Carbon In Ferrite Research Articles

Redistribution of carbon caused by butterfly defects in bearing steels

Butterfly defects initiate from inclusions in the subsurface of steel bearing components subject to rolling contact. The white etching matter (WEM) microstructure is a characteristic of butterflies and is related to the dissolution of carbides and thus generally believed to be enriched with carbon, in supersaturated solid solution, relative to the parent microstructure. Here, several butterflies are investigated using wavelength dispersive spectroscopy (WDS), soft x-ray emission spectroscopy (SXES) and electron microscopy (EM). Contrary to established thinking, in all cases investigated the butterfly-neighbouring WEM was found to be depleted in carbon, relative to parent material, by around 27% (measured in counts). Furthermore, the carbon level was shown to be lower than the matrix itself, suggesting that solute carbon is also expelled from the WEM during its formation, possibly due to the low level of solubility of carbon in ferrite. This was observed in both AISI 52100 and 18NiCrMo14-6 bearing steels. In spite of this depletion, nano-indentation found that WEM in both alloys was ~17% harder than the parent material. This may explain the strings of micro-voids observed near the WEM-parent interface, which appear to play a role in the growth of the butterfly cracks. It is suggested that the increased hardness of the WEM is mainly due to microstructural changes, rather than changes in solute carbon concentration.

Atomic-scale investigations of isothermally formed bainite microstructures in 51CrV4 spring steel

Atomic-scale investigation was performed on 51CrV4 steel, isothermally held at different temperatures within the bainitic temperature range. Transmission electron microscopy (TEM) analysis revealed three different morphologies: lower, upper, and inverse bainite. Atom Probe Tomography (APT) analysis of lower bainite revealed cementite particles, which showed no evidence of partitioning of substitutional elements; only carbon partitioned into cementite to the equilibrium value. Carbon in the bainitic ferrite was found to segregate at dislocations and to form Cottrell atmospheres. The concentration of carbon remaining in solution measured by APT was more than expected at the equilibrium. Upper bainite contained cementite as well. Chromium and manganese were found to redistribute at the cementite-austenite interface and the concentration of carbon in the ferritic matrix was found to be lower than the one measured in the case of lower bainite. After isothermal treatments close to the bainite start temperature, another austenite decomposition product was found at locations with high concentration of Mn and Cr, resembling inverse bainite. Site-specific APT analysis of the inverse bainite reveals significant partitioning of manganese and chromium at the carbides and at the ferrite/martensite interfaces, unlike what is found at isothermal transformation products at lower temperatures.

Solubility of Boron and Carbon in Ferrite of the Fe-B-C System Alloys

Investigation was carried out for Fe-B-C alloys with carbon content of 0.0001–0.01 % (wt.) and boron content of 0.0001–0.01 % (wt.), the rest is iron. To determine the structural state of alloys we use the microstructure analysis, X-ray microanalysis and X-ray structure analysis. The level of microstraines, dislocation density and the coercive force of ferrite is determined, and it is shown that structure imperfection grows with boron content increase in the alloy. The obtained results enable to suggest that boron atoms in a solid solution of α-iron occupy substitutional-interstitial positions depending on boron content. In the paper it is shown experimentally, that at room temperature solubility limit of boron and carbon in the ferrite is 0.00012 % (wt.) and 0.006 % (wt.). When boron and carbon content increases further, the following phases are formed: Fe2B, Fe3(CB) and Fe23(CB)6. In this paper by means of quasi-chemical method we obtain for the first time temperature dependence of the free energy for α-iron solid solution, as well as solubility limit of carbon and boron. Maximum mass fraction of carbon may be up to 0.016 % (wt.), and maximum boron mass fraction – up to 0.00025 % (wt.). At room temperature the boron solubility limit in ferrite is 0.0001 % (wt.), and carbon one is 0.004 % (wt.). The calculated numerical values of the solubility of boron and carbon in ferrite of the Fe-B-C system alloys are less than that of the experimental results. This can be explained by the fact that boron atoms interact more actively with structure imperfections than carbon atoms. At high temperatures the solubility of carbon and boron in given phase increases.

