Transformation Diagrams Research Articles

Kinetics of Isothermal Phase Transformations in Premium and Standard Rail Steels

Recently published articles have enhanced the importance of understanding phase transformations in rail eutectoid steels in an attempt to improve their manufacturing and welding processes. Herein, the kinetics of isothermal phase transformations of three eutectoid steels used in railways around the world are evaluated. Computer simulations are performed to evaluate the effect of steel chemical compositions on phase transformations considering thermodynamic equilibrium conditions. Austenitizing temperatures under continuous heating and the time intervals of isothermal austenite decomposition are measured by dilatometry. The time–temperature–transformation diagrams are determined. Qualitative and quantitative microscopy techniques are applied to characterize the phase transformation products. It is shown that by applying a well‐planned heat treatment, it is possible to obtain microstructural characteristics for a standard steel as refined as those achieved for premium steels. Empirical equations proposed in the literature to predict the Bs temperatures are applied, and the experimental values are compared. Using the dilatometry data, it is possible to determine the global activation energy for austenite–bainite transformation. Results show that premium steels have higher overall activation energy for austenite–bainite decomposition than what occurs in standard. This article presents unpublished data useful for steel researchers as well for rail manufacturers and welders.

Effects of austenite deformation on continuous cooling transformation of the pearlite heat-resistant steel

ABSTRACT Continuous cooling transformation (CCT) diagrams of undeformed and 40% compressively deformed pearlite heat-resistant steels at 950°C were obtained by a combined method of dilatometry and metallography. The microstructure and hardness of the steel under these two experimental conditions were compared and analysed. The effects of austenite deformation on the CCT of pearlite heat-resistant steel are discussed. The CCT diagrams show that the austenite deformation extends the ferrite and pearlite transformation fields and increases the critical cooling rates to achieve the ferrite and pearlite microstructure, which causes the CCT curve to shift to the left. However, austenite deformation suppresses the transformation of bainite and martensite. Moreover, it reduces the hardness of the tested steel at the same cooling rate and grain size of the ferrite at low cooling rates.

Continuous Cooling Transformation Diagrams of 2.25Cr-1Mo-0.25V Submerged-Arc Weld Metal and Base Metal

The transformation behavior and microstructural evolution during continuous cooling within the heat affected zone between the weld beads of a 2.25Cr-1Mo-0.25V all-weld metal and the corresponding 2.25Cr-1Mo-0.25V base metal were investigated by means of dilatometer measurements, optical and scanning electron microscopy. Furthermore, macro-hardness measurements were conducted and the ferrite phase fraction was analyzed from optical microscopic images using an imaging processing program. Thereupon a continuous cooling transformation (CCT) diagram for the 2.25Cr-1Mo-0.25V base metal and three welding CCT diagrams with different peak temperatures were constructed to realistically simulate the temperature profile of the different regions within the heat affected zones between the weld beads of the multi-layer weld metal. The microstructural constituents which were observed depending on the peak temperature and cooling parameters are low quantities of martensite, high quantities of bainite and in particular lower bainite, coalesced bainite and upper bainite as well as ferrite for the welding CCT diagrams. Regarding the base metal CCT diagram, all dilatometer specimens exhibited a fully bainitic microstructure consisting of lower bainite, coalesced bainite and upper bainite. Only the slowest cooling rate with a cooling parameter of 50 s caused a ferritic transformation. Nevertheless, it has to be emphasized that the distinction between martensite and bainite and the various kinds of bainite was only possible at higher magnification using scanning electron microscopy.

Atom-probe tomographic and dilatometric studies of phase-transformations after inter-critical annealing of a low-carbon dual-phase steel

