Continuous Cooling Transformation Diagrams Research Articles

Experimental Determination of Continuous Cooling Transformation (CCT) Diagrams for Dual-Phase Steels from the Intercritical Temperature Range

The phase transformation kinetics under continuous cooling conditions for intercritical austenite in a cold rolled low carbon steel were investigated over a wide range of cooling rates (0.1–200 ∘ C/s). The start and finish temperatures of the intercritical austenite transformation were determined by quenching dilatometry and a continuous cooling transformation (CCT) diagram was constructed. The resulting experimental CCT diagram was compared with that calculated via JMatPro software, and verified using electron microscopy and hardness tests. In general, the results reveal that the experimental CCT diagram can be helpful in the design of thermal cycles for the production of different grades of dual-phase–advanced high-strengh steels (DP-AHSS) in continuous processing lines. The results suggest that C enrichment of intercritical austenite as a result of heating in the two phases (ferrite–austenite) region and C partitioning during the formation of pro-eutectoid ferrite on cooling significantly alters the character of subsequent austenite phase transformations.

FEATURES OF KINETICS OF DESTROYING AUSTENITE AND THE REGULARITIES OF FORMATION OF THE c82dcrv STEEL STRUCTURE DURING CONTINUOUS COOLING

Formulation of the problem. Since the cooling of rolled metal under deformation-thermal treatment in a real production process occurs not in isothermal conditions, but with a continuous temperature decrease, the study of the decay of austenite under continuous cooling is of great practical importance. Purpose . Investigation of the kinetics of the decay of austenite, as well as the regularities in the formation of the structure of C82D CrV steel during continuous cooling with different rates from an elevated heating temperature. Results. The kinetics of the transformations was studied and a continuous cooling transformation diagram (CCTD) of the decomposition of supercooled austenite of C82D CrV steel (EN ISO 16120-2:2011) alloyed chromium and vanadium from the heating temperature 1040 °C was constructed. When constructing the TCD, the differential thermal analysis method was used, using a reference sample. The most effective intervals of air cooling rates have been established, which make it possible to form at least 90 % of sorbit in the rolled product structure, exclude the release of the excess phase (cementite secondary), and the appearance of structures formed by intermediate and shear mechanisms. The results of the research have been industrialized when developing the cooling mode for wire rod with a diameter of 8.0...12.0 mm from steel C82D C rV on the Stelmor line in the flow of a continuous fine-wire mill 320/150: the temperature of the rolling formation on the Stelmor line must be at least 1040 °C, further accelerated air cooling of the metal turns on the conveyor should be carried out at a rate of not less than 13 °C/sec and not more than 20 °C/sec to a temperature range of 570...540 °C followed by quasi-isothermal exposure under heat-insulating covers.

Characterization of an ASTM A335 P91 ferritic-martensitic steel after continuous cooling cycles at moderate rates

In this contribution some aspects of the behavior of the ASTM A335 P91 (9Cr1MoVNbN) steel subjected to continuous cooling cycles under fixed austenitization conditions are studied. Representative samples of the structures obtained after cooling at moderate rates (i.e. pure martensitic and mixed martensitic-ferritic) were analyzed in order to incorporate information to the Continuous Cooling Transformation (CCT) diagram of this material. The characterization was carried out by means of scanning and transmission electron microscopy (TEM), X-ray diffraction and Mössbauer spectroscopy. For the working conditions here employed, the results showed the existence of retained austenite within both, the pure martensitic and the mixed martensitic-ferritic domains of cooling rates, adding important information to the CCT diagram since retained austenite presence could be detrimental to the mechanical properties of the steel. Second phase precipitates, being relevant to fix the mechanical behavior of the material, were identified by means of TEM on carbon replicas. Mössbauer spectroscopy was very helpful to detect a low volume fraction of cementite-type phase in all of the samples, considering that it could not be conclusively determined by XRD due to orientation effects.