Enhancement of mechanical properties of low carbon dual phase steel via natural aging

The natural aging behavior of a low carbon dual phase (DP) steel was studied and the effect of martensite content was taken into account. The enhancement of strength and hardness by aging at room temperature was related to quench aging via the formation of fine precipitates in the ferrite grains. It was revealed that increasing the intercritical annealing temperature diminishes this precipitation hardening effect, which was related to the decreased degree of supersaturation of carbon in ferrite. The maximum quench aging effect was observed by annealing the ferritic-pearlitic steel at a subcritical temperature close to the eutectoid temperature, where the solute carbon content of ferrite reaches the maximum value. However, in contrast to aged DP steels, the tensile stress-strain curves showed yield-point phenomenon. The aging effect was rationalized by the Ashby-Orowan relationship and the effect of the silicon in the chemical composition of the studied steel was discussed.

Carbon diffusion in supersaturated ferrite: a comparison of mean-field and atomistic predictions

Hillert's mean-field elastic prediction of the diffusivity of carbon in ferrite is regularly used to explain the experimental observation of slow diffusion of carbon in supersaturated ferrite. With increasing carbon supersaturation, the appropriateness of assuming that many-body carbon interactions can be ignored needs to be re-examined. In this work, we have sought to evaluate the limits of such mean-field predictions for activation barrier prediction by comparing such models with molecular dynamics simulations. The results of this analysis show that even at extremely high levels of supersaturation (up to 8 at% C), mean-field elasticity models can be used with confidence when the effects of carbon concentration on the energy of carbon at octahedral and tetrahedral sites are considered. The reasons for this finding and its consequences are discussed.

Modeling decarburization kinetics of grain-oriented silicon steel

A mathematic model has been presented to predict the decarburization kinetics of grain-oriented silicon steel sheet in the gas mixture of N2-H2-H2O during annealing. This model is based on the carbon flux balance between the oxidation reaction at the surface and the carbon diffusion inside the steel sheet. It can be numerically solved to quantify the influences of annealing temperature and atmosphere on decarburization kinetics when the boundary conditions are properly determined. In case that a humid gas mixture is employed during annealing, the most influential process parameter is temperature rather than compositions of the gas mixture, because the diffusion of carbon in ferrite is the rate-limiting step. Therefore, a higher temperature is required for the efficient decarburization of the thicker silicon steel sheet using the industrial continuous annealing production line.

A method to study interface diffusion of arsenic into a Nb-Ti microalloyed low carbon steel

A novel diffusion couple method was used to investigate the interface diffusion of arsenic into a Nb-Ti microalloyed low carbon steel and its effects on phase transformation at the interface. It is discovered that the content of arsenic has great effect on grain growth and phase transformation at high temperature. When the arsenic content is no more than 1wt%, there is no obvious grain growth and no obvious ferrite transitional region formed at the diffusion interface. However, when the arsenic content is no less than 5wt%, the grain grows very rapidly. In addition, the arsenic-enriched ferrite transitional layer forms at the diffusion interface in the hot-rolling process, which results from a slower diffusion rate of arsenic atoms than that of carbon in ferrite.

Quantitative Analysis of Carbon in Ferrite in Dual-Phase Steels by Mechanical Loss Measurements

To establish the method for determining the amount of carbon in the ferrite phase in ferrite + martensite dual-phase low-alloy steels, mechanical loss measurements have been performed on a series of Fe–C alloys with varying constitution. The observed mechanical loss spectra of two-phase alloys turned out to be simple superposition of those of single phase alloys, of ferrite and of martensite. The concentrations of carbon in solution evaluated from the magnitude of the Snoek relaxation in the two-phase alloys agree well with those expected from the Fe–C phase diagram. It is thus possible to selectively analyse the carbon dissolved in the ferrite phase in the complex structure, at least in simple binary alloys.