Phase-transformations and microstructural evolution during and after inter-critical annealing of a low-alloy, dual-phase steel are studied utilizing dilatometric measurements, and microstructural characterization methods, including scanning electron microscopy, synchrotron X-ray diffraction, and atom-probe tomography. Dilatometric measurements reveal that the sample contains ~72% austenite, after annealing for 5-min at 871 °C, which agrees with the water-quenched microstructure, and ~90% after further annealing to 1 h, which agrees with thermodynamic predictions. A continuous-cooling transformation (CCT) diagram is constructed, within the cooling-rate range between 1 °C/s and 70 °C/s. The incubation time for the ferrite-transformation is <1 s, after inter-critical annealing, and the secondary-phases are mainly pearlite for cooling-rates of <5 °C/s, and martensite for cooling-rates >10 °C/s, which is confirmed utilizing scanning electron-microscope observations. The microhardness increases as the cooling rate increases, and an abrupt slope-change is explained by the major secondary-phase changing from martensite to pearlite between 5 °C/s and 10 °C/s. Atom-probe tomography demonstrates the coexistence of martensite and pearlite as secondary-phases for 30 °C/s and a switch in austenite-ferrite transformation mechanisms from para-equilibrium (70 °C/s) to partitioning local-equilibrium (30 °C/s). A microstructural map is constructed to reveal the austenite decomposition products in the cooling-rate range between 1 °C/s and 70 °C/s. Based on composition measurements of the matrix, utilizing APT, the effects of cooling-rates on solid-solution-strengthening are estimated, and the results demonstrate that solid-solution strengthening, especially carbon-redistribution, accounts for the majority of the strength variations due to different cooling-rates.

Adjustment of Isothermal Transformation Diagrams Using Finite-Element Optimization of the Jominy Test

A practical method for adjusting and optimizing isothermal transformation (IT) diagrams using the Jominy test is presented. The method is based on a finite-element optimization procedure, which iteratively minimizes the error between the target phase fractions and the corresponding finite-element solutions at the sample points using an optimization tool. A standard Jominy test of AISI 52100 bearing steel is used to investigate the feasibility and reliability of the method. Three optimization parameters for each IT diagram curve are mathematically applied to the modified Kirkaldy model. These parameters are the design variables in the optimization. The curves obtained from the modified Kirkaldy model are used as the initial guesses in the optimization and they approach the experimental IT diagram by minimizing the error. Good agreement is observed between the optimized diagram and the experimental diagram reported in the literature. The predicted phase fractions using the experimental IT diagram, the IT diagram obtained from the modified Kirkaldy model, and those obtained from the optimized model are compared and demonstrate that the adjustment or optimization procedure significantly improves the accuracy of the predicted phase fraction of the model. The applicability of the method is investigated in a practical case study.

Metallic Glacial Glass Formation by a First-Order Liquid-Liquid Transition.

The glacial phase, with an apparently glassy structure, can be formed by a first-order transition in some molecular-glass-forming supercooled liquids. Here we report the formation of metallic glacial glass (MGG) from the precursor of a rare-earth-element-based metallic glass via the first-order phase transition in its supercooled liquid. The excellent glass-forming ability of the precursor ensures the MGG to be successfully fabricated into bulk samples (with a minimal critical diameter exceeding 3 mm). Distinct enthalpy, structure, and property changes are detected between MGG and metallic glass, and the reversed "melting-like" transition from the glacial phase to the supercooled liquid is observed in fast differential scanning calorimetry. The kinetics of MGG formation is reflected by a continuous heating transformation diagram, with the phase transition pathways measured at different heating rates taken into account. The finding supports the scenario of liquid-liquid transition in metallic-glass-forming liquids.

Conceptual diagram of digital transformation of the operator company in the transport services market

Rationale: The Russian transport system is losing its competitive advantage due to the slow response to digital calls. Thus, for subjects of the market of transport and logistics services, it is relevant to have recommendations for adapting a business model when moving from an analogue economy to a digital one.&#x0D; Purpose: To develop a conceptual diagram of the digital transformation of the business model of the operator company.&#x0D; Methods: &#x0D; &#x0D; сomparison of the principles of the five business areas of the operator before and after the transition to the digital economy;&#x0D; analysis of the behavior of companies in the transport market;&#x0D; summarizing the results of the study and assessing the possibility of their application to other organizations in this market segment;&#x0D; case method Development of measures for the effective development of multimodal transportation of the Asia-Pacific Region (INTERTRAN));&#x0D; an integrated approach is the search for new points (channels) of interaction with a dynamic network of customers through the implementation of industrial logistics projects.&#x0D; &#x0D; Results: &#x0D; &#x0D; the status of five areas of the business strategy of the operator company was determined before and after the transition from the analogue era to the digital one;&#x0D; the conditions are formulated under which the implementation of the business process "sale of transport and logistics services" will provide the sales department of the operator company to develop "proactive sales";&#x0D; recommendations are proposed for adapting a value proposition for the provision of transport and logistics services.&#x0D; &#x0D; Conclusions: The novelty of the proposed conceptual scheme for adapting the business model of the operator company during the transition from the analogue economy to the digital one lies in the initial definition of the stage of digital transformation of each of the five areas of the operators business strategy, and then in the selection of a specific digital technology for implementation in a specific business-process.

