Cementite Precipitation Research Articles

ε-Carbide in alloy steels: First-principles assessment

There is now a large number of sophisticated steels which rely on silicon as an alloying addition with the purpose of avoiding the precipitation of cementite. However, there is also evidence that the silicon can enhance the formation of e-carbide; the mechanism of this effect is not understood and the absence of appropriate thermodynamic data makes it impossible to conduct calculations. We report here some ab initio calculations which throw light on both of these issues and suggest novel experiments.

Formation of Martensite Austenite Constituent in Continuously Cooled Nb-Bearing Low Carbon Steels

Fe-0.15%C-1.5%Mn-0.2%Si (Nb-free alloy) and Fe-0.15%C-1.5%Mn-0.2%Si-0.03%Nb (Nb-added alloy) were continuously cooled to room temperature at constant cooling rates in the range from 0.1 to 20K/s. At lower cooling rates, such as 0.1K/s, the Nb addition retards the ferrite transformation, resulting in a decrease in the transformation temperature and an increase in the volume fraction of bainite. The fraction of martensite-austenite constituent (MA) increases by the Nb addition and the largest fraction of MA, about 0.5 %, is observed in the Nb-added specimen cooled at 5K/s. In the specimens cooled at 5K/s, relatively coarse bainite without cementite precipitation is formed near the austenite () grain boundary in both alloys. Most of MA is localized between such relatively coarse bainitic ferrite (BF). On the other hand, MA is hardly observed in the bainite formed with cementite precipitation in  grain. Based on microstructure observation of the continuously cooled specimens down to intermediate temperatures followed by quenching, it is concluded that small-sized untransformed  near  grain boundary partly remains as MA whereas relatively larger untransformed  in the  grain decompose into bainite with cementite precipitation.

Effect of Quenching Rate on Hardness and Microstructure of Hot-Stamped Steel

The effect of cooling rate on hardness and microstructure of the hot-stamped boron steel with 0.2 mass% carbon was investigated. After sheet specimen with a thickness of 1.6 mm or 1.2 mm was heated up to 900°C for 4 min, it was press formed and simultaneously quench hardened with dies, or water-quenched. Simulated hot-stamping test was also carried out at various cooling rates. The Vickers hardness of quenched specimens was measured. The microstructure on the cross-section of quenched specimens was observed with optical microscope and transmission electron microscope. The microstrucure of hot-stamped specimen was composed of auto-tempered-martensite and was softer than water-quenched specimen which consisted of lath-martensite. Tempered martensite was distinguished from bainite by observation of cementite precipitation morphology. Cooling rate below the Ms point affects hardness significantly, even if cooling rate is higher than the upper critical cooling rate. Decrease in hardness caused by auto-tempering was formulated with the tempering parameter in which was taken account of integration of tempering effect.

Toughness deterioration in advanced high strength bainitic steels

Carbide free bainitic steels alloyed with manganese have achieved the highest strength and toughness combinations to date for bainitic steels. Ultimate tensile strengths ranging from 1600 to 1800 MPa were achieved while keeping a total elongation higher than 10%. Their toughness at room temperature matches tempered martensitic steels, known to be the best-in-class regarding this property. This improvement in toughness is achieved suppressing the precipitation of cementite during bainite formation by alloying the steel with about 1.5 wt% of silicon. However, it has been observed that strongly orientated martensite bands, associated to inhomogeneous manganese redistribution during solidification, lead to a remarkable deterioration in toughness in these advanced bainitic steels. The stress concentration associated with highly heterogeneous hardness distribution in the microstructure contributes to the premature crack nucleation.

Microstructural evolution of martensitic 100Cr6 bearing steel during tempering: From thermoelectric power measurements to the prediction of dimensional changes

Tempering of a martensitic 100Cr6 (AISI52100) bearing steel during isothermal treatments is studied using thermoelectric power, transmission electron microscopy (TEM), X-ray diffraction and dimensional analysis. Dimensional changes occurring during tempering are due to: (i) the precipitation of ε -carbides, (ii) the decomposition of retained austenite, (iii) the precipitation of cementite and (iv) the recovery of the dislocation structure and coarsening of martensite lathes. Analysis of thermoelectric power evolutions, measured during isothermal ageing treatments, and assisted by TEM characterization, leads to a quantitative estimation of austenite and precipitate volume fraction. These data are used to predict dimensional changes, and these are in very good agreement with dimensional measurements.