Application of Advanced Experimental Techniques for the Microstructural Characterization of Nanobainitic Steels

A 0.79C-1.5Si-1.98Mn-0.98Cr-0.24Mo-1.06Al-1.58Co (wt%) steel was isothermally heat treated at 350°C bainitic transformation temperature for 1 day to form fully bainitic structure with nano-layers of bainitic ferrite and retained austenite, while a 0.26C-1.96Si-2Mn-0.31Mo (wt%) steel was subjected to a successive isothermal heat treatment at 700°C for 300 min followed by 350°C for 120 min to form a hybrid microstructure consisting of ductile ferrite and fine scale bainite. The dislocation density and morphology of bainitic ferrite, and retained austenite characteristics such as size, and volume fraction were studied using Transmission Electron Microscopy. It was found that bainitic ferrite has high dislocation density for both steels. The retained austenite characteristics and bainite morphology were affected by composition of steels. Atom Probe Tomography (APT) has the high spatial resolution required for accurate determination of the carbon content of the bainitic ferrite and retained austenite, the solute distribution between these phases and calculation of the local composition of fine clusters and particles that allows to provide detailed insight into the bainite transformation of the steels. The carbon content of bainitic ferrite in both steels was found to be higher compared to the para-equilibrium level of carbon in ferrite. APT also revealed the presence of fine C-rich clusters and Fe-C carbides in bainitic ferrite of both steels.

Kinetic study on the coarsening behaviour of equilibrium phases in Nb alloyed ferritic stainless steels at 700 °C

Nb alloyed ferritic stainless steel is an attractive material to be used in automobile exhaust systems. Recently, in some published experimental work it was reported that coarsening rate of Laves phase (Fe 2Nb) can be higher than NbC in Nb alloyed ferritic stainless steels during aging at 700 °C. This observation was attributed to the fact that NbC has a more coherent interface with ferrite than has Laves phase. We explore this conclusion and find that the real reason for the smaller coarsening rate of NbC is the incredibly low solubility of carbon in ferrite.

Application of SIMS nano-analysis to the development of new metallurgical solutions

One of the reasons for brittleness of Fe–Al–Mn–C alloys developed at ArcelorMittal is the content of carbon in ferrite. The carbon in solid solution is detrimental to ductility because the C atoms are assumed to reduce the mobility of the edge dislocations. This dislocation pinning produces a twinning mechanism and leads to fracture. In order to reduce the brittleness of these materials which is due to the reduction of the carbon in solid solution in the ferrite, experimental measurements of the low carbon level (below 0.05 wt%) was done using secondary ion mass spectrometry (SIMS). To this end, the carbon distribution has been investigated on a Fe–Al–Mn–C grade after different thermal treatments (water quench and slow cooling rate). This paper shows that the SIMS nano-analysis is a well-suited tool to analyse the carbon in solid solution in the ferrite. On the basis of these analyses, it is possible to define thermal treatment conditions necessary to improve the ductility of the material.

Carbon–carbon interactions in iron

Carbon atoms occupy interstitial sites in iron so that the configurations of any solid-solution at constant composition depend solely on the distribution of carbon atoms on the interstitial sub-lattice, this in turn being influenced by interactions between carbon atoms in close proximity. The carbon–carbon interaction energy, which influences the distribution of carbon atoms, is reviewed with a view to understanding the nature of the interaction and to highlight some recent developments in the subject. It appears that the C–C interaction energy for ferrite cannot be deduced from the thermodynamic data currently available, primarily because of the very low solubility of carbon in ferrite. On the other hand, there is ample evidence to support the view that the corresponding energy for austenite is consistent with a strong repulsion between near neighbour pairs of carbon atoms.

