Transformation Diagrams Research Articles

Simulation of phase transformations in high carbon pearlite steel at various cooling rates

The paper reports on the experimental research into the austenite decomposition in high carbon steel at a constant temperature and rates of cooling. The study was carried out by the methods of dilatometry, micro-durometry, optical and scanning microscopy. From the dilatometric curves it is apparent they are sufficiently presented by the Kolmogorov-Avrami equation in a temperature range from 700 to 550 °С, and by the logistic function for temperatures ranging 500 to 250 °С. The data of the dilatometric analysis were used to draw isothermal and continuous cooling transformation diagrams of phase conversions. It has been pointed out an isothermal diagram to comprise four C-curves of pearlite and bainite transformations, which are approximated by second- and third-degree polynomials. The critical temperatures of austenite, bainite, and martensite transformations have been determined, being 711, 550 and 196 °С, respectively. Considering the data obtained experimentally, a mathematical model of austenite decomposition has been developed for a constant cooling rate, and volume fractions of structure-phase components estimated theoretically, demonstrating the congruence with experimental results. The suggested mathematical model can be adopted to calculate a structure-phase composition in industrial differential heat treatment of rails. The authors are grateful to Vladimir Sarychev, Egor Polevoy and Victor Gromov who took an active part in the preparation of the manuscript. This work is supported by the Russian Foundation for Basic Research (RFBR) [project number 19-32-60001] and President grant for State Support to young researches [grant number МK-118.2019.2].

Continuous Cooling Transformation Behaviour and Bainite Transformation Kinetics of 23CrNi3Mo Carburised Steel

In this study, the phase transformation behaviour of the carburised layer and the matrix of 23CrNi3Mo steel was comparatively investigated by constructing continuous cooling transformation (CCT) diagram, determining the volume fraction of retained austenite (RA) and plotting dilatometric curves. The results indicated that Austenite formation start temperature (Ac1) and Austenite formation finish temperature (Ac3) of the carburised layer decreased compared to the matrix, and the critical cooling rate (0.05 °C/s) of martensite transformation is significantly lower than that (0.8 °C/s) of the matrix. The main products of phase transformation in both the carburised layer and the matrix were martensite and bainite microstructures. Moreover, an increase in carbon content resulted in the formation of lamellar martensite in the carburised layer, whereas the martensite in the matrix was still lath. Furthermore, the volume fraction of RA in the carburised layer was higher than that in the matrix. Moreover, the bainite transformation kinetics of the 23CrNi3Mo steel matrix during the continuous cooling process indicated that the mian mechanism of bainite transformation of the 23CrNi3Mo steel matrix is two-dimensional growth and one-dimensional growth.

The formation of a mixed martensitic/bainitic microstructure and the retainment of austenite in a medium-carbon steel during ultra-fast heating

The current work focuses on the microstructure evolution of a medium-carbon steel during an ultra-fast heating cycle. Electron microscopy methods such as Scanning Electron Microscopy, Electron BackScatter Diffraction and Transmission Electron Microscopy are utilized for the characterization of microstructural constituents of the studied steel samples. The applied heating rate during this treatment was 100 °C/s and the heating temperature was 930 °C, slightly above the Ac3 temperature, and subsequent quenching in water. It is found that the microstructure consists of martensite (60 %), lower bainite (37 %) and retained austenite (3%). The formation of bainite during this treatment is attributed to the heterogeneity of the chemical composition in the microstructure due to the lack of time for carbon to diffuse from cementite in long distances. Regions that are located away from the former pearlitic colonies are not enriched in carbon and manganese along the austenitization process and, according to the Controlled Cooling Transformation diagram, will form bainite upon cooling. The retainment of austenite in films is observed between the bainitic plates and on the martensitic lath boundaries, where carbon and manganese are segregated. Such microstructures are utilized by the current 3rd Generation Advanced High Strength Steels for the optimization of the mechanical properties.

Finite element and experiment analysis of welding residual stress in S355J2 steel considering the bainite transformation

This paper investigated the residual stress distribution and evolution in single-pass bead-on-plate TIG welding of S355J2 steel using the coupled thermo-metallurgical-mechanical model. The metallurgical calculation was based on parameterizing the continuous heating transformation and continuous cooling transformation diagrams. The simulated phase fraction in welding joints was validated against the dilatometric curves. The predicted results of welding residual stress revealed that the solid-state phase transformation incorporating undercooling austenite and transformation strain had significant consequences on the welding residual stress. The residual stress measurement was obtained by the XRD and relaxation methods. The good agreements between the simulated and experimental results indicated that the undercooling austenite and the bainitic transformation strain were crucial for predicting welding residual stress. The accurate thermo-metallurgical-mechanical model contributed to controlling the residual stress distribution reasonably by numerical method.