Microstructural and Dilatational Changes during Tempering and Tempering Kinetics in Martensitic Medium-Carbon Steel

The microstructural and linear strain changes of martensitic medium-carbon steel were investigated during continuous heating to 600 °C at different rates using dilatometry and transmission electron microscopy (TEM). The precipitation of transition e-carbides between 70 °C to 240 °C and the precipitation of cementite between 200 °C to 450 °C were observed depending on the heating rate. The measured strain changes during tempering stages 1 and 3 were converted to the fraction of tempered martensite based on the theoretical iron atomic volume change between martensite and tempered martensite. The presegregation amount of carbon before tempering was calculated to be about 0.16 wt pct by comparing the theoretical strain change with the measured strain change. Tempering kinetic models were developed using the tempered martensite fractions converted from the measured strain changes during tempering stages 1 and 3. The kinetics models exhibit a good correlation with the experimentally measured tempering kinetics.

In situ TOF neutron diffraction during phase transformation in an engineering steel

Phase transformations on heating and cooling or during thermo-mechanically controlled processing (TMCP) for a low alloyed steel were studied by means of in situ neutron diffraction. Cylindrical specimens were quenched after austenitization and hence the starting microstructure was martensite (bct). Tempering behavior accompanying cementite precipitation and ferrite (bcc) to austenite (fcc) reverse transformation were observed upon heating. During cooling, austenite to ferrite and pearlite transformations were observed in diffraction profiles, where nearly equilibrium microstructural changes were confirmed. In TMCP simulation measurements, plastic deformation of austentite at an elevated temperature was found to accelerate the ferrite transformation.

Substitutional solution of silicon in cementite: A first-principles study

Cementite precipitation from austenite in steels can be suppressed by alloying with silicon. There are, however, no validated thermodynamic data to enable phase equilibria to be estimated when silicon is present in cementite. The formation energies of Fe3C, (Fe11SiFe4c)C4 and (Fe11SiFe8d)C4 have therefore been estimated using first-principles calculations based on the total energy all-electron full-potential linearized augmented plane-wave method within the generalized gradient approximation to density functional theory. The ground state properties such as lattice constants and bulk moduli have also been calculated. The calculations show that (Fe11SiFe4c)C4 and (Fe11SiFe8d)C4 have about 52.06 kJ mol−1 and 37.17 kJ mol−1 greater formation energy, respectively, than Fe3C. The formation energy for hypothetical cementite Si3C has also been calculated to be about 256 kJ mol−1. Silicon substitution significantly reduces the magnetic moments at the Fe(4c) site for both (Fe11SiFe4c)C4 and (Fe11SiFe8d)C4, irrespective of the Si substitution sites. The calculated electronic structures indicate that the magnetic moment reduction at the Fe(4c) site by the Si substitution at 4c site is indirect through the neighboring carbon atom, whereas at the 8d site it is direct.

A thermodynamic model for carbon trapping in lattice defects

A thermodynamic model in the framework of the CALPHAD method is proposed to describe the distribution of interstitial solute (carbon) atoms in bcc-iron during tempering of martensite. In the model, the crystal defects are introduced as a separate sublattice, which is a highly favored lattice position for carbon compared to undisturbed interstitial sites or precipitate sites in cementite. With the model, which is convenient for multicomponent, multiphase systems, the fraction of carbon atoms trapped by the defects during tempering can be calculated. It is demonstrated that, when martensite is heavily dislocated, the precipitation of cementite on tempering is greatly suppressed because the high dislocation density provides a sufficient number of stressed sites which can trap most of the carbon atoms. The calculations for an alloy steel show that, unlike cementite, the defect density and carbon trapping phenomenon influence the precipitation of stable carbides only marginally.

Influence of silicon on cementite precipitation in steels

The mechanism by which relatively small concentrations of silicon influence the precipitation of cementite from carbon supersaturated austenite and ferrite are investigated. It is found that one condition for the retardation of cementite is that the latter must grow under para-equilibrium conditions, i.e. the silicon must be trapped in the cementite. However, this is not a sufficient condition in that it can only be effective in retarding the transformation rate if the overall driving force for the reaction is not large. It is demonstrated that the experience that silicon retards the tempering of martensite requires the presence of lattice defects which can reduce the amount of carbon available for precipitation and the associated driving force.

