Constrained Carbon Equilibrium Research Articles

Synergistic regulation of microstructure, properties, and abrasive wear mechanism of a medium carbon dual phase steel for plowshares

In this study, a new medium carbon dual phase (DP) steel for plowshares is prepared. The quenching and partitioning process of the medium carbon steel is designed and optimised using the constrained carbon equilibrium model. The optimised process parameters are quenching temperature of 880 °C, quenching bath temperature of 190 °C and partitioning temperature of 350 °C. The microstructure of the acicular martensite matrix with 13% retained austenite was prepared, so that the good mechanical properties of tensile strength of 2007 MPa, yield strength of 1689 MPa, elongation of 11% and impact energy of 12 J are obtained. The multi-scale characterisation and abrasive wear experiments are carried out to investigate its microstructural evolution and wear mechanism under simulated working conditions. The results show that the coordinated deformation of retained austenite and martensite, as well as the trip effect, endows it with ultra-high strength and good plasticity. The recurring process of formation and spalling of the hardened layer is the main wear pattern of this medium carbon DP steel. The synergism of twinning formation and (Ti, Mo)C precipitation enhances wear resistance of this medium carbon DP steel.

Microstructure development of quenching and partitioning-processed martensitic stainless steels with different manganese content

This work presents an investigation of the microstructure development during the application of the quenching and partitioning (Q&P) process to two stainless steels with different Mn content. The results are compared with calculations based on the constrained carbon equilibrium theory, paying special attention to the presence of reactions competing for the carbon available for partitioning and to the effect of alloying element segregation. Results show that chromium carbides must be considered when accounting for the carbon available for austenite stabilisation. Moreover, manganese/chromium segregation bands play an important role in the microstructure development, particularly in martensite formation, with important consequences in the microstructure development during the following processing steps.

Enhancing strength-ductility combination in a novel Cu–Ni bearing Q&P steel by tailoring the characteristics of fresh martensite

The present work aims to study the effect of quenching temperature on phase transformation, microstructural characteristics and mechanical properties of a novel Cu–Ni bearing Q&P steel based on combining the experiments and modeling. In this study, a new constrained carbon equilibrium (CCE) model was proposed by considering the effect of intercritical ferrite and multi-element synergistic partitioning. The optimum quenching temperature and corresponding retained austenite (RA) fraction can be reflected in the new CCE model, which is in good agreement with XRD results. Furthermore, the significant role of fresh martensite in the tensile strength and fracture behavior were elucidated. As the volume fraction of fresh martensite increases from 9.8% to 57.8%, the strengthening contribution to the tensile strength increases from 13.7% to 68.8%. Meanwhile, the fracture mechanism transforms from ductile into quasi-cleavage fracture due to an increased amount of fresh martensite and the absence of tempered martensite. The superior mechanical properties, ultimate tensile strength of 1152 MPa, yield strength of 797 MPa, total elongation of 25%, and the product of strength and elongation (PSE) up to 28.8 GPa% were obtained in QP-200, which is largely due to the combined effect of the better deformation ability of a considerable amount of tempered martensite and the continuous TRIP effect provided by multiscale RA with different morphologies.

Austenite carbon enrichment and decomposition during quenching and tempering of high silicon high carbon bearing steel

The addition of Si to steels is a well stablished method to delay cementite precipitation, allowing for carbon partitioning from martensite to retained austenite during tempering. It has been argued that carbon enrichment and stabilization of austenite leads to increased ductility and toughness. This has been the main motivation for the development of novel heat treatments, such as quenching and partitioning. High carbon steels can also benefit from improved ductility provided by the presence of stabilized retained austenite. However, the process of carbon partitioning is less understood due to the increased tendency for competitive carbide formation with increasing carbon content. The present work investigates the austenite carbon partitioning and austenite decomposition phenomena in a modified 1.82 wt.% Si hypereutectoid bearing steel during tempering. Dilatometry, in-situ and ex-situ synchrotron X-ray diffraction, 3D atom probe tomography, scanning electron microscopy, and hardness measurements were used. The results are discussed based on different equilibrium states between α' and carbides. It was found that carbon partitioning towards retained austenite occurs for several minutes without significant phase decomposition at temperatures lower than 300 °C. A transition temperature between prevalent austenite carbon enrichment and austenite decomposition occurs at 350 °C. Secondary cementite precipitation inside martensite, and at the α'/γ interfaces, is observed during tempering at temperatures above 400 °C. Results from constrained carbon equilibrium modeling with carbide presence indicate that homogeneously dispersed spheroidized primary cementite has little influence in the carbon partitioning phenomenon.

