Continuous Cooling Transformation Curve Research Articles

Effect of Mo and Cr on the Microstructure and Properties of Low-Alloy Wear-Resistant Steels.

Low-alloy wear-resistant steel often requires the addition of trace alloy elements to enhance its performance while also considering the cost-effectiveness of production. In order to comparatively analyze the strengthening mechanisms of Mo and Cr elements and further explore economically feasible production processes, we designed two types of low-alloy wear-resistant steels, based on C-Mn series wear-resistant steels, with individually added Mo and Cr elements, comparing and investigating the roles of the alloying elements Mo and Cr in low-alloy wear-resistant steels. Utilizing JMatPro software to calculate Continuous Cooling Transformation (CCT) curves, conducting thermal simulation quenching experiments using a Gleeble-3800 thermal simulator, and employing equipment such as a metallographic microscope, transmission electron microscope, and tensile testing machine, this study comparatively investigated the influence of Mo and Cr on the microstructural transformation and mechanical properties of low-alloy wear-resistant steels under different cooling rates. The results indicate that the addition of the Mo element in low-alloy wear-resistant steel can effectively suppress the transformation of ferrite and pearlite, reduce the martensitic transformation temperature, and lower the critical cooling rate for complete martensitic transformation, thereby promoting martensitic transformation. Adding Cr elements can reduce the austenite transformation zone, decrease the rate of austenite formation, and promote the occurrence of low-temperature phase transformation. Additionally, Mo has a better effect on improving the toughness of low-temperature impact, and Cr has a more significant improvement in strength and hardness. The critical cooling rates of C-Mn-Mo steel and C-Mn-Cr steel for complete martensitic transition are 13 °C/s and 24 °C/s, respectively. With the increase in the cooling rate, the martensitic tissues of the two experimental steels gradually refined, and the characteristics of the slats gradually appeared. In comparison, the C-Mn-Mo steel displays a higher dislocation density, accompanied by dislocation entanglement phenomena, and contains a small amount of residual austenite, while granular ε-carbides are clearly precipitated in the C-Mn-Cr steel. The C-Mn-Mo steel achieves its best performance at a cooling rate of 25 °C/s, whereas the C-Mn-Cr steel only needs to increase the cooling rate to 35 °C/s to attain a similar comprehensive performance to the C-Mn-Mo steel.

Effect of Cooling Process on Microstructure and Properties of Low Carbon Bainite Steel

This article used Mn-Mo-Cr-B low-carbon bainitic steel as the experimental material. The continuous cooling transformation curve of the steel during continuous cooling was determined using a Gleeble-1500D thermal simulation test machine, and a corresponding phase transformation model for bainitic steel during continuous cooling was established. The influence of different cooling rates and final cooling temperatures on the microstructure and mechanical properties of the steel was investigated. Employing metallography, SEM, and EBSD techniques, the microstructure, crystallographic orientation, and grain boundary angle distribution of the low-carbon bainitic steel were explored, and their relationship with the steel's strength and toughness was studied. The research findings reveal that varying cooling rates and final cooling temperatures impact the phase transformation process and microstructure of the steel, consequently affecting its mechanical properties indirectly. With increasing cooling rate, the diffusion and fineness of martensite increase, and the quantity of lath bainite grows while the laths become finer. Elevated final cooling temperatures lead to larger martensitic-austenitic (MA) islands and reduced lath bainite quantity, causing the laths to become wider. Through analysis of the substructure of bainitic steel, it was determined that the bainite organization in the tested steel comprises primary austenite grains, lath packet, and lath block in succession. Lath packets are composed of lath blocks with different orientations, where lath size predominantly controls strength. Finer lath size corresponds to higher strength, and the influence of substructure on toughness is comparatively minor.

