Growth Of Cementite Research Articles

Role of sulphur in carburization, carbide formation and metal dusting of iron

AbstractThe kinetics and mechanisms of reactions on iron in CH4–H2 and CO–H2–H2O mixtures have been studied, also in the presence of H2S, to understand carburization, iron carbide formation, metal dusting and carbon deposition. Rate equations and mechanisms of the carburization in CH4–H2 and CO–H2–H2O have been elucidated. Both reactions are retarded by adsorbed sulphur, the rates becoming inversely proportional to the sulphur activity aS ∼ p(H2S)/p(H2) with increasing aS. At carbon activities aC > 1, cementite growth can be started in both gas mixtures on iron but the decomposition of this unstable carbide gives rise to a corrosion process called ‘metal dusting’, i.e. a disintegration of iron and steels to a dust of metal particles and graphitic carbon. This disintegration can be prevented by adsorbed sulphur, which hinders the nucleation of graphite. The stabilizing effect of sulphur on cementite and higher carbides such as Hägg carbide allows fundamental studies to be carried out on their thermodynamics, non‐stoichiometry and diffusional growth mechanisms, and the presence of sulphur will allow iron carbide production in a process of great present interest—the direct reduction of iron ores in carburizing gas mixtures. Kinetic studies in the system Fe–C–S are supplemented by AES and low‐energy electron diffraction investigations on the adsorption and segregation of sulphur on iron and cementite. Copyright © 2002 John Wiley & Sons, Ltd.

Detailed study of the transformation mechanisms in ferrous TRIP aided steels

Development of TRIP aided ferrous alloys is one answer to the demand for weight decrease in the automotive industry. The microstructure of hot rolled and cold rolled TRIP steels is quite complex and the optimisation of such steel products requires a detailed understanding of the mechanisms of phase transformation, during thermomechanical treatment as well as during mechanical testing or metal forming.We present in this paper the results obtained at Irsid concerning the study of austenite stabilisation through bainitic transformation during thermal treatment and its transformation into martensite during mechanical testing.First of all, the characterisation methods are presented. An effort has to be put on this point due to the refinement of the microstructure of TRIP steels, especially the size of austenite and martensite islands. Carbon replicas for the observation by means of transmission electron microscopy (TEM) are used to analyse the morphological features of the microstructure ‐ nature of the constituents, size and shape ‐ and the composition of cementite particles present in the steels. The mean value for this carbon content in retained austenite is deduced from X‐ray diffraction measurements.Then the kinetics of bainitic transformation are discussed as well as cementite precipitation. The typical composition of the steel studied is 0.5 % C, 1.5 % Mn. The use of 0.5 % C steels facilitates the study of bainitic transformation by avoiding the ferrite formation usually occurring in TRIP steels. Cementite nucleation appears at the ferrite/austenite interface without any partitionning of substitutional elements. To satisfy thermodynamic equilibrium at the interface, the silicon content on the cementite side is very low and high on the austenite side. Then, carbon diffusion towards austenite is delayed and, as a consequence, cementite growth is also delayed. As the diffusion kinetics are low at 400 °C, cementite keeps this “non partitioned” composition, even after 3 hours holding. At 500 °C, diffusion kinetics are higher and cementite composition approaches that predicted by equilibrium.Finally, the stability of retained austenite during mechanical testing is studied. Before and after mechanical testing the morphological characteristics of the microstructure (austenite island size and elongation) are analysed by TEM replicas and image analysis. There is a high density of very small austenite islands but they represent only a small fraction of the total retained austenite. These results confirm and quantify the size effect on austenite stabilisation during deformation.

Nanocrystallization in Fe-C Alloys by Ball Milling and Ball Drop Test.

Microstructural evolution and nanocrystallization in various carbon steels by ball milling and ball drop test has been studied. In ball milling, nanocrystallization was observed in all the carbon steels irrespective of the carbon content (up to 0.9 mass% C) or starting microstructure (ferrite, martensite, pearlite or spheroidite). In ball drop test, nanocrystallization was observed in high carbon steels or ultrafine grained low carbon steels. It is realized that high strength before ball drop test is required for the nanocrystallization. The nanocrystalline structure obtained by ball milling and ball drop test has similar microstructure with dark smooth contrast. The morphologies such as pearlite lamellar, spheroidite cementite, ferrite grain boundary disappeared by nanocrystallization. The boundary between the nanocrystalline and work-hardened regions is quite sharp. The hardness of the nanocrystalline region is about two times higher than that of work-hardened region. The annealing of nanocrystalline region shows substantially slow grain growth and re-precipitation of fine cementite. This annealing behavior is quite different from the work-hardened region which is characterized by recrystallization and fast grain growth. From the present study, it was confirmed that the nanocrystalline structure produced by ball milling and ball drop test has quite similar in structure, hardness and annealing behavior although the number of deformation applied is substantially different.