Heat Treatment of Sintered Valve Seat Inserts

The characterization of sintered valve seat inserts (VSIs) after being subjected to different heat treatment operations has been carried out. The VSIs were obtained from three different alloys by mixing iron powder with AISI M3:2, AISI M2 high-speed steels, and AISI D2 tool steel. After sintering, the VSI were quenched in air followed by double tempering at seven different temperatures. The cooling rate during air quenching was measured by means of a thermocouple type k attached to a data acquisition system. The characterization of the mechanical and physical properties of the VSIs was achieved by measuring relative density, apparent hardness and crush radial strength. The resulting microstructures for the sintered parts were interpreted using the isothermal and continuous cooling transformation diagrams for similar alloys. The VSI obtained with AISI M3:2 and AISI M2 high-speed steels after air quenching and double tempering at 600 oC showed the best results in terms of apparent hardness and crush radial strength.

In-situ investigation of phase transformation behaviors of 300M steel in continuous cooling process

Comprehensive understanding of continuous cooling transformation behaviors is important in heat treatment of steels, but there is still a lack of research in the phase transition behaviors of 300M steel, which would greatly hinder the microstructure controlling and finally affect the application of this material. In this work, in-situ observations by high temperature confocal laser scanning microscope (HTCLSM) was introduced by considering its superiorities in clarity, accuracy, and directness, to systemically investigate the microstructure evolutions of 300M steel under various cooling rates (0.01–100 °C/s). Pearlite, bainite, and martensite were observed to form at the cooling rate range of 0.01–0.15, 0.03–1, and 0.3–100 °C/s, respectively. A continuous cooling transformation diagram was constructed based on the in-situ observations, verified by metallography, and compared with dilatometry. Results showed that the transition temperatures by in-situ observations agreed well with the results of dilatometry, while the transition starting temperatures by in situ observation were lower and the phase transition termination temperatures were higher. Finally, models accurately describing the relationships between Vickers hardness, retained austenite content, and cooling rates were established, and the phase transformation mechanisms and kinetics were analyzed. Our finding not only understands fundamentally the transformation mechanisms of different microstructures, but also provides a useful reference for practical microstructure control in heat treatment of 300M steel.

Multiphase bainite - martensite steels: The significant impact of niobium microalloying on structure and mechanical behavior

We underscore here that microalloying of multiphase steels with niobium (Nb) introduces significant changes in phase transformation through modification of continuous cooling transformation diagram, refinement of phases and consequently mechanical properties, inclusive of crack propagation. Minor additions of niobium enlarged the bainite region, increased the martensite start temperature and refined the bainitic laths. This led to increase in the crack propagation threshold (ΔKth) of Nb-bearing steel to 13.68 MPa m1/2. In a similar manner, tensile strength and impact strength were enhanced. The Nb-induced microstructural changes were consistent with thermal simulations. The study demonstrates that microalloying with Nb is a viable approach to enhance the mechanical properties of multiphase high-strength steels, an aspect not widely explored in multiphase steels.

Simulation Kinetics of Austenitic Phase Transformation in Ti+Nb Stabilized IF and Microalloyed Steels

In the present study, the influence of cooling rates (low to ultrafast) on diffusion controlled and displacive transformation of Ti-Nb IF and microalloyed steels has been thoroughly investigated. Mechanisms of nucleation and formation of non-equiaxed ferrite morphologies (i.e., acicular ferrite and bainitic ferrite) have been analyzed in details. The continuous cooling transformation behavior has been studied in a thermomechanical simulator (Gleeble 3800) using the cooling rates of 1-150 °C/s. On the basis of the dilatometric analysis of each cooling rate, continuous cooling transformation (CCT) diagrams have been constructed for both the steels to correlate the microstructural features at each cooling rate in different critical zones. In the case of the IF steel, massive ferrite grains along with granular bainite structures have been developed at cooling rates > 120 °C/s. On the other hand, a mixture of lath bainitic and lath martensite structures has been formed at a cooling rate of ~ 80 °C/s in the microalloyed steel. A strong dependence of the cooling rates and C content on the microstructures and mechanical properties has been established. The steel samples that were fast cooled to a mixture of bainite ferrite and martensite showed a significant improvement of impact toughness and hardness (157 J, for IF steel and 174 J for microalloyed steel) as compared to that of the as-received specimens (133 J for IF steel and 116 J for microalloyed steel). Thus, it can be concluded that the hardness and impact toughness properties are correlated well with the microstructural constituents as indicated by the CCT diagram. Transformation mechanisms and kinetics of austenitic transformation to different phase morphologies at various cooling rates have been discussed in details to correlate microstructural evolution and mechanical properties.