Isothermal crystallization kinetics of (Cu60Zr25Ti15)99.3Nb0.7 bulk metallic glass

This paper reports the crystallization kinetics of (Cu60Zr25Ti15)99.3Nb0.7 bulk metallic glass under isothermal conditions. Differential scanning calorimetry (DSC) has been employed for isothermal annealing at ten different temperatures prior to the onset of crystallization (To) temperature. X-ray diffraction and transmission electron microscopy have been used to confirm the amorphous structure of the as cast sample. Crystallized volume fractions (x) are calculated from the exothermic peaks of DSC scans. Crystallized volume fractions (x) against time show sigmoidal type of curves as well as the curves become steeper at higher annealing temperatures. Continuous heating transformation diagram has been simulated to understand the stability of the bulk metallic glass. Crystallization kinetics parameters are calculated using Arrhenius and Johnson–Mehl–Avrami equations. Activation energy (Ea) and Avrami exponential factor (n) have exhibited strong correlation with crystallized volume fraction (x). The average activation energy for isothermal crystallization is found to be 330 ± 30 kJ/mol by Arrhenius equation. Nucleation activation energy (Enucleation) is found to be higher than that of growth activation energy (Egrowth). The Avrami exponential factor (n) indicates about the diffusion controlled mechanism of the nucleation and three-dimensional growth.

Effect of C-Mn ratio on the maximum hardness and toughness in TMCP steels with an identical carbon equivalent

In this study, the mechanical properties of two types of thermo-mechanically controlled processed (TMCP) steels with the same carbon equivalent but different alloying constituents of C and Mn were examined. Tensile, impact, hardness, and through-thickness tensile tests were employed for this examination. The taper hardness test was conducted in the weld heat-affected zones (HAZs) by using common onsite welding methods. The results revealed that the two distinct steel plates exhibited different microstructures in the base metal; however, their maximum hardness and microstructures in the HAZs were similar within the heat input range (10–30kJ/cm) that is often used at manufacturing sites. Moreover, the continuous cooling transformation diagrams for the HAZs and the base metal used for welding the TMCP steels were simulated to validate the experimental results for the microstructure. The drop weight test and weld bead bend test were employed to compare the toughness related with the weldability of the two steel plates. The steel with composition of 0.08% C-1.5% Mn exhibited lower nil-ductility transition temperature and better crack arrestability than that with the composition of 0.15% C-1.1% Mn.

Optimization of Niobium Content in Direct Quenched High-Strength Steels

This paper focuses on understanding the effect of niobium content on the phase transformation behavior and resultant mechanical properties of thermomechanically rolled and direct-quenched low carbon steels containing 0.08 wt.% carbon. Investigated steels contained three different levels of niobium: 0, 0.02 and 0.05 wt.%. The continuous cooling transformation (CCT) diagrams covering cooling rates in the range 3–96 °C/s constructed based on the dilatometer studies showed only a minor effects of Nb on the phase transformation characteristics. In addition, experiments were performed for reheating and soaking the slabs at 1050–1200 °C and the results revealed that for these low-carbon steels, Nb failed to prevent the austenite grain growth during slab reheating. In the case of hot rolling trials, two different finish rolling temperatures of 820 °C and 920 °C were used to obtain different levels of pancaking in the austenite prior to direct quenching. The resultant microstructures were essentially mixtures of autotempered martensite and lower bainite imparting yield strengths in the range 940–1070 MPa. The lower finish rolling temperature enabled better combinations of strength and toughness in all the cases, predominantly due to a higher degree of pancaking in the austenite. The optimum level of Nb in the steel was ascertained to be 0.02 wt.%, which resulted not only in marginally higher strength but also without any significant loss of impact toughness.