Critical analysis and determination of the solubility limit of carbon in ferrite

In spite of an extensive bibliography available, the solubility limit of carbon in ferrite is still uncertain (between 50 and 100 ppm at 600°C for example), and this is not sufficient for the improvement of steel processing. Therefore we have tried to establish as accurately as possible the solubility of carbon between 500 and 750°C in a low carbon aluminium killed steel using a thermoelectric power protocol, which enables to calculate the amount of free interstitials in a ferritic matrix from the amount of interstitials that segregate on dislocations after strain. The solubility limit has been determined at ± 2 ppm between 550 and 730°C, and described by the relation: C(%wt) = 6.63 exp(-11.8 kcal/mol/RT). At a time when metallurgical phenomena are more and more simulated, we believe that a similar procedure should be used for other experimental studies providing basic data for modelling.

Structure analysis of ferrite in deformed pearlitic steel by means of X-ray diffraction method with synchrotron radiation

Structure of ferrite in drawn pearlitic steel containing 0.9 mass% C was investigated with X-ray diffraction technique using synchrotron radiation. In the detailed analysis of diffraction peaks of ferrite during the drawing process, it was experimentally revealed that the transformation of ferrite from bcc to bct due to the supersaturation of carbon in ferrite of heavily drawn pearlitic steel.

Cementite decomposition in heavily drawn pearlite steel wire

Heavily drawn pearlitic wires have been investigated extensively over the years for its unusual strainhardening behavior as well as for the high strength with acceptable level of ductility. They are widely used for tire cord, springs, wire rope, and suspension bridge cable. They are typically produced by drawing wire of approximately eutectoid composition to an intermediate diameter, patenting to produce a fine pearlitic microstructure, and then cold drawing to strains between 1.5 and 5.0. Strength increases as a function of strains, and tensile strength exceeding 5 GPa was recently reported in a fine steel cord developed in a laboratory [1]. Previous transmission electron microscopy (TEM) observations have revealed a number of unique microstructural features associated with the wire drawing process [2-4]. The pearlitic lamellar distance decreases with drawing strain, and strengths are related with the lamellar distance with Hall-Petch like relationship [2]. Despite the limited ductility of monolithic cementite crystals, the cementite lamella in pearlitic wire co-deform with ferrite [5-7], but they appear to fragment into planar arrays of small particles. Recent high resolution electron microscopy study by Languillaume et al. [8] suggested that these fragmented cementite eventually dissolves into ferrite to reduce the surface energy associated with the nanoscale fragmented particles. Recent atom probe field ion microscopy (APFIM) [9-11] and three dimensional atom probe (3DAP) studies [12,13] indicate that substantial amount of carbon is dissolved in the ferritic phase in heavily deformed pearlite. Internal friction [14] and Mossbauer[15,16] experiments also suggest a substantial proportion of the cementite (from 20 to 50 volume percent) dissolves during deformation at room temperature. This phenomenon is interesting since cementite is stable at room temperature and the solubility of carbon in ferrite is quite low. In our previous APFIM and TEM studies [11,13], we reported the microstructures of pearlitic steel wires of maximum strain of 4.2 with tensile strength of 3,930 MPa. In this specimen, cementite was present as fragmented nanoscale particles, and evidence for partial dissolution of cementite was also observed. It is interesting to see how these microstructure change in a pearlitic steel wire with a higher strain. Hence, in the present study, we have characterized the microstructure of a pearlitic steel wire with a higher strain as much as 5.1 with tensile strength of 5,170 MPa.