Continuous Heating/Cooling Transformation Kinetics of a Novel CrNiMoV Steel for Large Structural Parts

Continuous heating transformation (CHT) and continuous cooling transformation (CCT) characteristics as well as phase transformation kinetics of a novel Cr–Ni–Mo–V steel for large structural parts are studied using a Gleeble‐3500 thermal–mechanical simulator. CCT curves are obtained. The results show that carbides in initial structure can be completely dissolved during CHT as the heating rates are not greater than 0.2 °C s−1. The final microstructure exhibits obvious sensitivity to cooling rate. It contains martensite with or without bainite. Besides, kinetic models of austenitization are established, and it is confirmed that diffusion is the major mechanism of austenitic transformation in steel during CHT. Meanwhile, bainitic phase transformations kinetic model is developed based on the J–M–A model. The fitted Avrami indexes n combined with transmission electron microscope reveal that the bainite nucleation sites of the steel are mainly located at the prior austenite grain boundaries, and the growth of bainite grains mainly follows a 2D mode. The kinetics of martensitic transformation is better modeled based on the K–M equation. A deep understanding of kinetics and microstructure characteristics during CHT and CCT is of great significance to regulate the final performance of the novel steel under development.

Study on induction hardening performance of 34CrNi3MoA steel crankshaft

The evolution of the temperature field, microstructure field, and residual stress field of a 34CrNi3MoA steel marine diesel engine crankshaft during medium-frequency induction hardening was studied based on an electromagnetic-thermal-transformation-stress coupled numerical model, which considers the effect of internal stress induced by transformation induced plasticity on residual stress. Using the equal conversion rate method, the austenitizing region of the crankshaft was determined during the induction heating stage. In the quenching stage, the parameters of the phase transformation model are derived from the continuous heating expansion curve and the continuous cooling transformation curve, and the phase transformation kinetics equation is used to analyze the phase transformation process of the crankshaft. The results indicate that extending the heating time can enhance the uniformity of the surface temperature of the crankshaft and the thickness of the hardened layer. The simulation results are validated by measurements of hardened layer, hardness and residual stress, and the simulation results are in good agreement with the experimental results.

Drawing fracture behavior and mechanism analysis of ER50-6G welding wire steel

In this paper, we investigated the drawing fracture behavior of ER50-6G welding wire steel and analyzed the factors affecting the fracture, such as continuous casting slag and inclusion, billet structure segregation, Ti inclusion, surface defect inheritance, metallographic structure type, fracture morphology, and so on. OLYMPUS GX53 optical microscope was used to observe and analyze the macrostructure of the cross-section and longitudinal section of the continuous casting billet and the original quality state of the wire rod surface. The continuous cooling transformation curve (CCT curve) of the steel was measured using a DIL805A thermal dilatometer and a Gleeble-3500 thermal simulation test machine. The microstructures of the non-fracture area and drawing fracture of wire rod samples were observed through a ZEISS-ULTRA scanning electron microscope. The results show that the austenite starting transformation temperature (Ac1) and the austenite finishing transformation temperature (Ac3) of ER50-6G welding wire steel are 732 °C and 865 °C, respectively. Under the actual cooling conditions of online production, the strip structure is composed of ferrite (F), pearlite (P), and a small amount of bainite (B). There is a small amount of slag and inclusion in the continuous casting billet; in addition, Al2O3 inclusions and TiC (or Ti(C,N)) inclusions can be observed in the wire rod. The scratch depth of the wire rod surface is 24∼62 μm and the width is 31∼50 μm. With the increase in drawing cumulative deformation, the Al2O3 and TiC (or Ti(C,N)) inclusions that are not easy to deform lead to drawing fracture and brittle fracture. In addition, a small amount of bainite in the wire rod microstructure causes the fracture of the wire with a fine diameter, whereas, the limited-depth scratches on the steel surface do not have an obvious effect on the drawing fracture.

Decomposition of γ-Fe in 0.4C–1.8Si-2.8Mn-0.5Al steel during a continuous cooling process: A comparative study using in-situ HT-LSCM, DSC and dilatometry

