Continuous Cooling Transformation Research Articles

Continuous cooling transformation behaviour of 30CrMo steel simulated by CSP process

ABSTRACT The continuous cooling transformation (CCT) diagram of 30CrMo steel simulated by compact strip production process was constructed. And the effect of austenite deformation on the CCT behaviour was investigated. The results showed that the microstructure gradually transformed from proeutectoid ferrite and pearlite to bainite and martensite with the increase of cooling rate, accompanying a refined microstructure and an increasing hardness. When the cooling rate was lower than 0.1°C s−1, the desired microstructure of ferrite and pearlite could be obtained. And the austenite deformation strongly promoted the formation of proeutectoid ferrite, which led to a decreasing hardness of deformed steel. When the cooling rate was larger than 10°C s−1, the fully martensite microstructure was obtained. And the austenite deformation inhibited martensite transformation, resulting in a decrease of the martensite transformation starting (M s) temperature from 391°C to 368°C.

A New General Methodology to Create Continuous Cooling Transformation Diagrams for Materials Fabricated by Additive Manufacturing

Abstract Additive manufacturing (AM) is a manufacturing method that can build high-strength materials layer-by-layer to form complex geometries. Previous studies have reported large variations in the mechanical properties of materials made by this process. One of the key factors that may contribute to variations within and among parts made by this process is a difference in the material’s microstructural phase and composition. A continuous cooling transformation (CCT) diagram is a useful tool that can be used with a thermal model for microstructure design and manufacturing process control. However, traditional CCT diagrams are developed based on slow and monotonic cooling processes such as furnace cooling and air cooling, which are greatly different from the repetitive heating and cooling processes in AM. In this study, a new general methodology is presented to create CCT diagrams for materials fabricated by AM. We showed that the effect of the segmented duration within the critical temperature range, which induced precipitate formation, could be cumulative. As multiple cooling processes occurred in a short time, and the temperature drops at a high cooling rate, a constant average cooling rate was assumed when constructing the CCT diagram. Inconel 718 parts fabricated by selective laser melting were analyzed. The accumulated duration required for γ′/γ″ precipitate formation was found to shift from at least 432 s to about 83.38 s. The large difference in the duration (around 518%) highlights the importance of creating CCT diagrams specifically for AM materials. The key factor contributing to phase transformation was identified as the accumulated duration within the critical temperature range. The presented methodology demonstrated the capability of combining a thermal model and experimental observation to quantitatively predict phase transformation and could be used to design microstructures and control AM processes.

Microstructure and Phase Transformations in High-Strength Bainitic Forging Steel

In this work, the microstructural evolution and the phase transformations on a 0.22 wt.% carbon bainitic forging steel are investigated using a deformation dilatometer. The microstructure evolution was characterized using light microscope (LM), scanning electron microscope (SEM), transmission electron microscope (TEM), x-ray diffraction (XRD), and electron backscatter diffraction (EBSD). It was tried to predict the phase transformations via a continuous cooling transformation (CCT) diagram calculated by JMatPro using the results of prior austenite grain size measurement. In addition, critical temperatures were calculated using Thermo-Calc, JMatPro, and empirical formulations. Based on the dilatometric tests, hardness, and microstructural investigation, a deformation-CCT diagram was plotted. It was found that a bainitic microstructure can be obtained at cooling rates from 0.15 to 8 K/s. The microstructures varied from a mixture of bainite and blocky martensite/austenite islands at lower cooling rates to a bainitic ferrite with martensite at higher cooling rates. Finally, an industrially hot-forged sample cooled at 1 K/s was investigated. The microstructural evolution showed a microstructure of carbide-free bainitic ferrite along with film and island of retained austenite.

Microstructure evolution and impact toughness variation for high strength steel multi-pass weld metals with various cooling rates

Thermally simulated experiments on high-strength weld metals were performed. Based on the results, an extended continuous cooling transformation (CCT) curve with microstructure, hardness, and impact toughness can be proposed. The microstructures, hardness, high-angle grain boundaries, impact toughness, and fracture surfaces of the weld metals treated with different cooling rates were systematically compared and analyzed. The microstructure of the original weld metal corresponds to a cooling rate of 5 °C/s and is composed of uniform granular bainite and a small amount of acicular ferrite, which is similar to that of the specimens with cooling rates of 3−30 °C/s. The increase in the cooling rates leads to the evolution of the microstructure from quasi-polygon ferrite to granular bainite. The proportion of large angle grain boundaries increases with the increasing cooling rate, which can increase the resistance of crack propagation and induce a higher impact toughness. The impact toughness at room temperature is in the range of 80–131.81 J. Various cooling rates lead to various impact toughness. Except for the cooling rate of 5 °C/s, the other cooling rates decrease the impact toughness. Moreover, at the lowest cooling rate, the decrease in the impact toughness reaches 63 %. The impact toughness at -50 °C is in the range of 34–109.86 J. The impact toughness of the weld metals without thermally simulated processes at -50 °C is 88.6 J. The reheating process decreases the impact toughness from 88.6 J to 34 J, and the decrease rate of the impact toughness at -50 °C reaches 38 %.