Metallurgical and Mechanical Properties of Fusion Zones of TRIP Steels in Laser Welding

Transformation induced plasticity (TRIP) steels are a promising solution for the production of cars with low body mass because of the combination of high strength and high plastic strain capacity that they offer. Si and Al are two important alternatives for alloying of TRIP steels in order to suppress carbide precipitation in the bainite holding temperature range during steel manufacture. Weldability of TRIP steel is one of the key factors governing its application in auto industry. In this paper, Al-alloyed TRIP steel was investigated with the diode laser welding process in terms of fusion zone metallurgical and mechanical properties, with Si-alloyed TRIP steel also included for comparison. It was found that the fusion zone of the Al-alloyed steel has a multiphase microstructure, containing skeletal ferrite, bainitic ferrite, martensite and retained austenite of two different morphologies. In contrast, the Si-alloyed steel fusion zone consists almost entirely of martensite. The high martensite content results in low fusion zone ductility in the Si-alloyed steel, only providing half the tensile elongation of the Al-alloyed steel. The Si-alloyed steel shows a greater decrease of the strength‐ductility balance (ultimate tensile strength times elongation) due to welding, i.e., 62.9% compared to 45.2% for the Al-alloyed steel in quasi-static tensile testing. High strain rate tensile testing with a Hopkinson Bar apparatus shows no significant effect of strain rate on the fusion zone ductility for either steel. The fusion zone of the Al-alloyed steel does not exhibit a detectable TRIP effect probably due to the low carbon content in the retained austenite. Al and Si are both relevant as agents to suppress cementite precipitation, but they are found to exert very different influences on steel weldability.

Redistribution of alloying elements during tempering of a nanocrystalline steel

Redistribution of alloying elements during tempering of a novel nanocrystalline steel consisting of a mixture of lower bainitic ferrite and carbon-enriched retained austenite has been analyzed by atom probe tomography. Three physical processes, namely redistribution of substitutional solute across the austenite/bainitic ferrite interface before retained austenite decomposition, redistribution of solute across the lower bainite cementite/ferrite interface, and the precipitation of transition carbides and cementite, have been observed. Results suggest that retained austenite decomposes during tempering before full equilibrium is reached at the interface. Moreover, cementite precipitates from supersaturated ferrite via a paraequilibrium transformation mechanism.

Characterization of the Carbon and Retained Austenite Distributions in Martensitic Medium Carbon, High Silicon Steel

The retained austenite content and carbon distribution in martensite were determined as a function of cooling rate and temper temperature in steel that contained 1.31 at. pct C, 3.2 at. pct Si, and 3.2 at. pct noniron metallic elements. Mossbauer spectroscopy, transmission electron microscopy (TEM), transmission synchrotron X-ray diffraction (XRD), and atom probe tomography were used for the microstructural analyses. The retained austenite content was an inverse, linear function of cooling rate between 25 and 560 K/s. The elevated Si content of 3.2 at. pct did not shift the start of austenite decomposition to higher tempering temperatures relative to SAE 4130 steel. The minimum tempering temperature for complete austenite decomposition was significantly higher (>650 °C) than for SAE 4130 steel (∼300 °C). The tempering temperatures for the precipitation of transition carbides and cementite were significantly higher (>400 °C) than for carbon steels (100 °C to 200 °C and 200 °C to 350 °C), respectively. Approximately 90 pct of the carbon atoms were trapped in Cottrell atmospheres in the vicinity of the dislocation cores in dislocation tangles in the martensite matrix after cooling at 560 K/s and aging at 22 °C. The 3.2 at. pct Si content increased the upper temperature limit for stable carbon clusters to above 215 °C. Significant autotempering occurred during cooling at 25 K/s. The proportion of total carbon that segregated to the interlath austenite films decreased from 34 to 8 pct as the cooling rate increased from 25 to 560 K/s. Developing a model for the transfer of carbon from martensite to austenite during quenching should provide a means for calculating the retained austenite. The maximum carbon content in the austenite films was 6 to 7 at. pct, both in specimens cooled at 560 K/s and at 25 K/s. Approximately 6 to 7 at. pct carbon was sufficient to arrest the transformation of austenite to martensite. The chemical potential of carbon is the same in martensite that contains 0.5 to 1.0 at. pct carbon and in austenite that contains 6 to 7 at. pct carbon. There was no segregation of any substitutional elements.