Two methods for calculating the stress-strain state of shape memory alloy constructions taking into account tension-compression asymmetry

Two methods for calculating the phase-structural deformations of shape memory alloy (SMA) structures under complex stress conditions are considered. They both are based on the one-dimensional phenomenological model, which is built upon the relationship between the direct transformation and martensitic inelasticity diagrams, which makes it possible to uniformly describe strains in the phase and structural transformations, since both of the strain components are associated with the formation of oriented martensite. The ability of the model to describe a number of basic macromechanical effects caused by martensitic transformations in SMA was shown in our previous work. After the generalization to the case of a complex stress state it can successfully be used for solving certain engineering problems. The generalization of the model can be accomplished in two ways. The first method involves the construction of three-dimensional constitutive relations, proceeding from the previously developed one-dimensional relations and some simplifying hypotheses, and the numerical implementation of these relations by the finite element method. The second is the structural method, applicable to structures, in which the stress-strain state is described by one kinematic and one force parameter. This method suggests the use of structural diagrams of direct transformation and martensitic inelasticity, which are similar to the corresponding material diagrams, but establish the dependence of the phase-structural component of the kinematic parameter on the force parameter (not the dependence of phase-structural strains on the stress). Although the structural method is associated with the necessity to experimentally determine the structural diagrams, it has the advantage of significantly reducing the computational costs. Additionally, the article presents a comparison of two methods for describing the tension-compression asymmetry, and also develops a method taking finite deformations into account.

Dilatometric study of the phase transformations under conditions of recrystallized and non-recrystallized austenite in 3Mn\u20131.5Al steel

This work presents the results of prior austenite state on the phase transformation behavior in a medium manganese steel alloyed with Al. The austenite was plastically deformed at two different temperatures. The first was at 1050 °C to ensure its recrystallization before cooling. The second treatment included deformation at 900 °C to keep high dislocation density in the austenite. The analysis of recrystallization process or its lack on the phase transformation behavior was analyzed. The study included thermodynamic calculations to analyze proper conditions of selected heat treatments. The dilatometric analysis of the phase transitions dependence on deformation temperatures was carried out. Deformation continuous cooling transformation diagrams were formed on this basis. The metallographic investigations were performed to determine microstructure constituents after cooling. The investigation proved the presence of ferrite untransformed during the austenitization step at 1100 °C. The dominant phase was bainite which was kept present up to 100 °C s−1 cooling rate. The amount of martensite increased with increasing the cooling rate. For the non-recrystallized austenite, more bainite was present in the microstructure for higher cooling rates compared to the recrystallized one. This was the result of higher density of preferable places for bainite nucleation in the non-recrystallized austenite. The Vickers hardness measurements were conducted after the applied heat treatments. The hardness of steel increased together with applying the higher cooling rates, which corresponded to the higher martensite amount. These values were higher for the non-recrystallized austenite because of higher dislocation density.

Prediction of the Properties of Alloyed Steels with Consideration for Phase Transformations

This article describes a prediction of the mechanical properties of a material based on simulation of phase transformations during fabrication of a structure. Application of hardness has been substantiated as reliable and reproducible parameter interrelating the mechanical properties of alloyed steel with its chemical and phase compositions. Regression models for prediction of the hardness of phase components by chemical composition are presented. A simulation procedure of steel phase transformations is proposed for a preset thermal cycle as are methods of determination of the required parameters by published diagrams of austenite phase transformations.

