Carbon Content In Austenite Research Articles

Transformation character of ferrite formation by a ledge mechanism under a mixed-control model

A mixed-control model was developed to study the transformation character of ferrite formation by a ledge mechanism. A numerical two-dimensional diffusion-field model was combined to describe the evolution of the diffusion field ahead of the migrating austenite/ ferrite interface. The calculation results show that the bulk diffusion-controlled model leads to a deviation from experimental results under large solute supersaturation. In the mixed-control model, solute supersaturation and a parameter Z together determine the transformation character, which is quantified by the normalized concentration of carbon in austenite at the austenite/ferrite interface. By comparing with experimental data, the pre-exponential factor of interface mobility, M 0, is estimated within the range from 0.10 to 0.60 mol·m·J−1·s−1 for the alloys with 0.11wt%–0.49wt% C at 700–740°C. For a certain Fe-C alloy, the trend of the transformation character relies on the magnitude of M 0 as the transformation temperature decreases.

Phase field simulation of the carbon redistribution during the quenching and partitioning process in a low-carbon steel

Phase-field modelling is used to simulate the quenching and partitioning process in a low-carbon transformation-induced plasticity (TRIP) steel, in order to understand the carbon redistribution in the microstructure during the heat treatment. The simulations show that, depending on local characteristics of the microstructure, including phase distributions and carbon-concentration gradients, different features in the carbon evolution during the partitioning step occur that are physically and practically relevant, but are not accessible for experimental observation. The overall carbon partitioning from martensite to austenite occurs not only by direct diffusion from martensite to austenite, but also through the bulk ferrite grains. The simulations also show interface migration driven by the free-energy difference between austenite and martensite, which affects the fractions of phases and the dimensions of the austenite grains. The carbon content of individual austenite, martensite and ferrite grains as well as average values are analysed, showing that the carbon concentration within the austenite grains is strongly inhomogeneous at short partitioning times, which contributes to a variable mechanical stability of individual austenite grains, affecting the occurrence of TRIP.

Wear Mechanism of ADI

In this research wear mechanism of ADI under different intensity of loading with different hardness have been investigated. To study of wear behavior, a series of austempered specimens with optimum mechanical properties were used for wear tests. Dry sliding wear tests were carried out in pin-on-ring wear tester machine at speed of 0.5 ms-1 and loaded with normal loads of 100,200,300 and 400 N. Scanning electron microscopy for microstructure and wear surface analysis was used. To determine the austenite volume fraction and the percentage of carbon content in austenite, X-ray diffraction analysis was used. Results show that the role of retained austenite at wear properties of ADI is dependent on loading intensity and austenite carbon content.

Wear Mechanism of ADI

In this research wear mechanism of ADI under different intensity of loading with different hardness have been investigated. To study of wear behavior, a series of austempered specimens with optimum mechanical properties were used for wear tests. Dry sliding wear tests were carried out in pin-on-ring wear tester machine at speed of 0.5 ms-1 and loaded with normal loads of 100,200,300 and 400 N. Scanning electron microscopy for microstructure and wear surface analysis was used. To determine the austenite volume fraction and the percentage of carbon content in austenite, X-ray diffraction analysis was used. Results show that the role of retained austenite at wear properties of ADI is dependent on loading intensity and austenite carbon content.

Derivation and Application for Calculation of Carbon Content in Austenitizing of Cast Iron

In this paper, a formula for the calculation of carbon content during austenitizing of cast iron was deduced, considering the effect of silicon content. According to this formula, carbon content of austenite at a certain austenization temperature for a cast iron with given composition can be easily calculated, and the austenization temperature for getting the expected carbon content in the austenite can also be determined. Besides, according to the relationship between austenization temperature Tx and the according carbon content Cax, and considering the effect of silicon content, the carbon content of the austenite in the commonly used cast iron during heat treatment was calculated. The formula can be as a theoretical basis for determined austenization temperature and carbon content in austenite during heat treatment of cast iron, in particular, can play an important role in heat treatment of austempered ductile iron. Keywords: cast iron heat treatment; diffusion of carbon; carbon content in austenite