Influence of Continuous Annealing Parameters on the Mechanical Properties of Cold Rolled Ultra High Strength Steels

Two C-Si-Mn cold rolled ultra high strength steels are investigated to study the effect of cooling parameters on the mechanical properties. The fast cooling start temperature and fast cooling end temperature as well as overageing temperature have significant influence on the yield and tensile strength of the experimental steels. It is found that tensile strength as high as 980Mpa can be achieved when the cooling speed exceeds 50°C/s. The yield strength and tensile strength can also be increased by increasing the fast cooling start temperature and meanwhile maintain relatively high fast cooling speed. The strength is significantly reduced if the steel is overaged at temperature of 350°C or above. Microstructure study reveals obvious decomposition of martensite and precipitation of cementite at high overageing temperatures. Other than these, it is found that the high Si steel has better elongation and lower YP/TS ratio than low Si steel at identical tensile strengths.

Efficiency Improvement of Vacuum Carburizing by Using Natural Gas

Acetylene and ethylene are frequently used in vacuum carburizing in Japan. In this study the natural gas which is available from the lifeline is applied to vacuum carburizing. The gas composition inside the furnace was analyzed by the gas chromatography in order to examine the carbon infiltration mechanism. Unsaturated hydrocarbon gases (such as acetylene and ethylene) are generated from the natural gas. The effect of acetylene concentration in the furnace on the carbon infiltration rate was investigated. The carbon amount which infiltrates into the steel increases, as acetylene concentration in furnace increases. It is possible that carbon concentration of specimen surface increases to the cementite precipitation concentration in the short term, when natural gas flow rate increases in the initial carburizing stage. After that, carbon concentration of specimen surface does not decrease, even if the natural gas flow decreases, because carbon atoms which are consumed for diffusion to inside are sufficiently supplied. By using this method, inhibition of soot generation, reduction of process gas and shortening of the carburizing period are possible. The carbon concentration profile of the vacuum carburized specimen was compared with the simulation.

Quench and Partitioning Steel: A New AHSS Concept for Automotive Anti‐Intrusion Applications

A new type of high strength, high toughness, martensitic steel, based on a newly proposed Quench and Partitioning (Q&P) process, is presented. This high strength martensitic grade is produced by the controlled low temperature partitioning of carbon from as‐quenched martensite laths to retained inter‐lath austenite under conditions where both low temperature transition carbide formation and cementite precipitation are suppressed. The contribution focuses on both the current understanding of the fundamental processes involved and includes a discussion of the technical feasibility of large‐scale industrial production of these steels as sheet products. The Q&P process, which is carried out on steels with a lean composition, should be implemented easily on some current industrial continuous annealing and galvanizing lines. In addition, martensitic Q&P sheet steel is characterized by very favourable combinations of strength, ductility and toughness, which are particularly relevant for high strength anti‐intrusion automotive parts.

Influence of diffusional stress relaxation on growth of stoichiometric precipitates in binary systems

Precipitation is usually connected with a significant misfit strain which may drastically influence the precipitation kinetics. Very recently it was demonstrated by Svoboda et al. [Svoboda J, Gamsjäger E, Fischer FD. Philos Mag Lett 2005;85:473] that the stress fields due to misfit strains can be effectively relaxed by the diffusive transport of vacancies in the matrix and their annihilation or generation at the precipitate/matrix interface. This idea has provided motivation to develop a new model for the simultaneous precipitate growth and misfit stress relaxation in binary systems. The actual state of the precipitate is described by its effective radius and by the thickness of the layer of the matrix atoms deposited at the precipitate/matrix interface. For these parameters the evolution equations are derived by application of the thermodynamic extremal principle. The model is applied to the Fe–C system by considering the growth of a cementite precipitate in the ferritic matrix. The influence of the stress relaxation on the precipitate growth kinetics is demonstrated.