Mathematical Research of the Phase Transformation Kinetics of Alloyed Steel

Regulation of the process parameters allows obtaining the required properties of the metal. A computer simulation of technological processes with allowance for structural and phase transformations of the metal forms the basis for the proper choice of those parameters. The purpose of the paper is to study the main diffusion and diffusion-free transformation processes in alloyed steels during heating and cooling using the methods of mathematical modeling. A comparative analysis of the kinetic equations of phase transformations including the Kolmogorov–Avrami and Austin–Rickett equations that describe in different ways the dependence of the diffusion transformation rate on the time and the attained degree of transformation is carried out. It is shown that the Austin–Rickett equation is equivalent to the Kolmogorov–Avrami equation with a smooth decrease of the Avrami exponent during the transformation process. The advantages of the Kolmogorov–Avrami equation for modeling the kinetics of ferrite-pearlite and bainite transformations and the validity of this equation for modeling the kinetics of martensite transformations during tempering are shown. The parameters for describing the tempering process of steel 35 at different temperatures are determined. The proposed model is compared with equations based on the Hollomon–Jaffe parameter. The diagrams of martensitic transformation of alloyed steels and the disadvantages of the Koistinen–Marburger equation used to describe them are analyzed. The equations of the temperature dependence of the degree of transformation, similar to the Kolmogorov–Avrami and Austin–Rickett equations, are derived. The equations contain the minimum set of the parameters that can be found from published data. An iterative algorithm for determining parameters of the proposed model providing the minimum rms deviation of the constructed dependence from the initial experimental data is developed. The dependence of the accuracy of approximation on the temperature of the onset of transformation is presented. The complex character of the martensitic transformation development in alloyed steels is revealed. The advantage of using equations of the Austin–Rickett type when constructing models with a limited amount of experimental data is shown. The results obtained make it possible to extend the approaches used in the modeling of diffusion processes of austenite decomposition to the description of the processes of martensite formation and decomposition in alloyed steels.

Laser additive manufacturing of metallic glasses: issues in vitrification and mechanical properties

Abstract Bulk metallic glasses (BMGs), on account of their attractive properties, have now begun to witness a few commercial applications, e.g. in coatings and micro-gears. Additive manufacturing (AM) or 3D printing, although established for crystalline alloys, has only recently been used for synthesizing BMG components. The issues arising in 3D printing of BMGs are of current relevance, and this review focuses on the key scientific aspects, namely vitrification (or crystallization) during printing, mechanical properties of printed glassy alloys and the use of AM in identifying newer BMGs. Available data on crystallization during printing of a variety of BMGs are analysed in terms of schematic time–temperature–transformation diagrams and the complex interplay between thermal cycles, the presence of quenched-in nuclei in the glass and oxygen contamination in a way that is hoped to be broadly applicable to most alloy systems. Also reviewed are three key factors influencing mechanical properties of printed BMGs, i.e. porosity, crystallinity and oxygen contamination and thereby potential strategies for improvement are suggested. The review concludes with a discussion on the use of AM for combinatorial alloy development aimed at identifying better glass-forming compositions, which may in turn facilitate greater use of AM in manufacturing glassy components with desired properties.

Isothermal Solid‐State Transformations of Inconel 718 Alloy Fabricated by Selective Laser Melting

Inconel 718 samples have been fabricated by selective laser melting (SLM) and subjected to a series of heat treatments to study the isothermal precipitation behaviors of δ, γ′, and γ″ phases. A time–temperature–transformation (TTT) diagram for as‐built Inconel 718 is finally plotted. The morphology, size, and distribution of δ phase are studied, and the corresponding transformation kinetics is analyzed by Avrami equation. The results demonstrate that the transformation rate at 900 °C is the highest, though the nose of the TTT curve occurs at 860 °C after an incubation period of 10 min. As the aging temperature increases (620–820 °C), the precipitation rate of γ′/γ″ increases and the time to reach peak hardness decreases. The nose of the TTT curve for γ′/γ″ precipitation occurs at 820 °C after only 32 s. Nevertheless, when aged above 720 °C, the peak hardness decreases. In addition, the age hardening can be described by Avrami equation when δ phase is absent (620–770 °C). In summary, SLMed Inconel 718 exhibits a faster precipitation kinetics compared with wrought material, whereas their phase transition temperatures are similar.

Effects of Austenitization Temperature and Pre-Deformation on CCT Diagrams of 23MnNiCrMo5-3 Steel.

The combined effect of deformation temperature and strain value on the continuous cooling transformation (CCT) diagram of low-alloy steel with 0.23% C, 1.17% Mn, 0.79% Ni, 0.44% Cr, and 0.22% Mo was studied. The deformation temperature (identical to the austenitization temperature) was in the range suitable for the wire rolling mill. The applied compressive deformation corresponded to the true strain values in an unusually wide range. Based on the dilatometric tests and metallographic analyses, a total of five different CCT diagrams were constructed. Pre-deformation corresponding to the true strain of 0.35 or even 1.0 had no clear effect on the austenite decomposition kinetics at the austenitization temperature of 880 °C. During the long-lasting cooling, recrystallization and probably coarsening of the new austenitic grains occurred, which almost eliminated the influence of pre-deformation on the temperatures of the diffusion-controlled phase transformations. Decreasing the deformation temperature to 830 °C led to the significant acceleration of the austenite → ferrite and austenite → pearlite transformations due to the applied strain of 1.0 only in the region of the cooling rate between 3 and 35 °C·s−1. The kinetics of the bainitic or martensitic transformation remained practically unaffected by the pre-deformation. The acceleration of the diffusion-controlled phase transformations resulted from the formation of an austenitic microstructure with a mean grain size of about 4 µm. As the analysis of the stress–strain curves showed, the grain refinement was carried out by dynamic and metadynamic recrystallization. At low cooling rates, the effect of plastic deformation on the kinetics of phase transformations was indistinct.