Continuous cooling transformation (CCT) diagrams represent roadmaps for producing all heat-treatable steels. CCT curves provide valuable information on the solid-state phase transformation sequence, depending on the defined cooling strategies, the alloying concept of the steel and previous processing steps. The experimental characterization of CCT diagrams is usually done on a laboratory scale applying thermal analysis of dilatometry. In current research studies, however, also other in-situ methods such as high-temperature laser scanning confocal microscopy (HT-LSCM) or differential scanning calorimetry (DSC) are frequently used to investigate phase transformations during thermal cycling. In the present study, HT-LSCM observations and DSC analysis are critically compared with dilatometry results by investigating the CCT diagram of a 0.4%C-1.8%Si-2.8%Mn-0.5%Al (in mass pct.) advanced steel grade. Furthermore, classical examinations by optical microscopy and hardness measurements were performed to support the analysis. In general, very good consistencies between all experimental techniques were identified in determining the transformation start temperature for pearlite, bainite and martensite. The optical microscopy confirmed the observed phase transformations and the results correlated with the measured hardness response. Based on the results, coupling of HT-LSCM and DSC is considered as a valuable novel approach to plot CCT diagrams in future research.

Research on 20CrMnTi gear steel continuous cooling transformation behavior

Mechanical properties are essential for gear steel, so as the microstructure. The gear steel’s continuous cooling transformation behavior has influence on both. The performance of 20CrMnTi gear steel has a lot of room for improvement, if the continuous cooling transition behavior is clear. By using the expansion method and microstructure observation, the transformation behavior of 20CrMnTi gear steel under various cooling rates has been investigated. Through these experiments, the 20CrMnTi gear steel continuous cooling transformation curves can be getted. Through this curve, the microstructure of 20CrMnTi gear steel under different conditions can be determined. The research results of 20CrMnTi gear steel are very helpful to the industrial production of gear steel.

Simulated Coarse Grain Heat‐Affected Zone Transformation Behavior of Newly Developed Zr–Ti–RE Deoxidized High‐Strength Low‐Alloy Steels with Extremely High Impact Toughness

The continuous cooling transformation behavior of the Zr–Ti–RE deoxidized high‐strength low‐alloy steel in the simulated coarse grain heat‐affected zone at different cooling rates (corresponding to different t8/5) is investigated by a Gleeble 3800 thermomechanical simulator in combination with microstructure and mechanical properties analysis. The continuous cooling transformation curve in the simulated heat‐affected zone is plotted, and the transformation law of M/A constituents is systematically investigated. It is found that with the increase of the cooling rate, the dominated microstructure is transformed from pearlite + ferrite into granular bainite and, finally, into lath bainite with the further refinement of M/A constituents. The steel exhibits excellent V‐notch Charpy impact properties at −20 °C under typical heat inputs (20, 100, 500 kJ cm−1). Moreover, acicular ferrite formed on spherical Zr–Ti–RE composite inclusions are observed at the heat input of 100 kJ cm−1. The low‐temperature impact absorbed energy gradually decreases with the increasing heat input due to the coarsening of the microstructure, larger effective grain size, and the reduction in the number of high‐angle variant pairs.

The Study of Phase Transformation Behaviors for 38MnB5Nb Ultra High-Strength Steel by CCT Curves and TTT Curves

To elucidate the phase transformation behaviors for 38MnB5Nb ultra high-strength steel, the continuous cooling transformation (CCT) and time-temperature-transformation (TTT) curves were determined by the thermal expansion method with different cooling rates and isothermal temperatures after complete austenization. To be more accurate, the microstructure was observed and the hardness was tested. The results showed that the starting and ending transformation temperatures of austenite during heating are 748 °C and 805 °C, respectively. Bainite’s start temperature is between 540 °C and 550 °C, while martensite’s start temperature is about 310 °C. The critical cooling transformation rate is between 10 °C/s and 15 °C/s. The results showed that the microstructures are severely related to the cooling rate during the continuous cooling process and are related to isothermal temperatures during the isothermal process. The relationship between hardness and the microstructure was investigated and hardness is severely related to the microstructure. Based on the results of CCT curves compared with the conventional 22MnB5 hot stamping steel, the studied 38MnB5Nb steel is more beneficial for selective cooling processes.

Investigation of the Thermodynamic Transformation and Rare Earth Microalloying of a Medium Carbon-Medium Alloy Steel for Large Ball Mill Liners.