Investigation of Hardness Change for Spot Welded Tailored Blank in Hot Stamping Using CCT and Deformation-CCT Diagrams

When an outer panel of a B-pillar is manufactured with the hot stamping process, reinforcements are spot welded on its inner side. Before reinforcements are added, the microstructure of the outer panel is martensite. However, reheating during spot welding changes the martensite to ferrite, which has a lower hardness in the heat-affected zone than in other areas. If spot welding is conducted before hot stamping for making a spot welded tailored blank, the microstructure in the spot welded tailored blank after hot stamping is martensite. This sequence of processes avoids hardness reduction due to spot welding. In this study, the hardness and microstructure around spot welded parts of the tailored blank were investigated. The results clearly showed that areas close to the spot welded parts are severely stretched during hot stamping. In addition, stretching suppresses the martensitic phase transformation and reduces the hardness. To characterize this phenomenon, a simulation was conducted that considered the effects of pre-strain on the phase transformation. A continuous cooling transformation (CCT) diagram and a deformation continuous cooling transformation (DCCT) diagram were made in order to quantify the effect of the cooling rate and pre-strain on the phase transformation and hardness. The hardness was then calculated using the experimentally measured CCT and DCCT diagrams and the finite element analysis results. The calculated hardness was compared with the experimental hardness. Good agreement was found between the calculated and experimental results.

Induction Hardening of External Gear

Problems and solution of gear induction hardening are described. Main attention is paid to the parameters of heating and cooling systems. ELTA 7.0 program has been used to obtain the required electrical parameters of inductor, power sources, resonant circuits, as well as to choose the quenching media. Comparison of experimental and calculated results of investigation is provided. In order to compare advantages and disadvantages of single- and dual-frequency heating processes, many variants of these technologies were simulated. The predicted structure and hardness of steel gears are obtained by use of the ELTA data base taken into account the Continuous Cooling Transformation diagrams.

Evolution of microstructure and microhardness of the weld simulated heat-affected zone of Ti-22Al-25Nb (at.%) alloy with continuous cooling rate

The influences of continuous cooling rate on microstructure and microhardness of the weld simulated heat-affected zone (HAZ) of Ti-22Al-25Nb (at.%) alloy were investigated in this study. The continuous cooling process of the weld HAZ of Ti-22Al-25Nb alloy was simulated using a Gleeble3500 thermo-mechanical simulator. The phase constituent investigated by X-ray diffraction showed that the weld simulated HAZs were all composed of lath-shaped O phase, equiaxed α2 particle and β/B2 matrix at different continuous cooling rates. Microstructural observations by scanning electron microscopy indicated that the size of O phase significantly decreased and the shape of α2 particle did not obviously change with the increase of continuous cooling rate. Quantitative analysis presented that with increasing continuous cooling rate the volume fraction of O phase decreased, while that of α2 and β/B2 phases increased. Phase transformation temperatures were determined by using dilatometric analysis and the weld simulated HAZ continuous cooling transformation diagram was determined. The microhardness of the weld simulated HAZ increased with the increase of continuous cooling rate.

Effect of Slag Composition on the Crystallization Kinetics of Synthetic CaO-SiO2-Al2O3-MgO Slags

The crystallization kinetics of CaO-SiO2-Al2O3-MgO (CSAM) slags was studied with the aid of single hot thermocouple technique (SHTT). Kinetic parameters such as the Avrami exponent (n), rate coefficient (K), and effective activation energy of crystallization (E A ) were obtained by kinetic analysis of data obtained from in situ observation of glassy to crystalline transformation and image analysis. Also, the dependence of nucleation and growth rates of crystalline phases were quantified as a function of time, temperature, and slag basicity. Together with the observations of crystallization front, they facilitated establishing the dominant mechanisms of crystallization. In an attempt to predict crystallization rate under non-isothermal conditions, a mathematical model was developed that employs the rate data of isothermal transformation. The model was validated by reproducing an experimental continuous cooling transformation diagram purely from isothermal data.