Continuous cooling transformation behaviour and toughness of heat-affected zones in an X80 line pipe steel

A continuous cooling transformation (CCT) diagram was developed using thermal simulation techniques to replicate the coarse-grain heat-affected zone (CGHAZ) of an X80 line pipe steel. Specimens were heated to a peak temperature of 1350 °C with a 1 s hold time, followed by cooling at several rates between 7.6 and 356 °C/s. The thermo-mechanical specimen design was compatible with sub-size instrumented Charpy impact testing, which allowed the toughness to be evaluated. Following dilatometry measurements during continuous cooling, specimens were prepared and examined using optical and scanning-electron microscopy. In addition, the initial CGHAZ microstructure (upper bainite) was reheated to a peak temperature of 850 °C for 1 s (double cycled) and cooled at three different cooling rates (10, 5 and 2 °C/s). This allowed the influence of intercritical reheating, which occurs in multi-pass welds to be investigated in terms of microstructure, tensile strength and notch toughness. It was observed that the microstructure consisted of effective grain (matrix) and untempered MA when cooling at 10 or 5 °C/s. However, when reheating was followed by a cooling rate of 2 °C/s, the microstructure consisted of a combination of effective grain and a more benign distribution of tempered MA, which provided better low temperature impact toughness.

Statistical Modeling for Prediction of CCT Diagrams of Steels Involving Interaction of Alloying Elements

The interaction of different alloying elements has a significant impact on the mechanical and microstructural properties of steel products due to the thermodynamic and kinetic effect. This article presents a statistically developed and validated model for austenite decomposition during cooling, based on a set of experimental continuous cooling transformation diagrams available in literature. In the model, two-way interactions of the alloying elements are included as add-on terms, and the procedure for the analysis ensures there is no overfitting. The model can be used to predict phase transformation temperatures and critical cooling rates for the formation of polygonal ferrite, bainite or martensite for the production of steel.

Special Features of Transformations of Supercooled Austenite in Modern Structural Steels

Dilatometric, metallographic and durometric studies of transformations of supercooled austenite in modern commercial and experimental alloy steels under continuous cooling from the austenitization temperature at constant rates of 0.1 – 30 K/sec are performed. Thermokinetic diagrams of transformation of supercooled austenite are plotted and analyzed. The effect of alloying on the temperature and time intervals of formation of structural components in the steels is determined. Statistical analysis of the data obtained is performed to find the dependences of the critical temperatures Ac1 and Ac3 and of the critical quenching rate on the chemical composition of the steel. The behavior of the hardness of the formed structure as a function of the cooling rate is determined. Equations relating the coefficients of the function of logarithmic hardness and the chemical composition of the steels are suggested.

Effect of molybdenum, aluminium and boron on the phase transformation in 4 ​wt.–% manganese steels

The transformation behavior of medium manganese steels after forging has been characterized using dilatometry in order to investigate the influence of the alloying elements. The alloys contain 4 ​wt.–% manganese, 0.5 ​wt.–% silicon, 0.035 ​wt.–% niobium and varying amounts of carbon (0.15–0.19 ​wt.–%), aluminum (0.025–0.5 ​wt.–%), molybdenum (0.02–0.2 ​wt.–%), titanium (0–0.02 ​wt.–%), and boron (0–0.06 ​wt.–%). The continuous cooling transformation (CCT) diagrams and a multiple linear regression (MLR) formula for calculating the martensite start (Ms) temperature is presented. The results are critically compared to a variety of formulae available in the literature. It is demonstrated within this study that molybdenum increases the Ms-temperature, silicon has no mandatory influence on the Ms-temperature and boron not only decreases the Ms-temperature but also increases the critical cooling rate. The new formula enables the precise setting of annealing parameters.