Simulation of paraequilibrium growth in multicomponent systems

A methodology to simulate paraequilibrium (PE) growth in multicomponent systems using the DIC-TRA (Diffusion-Controlled Transformation) software is presented. For any given multicomponent system containing substitutional and interstitial elements, the basic approach is to define a hypothetical element Z, whose thermodynamic and mobility parameters are expressed in terms of the weighted average (with respect to site fraction) of the thermodynamic parameters and mobilities of the substitutional alloying elements. This procedure facilitates the calculation of PE phase diagrams and the PE growth simulations directly in the Thermo-Calc and DICTRA software, respectively. The results of two distinct case studies in multicomponent alloys are presented. In the first example, we simulate the isothermal growth of PE cementite in an Fe-C-Co-Cr-Mo-Ni secondary hardening steel during tempering. This is of practical importance in modeling the carbide precipitation kinetics during secondary hardening. In the second example, we have presented the results of PE ferrite growth during continuous cooling from an intercritical temperature in an Fe-Al-C-Mn-Si low-alloy steel. This is of importance to the design of triple-phase steels containing an austenite that has optimum stability, to facilitate stress-induced transformation under dynamic loading. The results of both simulations are in good accord with experimental results. The model calculations do not consider any resistive or dissipative forces, such as the interfacial energy, strain energy, or solute drag, and, as a result, the interface velocities represent an upper limit under the available chemical driving force.

Carburization and Metal Dusting on Iron

Fundamental studies have been conducted on the kinetics and mechanisms of reactions of iron CH4-H2 and CO-H2-H2O mixtures, also in the presence of H2S, to understand carburization, iron carbide formation, metal dusting and carbon deposition. The rate equations and mechanisms of the surface reaction in carburization of iron are presented, the carburization in CH4-H2 is much slower than in CO-H2. Both reactions are retarded by adsorbed sulfur, the carburization rate becomes inversely proportional to the sulfur activity aSpH2S/pH2 with increasing aS. At carbon activities aC>1 cementite growth can be started in both gas mixtures on iron, however, the decomposition of this unstable carbide gives rise to a corrosion process 'metal dusting', i.e. the material disintegrates to a dust of metal particles and graphitic carbon. This disintegration can be prevented by adsorbed sulfur which hinders the nucleation of graphite. The stabilizing effect of sulfur on cementite and higher carbides such as Hägg carbide, allows fundamental studies about their thermodynamics, non-stoichiometry and diffusional growth mechanisms, on the other hand the presence of sulfur will allow iron carbide production in the reduction of iron ores in carburizing gas mixtures.

Effects of silicon, nickel, and vanadium on impact toughness in spring steels

The effects of alloying elements silicon, nickel, and vanadium, and the tempering temperatures on the mechanical properties, especially impact toughness, in 0.6C–(1.0–2.5)Si–(0.7–1.8)Ni–(0.1–0.2)V steels (wt-%), were investigated by performing hardness tests, impact tests, and TEM examination. The results obtained showed that the addition of silicon up to 2.5 wt-% shifted the embrittlement temperatures to higher temperatures, owing to the silicon retarding the formation and growth of cementite at boundaries. Additionally, it was found that the temperature of the peak in Charpy impact toughness v. tempering temperature curves coincided with the onset of the decomposition of retained austenite. The temperatures of the conversion of ε carbide to cementite and of the onset of tempered martensite embrittlement were not changed by the addition of nickel up to 2.0 wt-% in the present steels. However, the decomposition of retained austenite was delayed and the toughness was increased over all the tempering temperatures. The effect of the vanadium addition in the present steels was limited to the increase in hardness and impact toughness.

Effects of silicon, nickel, and vanadium on impact toughness in spring steels

The effects of alloying elements silicon, nickel, and vanadium, and the tempering temperatures on the mechanical properties, especially impact toughness, in 0.6C–(1.0–2.5)Si–(0.7–1.8)Ni–(0.1–0.2)V steels (wt-%), were investigated by performing hardness tests, impact tests, and TEM examination. The results obtained showed that the addition of silicon up to 2.5 wt-% shifted the embrittlement temperatures to higher temperatures, owing to the silicon retarding the formation and growth of cementite at boundaries. Additionally, it was found that the temperature of the peak in Charpy impact toughness v. tempering temperature curves coincided with the onset of the decomposition of retained austenite. The temperatures of the conversion of ε carbide to cementite and of the onset of tempered martensite embrittlement were not changed by the addition of nickel up to 2.0 wt-% in the present steels. However, the decomposition of retained austenite was delayed and the toughness was increased over all the tempering temperatures. The effect of the vanadium addition in the present steels was limited to the increase in hardness and impact toughness.

