makes possible to change its technological and mechanical properties in a wide range.

Kinetics of structural phase transitions in the Fe-=SUB=-80.5-=/SUB=-Ga-=SUB=-19.5-=/SUB=- alloy

This work is devoted to the Monte Carlo simulation of the kinetics of phase transitions of the order-disorder type in the Fe80.5Ga19.5 alloy using the Blume--Emery--Griffiths Hamiltonian. The study of phase transformations was carried out in two stages: (1) different cooling rates (different number of Monte Carlo steps at a constant value of the binding energy); (2) isothermal annealing (the binding energy depends on the number of Monte Carlo steps at a fixed temperature). In the first case, a high cooling rate leads to one phase transition A2->D03+A2 at 700oC, while slow cooling leads to a transition A2->D03+A2 at 700oC through a mixture of phases A2, D03, B2 and L12 in the temperature range from 850 to 700oC. In the second case, an increase in the volume fraction of the L12 phase is shown with an increase in the duration of annealing at 750oC. Based on the data obtained, a thermokinetic diagram of phase transformations was constructed. Keywords: Monte Carlo method, Fe-Ga, phase transitions.

КИНЕТИКА ПРЕВРАЩЕНИЙ АУСТЕНИТА В ВЫСОКОХРОМИСТЫХ СПЛАВАХ ЖЕЛЕЗА ПРИ НЕПРЕРЫВНОМ ОХЛАЖДЕНИИ

The work develops the models of two principal diffusional transformations in high-chromium iron alloys on continuous cooling, viz. precipitation of secondary carbides (Cr, Fe)7C3 from aus-tenite and pearlitic decomposition of the latter. Literature experimental data for alloys of various composi-tion were used for analysis. Chemical composition of austenite by the end of austenitizing holding was de-termined using previously developed models, and then the exact expression describing isothermal precipita-tion of carbides from supercooled austenite was found based on the general equation following from the ki-netic theory of phase transformations. It was found out that the time exponent n in that expression equals to 3/2, which corresponds to the case of preferred precipitation of very fine carbide particles. For transition to the continuous cooling condition the C-curve of isothermal carbide precipitation was approximated by a square parabola. This permitted to find the relation between cooling rate and the amount of precipitated car-bide phase by means of the Scheil–Steinberg integral. Further comparison with the experimentally observed increase of martensitic point allowed to determine the numerical parameters of the model and their depend-ence on carbide composition of a given alloy. For the pearlitic transformation, a previously developed mod-el describing the parameters of temperature dependence of the Avrami equation coefficients on austenite composition was employed. That model was generalized for the continuous cooling case in the same was as above. As a result, CCT diagrams of diffusional austenite transformations in high-chromium alloys were calculated, that in general corresponded well the experimental ones.

Continuous Cooling Transformation (Cct) Diagrams of Β Ti-40nb and Tmzf Alloys and Influence of Cooling Rate on Microstructure and Elastic Modulus

Continuous Cooling Transformation (Cct) Diagrams of Β Ti-40nb and Tmzf Alloys and Influence of Cooling Rate on Microstructure and Elastic Modulus

Effect of cooling rate on the microstructures of three low carbon alloys with different manganese and molybdenum contents

Low-alloy 16 to 20MND5 steels are used for the production of nuclear reactor components. During manufacturing, austenitization is followed by a quench; different types of microstructures are formed during this step. Characterizing the impact of Mo and Mn and of cooling rate on these microstructures can help understand how mechanical properties will evolve during tempering and ageing. The impact of molybdenum and manganese, as well as the impact of the cooling rate, were studied on microstructures of three model alloys: FeCMo, FeCMn and FeCMoMn. This was done using continuous cooling transformation (CCT) diagrams and electron backscattering diffraction (EBSD) characterizations. FeCMoMn was found to be a good model for 16 to 20MND5 steels, based on its CCT diagram and hardness. The presence of molybdenum or manganese did not modify the misorientation angle/axis pairs of martensite. In bainitic microstructures however, the presence of Mn seemed to favor the presence of block boundaries with a misorientation about 59° [433]. On the prior austenitic grain (PAG) level, the impact of the cooling rate was rather continuous, from martensite to slowly cooled bainite, and the same regardless of the composition, with the presence of block and sub-block boundaries. The microstructure became coarser with decreasing cooling rate, with fewer crystallographic orientations per PAG.

