Carbon Enrichment In Austenite Research Articles

Stabilizing austenite in flash-annealed lean steel via laminate chemical heterogeneity

Retained austenite (RA) is especially valued in advanced high strength steels for its transformation-induced plasticity (TRIP) effect during deformation. However, the stabilization of RA in lean steels (C < 0.2, Mn < 2 wt%) is challenging due to the lack of austenite stabilization elements. In this study, a flash austenitization heat treatment approach was used to shorten the low-carbon TRIP steel processing time. This achieved a dual-phase microstructure composed of mass retained austenite and fine-grained ferrite in place of traditional bainite. Blocky, lamellar, and granular RAs were observed in the microstructure. Mn heterogeneity in the initial pearlitic cementite and ferrite was found to prevail in the retained austenite. Following the designed annealing route, phase-field simulations revealed that the ferrite transition kinetics accelerated in the Mn-depleted region which induced efficient carbon enrichment in austenite. The pronounced chemical heterogeneity of C and Mn co-resulted in the nonuniformity of the phase transition driving force from austenite to bainite and martensite, thus aiding the stabilization of austenite.

Carbon Enrichment of Austenite during Ferrite – bainite Transformation in Low-alloy-steel

The bainitic transformation kinetics and carbon enrichment of austenite during isothermal holding at 723–923 K were investigated for an Fe-0.1mass%C-0.5mass%Si-2.0mass%Mn alloy. The transformation progressed rapidly until approximately 50 s, after which transformation stasis was observed at 823 K. The carbon concentration of austenite increased as the transformation proceeded, and showed an almost constant value during stasis. It reached approximately 0.45–0.50% at 823 K, which corresponds to the carbon concentration at the T0’ composition with an additional strain energy of 100 J/mol associated with the transformation. After stasis, a slight increase in the ferrite or bainitic ferrite fraction was observed. The carbon concentration of austenite also increased and reached approximately 0.60%, clearly exceeding the carbon concentration at the T0 composition. These results imply that at the first stage, the bainite transformation occurs and shows the incomplete transformation, following which at the second stage, diffusional ferrite transformation proceeds. The additional strain energy associated with the transformation calculated from the carbon concentration at stasis due to the incomplete bainite transformation tends to decrease as the holding temperature increases. This indicates that strain relaxation due to the transformation occurred at higher holding temperatures.

Kinetics of Carbon Enrichment in Austenite during Partitioning Stage Studied via In-Situ Synchrotron XRD

The present study reveals the microstructural evolution and corresponding mechanisms occurring during different stages of quenching and partitioning (Q&P) conducted on 0.6C-1.5Si steel using in-situ High Energy X-Ray Diffraction (HEXRD) and high-resolution dilatometry methods. The results support that the symmetry of ferrite is not cubic when first formed since it is fully supersaturated with carbon at the early stages of partitioning. Moreover, by increasing partitioning temperature, the dominant carbon source for austenite enrichment changes from ongoing bainitic ferrite transformation during the partitioning stage to initial martensite formed in the quenching stage. At low partitioning temperatures, a bimodal distribution of low- and high-carbon austenite, 0.6 and 1.9 wt.% carbon, is detected. At higher temperatures, a better distribution of carbon occurs, approaching full homogenization. An initial martensite content of around 11.5 wt.% after partitioning at 280 °C via bainitic ferrite transformation results in higher carbon enrichment of austenite and increased retained austenite amount by approximately 4% in comparison with partitioning at 500 °C. In comparison with austempering heat treatment with no prior martensite, the presence of initial martensite in the Q&P microstructure accelerates the subsequent low-temperature bainitic transformation.

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.

Effect of austempering temperature on microstructure and tribological behaviour of hypoeutectic austempered ductile iron alloyed with copper

ABSTRACT In the present work, Cu alloyed ductile iron was isothermally treated at five different temperatures (280°C, 300°C, 320°C, 350°C, and 380°C) for 1 h to obtain an ausferrite microstructure composing a very fine ferrite phase and carbon enriched austenite. Microstructural characteristics exhibited a high ultimate tensile strength of 950–1600 MPa of ADI comparable to or even stronger than forged steels. The bulk hardness was 460–288 HV10, higher than pearlite, but lower than pure martensitic. The tribological behaviour of ADI was investigated using a 10 mm pin diameter during sliding. Its friction coefficient decreased almost by half as compared with ductile iron due to its ausferritic morphology and carbon enrichment. Under oxidative and adhesive conditions, an austempered microstructure with very fine ferrite and high carbon austenite is most wear-resistant due to its adequate strength, toughness, and lubricative behaviour. Wear resistance of ADIs can be improved through tailoring microstructure through austempering.