PHASE TRANSFORMATION IN Fe-Mo-C AND Fe-W-C STEELS—I. THE STRUCTURAL EVOLUTION DURING TEMPERING AT 700°C

Mechanism and kinetics of carbide transformation during tempering at 700°C have been studied in Fe-Mo-C and Fe-W-C steels (with up to 2.5% W or Mo) by transmission electron microscopy (TEM) and X-ray diffraction. The sequence of carbide formation is Fe3C→Mo2C→(Fe2MoC, M23C6) in molybdenum steels and Fe3C→M6C→M23C6 in tungsten steels. Increasing the alloying element level increases the rate of carbide replacement reaction. In Fe-Mo-C steels the Fe2MoC carbides nucleate preferentially at the Fe3C-α interface and grow into cementite, whereas in Fe-W-C steels the M6C carbides usually precipitate inside cementite giving rise to the in situ transformation Fe3C→M6C. The software DICTRA and THERMO-CALC were used to simulate cementite growth and to show the possibility of the in situ transformation. The M23C6 carbide is first confined to prior austenite grain boundaries and penetrates into the grains with increasing tempering time. During growth the M23C6 carbide absorbs surrounding pre-existing carbides. As a result, after tempering for 500 h, patches of two-phase areas with (M23C6 + α) or (M6C + α) are observed in tungsten steels, and patches of (M23C6 + α) or (Fe2MoC + α) in molybdenum steels. The alloying element partitioning between α and precipitated carbides was determined using TEM-EDS. It was established that the M23C6 carbide is stable at 700°C in both investigated steels. The Fe2MoC carbide is stable in the Fe-Mo-C system at this temperature. The MC carbide was not observed even after tempering for 3000 h. © 1997 Acta Metallurgica Inc.

Atom probe field ion microscopy study of the partitioning of substitutional elements during tempering of a low-alloy steel martensite

The partitioning behavior of Mn and Si at the cementite/ferrite interface during tempering of Fe-C-Si-Mn steel martensite has been studied by atom probe field-ion microscopy (APFIM). It has been shown that cementite can form without partitioning of Si and Mn during the early tempering stage at a low temperature. The atom probe compositional analysis shows no evidence of segregation or of concentration spikes of substitutional elements at the interface. This suggests that the early stage of cementite growth occurs by paraequilibrium mode and is controlled only by C diffusion in the matrix. In addition, significant C concentration fluctuations are measured in the as-quenched condition. The onset of partitioning of both Si and Mn occurs after prolonged time or by increasing the tempering temperature.

The role of ledges in the proeutectoid ferrite and proeutectoid cementite reactions in steel

The influence of interphase boundary ledges on the growth and morphology of proeutectoid ferrite and proeutectoid cementite precipitates in steel is examined. After reviewing current theoretical treatments of growth by the ledge mechanism, investigations that clearly document the presence and motion of ledges with thermionic emission electron microscopy (THEEM) and transmission electron microscopy (TEM) are reviewed. A fundamental distinction is made between two types of ledges: (1) mobile growth ledges whose lateral migration displaces the inter-phase boundary and (2) misfit-compensating structural ledges. Both types of ledges strongly affect the apparent habit plane and aspect ratio of precipitate plates. Agreement between measured growth rates of proeutectoid ferrite and cementite (plates and allotriomorphs) and predicted growth kinetics assuming volume diffusion-controlled migration of ledge-free disordered boundaries is shown to be consistently poor. Physically realistic growth models should incorporate the ledge mechanism. More accurate comparisons of the growth models with experimental data will need to account for observed ledge heights, interledge spacings, and ledge velocities. In this vein, the sluggish growth kinetics of cementite allotriomorphs observed in an Fe-C alloy are shown to be quantitatively consistent with a strong increase in interledge spacing with time.