Computational Simulation of Duplex Stainless Steel Continuous Cooling Transformation Curves Using DICTRA®

Despite of their excellent combination of high mechanical strength, toughness and corrosion resistance, duplex stainless steels (DSS) are susceptible to sigma phase formation, negatively affecting their superior properties. Sigma formation continuous cooling transformation (CCT) diagrams can be a useful tool to avoid sigma formation during cooling from solution treatment temperatures; however, non-isothermal information about sigma formation in DSS are scarce in literature. This work presents a methodology to simulate CCT diagrams in DICTRA® software, showing excellent adherence to literature data. The methodology here presented was also able to describe sigma phase formation behaviour for different DSS compositions.

Microstructural characterization of a 1100 MPa complex-phase steel

The increasing market demand for Advanced High Strength Steels (AHSS) is associated with their high mechanical strength and adequate toughness, as the automotive industry requires stronger materials with low aggregated cost. One suitable way to increase the mechanical properties of steels is to control their microstructure. The first step is to quantify the initial microstructure and then theorize how processing could further develop microstructure to achieve the desired mechanical properties. In the present work, a new industrially produced Complex-Phase (CP) steel with tensile strength of 1100 MPa was investigated and characterized in order to determine if the mechanical properties could be further improved. A Continuous Cooling Transformation (CCT) diagram was created using dilatometric treatments on cold-rolled samples removed from the industrial process prior to heat treatment. The industrial heat treatment was reproduced in the dilatometer to identify the phase transformations that occurred during processing. The industrially produced steel had its microstructure characterized using Light Optical Microscopy (LOM) in combination with Electron Backscatter Diffraction (EBSD). This allowed the quantification of the complex microstructure presented by this kind of steel. Scanning Electron Microscopy (SEM)/Energy Dispersive X-ray Spectroscopy (EDS) maps, and electron diffraction obtained by transmission electron microscopy (TEM) were used to corroborate the findings. The results showed a possibility to increase the mechanical properties using a new heat treatment that favors the formation of less ferrite and more bainite.

Energy analysis of transfer processes and their main characteristics in thermo mechanical damping systems

The article discusses the energy analysis of transfer processes in the damping system. The basic theoretical foundations based on the equations of the energy balance of the hydraulic shock absorber and the law of conservation of energy are presented. The proposed approach is associated with the development of a methodology and scheme for calculating the technical system of vibration damping. The schemes of interaction of the system through the phenomena of transfer and functioning of the vibration protection system with the environment are presented. It is shown that damper systems are based on the physical process of transformation of mechanical energy into thermal energy with subsequent dissipation into the environment. The total energy distribution in damping problems takes the following form the mechanical energy of motion is absorbed due to the hydraulic resistance of the liquid and turns into a dissipative component, which can reach 80% of the total energy. A mathematical model of the law of conservation of energy is presented which includes a dissipative function. The analysis of how it is possible to design work processes in a shock absorber due to energy dissipation and similarity criteria: Euler, Froude, Reynolds, etc. As a result of physical experiments, it was found that the movement of a fluid in hydraulic calibrated throttles gives rise to cavitation and various physical phenomena and accompanying processes, in which there is a significant change in the energy balance and energy dissipation in non-stationary modes of fluid movement.
 The dependence of the total power loss of the shock absorber under changing operating conditions, and the diagram of physical processes and energy transformations in the problems of damping, which are in dissipative processes, are given. The article describes the principles that can be used for the design of devices and modules of damper systems of a wide class with the possibility of energy recovery and accumulation by introducing a damper into the system, for example, a motor generator, an inductor with permanent magnets or a peso element in the design of a traditional telescopic shock absorber.