Combined effect of Cu partitioning and nano-size precipitates on improving strength-ductility balance of Cu bearing Q&P steel

Cu was intentionally added to the traditional Q&P steel for further improving comprehensive mechanical properties through Cu partitioning, nano-size precipitates and microstructural refinement. Substantial enrichment of C, Mn, Cu and Ni atoms in newly formed austenite during intercritical annealing effectively inhibited the formation of bainite during the partitioning stage. The final microstructure of Cu bearing Q&P steel (Cu-QP steel) was significantly refined because of the refined prior austenite grains. Moreover, the modified constrained carbon equilibrium (CCE) model considering intercritical ferrite, Cu partitioning and the refinement effect of grain size was newly developed to better predict retained austenite (RA) fractions at different annealing temperatures in Cu-QP steel and Cu free Q&P steel (base steel). Both experiments and modeling results indicated that Cu-QP steel exhibited stronger austenite retention ability. The discrepancy of solid solubility between α and γ phases resulted in 14–18 nm Cu-rich precipitates randomly dispersed at the grain boundaries and in the interior of tempered martensite and intercritical ferrite after partitioning holding. The outstanding mechanical properties with ultra-high yield strength of 706–962 MPa, tensile strength of 1065–1139 MPa, and total elongation of 22–26% were obtained after the intercritical Q&P process. More volume fraction of tempered martensite and/or fresh martensite was formed in Cu-QP steel as a substitute for softer bainite, which remarkably improved the tensile strength. High density of nano-size Cu-rich precipitates and the refined microstructure enabled an appreciable increase of yield strength, while larger-fraction of ultrafine RA with higher stability contributed to better ductility. • Multi-element synergistic partitioning inhibited the formation of bainite. • The partitioning process facilitated carbon partitioning and Cu-rich precipitation. • The modified CCE model was newly developed to better predict RA fractions. • Multi-scale RA with various morphologies was beneficial to stronger TRIP effect.

Effect of repeated laser scanning on the microstructure evolution of carbon-bearing martensitic steel manufactured by laser powder bed fusion: Quenching-partitioning drives carbon-stabilized austenite formation

Carbon-bearing martensitic steels have gained interest for their use in aerospace and defense applications due to their ultra-high strength and reasonable ductility. In addition, additive manufacturing (AM) is an effective manufacturing process for a high degree of control over geometry. Microstructure evolution during additive manufacturing should be importantly considered because it is closely related to the formation of defects, mechanical properties, and conditions of post-heat treatments. This study investigated the microstructure evolution of carbon-bearing martensitic steel during laser powder bed fusion (LPBF) to identify the effect of rapid solidification, solid-state transformation, and in-situ heat treatments on the formation of the as-built microstructure. As a result of investigation of a reconstructed prior austenite map by the Kurdjumov-Sachs orientation relationship, it was confirmed that the austenite is generated mainly at the columnar, martensite block, and lath boundaries. A Scheil-Gulliver model represented that carbon and molybdenum are segregated to the columnar boundary, and this could be the main reason why austenite showed high fraction at room temperature. The carbon concentration in the austenite is much higher than that of the martensite interior, which is significantly caused by carbon partitioning derived by repeated laser scanning. A constrained carbon equilibrium model was used to estimate the final fraction of austenite, and the results showed that the characteristics of the as-built microstructure can be reasonably estimated. The microstructural evolution mechanism introduced here is expected to allow successful LPBF of carbon-bearing martensitic steels.