Dissolution and precipitation of copper-rich phases during heating and cooling of precipitation-hardening steel X5CrNiCuNb16-4 (17-4 PH)

Continuous heating transformation (CHT) diagrams and continuous cooling transformation (CCT) diagrams of precipitation-hardening steels have the drawback that important information on the dissolution and precipitation of Cu-rich phases during continuous heating and cooling are missing. This work uses a comparison of different techniques, namely dilatometry and differential scanning calorimetry for the in situ analysis of the so far neglected dissolution and precipitation of Cu-rich phases during continuous heating and cooling to overcome these drawbacks. Compared to dilatometry, DSC is much more sensitive to phase transformation affecting small volume fractions, like precipitation. Thus, the important solvus temperature for the dissolution of Cu-rich phases was revealed from DSC and integrated into the CHT diagram. Moreover, DSC reveals that during continuous cooling from solution treatment, premature Cu-rich phases may form depending on cooling rate. Those quench-induced precipitates were analysed for a broad range of cooling rates and imaged for microstructural analysis using optical microscopy, scanning electron microscopy and transmission electron microscopy. This information substantially improves the CCT diagram.

Effect of cooling rate on α variant selection and microstructure evolution in a near β Ti–5Al–3Mo–3V–2Cr–2Zr–1Nb–1Fe alloy

Abstract The microstructure evolution and selection of α variants in a new near β titanium alloy Ti-5321 (Ti–5Al–3Mo–3V–2Cr–2Zr–1Nb–1Fe) were thoroughly investigated by dilatometry under continuous cooling conditions accompanied with OM, XRD, SEM, EBSD and TEM. Cooling rates from β region to room temperature significantly influenced the microstructure based on the competitive growth of αGB, αWGB and αWI. The low (0.01 °C/s and 0.1 °C/s), intermediate (0.5 °C/s and 1 °C/s) and high (2 °C/s, 4 °C/s and 5 °C/s) cooling rates resulted in bi-lamellar, basket-weave and single metastable-β microstructures, respectively. Moreover, the transformed microtexture was also found in the cooling process at low cooling rate. In addition, a continuous cooling transformation (CCT) diagram obtained by the dilatometric analysis revealed the sensibility of the β→α phase transformation temperature range to the cooling rate. EBSD analysis revealed that microtexture formed by α colonies with the similar orientations, occurred in the adjacent β grains and maintained Burgers orientation relationship (OR) with at least one of the adjacent β grains. Two typical cases, i.e. the single-BOR α colonies (the colonies which maintain Burgers OR with β grain only on one side) and the double-BOR α colonies (α colonies which maintain Burgers OR with both the adjacent β grains) were studied. The single-BOR α colonies corresponded to the different α variants from the parent β grain. The double-BOR α colonies occurred in which the adjacent β grains shared nearly common {110}β plane, moreover, the double-BOR α colonies could possess the identical orientation in the adjacent β grains which have a 60° rotation about a common β direction.

Adjusting the melting and crystallization behaviors of ferronickel slag via partially replacing of SiO2 by B2O3 for mineral wool production

Utilization of ferronickel slag as a raw material for mineral wool production not only conserves natural resources, but also achieves the sustainable development for the ferronickel industry. In order to utilize the ferronickel slag more efficiently, the melting and crystallization behaviors of the slag with the partial replacement of SiO2 by B2O3 were investigated in this paper. The results show that the initial and complete melting temperatures decreased with B2O3 substitution for SiO2, which indicates that the addition of B2O3 can reduce the energy consumption during the melting process. The incubation time of the slag in the Time Temperature Transformation (TTT) diagram increased, whereas the initial crystallization temperature in the Continuous Cooling Transformation (CCT) diagram decreased when SiO2 was substituted by B2O3. In addition, the main crystalline phases precipitated in the slags were columnar MgSiO3 and Mg2SiO4. The weakening of crystallization ability caused by the substitution of B2O3 for SiO2 will be beneficial for the improvement of fiber quality when the ferronickel slag is used as a raw material for mineral wool production.