In-situ microstructural evolution during quenching and partitioning of a high-carbon steel by high-temperature X-Ray Diffraction

Carbon partitioning from martensite to austenite is essential for austenite stabilization during quenching and partitioning (Q&P), while a few competitive phenomena, such as bainitic transformation and carbide precipitation, alter the microstructural evolution. So, there is a need of using in-situ in combination with ex-situ characterisation techniques to understand the C partitioning at high temperature in relation to simultaneous competitive phenomena that might occur during the partitioning stage.In this study, microstructural evolutions of a medium carbon steel ( 0.6C–1.6Si–1.25Mn–1.75Cr wt%) during Q&P treatment were investigated by using an in-situ High-Temperature X-Ray Diffraction (HTXRD) equipment at three partitioning temperatures. Results confirmed that carbon enrichment of austenite at 280 and 400 ℃ originates from partial carbon depletion from martensite and bainitic transformation, while partitioning at 500 ℃ results in the complete depletion of carbon from initial martensite and ferrite formation. Short diffusion distance (~0.13 µm) of carbon at 280 ℃ caused a poor carbon homogenization of austenite and formation of 8 vol% fresh martensite after final quenching. High Si content of the steel stabilized transitional carbides and, concurrently, suppressed Fe3C formation during Q&P. The outcome of this study could contribute to the design of suitable chemistry and process parameters for producing quenched and partitioned steels.

Effect of Si on bainitic transformation kinetics in steels explained by carbon partitioning, carbide formation, dislocation densities, and thermodynamic conditions

The effect of Si addition on the evolution of bainitic transformation, carbon diffusion, carbide formation, and dislocation density in steel was investigated using in-situ high-energy X-ray diffraction (HEXRD). Alloys Fe-0.4C-1.7Mn (in wt%) with 1–4 wt% Si were austenitized at 1273 K and then isothermally heat treated at 573, 623, and 673 K. According to the HEXRD results, increasing Si content reduces the bainitic transformation kinetics and causes the incompleteness of the bainitic transformation to occur at lower bainite volume fraction. This is because i) Si retards carbide formation, impeding the eutectoid bainitic transformation, and leads to the accumulation of carbon at the migrating interface; ii) Si leads to higher strain energy and more dislocations in the austenite that also hinders the migration of the interface. Carbide formation was observed to occur prior to the incomplete transformation stage. During further isothermal holding, the decrease in dislocation density due to dislocation annihilation had little effect on carbide formation or carbon diffusion. Finally, the Si content has a minor effect on the calculated T 0 , T 0 ’, and WBs lines. The measured carbon content in carbon enriched austenite agrees well with WBs and T 0 but not with T 0 ’. • Bainitic transformation was systematically studied by in-situ HEXRD and microscopy. • Si affects transformation by solute hardening and retardation of carbide formation. • Carbide formation is observed prior to incomplete transformation stage. • Dislocation annihilation has insignificant effect on carbide formation. • The final carbon content in retained austenite agrees with T 0 and WBs predictions.

In Situ Synchrotron X-ray Diffraction and Microstructural Studies on Cold and Hot Stamping Combined with Quenching &amp; Partitioning Processing for Development of Third-Generation Advanced High Strength Steels

A novel combined process of Cold Stamping (CS) and Hot Stamping (HS) with Quenching and Partitioning (Q&amp;P) treatment applied to advanced TRIP-assisted steel has been conducted by thermomechanical simulation to evaluate the influence of CS or HS in the Q&amp;P processing. With this purpose, Q&amp;P, CSQ&amp;P, and HSQ&amp;P cycles were designed to obtain multiphase microstructures containing ferrite, martensite, bainitic-ferrite, and the maximum retained austenite (RA) fraction after the processes. The objective was to investigate the effects of the variables involving the heat treatments, such as the intercritical austenitization temperature, the isothermal and non-isothermal deformation, the amount of deformation, and the temperature and partitioning times, and to analyze their influence on the microstructural and mechanical responses. Time-resolved X-ray diffraction using synchrotron radiation was undertaken in a thermomechanical simulator coupled to the synchrotron light source to understand the influence of time, temperature, and strain on the level of carbon enrichment in austenite. In addition, the in situ austenite transformation kinetics and lattice parameter evolution were tracked, making it possible to optimize the RA fraction at room temperature after Q&amp;P processing. The newly developed combined process is promising as the transformation-induced plasticity phenomenon during deformation can contribute to the formability and energy absorption. The results also indicate that the deformation of austenite promotes the ferrite transformation while suppressing the bainite transformation. It was possible to plot the results in an elongation-mechanical strength diagram, coupled to material property charts, also known as, ‘banana curve’, allowing us to identify and correlate the thermal or thermomechanical treatment conditions that led to an increase in ductility or strength according to the volume fractions of the resulting phases. Comparing the results for the HSQ&amp;P treatments, it was observed that isothermal strains at higher temperatures (≥800 °C) are more advantageous to increase mechanical strength, while non-isothermal strains (starting at 750 °C) are suggested if the objective is the increase in ductility, with mechanical strength being slightly sacrificed.