Influence of carbon concentration and reaction temperature upon bainite morphology in Fe-C-2 Pct Mn alloys

Isothermal transformation of austenite to bainite was studied by optical, replication, and thin foil transmission electron microscopy (TEM) in both hypo- and hypereutectoid Fe-C-2 wt pct Mn alloys (and a single 3 pct Mn alloy) containing from 0.1 to 1.37 wt pct C in order to characterize and explain the transitions which occur in bainite morphology as a function of carbon content and reaction temperature. A “morphology map” was constructed showing the temperaturecarbon composition(T-x) regions in which four different bainite morphologies predominate: upper bainite, lower bainite, nodular bainite, and inverse bainite. Calculations of the volume free energy changes associated with the nucleation of ferrite, †G v α , and cementite, †G v c , and the parabolic rate constant for growth of ferrite, αα, and cementite, αc, were performed in order to explain the observed morphological transitions. In hypoeutectoid alloys, where |†G v α | ≫ |ΔG v c | and αα ≫ α c , the Widmanstatten ferrite-dominated morphologies of upper and lower bainite predominate. The upper-to-lower bainite transition appears to be associated with the emergence of edge-to-face sympathetic nucleation at high values of |ΔG v α |. In hypereutectoid alloys, the ratios ΔG v α /ΔG v c and αα/αc are considerably smaller; hence, cementite can compete much more readily with ferrite during both nucleation and growth, resulting in the formation of nodular bainite. With decreasing temperature in the hypereutectoid regime,ΔG v α /ΔG v c , and especially αga/αc, increase, resulting in the replacement of nodular bainite by lower bainite at temperatures below about 250 °C to 275°C.

Graphite Morphology Control in Cast Iron

ABSTRACTGraphite morphology in cast iron is analyzed in terms of the growth kinetics of graphite crystals in liquid iron. At small driving forces, i.e., low supersaturation or small kinetic undercooling, graphite growth is characterized by faceted growth, resulting in flake, compacted and spherulitic graphite morphologies. However, at large driving forces, there is a transition from facted to non-faceted growth, resulting in a dendritic growth morphology.Flake morphology is rationalized in terms of impurity dependent crystal growth mechanisms, whereas a spherulitic morphology is attributed to a defect controlled spiral growth mechanism. Compacted graphite morphology is considered as a transition between flake and spherulitic morphology.A thermodynamic approach is used to inter-relate the residual concentrations of impurities of technological interest, i.e. S and 0, as a function of the residual concentration of the reactive elements, Mg, Ca, and Ce in a typical cast iron melt at 1500'C and atmospheric pressure. Such a diagram that quantitatively relates graphite morphology in thick cast iron sections to soluble concentrations of impurities is referred to as a graphite morphology control diagram.In thin section castings that freeze at faster cooling rates and large kinetic undercoolings, the basal spiral growth mechanism dominates over the impurity controlled prism growth mechanism, leading to deviations from predictions based simply on the graphite morphology control diagram. In the ase of compacted graphite, where growth on both the prism and basal faces is involved, the degree of nodularity increases with the cooling rate, giving rise to section sensitivity.At large undercoolings, the prevention of the nucleation and growth of cementite is an essential feature of graphite morphology control. It is estimated that the mobility of the cementite interface exceeds that of the prism interface in flake graphite growth by an order of magnitude and that of the basal interface in spherulitic graphite growth by three orders of magnitude. In practice, the driving force for graphite growth is raised selectively through the addition of graphite stabilizing elements, such as silicon, which raise the temperature of the graphite eutectic and depress the temperature of the carbide eutectic. Kinetic growth undercooling can be decreased by increasing the number of heterogeneous nuclei for graphite growth through inoculation. The application of the above concepts for the control of graphite morphology in shaped automotive castings is discussed.

The effect of phosphorus content on grain boundary cementite formation in AISI 52100 steel

Intergranular fracture surfaces of high phosphorus (0.023 wt pct P) and low phosphorus (0.009 wt pct P) AISI 52100 steels were investigated by Auger Electron Spectroscopy (AES). Cementite, identified by composition and Auger peak shape, was found to form on austenite boundaries in specimens oil quenched from 960 °C to room temperature as well as in specimens quenched from 960 °C and isothermally held at temperatures between Acm and A1. Phosphorus segregates to austenite boundaries during austenitizing and accelerates cementite formation on the austenite boundaries. Concentration profiles obtained by AES during ion sputtering showed that phosphorus may be incorporated in the first-formed cementite and concentrates at cementite/matrix interfaces in later stages of cementite growth. The amount of interphase P segregation in the later stages is proportional to bulk alloy P concentration in accord with McLean’s theory of grain boundary segregation in dilute alloys and appears to approach equilibrium at high reaction temperatures (785 °C). At lower reaction temperatures (740 °C), the interphase segregation is lower than expected, a result that may be attributed to reduced diffusivity of P at the lower reaction temperature.