A new approach to estimate the curing mode of thermosetting polymer films with regard to physical aging and slow chemical processes

AbstractWe propose an original method to determine the temperature and time required for additional curing of physically aged partially cured glass‐shaped systems, based on the example of epoxy amine systems. The approach consists of the plotting of temperature–time–transformation (TTT) diagrams, supplemented with lines, which illustrate the shift in glass transition temperature (Tg) as a result of the processes of physical aging (enthalpy relaxation) and with isoconversion curves. Then we should determine Tg of partially cured and aged sample at a certain temperature. Having known this Tg value, the position of the isoconversion curve at a given point and Tg,∞ as well, we can determine the postcuring time at T = Tg,∞ + 5 K. Thus, the method provides information about the temperatures and the duration of these two consecutive steps. The TTT diagrams are based on experimental results obtained by differential scanning calorimetry in dynamic mode with preliminary heated samples in an external thermostat. The proposed approach based on epoxy amine systems allows the prediction of the optimal curing mode for thermosetting systems to provide the highest degree of cure. Our method is applicable to a wide range of thermosetting polymer systems that complete transition to a glassy state before full conversion is achieved.

Thermo-Mechanical Simulation of Hybrid Welding of DP/AISI 316 and TWIP/AISI 316 Dissimilar Weld.

In this paper, hybrid laser-MAG (metal active gas) welding of twinning-induced plasticity (TWIP) and dual-phase (DP) steels with austenitic stainless steel (AISI316) was simulated by means of the finite element method. A thermo-mechanical model, which uses a 3D heat sources, was developed using the software Simufact Welding. The calculated dimensions, shape and distortion of the weld were compared with the experimental results, thence the model was validated. The metallurgical transformations for the DP steel were evaluated using the continuous cooling transformation (CCT) diagram and the calculated cooling rate. The numerical model predicted accurately the shape of the molten pool, the thermal cycles as well as the geometrical distortion of the butt weld. Therefore, the numerical model showed a good reliability and its potential for further development.

PRODUCTION OF A DUAL PHASE STEEL FROM A LOW CARBON MICROALLOYED STEEL

A dual phase steel was manufactured with the chemical composition established for a microalloyed steel, in order to take advantage of the hardening mechanisms of both steels. The treatment temperature was established based on the critical transformation temperatures obtained by dilatometric techniques and simulated phase transformation diagrams. The specimens were hot rolled in a six continuous passes with a total rolling time of 65 seconds up to 47% thickness reduction. Optical microscopy and scanning electron microscopy were used for microstructural characterization in order to obtain the average ferritic grain diameter and volume fraction of ferrite and martensite. Vickers microhardness for each phases was determined and tension tests were performed to obtain the ultimate tensile strenght, 0.2% offset yield strength, percent reduction in area and percent elongation. Microstructures composed of ferrite and martensite were obtained, with a 35.98% average percent of martensite. The average ultimate tensile strenght values and and percent elongation were 719 MPa and 22%, respectivelly. The use of simulated time-temperature-transformation and continuous-cooling-transformation diagrams, as well as the thermal history of thermomechanical and intercritical processes, proved to be useful in the development of processing routes for the manufacture of dual-phase steels with a specific chemical composition. The precipitation mechanisms present in microalloyed steels enhance the microstructure refinement in a dual phase condition.

Microstructure and Properties Formed at Different Cooling Rates of Low Carbon Alloy Steel for Welding Wire Production

Physical simulation of steel Mn3Ni1CrMo continuous cooling with different speeds from austenitic state was performed using GLEEBLE 3500 complex. The phase transformations are analyzed and the effect of the cooling rate on the structure and hardness is investigated. A continuous cooling transformation diagram of the undercooled austenite decomposition is constructed. It was concluded that it is possible to reduce the hardness of the hot-rolled billet by reducing the cooling rate compared to the existing in the processing at the STELMOR line of PJSC “MMK”, and this will eliminate the heat treatment of welding wire on the hardware processing.