Universality of quenching-partitioning-tempering local equilibrium model

• Microstructural amount and C γ in low-C and medium-C Q-P-T steels were quantitatively characterized. • The QPT-LE model is a novel tool for the design of Q&P/Q-P-T steels. • The universality of QPT-LE model was firstly verified. The design of metastable retained austenite (RA) and its stability have always played a vital role in developing advanced high strength steels (AHSSs). The quenching-partitioning-tempering local equilibrium (QPT-LE) thermo-kinetic model was established by us, with consideration of carbide precipitation and interface migration. As a tool for designing AHSSs, the above model can well predict the microstructural amount and the carbon content in RA ( C γ ) in a high-C quenching-partitioning-tempering (Q-P-T) steel. In this work, QPT-LE model predicates the microstructural amount and C γ in low-C and medium-C Q-P-T steels more accurately than constrained carbon equilibrium (CCE) model without consideration of carbide precipitation and interface migration, and more accurately than quenching and partitioning-local equilibrium (QP-LE) model without consideration of carbide precipitation, which verifies the universality of QPT-LE model. The advantages of QPT-LE model are also discussed.

On competing reactions and austenite stabilization: Advanced model for exact microstructural prediction in Q&P steels with elevated Mn-content

In the case of Quenching & Partitioning (Q&P) steels, striving for large retained austenite (RA) fractions, the exact selection of the process temperatures is required. In this context, the quenching temperature (TQ) is known as a crucial parameter for setting the appropriate microstructure. In recent years, high research effort has therefore been put in the development of models that forecast the microstructural evolution during Q&P processing. In particular, the constrained carbon equilibrium (CCE) methodology is extensively widespread, although it represents a simplified state and therefore exhibits large deviations compared to experimentally obtained results. Hence, the aim of this work was to develop a new model that considers the experimentally observed competing reactions occurring during Q&P processing, including the formation of bainitic ferrite (αB) and incomplete C-partitioning into the remaining austenite (γremain). Moreover, the mechanical stabilization of γremain due to the occurrence of compressive stresses was also taken into account. Furthermore, a relation between C-enrichment in γremain and RA stability against strain-induced martensitic transformation (SIMT) could be established. The findings were summarized in a model, which allows for the accurate prediction of the phase fractions and the mechanical stability of RA as a function of the applied TQ in case of Q&P steels with an example for lean medium Mn compositions. The basic principles of this model, taking into account the competing reactions and mechanical stabilization of RA, can be however applied to all RA containing advanced high strength steels (AHSS), in which RA is primarily stabilized by C-enrichment during heat-treating.

Kinetics of Carbon Partitioning of Q&P Steel: Considering the Morphology of Retained Austenite

The diffusion of carbon atoms from martensite to retained austenite (RA) is controlled by the carbon partitioning kinetics when the quenching and partitioning (Q&P) process is conducted. The RA is divided into film-like and blocky ones in morphology. This research aims to study the influence of the morphology of RA on the kinetics of carbon partitioning mainly by developing a numerical simulation. A one-step Q&P process was modeled at the partitioning temperature of 330–292 °C, with a partitioning time ranging from 10−6 to 5 × 103 s. The finite element method was employed to solve the carbon diffusion equation. A thermomechanical simulator Gleeble-3500 was used to conduct the corresponding Q&P heat treatment, and the RA was examined by X-ray diffraction. The results show that the film-like RA will be enriched in carbon within a short time at first, followed by a decrease in carbon concentration due to the massive absorption of carbon by blocky RA, leading the stable film-like RA to become unstable again. The end of the kinetics of carbon partitioning was the concentration determined by the constrained carbon equilibrium (CCE) model, provided that the CCE condition was employed in this study. It took quite a long time (thousands of seconds) to complete the carbon partitioning globally, which was influenced by the partitioning temperature.