Study on the Austemperability of Thin-wall Ductile Cast Iron Produced by High-Pressure Die-casting

The production of thin-wall ductile iron (TWDI) by high-pressure die-casting (HPDC) is complex because of several metallurgical and microstructural challenges. The present work aims to evaluate the austemperability of components (4 mm thickness) produced by HPDC process. The graphitization kinetics, the pearlite formation during continuous cooling, and the effect of austempering on the evolution of the ausferritic microstructure were investigated using dilatometric tests, microstructural analysis as well as Vickers hardness tests and tensile tests. Results show that components exhibit a brittle behavior because of white structures, small shrinkage cavities, and microporosity in the as-cast condition. Graphitization at 1100 °C allows rapid formation of small graphite particles within a short time (40 s). The critical cooling time (t8/5) to avoid the formation of pearlite upon cooling was found to be 5 s at a martensite start temperature of 193 ± 14 °C. Austempering at 360 °C for 40 min results in an ausferritic microstructure with stable carbon-enriched austenite which provides a high hardness (355 ± 4 HV10) and tensile strength (Rm = 709 ± 65 MPa). The results represent main criteria regarding the producibility of die-casted TWDI, which are helpful for future alloy and heat treatment design.

Evolution of microstructure during the "quenching and partitioning (Q&P)" treatment

Most studies on the Q&P process focus on enhancing the resultant amount of retained austenite. The quenching temperature (QT), which maximizes the amount of retained austenite is known as optimum quenching temperature (QTopt) and is the primary design parameter of the Q&P process. The results show that the existing models do not give a close estimate of the experimental QTopt and the corresponding amount of retained austenite. The detailed microstructural and dilatometry analysis showed bainite transformation, incomplete carbon partitioning from martensite and carbide precipitation during the partitioning treatment as well as segregation of the carbon atoms around dislocations in martensite, which were ignored in these models. In addition, existing empirical equations do not predict the progress of martensite transformation accurately. These factors were considered in this work to improve the existing models. The martensite transformation kinetics was deduced from the experimental dilatation curve and was fitted to existing empirical equations to get the model constants. The amount of bainite was calculated using lever rule after considering the carbon enrichment of austenite due to martensite decarburization during the partitioning treatment. The carbon concentration of bainitic-ferrite was taken based on the carbon content of this parent austenite from which bainite forms and the corresponding Zener ordering temperature with respect to the partitioning temperature. The total amount of carbon trapped at dislocations and defects was estimated using various empirical relations. The revised model, after considering these factors, gives a better prediction of QTopt and the corresponding amount of retained austenite.

Color Light Metallography Versus Electron Microscopy for Detecting and Estimating Various Phases in a High-Strength Multiphase Steel

In this study, fresh attempts have been made to identify and estimate the phase constituents of a high-silicon, medium carbon multiphase steel (DIN 1.5025 grade) subjected to austenitization at 900 °C for 5 min, followed by quenching and low-temperature bainitizing (Q&amp;B) at 350 °C for 200 s. Several techniques were employed using different chemical etching reagents either individually (single-step) or in combination of two or more etchants in succession (multiple-step) for conducting color metallography. The results showed that the complex multiphase microstructures comprising a fine mixture of bainite, martensite and retained austenite phase constituents were selectivity stained/tinted with good contrasting resolution, as observed via conventional light optical microscopy observations. While the carbon-enriched martensite-retained austenite (M/RA) islands were revealed as cream-colored areas by using a double-step etching technique comprising etching with 10% ammonium persulfate followed by etching with Marble’s reagent, the dark gray-colored bainite packets were easily distinguishable from the brown-colored martensite regions. However, the high-carbon martensite and retained austenite in M/RA islands could be differentiated only after resorting to a triple-step etching technique comprising etching in succession with 2% nital, 10% ammonium persulfate solution and then warm Marble’s reagent at 30 °C. This revealed orange-colored martensite in contrast to cream-colored retained austenite in M/RA constituents, besides the presence of brown-colored martensite laths in the dark gray-colored bainitic matrix. A quadruple-step technique involving successive etching with 2% nital, 10% ammonium persulfate solution, Marble’s reagent and finally Klemm’s Ι reagent at 40 °C revealed even better contrast in comparison to the triple-step etching technique, particularly in distinguishing the RA from martensite. Observations using advanced techniques like field emission scanning electron microscopy (FE-SEM) and electron back scatter diffraction (EBSD) failed to differentiate untempered, high-carbon martensite from retained austenite in the M/RA islands and martensite laths from bainitic matrix, respectively. Transmission electron microscopy (TEM) studies successfully distinguished the RA from high-carbon martensite, as noticed in M/RA islands. The volume fraction of retained austenite estimated by EBSD, XRD and a point counting method on color micrographs of quadruple-step etched samples showed good agreement.