Experimental study of in-line heat treatment of 1.0577 structural steel

In-line heat treatment is frequently used in rolling mills because it offers a significant improvement of rolled product mechanical properties with costs benefits. This method allows achieving required mechanical properties without necessity of additional alloying and rolled product reheating. Disadvantage of in-line heat treatment is fixed rolling velocity which is typically strong parameter in controlling of final cooling regime. Water flow rate, pressure, type, size and position of nozzles, water temperature are examples of parameters influencing cooling intensity and the Leidenfrost temperature. Laboratory experimental study is needed to design well controllable cooling system which allows keeping required cooling regimes for various product steel grades and dimensions. This paper describes experimental stages of cooling system designing procedure for improving structural steel 1.0577 mechanical properties. First experimental part began with building of cooling intensities (heat transfer coefficients - HTC) database for tested several nozzles configurations. Then required cooling regime was selected according to the continuous cooling transformation diagram. The target was obtaining harder (quenched) material with good ratio between elongation and strength. The final equalization temperature was set to 600 °C in the whole body. Numerical simulations of cooling followed based on the knowledge of heat transfer coefficients from database. Appropriate nozzle configuration was chosen and numerical results were experimentally validated using modified Jomminy test. A hardness was improved significantly up to thickness of 12 mm (275 HV under sprayed surface decreasing to 180 HV in 12 mm). When the required material structure and hardness verified appropriateness of cooling regime by previous tests, the first design of cooling section was done. Full scale sample was heat treated on a new experimental stand (Karusel) which was developed by HeatLab. It enabled to simulate real cooling process in laboratory conditions. The sample was heated to rolling temperature and moved through the intensive spray (surface temperature drop to 300 °C) and then through the soft spray because of water savings. The cooling stopped in required time and the sample tempered to the target equalization temperature of 600 °C in the whole body. Finally, the hardness was measured, tensile and Charpy pendulum tests were done to confirm design of cooling unit. The hardness of the original material was constant along depth – 160 HV. It was improved by heat treatment – decreasing from the sprayed surface (265 HV) to the depth of 12 mm (175 HV). The minimal yield strength of the heat treated material increased from 355 MPa 400 MPa (maximal up to 750 MPa – closed to the sprayed surface). The sample was cooled to -20 °C for Charpy pendulum test. The pendulum energy of heat treated material rapidly increased from 50 J to 150 J.

THE RELATIONSHIP BETWEEN CONTINUOUS COOLING RATE AND MICROSTRUCTURE IN THE HEAT AFFECTED ZONE (HAZ) OF THE DISSIMILAR WELD BETWEEN CARBON STEEL AND AUSTENITIC STAINLESS STEEL

This paper investigates the changed in the microstructure in HAZ of the dissimilar weld between carbon steel and 304 austenitic stainless steel. Continuous cooling transformation diagrams (CCT), peak temperature profiles and cooling rates can be used to predict the change in the microstructure of the HAZ during the welding process. Optical microscopy, X – ray, SEM and TEM were used to determine the phases which formed in HAZ of carbon steel and austenitic stainless steel. The results of this study indicate that grain size in HAZ depended on the temperature at that point could be reached during the welding. Fully Martensitic layer observed at the interface in carbon side due to the combination of the rapid cooling subsequent to weld and local chemical composition. Cooling rate played the rule on forming Widmanstatten ferrite, Bainite and Pearlite. On the other hand, microstructures and grain size in HAZ of austenitic stainless steel were not affected by temperature and cooling rate. Carbides precipitation (M23C6, M7C3), however, were found in the boundary of grains.