Fatigue Fracture of Pie Channel Weld of Sinter Screen: A Metallurgical Insight

Sintering is an agglomeration process to convert iron ore fines into a larger mass suitable to be charged directly into the blast furnaces. A screen separates the incoming iron ore raw materials based on their size fraction. These screens are supported by pie channels. One of these channels failed during operation that led to unwarranted interruption in production. Owing to the criticality of the matter, detailed failure investigation was conducted. It involved visual observations, micro- and macro-fractography to determine the mode of fracture, chemical analysis and microstructure examination using optical and scanning electron microscopes aided with energy-dispersive x-ray spectroscopy and electron probe microanalyzer. The analysis revealed incompatible austenitic stainless-steel filler metal was used for welding mild steel material along with poor welding workmanship as suggested by underfilling and presence of thick layer of welding slag. There was formation of un-tempered martensite in dilution zone (the interface layer between heat-affected zone/fusion zone). The formation of undesirable martensite in the dilution zone is correlated with the time–temperature transformation diagram where the nose of the C-curve got significantly shifted toward the right due to enrichment with chromium and nickel as revealed by electron probe microanalysis. It was concluded that the failure occurred in fatigue mode due to deficiencies in welding and recommendations to prevent failures are also provided.

Toughness properties at multi-layer laser beam welding of high-strength steels

The material characteristics of high toughness and high strength in steel are usually not available at the same time. However, it would be an advantage if high-strength steels would show high impact toughness also at lower temperatures for applications in critical surroundings. In this paper, an approach of multi-layer welding of high-strength steel is presented in order to increase the weld-metal toughness using wire material in combination with thermal cycle modifications. Promising interlocking microstructures were found after multiple tempering of the previously applied structure at homogeneously distributed material in the weld seam. It was found that short thermal cycles during laser processing lead to insufficient time for carbon diffusion, which leads to remaining ferrite structures in contrast to the prediction of welding transformation diagrams. The additionally applied heating cycles during multi-layer laser welding induce the formation of interlocking microstructures that help to increase the weld seam toughness.

Overview and critical survey of polyamide6 structural habits: Misconceptions and controversies

AbstractVarious aspects of the structural habits and associated thermodynamic properties of polyamide6 are critically examined with regard to controversies that appeared in literature mainly due to misconceptions or erroneous/ambiguous interpretations of experimental findings. The structural habits under concern are the crystallization capabilities and chain topology, hydrogen bonds and sheet‐like structure, crystalline polymorphism together with thermodynamic stability. Thermo‐mechanical properties are also discussed for the sake of complementary argumentation. Comparisons are made with polycaprolactone, that is, the polyester homolog of polyamide6 regarding chain structure, in order to evaluate the actual role of the H‐bonds on the structural habits. Additional comparisons with polyethylene and polypropylene are also made at several occasions for the sake of supporting argumentation on the structural behavior of polyamide6. A temperature–time–transformation diagram of polyamide6 is finally proposed from the gathering of plentiful literature data.

A multi-phase linear kinematic elastoplastic model for the HAZ of welded S355J2 steel under low-cycle fatigue

The aim of this paper is to develop a linear kinematic elastoplastic model for simulating the mechanical behavior of a heat-affected zone under low-cycle fatigue for welded S355J2 low-carbon steel. First, an experimental procedure is developed by means of a Gleeble machine for creating macroscopic tensile specimens with different homogeneous metallurgical compositions according to a welding continuous cooling transformation diagram. Then, cyclic tensile tests are carried out by prescribing different strain amplitudes up to 1%. By considering the stabilized behavior at mid-life, the yield stress and hardening modulus are identified as functions of the metallurgical composition by means of a linear mixture rule. Comparisons with numerical simulations are presented to show the efficiency of the multi-phase cyclic linear kinematic elastoplastic model proposed in this work.

Metastable Dendrite Morphologies in Aluminum Alloys

Cubic metallic alloys generally grow along 〈100〉 directions due to the anisotropy of the solid–liquid interfacial energy. Under rapid solidification conditions, dendrites may deviate from 〈100〉 and develop unusual morphologies. Here, Al-alloy droplets (Al-4.5Cu, Al-10Si, Al-1.9Fe, Al-33Cu, all in wt.%) were rapidly solidified using Impulse Atomization to study the microstructures forming at different cooling rates and undercoolings. Growth morphologies of Al-4.5Cu droplets were characterized using x-ray micro-tomography and EBSD. Al-dendrites were found to grow along either 〈100〉 or a more unusual 〈111〉 depending on the solidification conditions. Also, a transition from 〈111〉 to 〈100〉 in the same droplet was observed. These uncommon growth directions were also observed in other Al-alloys. In Al-1.9Fe droplets, a change in dendrite growth direction from 〈100〉 to 〈111〉 was observed, while 〈110〉 growth directions were detected in Al-10Si samples. These experimental observations will be related to their solidification conditions using Solidification Continuous Cooling Transformation diagrams.