Stiffener Design to Maintain Line Heating Efficiency during the Lifting Process Considering Phase Transformation.

In the shipbuilding industry, welding is the main technique used to join steel structures. There is a lifting process, post-welding, that can eliminate the correction effect of line heating. Line heating is reperformed after the lifting process. This can significantly delay the ship assembly process. Herein, we present a design method for installing a permanent stiffener to avoid the disappearance of the line heating effect during the lifting process. The change in physical properties due to heating and cooling of the line heating is calculated. The limiting stress, at which the effect of the line heating completely disappears, based on the inherent strain theory, is obtained. The phase fraction by the cooling rate is calculated using the continuous cooling transformation diagram and the Kiustinen–Marburgerm equation. Physical properties affected by the phase transformation are calculated, considering the physical properties and fraction of each phase. The square plate theory and superposition principle are used to construct a local model, with a stiffener, of the ship block. The stress caused by the shape of the stiffener and the distance between the stiffeners were calculated for the local model. The calculated stress and the limiting stress were compared to determine, for the expected line heating efficiency, the most acceptable stiffener design. Finally, to confirm the elimination of the problem, the designed stiffener is analyzed using the finite element method.

Revealing the crystallization kinetics and phase transitions in Mg65Zn30Ca5 metallic glass by nanocalorimetry

As a potential candidate for biomedical applications, Mg65Zn30Ca5 (at%) metallic glass has attracted considerable attention. To guide the development of MgZnCa alloys with improved mechanical and biomedical properties, it is necessary to understand the phase transitions of this metallic glass. In this study, controlling phase transitions in Mg65Zn30Ca5 metallic glass is realized by nanocalorimetry. The underlying thermodynamic and kinetic features at heating rates ranging from 102 to 105 K s−1 are therefore revealed. Upon rapid heating, an intermediate amorphous phase is detected. This metastable phase facilitates the formation of the Mg7Zn3 crystalline phase with a remarkably reduced reaction temperature and activation energy. With the combination of nanocalorimetry and structural characterization, the continuous heating transformation (CHT) diagram for Mg65Zn30Ca5 metallic glass is plotted, and crystallization trajectories under various heating conditions are distinguished. Meanwhile, the crystallization kinetics covering seven orders of magnitude is estimated, giving the growth rate of the MgZn crystalline phase in the undercooled liquid with temperatures ranging from Tg to Tm. Accordingly, the maximum crystal growth rate and corresponding temperature are estimated at umax= 0.49 m s−1 and Tu,max= 563 K, respectively. With the decrease in undercooling, the transition from kinetics-controlled crystallization to thermodynamics-controlled crystallization is illustrated, causing divergence of the estimated growth rate from the experimental results.

Microstructure and mechanical properties of heat affected zone of laser-MAG hybrid welded low carbon bainitic steel joints

In the study, laser-MAG hybrid welding of low carbon bainitic steel was conducted. The weld formation, microstructure and mechanical properties were investigated. The thermal simulation technology was used to investigate the microstructure evolution of the coarse grain heat affected zone (CGHAZ) under different t8/5 conditions. Meanwhile, the simulated heat affected zone continuous cooling transformation (SHCCT) diagram of low carbon bainitic steel was developed and the optimized heat input range (6–18 kJ/cm) was obtained. The microstructure in the CGHAZ were lath bainites (LB) with a little granular bainites (GB), and M-A constituents. The average tensile strength of the joint was 714 MPa, reached 92.5% of the base metal. The impact energy of the HAZ was 33.8 J, which was 66.0% of the base metal. The reduction of impact toughness in the HAZ attributed to easier propagation of crack leaded by the M-A constituents and parallel slats within LB.

Structural Metastability and Fermi Surface Topology of SrAl2Si2.