EBSD Characteristics of HSLA100 Steel Quenched in the 2-Phase Region

The grain and grain boundary characteristics of HSLA100 steel quenched in the 2-phase region were investigated by electron backscatter diffraction (EBSD).The results showed that the austenite amount was controlled by the 2-phase region heating temperature and the composition and morphology of transformation products were mostly depended on the carbon content in austenite. With the quenching temperature in 2-phase region increasing, the dislocation density in bainite decrease as a result of the block bainite coarsening and mergering, which makes the subgrains with much more low angle grain boundaries (2°~15°) decreasing and the massive ferrite with more high angle grain boundaries increasing gradually. Furthermore, the lath martensite or bainite with high angle grain boundaries and subgrains with low angle grain boundaries gradually increase owing to new austenite grain increasing and growing up. With the roles of above two interactions, some EBSD characteristics such as the packet size and the number fraction of high angle grain boundaries all have a peak present at 740°C

Finite-element simulation of quenching incorporating improved transformation kinetics in a plain medium-carbon steel

An experimental and numerical investigation was conducted to acquire more precise continuous cooling transformation kinetics for the quenching simulation of a plain medium-carbon steel by improving the conversion model of transformation strains to phase fractions. Previous conversion models have limited applications in plain medium-carbon steels with variable bainite start temperatures (B s) and martensite start temperatures (M s), which are cooling rate dependent and cannot be distinguished on dilatometric curves. Therefore, we propose new methods for determining variable B s and M s. More accurate diffusive transformation kinetics models were developed based on transformation kinetics data converted from continuous cooling dilatometric curves and considering variable B s. The martensitic transformation kinetics model was also improved using the quantitative relationship between the variable M s and the enriched carbon content in residual austenite during diffusive transformations. Finite-element simulation of quenching incorporating the improved transformation kinetics was performed to predict the temperature, microstructure, residual stress and distortion of a plain medium-carbon steel cylinder. The pearlite, bainite, and martensite fractions, axial and hoop stresses near the surface, and the degree of distortion calculated using the improved transformation kinetics exhibited markedly better agreement with the measured results than those calculated using the previous transformation kinetics with B s and M s held constant. Furthermore, a comparison of the two simulation results provided a detailed understanding of the effects of transformation kinetics on residual stress and distortion.

Quenching and partitioning treatment of a low-carbon martensitic stainless steel

► The amount of retained austenite was increased by Q&P treatment in 12Cr–0.1C steel. ► Ideal carbon concentrations in austenite and ferrite were calculated assuming CCE condition. ► The optimum partitioning treatment condition for 12Cr–0.1C steel was found. ► The strength–ductility balance of 12Cr–0.1C steel was improved by TRIP effect. Quenching and partitioning (Q&P) treatment was applied to a commercial low-carbon martensitic stainless steel, AISI Type 410 (Fe–12Cr–0.1C). The quench interruption temperature was optimized with consideration of the ideal carbon concentration in untransformed austenite after partitioning to lower the Ms temperature to room temperature. After partitioning at an appropriate temperature, a significant fraction of austenite was retained through the enrichment of carbon into the untransformed austenite. It was also suggested that the addition of silicon is not necessarily required for the Q&P treatment of 12Cr steel because of the retardation of carbide precipitation at the partitioning temperature owing to the large amount of chromium. Tensile testing revealed that the Q&P-treated material exhibited a significantly improved strength–ductility balance compared with conventional quench-and-tempered materials due to the transformation-induced plasticity (TRIP) effect by the retained austenite.

Effect of the carbide phase on the tribological properties of high-manganese antiferromagnetic austenitic steels alloyed with vanadium and molybdenum

Effect of special carbides (VC, M6C, Mo2C) on the wear resistance and friction coefficient of austenitic stable (Ms below −196°C) antiferromagnetic (TN = 40–60°C) steels 80G20F2, 80G20M2, and 80G20F2M2 has been studied. The structure and the effective strength (microhardness Hsurf, shear resistance τ) of the surface layer of these steels have been studied using optical and electron microscopy. It has been shown that the presence of coarse particles of primary special carbides in the steels 80G20F2, 80G20M2, and 80G20F2M2 quenched from 1150°C decreases the effective strength and the resistance to adhesive and abrasive wear of these materials. This is caused by the negative effect of carbide particles on the toughness of steels and by a decrease in the carbon content in austenite due to a partial binding of carbon into the above-mentioned carbides. The aging of quenched steels under conditions providing the maximum hardness (650°C for 10 h) exerts a substantial positive effect on the parameters of the effective strength (Hsurf, τ) of the surface layer and, correspondingly, on the resistance of steels to various types of wear (abrasive, adhesive, and caused by the boundary friction). The maximum positive effect of aging on the wear resistance is observed upon adhesive wear of the steels under consideration. Upon friction with enhanced sliding velocities (to 4 m/s) under conditions of intense (to 500–600°C) friction-induced heating, the 80G20F2, 80G20M2, and, especially, 80G20F2M2 steels subjected to quenching and aging substantially exceed the 110G13 (Hadfield) steel in their tribological properties. This is due to the presence in these steels of a favorable combination of high effective strength and friction heat resistance of the surface layer, which result from the presence of a large amount of special carbides in these steels and from a high degree of alloying of the matrix of these steels by vanadium and molybdenum. In the process of friction, there are formed nanocrystalline austenitic structures possessing high effective strength and wear resistance on the wear surface of these steels.