The isothermal thickening of cementite allotriomorphs in a 1.5Cr1C steel

The kinetics of isothermal thickening of cementite allotriomorphs in a l%C1.5%Cr steel (AISI 52100) were investigated by quantitative metallographic measurements. Specimens from high phosphorus (0.023%) and low phosphorus (0.009%) heats were fully austenitized and then held at various temperatures between A 1 and A cm for times up to 6 h. Thickening rates in the later stages were quite sluggish compared with those expected for carbon diffusion controlled processes in austenite. Growth effectively stopped far short of the expected equilibrium volume fractions of cementite. Auger ionsputtering analysis of the fracture surface of specimens isothermally treated to form grain boundary allotriomorphs of cementite showed carbon activities in austenite that were independent of the location in an austenite grain. Therefore, growth was controlled by a mechanism other than carbon diffusion. Structural observations made by transmission and scanning electron microscopy indicated that the carbide thickening occurred by lateral ledge migration on partially coherent interfaces between the cementite and the parent austenite. Cementite growth may proceed rapidly without Cr partitioning in the early stages, but the growth becomes sluggish in the later stages due possibly to Cr partitioning to and/or Si rejection from cementite.

Structure-property relations and the design of Fe-4Cr-C base structural steels for high strength and toughness

Some design guidelines for improving strength-toughness combinations in medium car-bon structural steels are critically reviewed. From this, quaternary alloy development based on Fe/Cr/C steels with Mn or Ni additions for improved properties is described. Transmission electron microscopy and X-ray analysis reveal increasing amounts of retained austenite in these alloys with Mn content up to 2 wt pct and Ni additions at 5 wt pct after quenching from 1100°C. A corresponding improvement in toughness properties is also found. Grain refining results in a further increase in the amount of retained austenite. In addition, the excellent combinations of strength and toughness in these quaternary alloys are attributed to the production of dislocated lath martensite from a homogeneous austenite phase free from undissolved alloy carbides. The question of thermal instability of retained austenite following tempering is considered in detail and it is shown that the decomposition of retained austenite is closely related to the ease of nucleation and growth of cementite. Thus, graphitizing alloying elements such as Ni are beneficial in postponing the decomposition of retained austenite.

Stress relaxation during the tempering of hardened steel

Stress relaxation during tempering was examined in hardened AISI 52100 steel and a series of plain-carbon steels by means of wedged split-ring specimens. Enhanced relaxation arising from the formation and growth of ϵ-carbide and cementite was found to be independent of the elastically applied stress at least within the range investigated. The percent stress relaxation increased with increasing carbon content of the as-quenched martensite, while the kinetics of relaxation,were essentially those associated with tempering. In addition to stress relaxation, a phenomenon of creepback was oblerved when a partially recovered sample was re-tempered in the unwedged condition. A mechanism for the enhanced relaxation during tempering is proposed. It is based on the generation of a plastic zone set up when a local volume change occurs, e.g., that accompanying the precipitation of a carbide particle in the martensitic matrix during tempering. This mechanism is not only consistent with the stress-relaxation results, but is also capable of explaining the creepback effect.

Cementite morphology in pearlite

Some characteristic growth patterns of cementite in pearlite, revealed by electron microscopy, are discussed in terms of the geometry of growth and of diffusion and surface energy factors. These include 1. (a) linear discontinuities in the lamellar structure, which are shown to be one possible origin of branching, 2. (b) curved and striated lamellae, 3. (c) acicular growth of cementite. It is concluded that lamellar precipitation of cementite has its origin in anisotropy of ferrite-cementite interfacial energy.

電子顕微鏡によるフェライト及びセメンタイトの研究

In this paper, the two kinds of precipitations in ferrite and cementite of iron or steel are described. These precipitations were observed by means of an electron microscope. (Reference Photo. 1 to Photo. 8).With these precipitations the following results were obtained by metallurgical considerations with respect to C, Si, Mn, P, S, N and O in iron or steel. (1) Ferrite contains many spherical precipitations, most of which are Fe3Si2 particles. (2) Fe3Si2 particles are about 550 Å in average. But this size grows in proportion as the Si content in iron or steel increases, and then they group with other kinds of particles. (3) Fe3Si2 particles are precipitated in a field of γ solid solution and their distribution is dense at the grain boundary but not in the interior of the crystal grains. (4) Cementite is a combination of Fe3C and Mn3C. (5) Spherical particles of MnSi are precipitated in a very small quantity from cementite. They are about 400 A on the average. (6) Growth of pearlite cementite is partially disturbed by the Fe3Si2 particles. (Reference Photo. 8). (7) As compared with ferrite of plain carbon steel, silico-ferrite contains much more Fe3Si2 particles.