Welding procedure qualification of Q345 grade fire-resistant steel based on Jmat-pro calculation

The welding process of two Q345 grade low Mo (about 0.25 wt.%) fire-resistant steels will be calculated and evaluated in this paper. The continuous cooling transformation (CCT) curve and isothermal transformation (TTT) curve of test steel were calculated by Jmat-pro software. The welding processes of steels were evaluated by measuring the continuous cooling transition (SH-CCT) curves of welding heat affected zone. The test results show that when the actual cooling time of Q1 steel is t 8/5>43s, cracking does not occur in the HAZ of welding and near the fusion zone, while when t 8/5<43s, cracking may occur in the HAZ or fusion zone; When the actual cooling time of Q2 steel is t 8/5>44s, cracking does not occur in the welding heat affected zone and the vicinity of the fusion zone, while when t 8/5<44s, cracking may occur in the heat affected zone or the fusion zone. In addition, Jmat-pro software has a certain accuracy for the calculation of Ac 3, but has a large error for the continuous cooling transformation (CCT) curve, especially the difference between the calculated value and the measured value of Q2 steel is larger, which only has reference value.

Optimization of the CCT Curves for Steels Containing Al, Cu and B

New continuous cooling transformation (CCT) equations have been optimized to calculate the start temperatures and critical cooling rates of phase formations during austenite decomposition in low-alloyed steels. Experimental CCT data from the literature were used for applying the recently developed method of calculating the grain boundary soluble compositions of the steels for optimization. These compositions, which are influenced by solute microsegregation and precipitation depending on the heating/cooling/holding process, are expected to control the start of the austenite decomposition, if initiated at the grain boundaries. The current optimization was carried out rigorously for an extended set of steels than used previously, besides including three new solute elements, Al, Cu and B, in the CCT-equations. The validity of the equations was, therefore, boosted not only due to the inclusion of new elements, but also due to the addition of more low-alloyed steels in the optimization. The final optimization was made with a mini-tab tool, which discarded statistically insignificant parameters from the equations and made them prudently safer to use. Using a thermodynamic-kinetic software, IDS, the new equations were further validated using new experimental CCT data measured in this study. The agreement is good both for the phase transformation start temperatures as well as the final phase fractions. In addition, IDS simulations were carried out to construct the CCT diagrams and the final phase fraction diagrams for 17 steels and two cast irons, in order to outline the influence of solute elements on the calculations and their relationship with literature recommendations.

A viscoelasticity model for polymers: Time, temperature, and hydrostatic pressure dependent Young's modulus and Poisson's ratio across transition temperatures and pressures

Capturing and fundamentally understanding the variation of a polymer's mechanical properties with time, temperature, and hydrostatic pressure is essential for accurate constitutive modeling. In this paper, a new linear viscoelastic constitutive model is developed that regards Young's modulus and Poisson's ratio as functions of time, temperature, and hydrostatic pressure. Modified logistic functions are employed, which provide appealing mathematical and physical simplicity. Aided by time/temperature/hydrostatic-pressure superposition, these functions can be transformed from the time domain to the temperature or pressure domain, or the converse. Our model is successfully benchmarked against numerous sets of experimental data. Our model is also correlated to polymer processing through continuous cooling transformation (CCT) diagrams. Since CCT diagrams help determine a polymer's microstructure, which in turn determines its macroscopic mechanical properties, our model can be used as a guide for tailoring the manufacturing process (e.g., controlling quenching temperature and speed) to obtain a targeted set of mechanical properties. Our new model can also be embedded within existing nonlinear viscoelastic constitutive frameworks to capture the mechanical behavior of polymers across a wide range of strain rates, temperatures, and pressures.

A Comparison Study of the Austenite Grain Growth and Its Transformation Behavior during Uniform Continuous Cooling of a Wrought and Selective Laser Melting 4340 Steels

Abstract This study compares austenite grain growth and its continuous cooling transformation (CCT) behavior between selective laser melting (SLM) 4340 and conventional wrought 4340 steels. Standard dilatometry tests were used to determine the austenite decomposition behavior at different cooling rates. The analysis of the data from the controlled cooling paths was used to generate CCT diagrams for the two steels investigated in this study. Advanced microstructural characterization techniques, Scanning Electron Microscopy and Orientation Imaging Microscopy, were employed to support the CCT diagrams. Electron Backscatter Diffraction (EBSD) and a special EBSD-Image Quality characterization technique were used to assess the percentage of the microstructural components observed. Three important observations were made: (1) the hardenability of the wrought 4340 steel was higher than the SLM 4340 steel; (2) the presence of granular bainite was observed after slow cooling conditions in both steels; (3) the SLM steel exhibited finer austenite grain size distribution than the wrought steel in the temperature range studied. MTEX, a MatLab program, was used to reconstruct the prior austenite grains. The SLM steel exhibited finer austenite grain size distribution than the wrought steel in the temperature range studied.