Lengthening Kinetics of Pro-Eutectoid Ferrite Ledge in Fe-C Alloy under Interface-Diffusion Mixed Control Mode

A mixed control mode is developed to model the ledge growth of pro-eutectoid ferrite, considering coupled effects of migration of austenite/ferrite interface and carbon diffusion in austenite. Carbon concentration of austenite at the austenite/ferrite interface increases from the bulk carbon concentration to a steady level, which is lower than that in local equilibrium, during the ferrite growth process. Correspondingly, ferrite grows rapidly at the beginning since all the driving force of ferrite transformation is dissipated on the interface migration. In the later stage of isothermal transformation, the growth rate of ferrite decreases towards a steady level since a part of driving force is dissipated on carbon diffusion in austenite. The effect of interface migration on ferrite growth rate by changing the interface mobility is emphatically discussed. In the case of the low interface mobility, the growth rate of ferrite is very small while the growth is dominated by the carbon diffusion ability in the case of large interface mobility. When a medium interface mobility is obtained, the growth rate of ferrite may reach a maximum value, which exceed the limitation of diffusion control and interface control modes. After comparing the modeled growth rate of ferrite with the experimental data of 0.11-0.49 wt% C alloy at 973-1113 K, the pre-expontential factor (M0) of interface mobility is estimated within the range of 0.1-1 mol m J-1 s-1, around the value 0.5 mol m J-1 s-1 theoretically estimated.

Heredity Characteristic From Hot Rolled Microstructure to Annealed Microstructure in High Strength TRIP Steels

Abstract The influence of hot rolled process on microstructure in TRIP (transformation induced plasticity) steel and the heredity characteristic from the hot rolled microstructure to annealed microstructure are investigated. The results show that there are two kinds of hot rolled microstructures at different coiling temperatures. One is composed of coarse grains of ferrite, pearlite and bainite, and the other is composed of small grains of ferrite, bainite and austenite. After annealing, the first kind of hot rolled microstructure is greatly refined, and volume fraction, and carbon content of austenite increase significantly. However, it has little changes in grain size, volume fraction and carbon content of austenite after the second kind of hot rolled sheet is annealed. There are also differences in distribution of retained austenite between the two annealed microstructures observed by EBSD and TEM technology. Retained austenite in the first annealed microstructure is distributed mainly on the inside of the polygonal ferrite in the form of spot, only little retained austenite is located on bainte ferrite boundary, however retained austenite in the second annealing microstructure is located in several places, such as inside of polygonal ferrite, polygonal ferrite boundary, and bainte ferrite boundary.

<i>In Situ</i> High Temperature X-Ray Studies on Bainitic Transformation of Austempered Silicon Alloyed Steels

The in-situ X-ray diffraction observations of the bainitic transformation of silicon alloyed steels were performed using the high temperature X-ray diffraction technique. The experimental results have shown that the volume fraction and carbon content of austenite remains a constant value which indicate that the transformation is almost finished after the early stages of austempering transformation. Asymmetry diffraction peaks are obtained for samples at the early stage of transformation due to a heterogeneous distribution of carbon in different regions of austenite and thus exists two types of austenite: low-carbon austenite (γLC) and the high-carbon austenite (γHC). The experimental results supports that the bainite growth is by a non-diffusive mechanism when austempering temperature is in the lower bainite transformation temperature.