Effects of Heating and Cooling Rates on Phase Transformations in 10 Wt Pct Ni Steel and Their Application to Gas Tungsten Arc Welding

10 wt pct Ni steel is a high-strength steel that possesses good ballistic resistance from the deformation induced transformation of austenite to martensite, known as the transformation-induced-plasticity effect. The effects of rapid heating and cooling rates associated with welding thermal cycles on the phase transformations and microstructures, specifically in the heat-affected zone, were determined using dilatometry, microhardness, and microstructural characterization. Heating rate experiments demonstrate that the Ac3 temperature is dependent on heating rate, varying from 1094 K (821 °C) at a heating rate of 1 °C/s to 1324 K (1051 °C) at a heating rate of 1830 °C/s. A continuous cooling transformation diagram produced for 10 wt pct Ni steel reveals that martensite will form over a wide range of cooling rates, which reflects a very high hardenability of this alloy. These results were applied to a single pass, autogenous, gas tungsten arc weld. The diffusion of nickel from regions of austenite to martensite during the welding thermal cycle manifests itself in a muddled, rod-like lath martensitic microstructure. The results of these studies show that the nickel enrichment of the austenite in 10 wt pct Ni steel plays a critical role in phase transformations during welding.

Effect of Dissolution and Precipitation of Nb on Phase Transformation, Microstructure, and Microhardness of Two High-Nb Pipeline Steels

Niobium (Nb) as an important microalloying element is widely applied in high strength pipeline steels. In this work, the continuous cooling transformation diagrams of two high-Nb steels with and without hot deformation were studied using a Gleeble 3500 thermal simulator. The amounts of dissolved Nb, undissolved Nb, and precipitated Nb were determined by inductively coupled plasma-atomic emission spectrometry. Results show that the increasing of Nb content in the high-Nb steels can restrain the prior austenite grain growth, dynamic, and/or static recrystallization; moreover, it can suppress polygonal ferrite transformation and promote acicular ferrite and bainite transformation, refining the microstructure and increasing the microhardness as a consequence. Nevertheless, the amplified Nb content in steels escalates trends of strain-induced Nb(C,N) precipitation. The increase in the amount of Nb(C,N) precipitates promote the polygonal ferrite and acicular ferrite transformation, while also decrease the microhardness. The results from this work show that the higher Nb content of 0.13% in the tested steel is unnecessary.

Continuous cooling transformation behavior and the kinetics of bainite formation in a bainitic–martensitic steel

Abstract The continuous cooling transformation diagram of a low carbon bainitic–martensitic steel was constructed using dilatometry and metallographic methods. It was found that as cooling rate increases, the structure changes from granular bainite to lath martensite. Three regions of different kinetic behavior were discerned for the bainitic–martensitic steel. One of the regions conformed to martensite formation and the other two comprised transformation to bainite. A non-isothermal type of Johnson–Mehl–Avrami–Kolmogorov kinetic equation of reaction rate was used to analyze the transformation behavior during continuous cooling of bainite formation. The Avrami exponent and activation energy values for different regions at cooling rates of 0.1 and 0.4 K s−1 varied from 1.5 to 4.7 and 71 to 84 kJ mol–1 respectively. Models obtained from such kinetic coefficients closely corresponded to experimental results.

Investigation of physically simulated weld HAZ and CCT diagram of HSLA armour steel

The phase transformation under various cooling rates and in different HAZ regions for high-strength armour steel was analysed by dilatometry. To develop a continuous cooling transformation (CCT) diagram, the samples were heated up to a peak temperature of 1250 °C to achieve a coarse-grained microstructure and then cooled down with a cooling time t 8/5 varying from 3 to 240 s. Analysis of dilatation curves revealed the austenite decomposition process, during which transformation temperatures were determined. The results showed martensitic transformations for all welding-relevant cooling times. Furthermore, to analyse different heat-affected subzones of the weld, the peak temperature was varied between 550 and 1250 °C at a constant cooling time t 8/5 of 6 s. The simulated coarse-grained heat-affected zone (CGHAZ) and fine-grained-heat affected zone (FGHAZ) showed only martensitic transformations with transformation temperatures below 400 °C. The steel exhibited an inhomogeneous hardness with hardening in the CGHAZ and FGHAZ and softening in the intercritical and subcritical HAZ. The physically simulated microstructure was validated by a real hybrid laser-arc weld microstructure.