Effect of Tempering Temperature on Microstructure and Uniformity of LP Steel Plate under HOP Process

Longitudinal profiled steel plate (LP steel plate) is a green steel product, which is widely used in steel structure fields such as ships and bridges. In this paper, the effect of Heat-treatment Online Process (HOP) on microstructure and uniformity of LP steel plate was investigated by thermal simulation test. The results show that the microstructure consists of grain boundary allotriomorphs, upper bainite, acicular ferrite and carbon-enriched austenite at 550°C. During the isothermal process, the austenite transforms into bainite, the acicular ferrite grows and coarsens, and the bainite laths gradually “fuse” to form large granular bainite. After online tempering at 650°C, the austenite transforms into ferrite, acicular ferrite and bainite undergo tempering transformation, in which the size of bainite decreases and some acicular ferrite transforms into polygonal ferrite. After tempering at different temperatures (650∼750°C), the microstructure consists of grain boundary allotriomorphs, coarsened acicular ferrite and a small amount of granular bainite. With the increase of tempering temperature, the tempering transformation degree of acicular ferrite and bainite is intensified. Elongated carbides at grain boundaries become short rods and particles, which decreases the size and aggregation of carbides. Compared with the traditional process, the difference of microstructure and hardness of LP steel plate with different thickness specifications after HOP process is reduced, and the hardness is higher, which is beneficial to improve the performance and uniformity.

Enhanced carbon enrichment in austenite through introducing pre-existing austenite as a ‘carbon container’ in 0.2C-2Mn steel: The significant impact on microstructure and mechanical properties

We describe here a novel approach of increasing the carbon in austenite via intercritical annealing and consequently derive the benefit of obtaining the desired microstructure and mechanical properties in a low carbon low manganese steel. The reversed austenite obtained by pre-annealing acted as a “carbon container”, and received carbon from cementite dissolved during pre-annealing. The reversed austenite was partly retained at room temperature and the retained austenite could further absorb carbon from martensite. As a result, the precipitation of cementite was significantly reduced during subsequent intercritical annealing and austenite was enriched with more carbon compared to the situation without pre-annealing. The morphology of reversed austenite changed from granular in the absence of pre-annealing to lath-like in pre-annealed steel. In addition, the size of retained austenite was refined by pre-annealing, which enhanced the stability of austenite and contributed to superior ductility of steel.

Influence of the cooling rate below Ms on the martensitic transformation in a low alloy medium-carbon steel

The influence of cooling rate below the martensite start temperature, Ms, on the kinetics of martensitic transformation in a medium carbon low alloy steel was determined using high resolution dilatometry, optical microscopy and X-ray diffraction techniques. A two-stage transformation was observed for slow cooling rates while at higher cooling rates, martensitic transformation occured through a single stage process. It is shown that the Koistinen–Marburger equation cannot adequately describe the observed two-stage transformation. A new equation is proposed in order to model the evolution of martensitic transformation by considering the influence of post Ms cooling rate. The method considers contributions from both, the initial austenite and the carbon enriched austenite. The underlying mechanisms are discussed and validated with experimental findings.

Competitive mechanisms occurring during quenching and partitioning of three silicon variants of 0.4 wt.% carbon steels

Quenching and partitioning (Q&P) treated steels are traditionally alloyed with silicon (Si), but its precise role on microstructural mechanisms occurring during partitioning is not thoroughly understood. In this study, dilatometric analysis has been combined with detailed microstructural characterization to unravel the competing mechanisms occurring during partitioning either in parallel or in succession. Three 0.4 wt.% carbon steels with varying Si contents were quenched to 150 °C for ~20% untransformed austenite, and partitioned for 10–1000 s in the temperature range 200–300 °C. The steel with low Si content (0.25 wt.%) exhibited substantial bainitic transformation during partitioning at 300 °C and only 4% retained austenite (RA) at room temperature (RT) even after 1000 s hold. In contrast, a high Si fraction (1.5 wt.%) enabled retention of ~18% austenite under similar conditions. While η-carbides precipitated within the martensite laths in the high-Si steel, cementite precipitated in the low-Si variant. Furthermore, carbide precipitation and growth were strongly suppressed by high Si content. Secondary martensite formation occurred from carbon-enriched austenite during final cooling, irrespective of Si-content. Results illustrate that Si retards austenite decomposition at higher partitioning temperatures but does not improve carbon partitioning at lower temperatures.