Advanced Bainitic and Martensitic Steels with Carbide-Free Microstructures Containing Retained Austenite

Recent decades have witnessed some remarkable advances in engineering steels driven by the need to respond to challenges posed, for example, by recovery and transmission of oil and gas, or enhanced vehicle safety and fuel economy. Foremost amongst these must surely be the extended application of carbon steels, achieved principally through ferrite grain refinement by the practice of microalloying coupled with controlled thermomechanical processing. Limitations to strengthening ferrite/pearlite structures further by grain refinement or precipitation, however, has focused attention back to acicular forms of microstructure. One of the most interesting advances in this area has been the development of bainitic steels, which have been almost dormant since the mid-20th century. This resurgence may partly be attributed to a better appreciation of the bainite transformation mechanism, and the experimental work for this which unexpectedly spawned some interesting bainitic microstructures which have seen further development and application. These are the so-called ‘carbide-free’ bainites, which employ alloying to replace carbides, principally cementite, with carbon-stabilized retained austenite. Particularly noteworthy has been the emergence of the transformation induced plasticity (TRIP) sheet steels with enhanced properties principally targeted for automotive use. It is worth mentioning also that a parallel development has produced similar microstructure in austempered ductile irons (ADI), another important ferrous alloy which has seen recent expanding interest in its application. Even more recently, as we proceed into the 21st century, the concept of employing steel microstructures containing carbon-enriched retained austenite, has been developed further by combining both alloying and novel heat treatment procedures to exchange ‘bainitic’ ferrite with ‘martensitic’ ferrite. Interestingly, this non-equilibrium ‘quenching and partitioning’ process route also offers the possibility to increase the retained austenite carbon concentration to very high levels, potentially revealing new and previously unobtainable properties.

Structure–property correlation in austempered alloyed hypereutectic gray cast irons

The austempering behavior of a series of hypereutectic alloyed gray iron compositions with carbon equivalent from 4.37 to 5.14 was studied to understand the influence of microstructure on its mechanical and wear properties. The alloying elements in the alloys included Ni, Mo, Cr and inoculation by micro-constitution of Ti, Nb and Ce. The alloys were austempered at 360 °C and upper bainitic type feathery ferrite was observed in the matrix. While the graphite content determined by optical metallography varied between 16 and 24 vol%. The volume of austenite determined by XRD analysis showed values between 20 and 26%. The ferrite lath size was determined using XRD peak broadening. The tensile property varying between 188 and 270 MPa, showed no significant variation with volume percentage of carbon or austenite in the ausferrite. However the wear rate varying between 0.5 and 2.6 × 10 −7 g/Nm, showed a decreasing trend with graphite content attributed to the higher lubricating effect of released carbon during sliding wear. The specific wear rate of hypereutectic alloys, increased with increasing ferrite lath size due to enhanced softer ferrite phase on the sliding surface. The wear rate was found to increase with volume of austenite, austenite carbon content and austenite lattice parameter, which is attributed to increased stability of austenite against strain induced martensite formation and the increased formation of bainitic carbides in the second stage tempering. The various technical aspects in correlating the microstructure with the mechanical and wear properties of hypereutectic austempered gray iron are described.

Influence of nodule count on residual stresses and distortion in thin wall ductile iron plates of different matrices

Surface characteristics of thin wall ductile iron (TWDI) parts, such as residual stresses, distortion and dimensional changes produced during casting and heat treatment, are relevant variables when it comes to production processes design. This work focuses on the effect of nodule count on residual stresses (RS) and of TWDI plates distortion. As-cast, ferritised and austempered samples were employed. The role played by two typical austempering temperatures (280 and 360 °C) and three significantly different nodule counts (265, 1200 and 1700 nodules (nod)/mm 2) are discussed by establishing microstructural changes, i.e., microstructure fineness, retained austenite volume fraction ( V γ %), and austenite carbon content ( C γ %). Besides, residual stresses profiles below the surface and dimensional changes are determined. Results show that all samples display compressive RS on the surface and neighbouring layers. Samples with high nodule count austempered at 280 °C lead to higher compressive RS and distortion as well as to an increase in the RS field below the surface. As-cast and ferritised plates exhibit noticeably lower compressive RS values.