Predictive integrated numerical approach for modeling spatio-temporal microstructure evolutions and grain size dependent phase transformations in steels

A computational modeling for predicting microstructure evolutions and mechanical properties of steels under thermo-mechanical-metallurgical process is established, for the first time, by integrating the finite element (FE) simulation, cellular automaton simulation (CA), and phase transformation kinetics. In this microstructural-integrated modeling, various recrystallization processes, such as dynamic recrystallization (DRX), meta-DRX, and static recrystallization (SRX), are formulated based on dislocation density based constitutive laws. With microstructure information provided by the CA modeling, the austenite grain size (AGS)-dependent phase kinetics in the form of continuous cooling transformation (CCT) diagram is applied for addressing the effect of AGS on transformations under various cooling conditions. The integrated numerical approach implemented in the FE software via user defined subroutines can simulate the morphology and size distribution of constituent grains, transformed fractions of various phases, hardness profiles and flow stresses after thermo-mechanical process with large plastic deformation. As a validation of the integrated modeling, the multiple oval-round pass hot rolling and subsequent cooling process are simulated for the seismic reinforcing steel bar and the predicted microstructure and mechanical properties are compared to those of experimental data.

Modelling of Delta Ferrite to Austenite Phase Transformation Kinetics in Martensitic Steels: Application to Rapid Cooling in Additive Manufacturing

In this paper, the high temperature transformation kinetics of delta ferrite to austenite (δ → γ) phase transformation is modeled by thermodynamic and diffusion calculations. It appears that, in martensitic steels, the δ → γ transformation is very fast (a few nanoseconds) as soon as the first austenite nuclei appears. Classically the austenitic phase will thus systematically be observed in the material during conventional elaboration processes. However, in powder metallurgy and additive manufacturing, it is possible to obtain sufficiently high quenching rates (up to 106 °C/s) so that the γ phase does not have time to appear. The calculations presented here allow to rationalize the understanding of the microstructures of powders and different additive manufacturing materials. They enable to understand why ferrite or martensite is sometimes obtained in the final microstructure. From the calculations made, an original CCT (Continuous Cooling Transformations) diagram starting from the δ phase is proposed. It allows to set up a strategy of grade design or process definition according to the final microstructure targeted.

Characterization and Prediction of Continuous Cooling Transformations in Rail Steels

This paper presents the characterization of austenite to pearlite transformation in rail steels (premium and standard) using dilatometry. The critical transformation temperatures were measured as a cooling rate function and the obtained microstructures were qualitatively and quantitatively characterized. Aiming to study the transformation kinetics, three equations proposed by technical literature were evaluated in order to predict the transformation evolution. Experimental and calculated CCT diagrams were compared. It was concluded once the kinetics of phase transformation in rail steels is well characterized, it can be used as reference to enable suitable heat treatment to obtain pearlitic microstructures for the standard steel as refined as those of premium. The minor prior austenite grain size of the standard steel favoured the pearlite nucleation, however its highest Mn, Nb and Si contents delayed the growth stage. All equations evaluated in order to predict the kinetics of austenite to pearlite transformation under continuous cooling were satisfactory.

Dilatometric analysis tool steel X153CrMoV12

The article deals with phase transformations and austenitizing behavior of the X155CrMoV12 tool steel. Dilatation analyses of a series of samples were performed at various cooling rates, chosen in the range from 10 °C·s-1 to 0.1 °C·s-1. Acquired experimental data were used for evaluation of dilatometric curves in order to map the temperature ranges of phase transformations of the austenite to pearlite, bainite or martensite. All experimental samples from dilatometric analyses were then subjected to microstructural analyses and hardness measurements to characterize the microstructure and hardness for every tested heat treatment regime. The second part of this article, entitled "EXPERIMENTAL DETERMINATION OF CONTINUOUS COOLING TRANSFORMATION DIAGRAM FOR HIGH STRENGTH STEEL X153CRMOV12", deals with these analyses of the cooling curve microstructure.

Continuous Cooling Transformation Behavior in Welding Coarse-Grained Heat-Affected Zone and Microstructure Evolution of G115 Novel Martensitic Heat-Resistant Steel

Continuous Cooling Transformation Behavior in Welding Coarse-Grained Heat-Affected Zone and Microstructure Evolution of G115 Novel Martensitic Heat-Resistant Steel

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.

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.

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.

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.

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.