Nature of hydrogen trapping sites in steels induced by plastic deformation

Thermal desorption spectroscopy (TDS) has been used to reveal the nature of defects acting as trapping sites of hydrogen. Hydrogen was charged to ferritic and eutectoid steels deformed to various degrees and then given annealing treatment. Desorption with a single peak appeared between room temperature and 600 K from ferritic steels. Under constant hydrogen charging conditions, the amount of desorption increased with strain. However, when the deformed samples were subjected to annealing at temperatures as low as 500 K, the increase of desorbed hydrogen no longer appeared. Vacancy clusters, which themselves annihilate in the course of TDS measurement, are the probable source of hydrogen desorption. When heavy deformation was given to ferritic steels, a two-step decrease of hydrogen desorption took place with increasing annealing temperature, corresponding to annihilation of vacancy clusters and decrease of dislocation density, respectively. The desorption with a single peak has two origins, one due to the annihilation of the trapping sites themselves and the other to desorption from stable sites. For heavily deformed eutectoid steel, an additional desorption peak centered at around 640 K appeared. The peak likely results from deformation-induced defects within the cementite phase or supersaturated carbon in ferrite. Two types of desorption, one due to the annihilation of trapping sites in the course of measurement and the other due to desorption from stable sites, should be discriminated. TDS using hydrogen as a tracer can be applied as a tool to investigate the various defects induced by plastic deformation.

The Role of Carbon in Enhancing Precipitation Strengthening of V-Microalloyed Steels

The present work has concentrated on the role of carbon in controlling the microstructure and strength of V-N structural steels containing 0.04-0.22%C. Carbon content has usually been considered not relevant to precipitation strengthening when the precipitation occurs in ferrite because of the very small carbon content in solution in ferrite at equilibrium. We demonstrate that the effective carbon for precipitation in ferrite is much greater than this during the period of phase transformation, which in turn has a great effect on precipitation strengthening. Such behaviour is explained on the basis that the activity of carbon in ferrite is abnormally high in the presence of under-cooled austenite and before cementite nucleation so that profuse nucleation of vanadium carbonitride is encouraged. This new mechanism for precipitation is particularly significant for medium carbon steels typically used for hot rolled bars and sections. The total carbon content of the steel also contributes to the yield strength by increasing the volume fraction of pearlite. It is shown that the contribution from pearlite may be stronger than generally recognised.

Strengthening Mechanisms in Vanadium Microalloyed Steels Intended for Long Products.

The present work has concentrated on the roles of vanadium, nitrogen and carbon in controlling the microstructures and strength of steels designed for hot rolled long products. Effects of cooling rate and additional microalloying with titanium have also been included. The degree of precipitation strengthening of ferrite at a given vanadium content depends on the available quantities of carbon and nitrogen. The nitrogen content of the ferrite is approximately the same as that of the austenite from which it forms, i.e. the total nitrogen content in steel. It was confirmed that nitrogen is a very reliable alloying element, increasing the yield strength of V-microalloyed steels by some 5 MPa for every 0.001% N, essentially independent of processing conditions. Carbon content, on the other hand, has usually been considered not relevant to precipitation strengthening when the precipitation occurs in ferrite because of the very small carbon content in solution in ferrite at equilibrium. We demonstrate that the effective carbon for precipitation in ferrite may be much greater than this during the period of phase transformation, which in turn has a great effect on precipitation strengthening. Such behaviour is explained on the basis that the activity of carbon in ferrite is abnormally high in the presence of under-cooled austenite and before cementite nucleation so that profuse nucleation of vanadium carbonitride is encouraged. This new mechanism for precipitation is particularly significant for medium carbon steels typically used for hot rolled bars and sections. The total carbon content of the steel also contributes to the yield strength by increasing the volume fraction of pearlite. It is shown that the contribution from pearlite is stronger than generally recognised.

Characterization of medium- and low-temperature carbides in a low-carbon steel by internal friction

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The influence of diffusion of carbon in ferrite as well as in austenite on a model of reaustenitization from ferrite/cementite mixtures in Fe-C steels

The kinetics of formation of austenite from ferrite and cementite mixtures has been modelled by assuming the local equilibrium at the planar phase interfaces. The exact solutions to the diffusion equations governing the volume diffusion of carbon in austenite and ferrite are presented. The concurrent motions of the two interfaces are calculated via solving a set of transcendental equations derived from the flux balance conditions. At low isothermal transformation temperatures, it is found that the time required for reaustenitization is slightly greater than the time previously calculated with no diffusion of carbon in ferrite.