The medium carbon-medium alloy steel was developed for the manufacture of large ball mill liners and sports equipment. In this study, the continuous cooling transformation curve of a novel type of medium carbon-medium alloy steel was measured with a thermal simulation machine; based on this curve, the hardening and tempering processes were optimized. The steel was then complex modified with alkaline earth and rare earth alloys. The mechanical properties of the treated steel were tested. The microstructure of the steel was analyzed by metallographic microscopy, X-ray diffraction, scanning electron microscopy and transmission electron microscopy, and the wear surface of the steel was analyzed by a three-dimensional morphometer. After high-temperature tempering, the microstructure transformed into tempered sorbite, which possesses good mechanical properties and can adapt to working conditions that require high strength and toughness. Rare earth or alkaline earth modification of the medium carbon-medium alloy steel promoted microstructural uniformity and grain refinement and improved the mechanical and anti-wear properties.

The Ferrite Layer on the Surface of Carbon Steel Obtained by Precisely Designed Cooling Rate Heat Treatment and Its Microstructure Analysis

This paper presents obtaining a single-phase ferrite layer with low content of carbon (the average thickness is about 156–246 µm) on the surface of 0.45% carbon steel by precisely designing the cooling rate during heat treatment, and its mechanical properties show a graded change in the cross-section. It may be achieved by preparing gradient/multilayer materials with more commonly utilized structures or a specific performance. Combining with phase identification by employing electron backscatter diffraction (EBSD) of the layer in this study is BCC ferritic phases. Based on the examination of Continuous Cooling Transformation (CCT) curves, the following conclusions are given. Under the cooling process with gradient temperature, a ferrite layer first forms on the outer lower temperature surface of the 0.45% carbon steel and subsequently develops by pushing the surplus carbon to the inner higher temperature austenite region. It is corroborated by the experimental findings of carbon contents dispersion acquired by electron probe microanalyzer (EPMA). Finally, the experimental findings of grain orientations and size distribution defined by electron backscatter diffraction (EBSD) are given as requirements for microscopic interpretation of the combination of excellent strength and bending capabilities of materials. Furthermore, the experimental findings of oxidation precisely specified the cooling rate during heat treatment of Cu coating samples, which are defined as criteria for identifying the production mechanism of the surface ferrite layer. It provides a theoretical explanation and direct experimental proof for creating the ferrite layer on the surface.

Physical simulation on Joining of 700 MC steel: A HAZ and CCT curve study

In the present work, coarse grain heat-affected zone (CGHAZ) was simulated by Gleeble 3800 thermo-mechanical simulator. A continuous cooling transformation (CCT) diagram was generated from the results of the dilatometer, hardness, and microstructure analysis. The heating rate of 100 °C s−1, the peak temperature of 1300 °C, the holding time of one second, and thirteen different cooling conditions representing the actual welding condition were chosen here for simulation where the cooling rate was controlled by t8/3. At slow cooling rate ferrite, cementite, pearlite, and bainite were obtained. At a medium cooling rate, ferrite, bainite, and a small amount of martensite were observed. At a fast-cooling rate i.e., 100 °C s−1 fully martensite was obtained. The obtained hardness values were 225 HV, 263 HV, and 342 HV for slower, medium, and fast cooling rates respectively. The increase in hardness value shows that the amount of non-diffusional phases increases with an increase in cooling rate. The CCT curve shows the range of cooling rate and phase transformation temperature of ferrite, bainite, and martensite.

The Effect of Stress on Ferrite Transformation During the Continuous Annealing of DP780 High-Strength Steel

Continuous cooling transformation curves for DP780 high-strength steel were drawn by combining measured thermal expansion curves, microstructures and hardness values. Annealing experiments for DP780 highstrength steel under three different stresses of 0 MPa, 10 MPa and 20 MPa were carried out using Gleeble 3500, and a transformation kinetics model of austenite to ferrite was established by considering the effect of stress at low cooling rates. Then, based on the finite element platform and secondary development, the continuous annealing process of DP780 high-strength steel under various stress states was simulated. The results show that the applied stress has significant effects on the ferrite transformation of DP780 high-strength steel. With increasing tensile stress, the transformation plasticity of ferrite becomes more obvious. The simulation results coincide with the measurement. It is thus proven that the numerical model can correctly reflect the deformation and transformation laws during the heat-treatment process and lay a foundation for further numerical simulations of the heat-treatment process of high-strength steel.