Fracture toughness characteristics of thermo-mechanically rolled direct quenched and partitioned steels

The propensity of the retained austenite to transform to martensite under external strain is known to induce plasticity (TRIP) in steels. If the retained austenite has a high volume fraction, it generally has blocky morphology and increases the tensile ductility via TRIP effect but can potentially deteriorate the yield strength. The present work demonstrates improved balance of strength and fracture toughness in a Si-containing 0.2%C steel having a refined martensitic matrix and evenly distributed and finely divided retained austenite (RA). Such a microstructure is obtained by thermo-mechanical rolling followed by direct quenching and partitioning (TMR-DQP) of the steel that results in establishing a constrained carbon equilibrium stabilising the austenite to room temperature. Transmission electron microscopy was used to characterise the nano-twinned martensite and the interlath nanometer thick austenite. An improved combination of yield strength, YS (1171 MPa), ultimate tensile strength, UTS (1419 MPa) and elastic-plastic fracture toughness, KJC (154 MPa √m) was achieved as compared to YS 1240 MPa, UTS 1654 MPa and KJC 111 MPa √m, respectively, for the fully martensitic counterpart. X-ray diffraction combined with Rietveld analysis revealed a reduction in the retained austenite from 9% away from the crack, which undergoes little strain, to 4 vol% near the crack, under higher strain, demonstrating strain induced martensitic transformation. The constrained nature of the austenite-to-martensite transformation within the rigid surrounding martensite is believed to increase the energy required to drive the crack forward, which raises the fracture toughness. Importantly, the results show that the uniformly distributed and nanoscale retained austenite is effective in imparting transformation-induced- plasticity at relatively small strength penalty.

Revealing carbide precipitation effects and their mechanisms during quenching-partitioning-tempering of a high carbon steel: Experiments and Modeling

Quenching-Partitioning-Tempering (Q-P-T), as a modified process of quenching and partitioning (Q&P), is a promising process to treat ultra-high strength steels with a good balance of strength and ductility. The essences of the Q-P-T process are to stabilize metastable austenite via carbon partitioning from martensite into austenite during partitioning and to strengthen the martensite matrix via nano-precipitation of micro-alloyed carbides during tempering. The competitive reactions, e.g. carbon segregation to dislocations, transition carbide precipitation and austenite decomposition, could occur during partitioning/tempering, which are expected to play a substantial role in the microstructures of the Q-P-T steels. In this contribution, the complex microstructure evolution during the Q-P-T processing of an Fe-0.67C-1.48Mn-1.53Si-0.038Nb steel was systematically characterized by various techniques. A concise QPT-LE (Local Equilibrium) thermo-kinetic model with dual interfaces (martensite/carbide and martensite/austenite) migration was established to predict the evolution of austenite fraction and its carbon content based on the consideration of competitive reactions mentioned above. In the QPT-LE model the effect of carbide precipitation is introduced, which is different from CCE (constrained carbon equilibrium) thermodynamic model and QP-LE thermo-kinetic model. Therefore, the QPT-LE model can be used to reveal the effects of carbide precipitation on the retained austenite fraction and its carbon content, while the prediction accuracy of carbide fraction can be further improved by considering the effect of carbon segregation to dislocation. In general, the QPT-LE model can better predict the experimental results compared with popular CCE model and QP-LE model.

Balanced Constrained Carbon Equilibrium Accompanied by Carbide Precipitation

The final carbon content of austenite in equilibrium with tempered martensite can be estimated by the so-called constrained carbon equilibrium in the presence of carbide (CCEθ) model. However, the linear predictions under CCEθ deviate from both the initial and the experimentally measured carbon content. A modified approach to the CCEθ model is proposed, which predicts an increase of the carbon content in austenite with the decrease of temperature below the onset of martensitic transformation.

Prediction and evaluation of optimum quenching temperature and microstructure in a 1300 MPa ultra-high-strength Q&P steel

The quenching and partitioning steel is the representative of the third generation of advanced high-strength steel. The effect of quenching temperature on the microstructure and mechanical property of ferrite-containing quenching and partitioning steel was studied by intercritical annealing quenching and partitioning processes. When preparing a test steel with a tensile strength of 1300 MPa and total elongation of 19%, it is found that the actual optimum quenching temperature was lower than that calculated according to the constrained carbon equilibrium. The results indicate that the martensite start temperature of the austenite was overestimated when considering the diffusion of carbon only. Austenite grain size which is affected by low temperature and the existence of ferrite during intercritical annealing influenced the optimum quenching temperature. A scheme considering the diffusion of various alloying elements and austenite grain size was proposed and verified. Using this scheme, the optimum quenching temperature of intercritically annealed quenching and partitioning steel with complex microstructures was well predicted.