&lt;i&gt;In Situ&lt;/i&gt; Neutron Diffraction Measurement during Bainite Transformation and Accompanying Carbon Enrichment in Austenite at iMATERIA, J-PARC MLF

The authors have developed the in situ neutron diffraction technique focusing on bainite transformation during austempering. Thanks to the features of time-of-flight type neutron diffraction, textures, phase fractions and lattice parameters can be simultaneously measured at high temperature. In this paper, the design of experimental equipment and analytical approach are mainly described.

Effect of Silicon Content on the Decomposition of Austenite in 0.4C Steel during Quenching and Partitioning Treatment

Although quenched and partitioned (Q&amp;P) steels are traditionally alloyed with Si, its precise role on microstructural mechanisms occurring during the partitioning process is not thoroughly investigated. In this study, a systematic investigation has been carried out to reveal the influence of Si on austenite decomposition, phase transformation and carbide precipitation during Q&amp;P treatment. Using a Gleeble thermomechanical simulator, three medium carbon steels with varying Si contents (0.25, 0.70 and 1.5 wt.%) were hot-rolled, reaustenitized, quenched into the Ms -Mf range, retaining about 20% austenite at the quench-stop temperature (TQ), and held for 1000 seconds above TQ in the temperature range of 200-300°C in order to better understand the mechanisms operating during partitioning. Dilatometric measurements combined with microstructural characterization using SEM-EBSD, TEM, and XRD clearly revealed the occurrence of various mechanisms. The effect of partitioning temperature/time on the hardness of the Q&amp;P samples was correlated with the microstructural features. Steel containing low Si content (0.25%) was incapable of promoting carbon enrichment of austenite during partitioning, leading to its continuous decomposition into isothermal martensite and/or bainite without any detectable austenite retained even holding at 300°C. In comparison, 1.5% Si content promoted retention of about 19% austenite under similar Q&amp;P conditions. Small fractions of bainite and high-carbon martensite formed during final cooling in both steels after partitioning at 200°C. Moreover, carbide precipitation was strongly retarded by high Si content.

Carbon Enrichment of Austenite during Ferrite-bainite Transformation in Low-alloy-steel

Carbon Enrichment of Austenite during Ferrite-bainite Transformation in Low-alloy-steel

In Situ Investigation of the Bainitic Transformation from Deformed Austenite During Continuous Cooling in a Low Carbon Mn-Si-Cr-Mo Steel

The effects of hot deformation on the bainitic transformation of a low carbon steel during continuous cooling were comprehensively studied through in situ high-energy synchrotron X-ray diffraction, dilatometry, and ex situ microstructural characterizations. The obtained results indicated that the prior deformation of austenite at 950 °C accelerates the bainite formation at the early stages. During the ongoing of the transformation, both the overall kinetics of bainite and carbon enrichment of austenite are lower in deformed austenite. The bainitic microstructure developed from deformed austenite is more refined and presents the same retained austenite content at room temperature with slightly lower carbon content when compared with the undeformed sample. Besides, a significant higher dilatation strain was measured during the bainitic transformation in the deformed sample, which can be explained by the crystallographic texture in hot deformed austenite. The evolution of the peak broadening of the {220}γ and {211}α reflections during bainitic transformation are discussed in detail.

Quench Temperature-Dependent Phase Transformations During Nonisothermal Partitioning

An attempt has been made to estimate the amount and composition of different phases formed during the simulated quenching and nonisothermal partitioning (Q&P) process in a dilatometer by matching the experimental dilation data with empirically determined dilation curve. The result highlights the carbon enrichment of austenite, as well as its partial transformation to secondary martensite and/or bainite, during the partitioning step. Also, an increase in quench temperature (QT) led to enhanced bainite or secondary martensite formation, as a result of reduced carbon enrichment of remaining austenite. Further Q&P experiments on bulk samples were carried out to understand the dependence of carbon diffusion and subsequent microstructure evolution with QT. Although the change in experimentally obtained retained austenite (RA) content with QT corroborates with the existing model predictions, the maximum amount of RA was observed at QT lower than predicted. The half-thickness of RA films increased with increasing QT, which substantiates the theoretical prediction of the diffusion distance of carbon atoms in austenite.