Phase Transformation of M2 High Speed Steel during Semi-Solid Cooling and Conventional Cooling

In this paper, the fundamental microstructure evolution of M2 high speed steel was investigated during semi-solid controlled cooling and conventional cooling, respectively. Semi-solid controlled cooling was conducted at 1260 °C with cooling rates from 0.1 to 10 °C/s, while conventional cooling was conducted at 1200 °C and 890 °C with different cooling rates. The continuous cooling transformation curves were plot according to the microstructure evolution. The results showed that microstructure transformation behavior of cooling structure in semi-solid temperature range was different from that of conventional process. For semi-solid specimen, the solid austenite dissolved more alloy elements, and the austenite stability was increased. The solid matrix was pearlite structure in the samples with cooling rate of 0.1 °C /s. When the cooling rate reached 1 °C/s, the granular pearlite disappeared and martensite lath was formed. The structure was relatively uniform, on which there were large carbide with regular shape. The solidified liquid phase showed a network shape surrounding the solid particles. The size of solid particles showed a decreasing trend with the increase of cooling rates. For conventional cooling process, the large eutectic M6C carbide and the small precipitated MC carbide could not be dissolved by austenitized at 890 °C. Increasing the austenitization temperature helped dissolving part of the carbides. The hardenability of M2 steel was high. The hardness has increased to a high level for both semi-solid and conventional specimens when cooling rate reached 1 °C/s. No obvious increase happened when cooling rate continued increasing.

Phase transition kinetics and microstructures of A508-3 steel under continuous cooling process

The phase transition kinetics of A508-3 steel during continuous cooling were studied by high temperature transition instrument which was based on thermodilatometry in this paper. The microstructures during continuous heating and cooling were also observed. The transition points were determined to be austenite formation temperatures, A c1 = 757 °C, A c3 = 823 °C, and martensitic transformation start and final temperatures, Ms = 382 °C, Mf = 222 °C at the cooling rate of 5 °C/s. The curves of Austenite transition fraction and time during continuous heating show a typical S shape. And the Austenite will show a maximum transition rate value at the middle temperature. The volume fraction of new phases translated from Austenite as functions of time and temperature during the cooling process can be obtained using the lever law. The pearlite transitions can occur in high temperature region at cooling rate smaller than 0.1 °C/s. And Bainite can be gotten at a lower temperature region. The S type curves can be used to describe the relationships between the transition volume fraction and time. Increasing the cooling rate enhances the volume fraction of Martensite at the cooling rate ≥5 °C/s. And only Martensite transition can be observed at the cooling rate > 10 °C/s. And Ms and Mf increase with increasing cooling rate. The Martensite transition kinetics equation with changing temperature were obtained. Finally, the continuous cooling transformation curves of A508-3 steel was constructed, which can be used to predict the microstructure and design the heat treatment technologies.

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

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

Formation mechanism of pearlite during thermal cycling in U75V steel rail repaired by laser directed energy deposition

Laser additive manufacturing is used to repair the damaged area of the rail, which is prone to martensite microstructure in the cladding layers, and it seriously affects the mechanical properties and threatens the safety of rail service. Therefore, in order to avoid the formation of martensite microstructure in the cladding layers, U75V steel rail cladding layer process experiment with different layers was carried out and the pearlite microstructure was successfully prepared by considering self-tempering characteristics. Moreover, combined with the finite element method to research the phase transformation behavior of U75V steel rail affected by thermal history, the mechanism of pearlite transformation in the process of laser directed energy deposition (LDED) repairing steel rail was studied. The results indicate that the cooling rate of the solid phase transformation stage is approximately 100–101 °C/s. Two indispensable conditions for pearlite formation in U75V steel repaired by LDED were revealed: the laser thermal accumulation must enable the cladding layers to maintain the valley temperature above the martensite start temperature and the cooling rate of the cladding layers in the solid phase transformation must be lower than the critical cooling rate of pearlite transformation. Based on this theory, pearlite microstructure can be obtained efficiently by means of preheating pretreatment and Co element doped alloying and optimizing the process parameters. Preheating pretreatment is beneficial to increase basic temperature, and Co-doped alloying can change continuous cooling transformation curve of U75V steel rail.