Modeling retained austenite in Q&P steels accounting for the bainitic transformation and correction of its mismatch on optimal conditions

Modeling retained austenite in quenching and partitioning (Q&P) steels remains a challenge, and the conventional ‘constrained carbon equilibrium’ (CCE) model fails to predict the optimal condition for achieving the maximal amount of retained austenite in various systems, which impedes the optimization of the Q&P process. One of the main limitations is that the possible decomposition of austenite to bainite during partitioning is completely ignored by the essential assumptions of the Q&P process and hence the associated CCE model. In this study, a CCET model that combines the conventional CCE model with the T0 model for the bainitic transformation and incorporates the effect of the isothermal bainitic transformation to describe the austenite stability during the Q&P process has been proposed. A detailed comparison between the experimental observation and model predictions demonstrated that the retained austenite could be better described by the CCET model, including its carbon content. The current model therefore provides a more accurate approach for tailoring the amount and stability of retained austenite after the Q&P processing of Fe-C-Mn-Si steels.

Isothermal Phase Transformations in a Low Carbon Steel During Single and Two-Step Partitioning

Single and two-step heat treatments were performed on hot-rolled 0.2C-2Mn-1.4Si-0.3Mo steel. The microstructure was characterized by means of optical and scanning electron microscopy and X-ray diffraction measurements. The results showed that the main microstructural components are tempered martensite, bainite, and retained austenite. The thermodynamic analysis showed that the constrained carbon equilibrium in the presence of cementite CCEθ model fits the experimental data for the isothermal heat treatment at a temperature slightly below the MS, despite the fraction of bainite in the microstructure. The existing models for predicting the equilibrium between martensite and austenite (i.e., constrained carbon equilibrium CCE and CCEθ) seems to overestimate the austenitic carbon content well below the MS. Equilibrium calculations between tempered martensite and austenite showed fair agreement with the experimental values for large martensite fractions. For small fractions, it is proposed that local equilibrium (LE) conditions during bainitic transformation govern the final stage of both single and two-phase heat treatments.

Response of retained austenite to quenching temperature in a novel low density Fe-Mn-Al-C steel processed by hot rolling-air cooling followed by non-isothermal partitioning

The quenching temperatures below Ms are difficult to uniformly control during large-scale processing of quenched and partitioned (Q&P) steels, which results in significant variation in tensile properties for the entire coil. The present study is aimed at obtaining a stable elongation for a wide range of quenching temperatures in hot rolled Q&P steels through microstructural design. Instead of cold rolling and annealing processes, a novel process involving hot rolling and air cooling followed by cooling in furnace was applied to a newly designed 0.25C-3Mn-2Al (wt.%) steel. The results indicated that a stable amount of retained austenite (RA) ∼ 19–23% was obtained during air-cooling to finish temperatures in the range of ∼280–360 °C, resulting in no loss in elongation at low quenching temperatures. The RA that was promoted by the transfer of carbon from ferrite/bainite reached up to 1.346 μm, whereas the RA embedded in martensitic laths was ∼40 nm. The two types of RA provided continuous TRIP effect during tensile deformation and the ultra-fine martensite lath of ∼100 nm ensured tensile strength greater than 1091 MPa. Finally, a stable total elongation of ∼19.2–20.5% in combination with tensile strength of ∼1091–1196 MPa was obtained over a wide quenching temperature range of 80 °C, which broadened the processing window for the production of hot rolled Q&P steels. In addition, the isothermal transformation during the initial partitioning stage was the underlying reason to obtain stable amount of RA. The CCE (Constrained Carbon Equilibrium) model combined with isothermal transformation was used to quantitatively calculate the volume fraction of RA.