Continuous cooling transformation behavior and impact toughness in heat-affected zone of Nb-containing fire-resistant steel

Simulated heat-affected zone continuous cooling transformation diagram was developed for advanced fireresistant steel. Over a wide range of cooling rates, corresponding to t8/5 from 6 s to 150 s, granular bainite was the dominant transformation constituent, while the morphology of less dominant martensite-austenite (M-A) constituent changed from film-like to block-type constituent; but the hardness remained similar to the average value of 190-205 HV (0.2). The start and finish transformation temperature was high at 700 °C and 500 °C, and is different from the conventional high strength low alloy steels. It is believed that the high-content (0.09 wt%) of Nb may promote bainite transformation at relatively high temperatures. Martenistic matrix was not observed at high cooling rate and the film-like M-A constituent and blocky M-A constituent with thin film of retained austenite and lath martensite were observed on slow cooling. Excellent impact toughness was obtained in the heat-affected zone with 15-75 kJ/cm welding heat input.

Simulated HAZ continuous cooling transformation diagram of a bogie steel of high-speed railway

Simulated HAZ continuous cooling transformation (SH-CCT) diagram presents the start and end points of phase transformation and the relationships of the microstructures of HAZ, temperature and cooling rates. It is often used to assess the weldability of materials. In this paper, a weathering steel Q345C which is widely used in the bogies manufacturing was studied. The cooling times from 800[Formula: see text]C to 500[Formula: see text]C ([Formula: see text]) were from 3 s to 6000 s, aiming to study the microstructures under different cooling rates. Different methods such as color metallography were used to obtain the metallography images. The results show that ferrite nucleates preferentially at the prior austenite grain boundaries and grows along the grain boundaries with a lath-like distribution when [Formula: see text] is 300 s. Austenite transforms into ferrite, pearlite and bainite with decreasing [Formula: see text]. Pearlite disappears completely when [Formula: see text] s. Martensite gradually appears when [Formula: see text] decreases to 30 s. The hardness increases with decreasing [Formula: see text]. The SH-CCT diagram indicates that the welding input and [Formula: see text] should be taken into consideration when welding. This work provides the relationships of welding parameters and microstructures.

NUMERICAL MODELING OF PHASE TRANSFORMATION DURING GRINDING PROCESS

The rapid heating and cooling in a grinding process may cause phase transformations. This will introduce thermal strains and plastic strains simultaneously in a workpiece with substantial residual stresses. The properties of the workpiece material will change when phase transformation occurs. The extent of such change depends on the temperature history experienced and the instantaneous thermal stresses developed. To carry out a reliable residual stress analysis, a comprehensive modelling technique and a sophisticated computational procedure that can accommodate the property change with the metallurgical change of material need to be developed. The objective of this work is to propose a simplified model to predict phase evolution during given temperature history for heating and cooling as encountered during grinding process. The numerical implementation of the proposed model is carried out through the developed FORTRAN subroutine called PHASE using the FEM commercial software Abaqus®/standard. Micro-structural constituents are defined as state variables. They are computed and updated inside the subroutine PHASE. The heating temperature is assumed to be uniform while the cooling characteristics in relation to phase transformations are obtained from the continuous cooling transformation (CCT) diagram of the given material (here AISI 52100 steel). Four metallurgical phases are assumed for the simulations: austenite, pearlite, bainite, and martensite. It was shown that at low cooling rates high percentage of pearlite phase is obtained when the material is heated and cooled to ambient temperature. Bainite is formed usually at medium cooling rates. Similarly at high cooling rates maximum content of martensite may be observed. It is also shown that the continuous cooling transformation kinetics may be described by plotting the transformation temperature, directly against the cooling rate as an alternative to the continuous cooling transformation diagram. The simulated results are also compared with experimental results of Wever [20] and Hunkle [21] and are found to be in a very good agreement. The model may be used for further thermo-mechanical analysis coupled with phase transformation during grinding process.