SrAl2Si2 crystallizes into either a semimetallic, CaAl2Si2-type, α phase or a superconducting, BaZn2P2-type, β phase. We explore possible transformations by employing pressure- and temperature-dependent free-energy calculations, vibrational spectral calculations, and room-temperature synchrotron powder X-ray diffraction (PXRD) measurements up to 14 GPa using a diamond anvil cell. Our theoretical and empirical analyses together with all reported baric and thermal events on both phases allow us to construct a preliminary P-T diagram of transformations. Our calculations show a relatively low critical pressure for the α-to-β transition (4.9 GPa at 0 K, 5.0 GPa at 300 K, and 5.3 GPa at 900 K); nevertheless, our nonequilibrium analysis indicates that the low-pressure low-temperature α phase is separated from a metastable β phase by a relatively high activation barrier. This analysis is supported by our PXRD data at ambient temperature and P ≤ 14 GPa, which shows an absence of the β phase even after a compression involving three times the critical pressure. Finally, we briefly consider the change in the Fermi surface topology when atomic rearrangement takes place via either transformations among SrAl2Si2 dimorphs or total chemical substitution of Ca by Sr in the isomorphous CaAl2Si2 α phase; empirically, the manifestation of such a topology modification is evident upon comparison of the evolution of the (magneto)transport properties of members of SrAl2Si2 dimorphs and α isomorphs.

Non-classical nucleation in vapor\u2013liquid\u2013solid growth of monolayer WS2 revealed by in-situ monitoring chemical vapor deposition

The very early nucleation stage of a transition metal dichalcogenide (TMD) was directly observed with in-situ monitoring of chemical vapor deposition and automated image analysis. Unique nucleation dynamics, such as very large critical nuclei and slow to rapid growth transitions, were observed during the vapor–liquid–solid (VLS) growth of monolayer tungsten disulfide (WS2). This can be explained by two-step nucleation, also known as non-classical nucleation, in which metastable clusters are formed through the aggregation of droplets. Subsequently, nucleation of solid WS2 takes place inside the metastable cluster. Furthermore, the detailed nucleation dynamics was systematically investigated from a thermodynamic point of view, revealing that the incubation time of metastable cluster formation follows the traditional time–temperature transformation diagram. Quantitative phase field simulation, combined with Bayesian inference, was conducted to extract quantitative information on the growth dynamics and crystal anisotropy from in-situ images. A clear transition in growth dynamics and crystal anisotropy between the slow and rapid growth phases was quantitatively verified. This observation supports the existence of two-step nucleation in the VLS growth of WS2. Such detailed understanding of TMD nucleation dynamics can be useful for achieving perfect structure control of TMDs.

Using Intercritical CCT Diagrams and Multiple Linear Regression for the Development of Low-Alloyed Advanced High-Strength Steels

The present work presents a theoretical and experimental study regarding the microstructure, phase transformations and mechanical properties of advanced high-strength steels (AHSS) of third generation produced by thermal cycles similar than those used in a continuous annealing and galvanizing (CAG) process. The evolution of microstructure and phase transformations were discussed from the behavior of intercritical continuous cooling transformation diagrams calculated with the software JMatPro, and further characterization of the steel by scanning electron microscopy, optical microscopy and dilatometry. Mechanical properties were estimated with a mathematical model obtained as a function of the alloying elements concentrations by multiple linear regression, and then compared to the experimental mechanical properties determined by uniaxial tensile tests. It was found that AHSS of third generation can be obtained by thermal cycles simulating CAG lines through modifications in chemistry of a commercial AISI-1015 steel, having an ultimate tensile strength of UTS = 1020–1080 MPa and an elongation to fracture of Ef = 21.5–25.3%, and microstructures consisting of a mixture of ferrite phase, bainite microconstituent and retained austenite/martensite islands. The determination coefficient obtained by multiple linear regression for UTS and Ef was R2 = 0.94 and R2 = 0.84, respectively. In addition, the percentage error for UTS and Ef was 2.45–7.87% and 1.18–16.27%, respectively. Therefore, the proposed model can be used with a good approximation for the prediction of mechanical properties of low-alloyed AHSS.