In situ x-ray observation of bainitic transformation of austempered silicon alloyed steel

The in situ x-ray diffraction observations of the bainitic transformation were conducted by using the high-temperature x-ray diffraction technique. The volume fraction and carbon content of austenite depend on the transformation temperature. The d{110} value of bainitic ferrite decreases with increasing austempering temperature, which is related to the decrease of carbon concentration in bainitic ferrite. Asymmetry diffraction peaks are obtained for samples at the early stage of transformation at any austempering temperatures. This asymmetry diffraction peak after the formation of bainitic ferrite could be attributed to a heterogeneous distribution of carbon in different regions of austenite and show that two types of austenite with different carbon contents, low-carbon austenite (γLC) and the high-carbon austenite (γHC), exist during the transformation. The microstructure after cooling down to room temperature is presented to show the effectiveness of the x-ray diffraction analysis.

Design of Composition in (Al/Si)-alloyed TRIP Steels

There is an increasing interest in the progressive substitution of Si by Al in TRIP steels in order to obtain alloys with excellent mechanical properties and improved coatability. In this paper, thermodynamic calculations have been carried out with the help of JMatProTM software in order to assess and compare the effects that Si and Al additions exert on the phase transformation, carbon enrichment and alloying element content of phases during continuous galvanizing of multiphase steels. These simulations have provided important implications regarding the optimal combination of Si and Al. It has been found that Al causes a more pronounced increase of A3 temperature and a wider extension of the intercritical range than Si. For a constant volume fraction of phases, the carbon content in austenite is similar for Al and Si-alloyed steels. However, ferrite in Al-alloyed is richer in carbon and consequently an increase in its strength could be expected. The hardenability of intercritically annealed austenite has been estimated for alloys with different combinations of Mn, Al and Si. Finally, simulated CCT diagrams predict for Al-alloyed steels a higher amount of new ferrite formed during cooling from intercritical annealing and the need of shorter isothermal holding times at 460°C. However, Si-TRIP steels would need faster cooling rates to prevent pearlite formation and longer isothermal holding times to complete the bainitic transformation and to obtain a microstructure with high retained austenite.

In-Situ High Temperature X-ray Studies of Austempering Transformation in High Silicon Cast Steel

The in-situ X-ray diffraction observations of the bainitic transformation of high silicon cast steel were performed using the high temperature X-ray diffraction technique. The volume fraction and carbon content of austenite depend on the transformation temperature. The experimental result has shown that the volume fraction of austenite ceases to a constant value which indicate that the transformation is almost finished after holding for about 1000 s. Asymmetry diffraction peaks are obtained for samples at the early stage of transformation due to a heterogeneous distribution of carbon in different regions of austenite and thus exists two types of austenite: low-carbon austenite (γLC) and the high-carbon austenite (γHC). The volume fraction of bulk austenite with low carbon decreases greatly at the early stage of transformation and then tends towards zero. The lattice parameter of both low-carbon and high-carbon austenite increases with the holding time due to the carbon partition from the supersaturated ferrite to the austenite. The experimental results supports that the bainite growth is by a diffusionless mechanism when austempering temperature is in the lower bainite transformation temperature range.

The effect of alloying elements on constrained carbon equilibrium due to a quench and partition process

Abstract The model proposed by Speer et al. to calculate the constrained carbon equilibrium is expanded to include any number of substitutional solutes and coded with the MatLab and Thermo-Calc programs. This model is used to evaluate the effect of Al, Cr, Cu, Mn, Mo, Ni and Si solutes on the final austenite carbon concentration of ternary Fe – X – C alloys. Comparison is also made with the carbon concentration measured experimentally in some Transformation Induced Plasticity steels.

Influence of isothermal bainitic processing on the mechanical properties and microstructure characterization of TRIP steel

The mechanical properties of transformation induced plasticity (TRIP) steel are strongly affected by the conditions of isothermal bainitic processing. The multiphase microstructure of TRIP steel under different conditions of isothermal bainitic processing was investigated using OM, SEM, XRD and TEM. The volume fraction of retained austenite and the carbon content in austenite were determined quantitatively using X-ray diffraction patterns. The relationship between mechanical properties and isothermal bainitic processing parameters was investigated. The stability of retained austenite was analyzed by the volume fraction of retained austenite and the carbon content in retained austenite. The experimental results show that the multiphase microstructure consists of ferrite, bainite and metastable retained austenite. To obtain good mechanical properties, the optimal conditions of isothermal bainitic temperature and holding time are 410-430°C and 180-240 s, respectively. After isothermal bainitic processing under the optimal conditions, the corresponding volume fraction of retained austenite is 5vol%-15vol%, which can provide enough retained austenite and plastic stability for austenite with high carbon content.