Phase-field analysis of quenching and partitioning in a polycrystalline Fe-C system under constrained-carbon equilibrium condition

Mechanical properties of steels are significantly enhanced by retained austenite. Particularly, it has been shown that a recently developed heat-treatment technique called Quenching and Partitioning (Q&P) stabilises austenite effectively. In the present work, the phase-field approach is adopted to simulate the phase transformation and carbon diffusion, which respectively accompanies the quenching and partitioning process of the polycrystalline Fe-C system. By incorporating the chemical driving-force from the CALPHAD database, the elastic phase-field model, which recovers the sharp-interface solutions, simulates the martensite (α′) transformation at three different quenching temperatures. The resulting martensite volume-fractions are in complete agreement with the analytical predictions. For the first time, in this study, the constrained carbon equilibrium (CCE) condition is introduced in the polycrystalline set-up to yield the predicted partitioning endpoints. Under the CCE condition, the carbon partitioning in two alloys of varying composition is analysed through the phase-field model which employs chemical potential as the dynamic variable. The volume fraction and distribution of retained austenite is determined from the carbon distribution and its temporal evolution during the partitioning is investigated. It is identified that in the initial stages of partitioning carbon gets accumulated in the austenite (γ) along the γα′-interface, owing to the substantial difference in the diffusivities and CCE endpoints. This accumulation stabilises the austenite adjacent to the interface. However, depending on the martensite volume-fraction and the alloy composition, the evolution of the stabilised austenite varies. Furthermore, the influence of the phase distribution on the kinetics of the temporal evolution of retained austenite is elucidated.

Assessment of the predictive capabilities of commercial simulation software packages for the analysis of the C redistribution during Q&P processing

The C enrichment of austenite is of great importance to different kinds of transformation-induced plasticity-aided steels. Particularly, quenching and partitioning – a two-step heat treatment process, which involves the partial transformation to martensite and the subsequent C partitioning to austenite and its concomitant stabilization – has received considerable attention in recent years. The present work assesses the applicability of commercial software packages (DICTRA and MatCalc) to predict the interplay of C enrichment of austenite and concomitant C depletion of martensite. Results are additionally compared to calculations based on the “constrained carbon equilibrium” (CCE) and, moreover, to experiments conducted for two exemplary material conditions. DICTRA simulations extend the CCE calculation in terms of time dependence and allow to account for the possible displacement of the phase boundary, but lack the implications of the martensitic structure. Predictions based on MatCalc more accurately reflect experimental observations. Data gained allows for the evaluation of the predictive capabilities of commercial simulation packages corresponding to a prerequisite for the industrial implementation of novel heat treatment strategies and the required process control.

Alloying-element interactions with austenite/martensite interface during quenching and partitioning of a model Fe-C-Mn-Si alloy

The alloying elements partitioning from martensite to austenite during quenching and partitioning was analysed by coupling in situ High Energy X-Ray Diffraction experiments and 3D atom probe tomography. A rapid and significant carbon partitioning from martensite to austenite without any partitioning of both Mn and Si was highlighted while the interface was immobile. After a relatively short time at 400 °C, a clear Mn partitioning occurs at the vicinity of the α′/γ interface. The analysis conducted indicates that manganese equilibrates its chemical potential during partitioning and raises the issue of the Constrained Carbon Equilibrium model applicability throughout the partitioning process.

Application of the Thermodynamic Extremal Principle to Massive Transformations in Fe-C Alloys

The thermodynamic extremal principle was applied to propose a model in which trans-interface diffusion from the product phase to the interface, from the interface to the parent phase and interface migration are integrated for diffusion-controlled phase transformations in Fe-C alloys. Compared with the classical models with either a local-equilibrium condition or a constrained carbon equilibrium condition, the current model is able to predict massive transformations in the two-phase region. Application to isothermal phase transformations showed that the phase transformation mode is independent of (dependent on) trans-interface diffusion when the initial composition is close to the T0 line (close to the α/α + γ boundary). Ascribed to the large solute diffusivity of C, the thickness of the spike upon massive transformations could be much larger than the atomic spacing and the diffusion-controlled phase transformations could be faster than the interface-controlled phase transformations. Three stages, i.e., the diffusive transformation, massive transformation and the soft impingement stages, were predicted for phase transformations upon continuous cooling, according to which the critical limit between diffusive and massive transformations was determined to be within the two-phase region, being consistent with the experimental results in ultra-low-carbon Fe-C alloys. The current work could be very useful to control diffusion-controlled phase transformations and modulate the mechanical properties of steels.