Tunable all-optical format conversion for BPSK to OOK based on highly nonlinear optical loop mirror

A scheme is proposed for tunable all-optical format conversion from BPSK to OOK signal without active phase locking under the highly nonlinear loop structure. The 20 Gbit/s BPSK is mapped to two-level amplitude signal by delay interferometer (DI). One carrier branch generated locally remodulates the amplitude information to phase based on cross-phase modulation (XPM) effect in the highly nonlinear fiber (HNLF). The other carrier propagates as a pump under symmetrical loop ensuring the stability of phase relationship. After interference and vector shifting in coupler, the wavelength-tunable OOK is obtained. Benefiting from the signal remodulation mechanism and symmetric loop setup, separate carrier extraction and additional phase-locking are not required. The feasibility of the scheme is demonstrated by constellation diagram, eye diagram and pattern transformation diagram in numerical simulations. At the BER of 10−3, the converted OOK has ∼0.7 dB and ∼1.9 dB optical signal-to-noise ratio (OSNR) improvement, respectively, compared to back-to-back (B2B) OOK under different quality of input signals. It is shown that the scheme optimizes the signal quality while converting signal formats. For low-quality BPSK with OSNR of 15 dB, the system indicates better noise tolerance. Furthermore, the scheme has potential scalability to bidirectional format conversion between BPSK and OOK.

Effect of Alloy Elements in Time Temperature Transformation Diagrams of Railway Wheels

The Heavy-Haul railroad wheels started to use higher wear resistance steels microalloyed with niobium, vanadium, and molybdenum [1]. During continuous cooling, these elements depress the temperature of the pearlite formation, producing smaller interlamellar spacing that increases the hardness of the steel, besides to favor the precipitation hardening through the formation of carbides [2, 3]. Also, they delay the formation of difusional components like pearlite and bainite during isothermal transformation. The effects of these alloy elements on microstructure during isothermal transformation were studied in this work using a Bähr 805A/D dilatometer. Three different compositions of class C railway wheels steels (two microalloyed and one, non microalloyed) were analyzed in temperatures between 200 and 700 °C. The microstructure and hardness for each isothermal treatment were obtained after the experiments. Comparing with non microalloyed steel (7C), the vanadium addition (7V steel) did not affect the beginning of diffusion-controlled reactions (pearlite and bainite), but delayed the end of these reactions, and showed separated bays for pearlite and bainite. The Nb + Mo addition delayed the beginning and the ending of pearlite and bainite formation and also showed distinct bays for them. The delays in diffusion-controlled reactions were more intense in the 7NbMo steel than in 7V steel. The V or Nb + Mo additions decreased the start temperature for martensite formation and increased the start temperature for austenite formation.

Influence of Chromium Content and Prior Deformation on the Continuous Cooling Transformation Diagram of Low-Carbon Bainitic Steels

The effect of chromium content and prior hot deformation of the austenite on the continuous cooling transformation (CCT) diagram of a newly developed low-carbon bainitic steel has been studied using dilatometer measurements conducted on a Gleeble 3800 simulator with cooling rates ranging from 2-80 °C/s. After austenitization at 1100 °C, specimens were either cooled without strain or given 0.6 strain at 880 °C prior to dilatometer measurements. The resultant microstructures have been studied using laser scanning confocal microscopy, scanning electron microscopy and macrohardness measurements. CCT and deformation continuous cooling transformation (DCCT) diagrams were constructed based on the dilatation curves, final microstructures and hardness values. Depending on the cooling rate, the microstructures of the investigated steels after cooling from the austenite region consist of one or more of the following microstructural components: lath-like upper bainite, i.e. bainitic ferrite (BF), granular bainite (GB), polygonal ferrite (PF) and pearlite (P). The proportion of BF to GB as well as the hardness of the transformation products decreased with decreasing cooling rate. The cooling rate at which PF starts to appear depends on the steel composition. With both undeformed and deformed austenite, increasing the chromium content led to higher hardenability and refinement of the microstructure, promoting the formation of BF and shifting the ferrite start curve to lower cooling rates. Prior hot deformation shifted the transformation curves to shorter times and higher temperatures and led to a reduction in hardness at the low cooling rates through the promotion of ferrite formation.