New Model for the Overall Transformation Kinetics of Bainite. Part 2: Validation

A new model for the overall kinetics of the bainite transformation has been validated experimentally. The new model, presented in the 1st part of this work, is based in the displacive mechanism for bainite transformation. Thus, the bainite transformation kinetics of three medium carbon-high silicon steels has been studied. Results show that the model, which does not consider the effect of carbide precipitation in the bainite kinetics, predicts with a high degree of agreement the time evolution of bainitic ferrite volume fraction, even when lower bainite is present at the microstructure. Data from two additional medium carbon-high silicon steels, frequently reported in the literature, have been also used for reinforcing the validation, obtaining, again, a high agreement between the kinetic results for bainite transformation predicted by the model and those obtained experimentally. (doi:10.2320/matertrans.47.2473) In the first part of this study, a new model for the kinetics of the bainite transformation has been proposed. The model is based on the displacive mechanism for bainite transforma- tion. An important characteristic of this new model is the complete separation between the kinetics of both nucleation events of bainitic ferrite subunits, in austenite grain bounda- ries and at subunits previously formed. This distinction is based in a geometrical conception of the development of the transformation and has led to the elimination of the autocatalysis factor, an obscure parameter used in former kinetics models. The model correctly predicts the effect of carbon and other alloying elements such manganese and cobalt on the transformation kinetics, as it was shown in the first part of this work. The precipitation of cementite between the subunits of bainitic ferrite during bainitic transformation can be sup- pressed by alloying the steel with about 1.5 mass% silicon, which has very low solubility in cementite and greatly retards its growth from austenite. 1-3) The carbon that is rejected from the bainitic ferrite enriches the residual austenite, thereby stabilising it down to room temperature. Consequently, an isothermal transformation in the range of the bainite trans- formation leads to a microstructure consisting of bainitic ferrite separated by carbon-enriched regions of austenite. Since the cementite precipitation between the plates of bainitic ferrite plates has not been considered in the modelling, steels with high silicon content are most adequate for the validation of the proposed model. However, cementite precipitation within bainitic ferrite plates (lower bainite), that can occur even in high silicon steels, has not been considered in the model. In this sense, the study of lower bainite kinetics is interesting in order to evaluate the reliability of the model predictions in the event of lower bainite formation. It has been shown in the 1st part of the study that if no other reaction interacts with the successive nucleation and growth of subunits of bainitic ferrite, the incomplete reaction phenomenon, by means of the T 0 0 curve, provides a method for the estimation of the maximum volume fraction of bainitic ferrite that can be formed, at a given temperature, in a steel, vbmax. Considering a steel with a nominal carbon content �

Coarse Columnar Structure of Transformation-Grown Ferrite in Pure Iron —On Wrought Iron and Sintered Iron—

When a nucleus of ferrite crystal is formed in cast-and-rolled or sintered iron, either of which contains less than approximately 50 ppm interstitial elements, namely, carbon and nitrogen, the ferrite crystal may grow into a coarse crystal joining many austenite crystals. Furthermore, such coarse ferrite crystals may compose a columnar macrostructure by unidirectional phase transformation under the condition accompanying a gradient of temperature. The formation of the macrostructure gives a maximum linear expansion of the sample that is numerically equal to the volume expansion at the same time. Even a small amount of carbon in pure iron can cause the condensation of carbon atoms or the formation of a fairly large number of minute cementite precipitates at ferrite/austenite phase boundaries. Both the condensation of carbon and the precipitation of cementite may cause the nucleation of new ferrite crystals, leading to the development of a fine-grained macrostructure. [doi:10.2320/matertrans.47.2449]

Evaluation of Displacive Models for Bainite Transformation Kinetics in Steels

Several kinetic models for bainite transformation have been widely applied in industry and research. The majority of these models, that do not consider the effect of cementite precipitation during bainite transformation, were validated in high silicon bainitic steels in order to avoid the interference of cementite precipitation during bainite formation. In this work, displacive models for bainite transformation have been validated in bainitic steels with different silicon content with the aim of evaluating their applicability on steels where cementite precipitation may play an important role on bainite formation. It has been found that these models fail in the calculus of the maximum volume fraction of bainite of lean silicon steels, but lead to a reasonable accuracy in high silicon steels. This is not surprising since cementite formation reduces the carbon content in the residual austenite, stimulating the formation of a further quantity of ferrite. Likewise, an imprecise estimation of the nucleation rate of bainite must be the reason for the poor correlation in the predictions of the bainite transformation kinetics in high silicon steels. This entails a better treatment of autocatalytic nucleation, still unresolved issue in the bainite transformation kinetics theory. [doi:10.2320/matertrans.47.1492]