Phase Transformation of ER70-Ti for Gas Shielded Wire

ER70-Ti is a high strength gas shielded welding wire steel, which is suitable for ships, bridges and other structures, and can be used for thick plate welding with high current. In the welding wire industry, ER70-Ti is a high-tech deep-processing product with high added value. In this study, the thermal expansion experiment of ER70-Ti wire rod was carried out. The critical temperature of ER70-Ti phase transformation was measured and the continuous cooling transformation curve (CCT curve) of undercooled austenite was drawn. The microstructure and hardness of the samples under different cooling rates were observed. The results show that Ac1 temperature of ER70-Ti sample was 690 °C, Ac3 temperature was 877°C, and Bs temperature was 575°C. When the cooling rate was low (0.1°C/s~2.5°C/s), the phase transformation products of ER70-Ti were equiaxed polygonal ferrite and granular bainite. With the increase of cooling rate, the grain size of ER70-Ti sample was refined and the bainite content increased from 53% to 85%. When the cooling rate was higher than 5°C/s, all the phase transformation products were bainite. The Vickers microhardness also increased with the increase of cooling rate, from 185HV to 325HV.

Phase transformation testing and modeling for hot stamping of boron steel considering the effect of the prior austenite deformation

Hot stamping of boron steels is a thermal-mechanical and phase transformation coupled process. The phase transformation of boron steels during continuous cooling is a complex process since the phase transformation is tightly coupled with plastic deformation during the hot stamping process. In the present work, the effects of hot deformation amount, deformation temperature, and strain rate on phase transformation are investigated by using a comprehensive set of methods including in-situ isothermal tensile tests combined with dilatometry and ex-situ microstructure characterization. The dilatation curves, phase fraction evolution curves, continuous cooling transformation (CCT), and deformed continuous cooling transformation (DCCT) diagrams are constructed. Finally, based on the testing results, a new non-isothermal kinetic model of phase transformation are proposed to introduce the effect of the austenite deformation on transformation during the continuous cooling. The proposed model is validated by testing Vickers hardness of DCCT specimens and hot stamping W-channel parts. The results demonstrate that the deformation accelerates the diffusive phase transformation, causing the CCT curves to move in the direction of shorter incubation time. But the acceleration of diffusion transformation is rapidly saturated as the plastic strain increases. Lower deformation temperature and higher strain rate are more favorable to the diffusion phase transformation, and the higher the critical cooling rate of the martensite transformation is needed. The proposed kinetic model not only accurately predicts the final phase fraction and hardness of the DCCT specimen, but also the distribution of mechanical properties of the hot stamping W-channel parts.

Analysis on hot deformation and following cooling technology of 25Cr2Ni4MoVA steel

The true stress–true strain curves of 25Cr2Ni4MoVA steel were obtained by uniaxial compression experiments at 850–1200 °C in the strain rate range of 0.001–10.0 s−1. And the dynamic continuous cooling transformation curves were obtained at the cooling rate range of 0.5–15.0 °C s−1 from the austenitization temperature of 1000 °C to the room temperature by pre-strain of 0.2 as well. The power dissipation map and the dynamic continuous cooling transformation diagram were constructed based on the data provided by these curves. Compared with the optical micrographs of the compressed samples, the full dynamic recrystallization region is located between 1000 and 1200 °C and at the strain rate range from 0.01 to 10.0 s−1 with the power dissipation efficiency not less than 0.33. In the full dynamic recrystallization region, the power dissipation efficiency increases and the dynamic recrystallization activation energy decreases with the temperature increasing. With the strain rate decreasing, the power dissipation efficiency increases firstly and then starts to decrease as the strain rate is less than 0.1 s−1, and dynamic recrystallization activation energy changes on the contrary. According to the dynamic continuous cooling transformation diagram, slow cooling is a better way for the hot-deformed piece with large size or complex shape to avoid cracking as the temperature of the piece is lower than 400 °C, and different cooling ways can be used for the hot-deformed piece with small size and simple shapes to obtain certain microstructure and meet good compressive properties.