Iron-carbon Martensite Research Articles

Thermodynamic Analysis of the Formation Process of Metastable Carbides in Iron–Carbon Martensite

Thermodynamic Analysis of the Formation Process of Metastable Carbides in Iron–Carbon Martensite

Reversible martensitic transformation, ageing and low-temperature tempering of iron–carbon martensite

Reversible martensitic transformation was observed in Fe-based high-carbon alloys at temperatures when none lattice defects (including divacancies) could diffuse. Formation and behavior of carbon-vacancy clusters was studied and discussed. Chemical composition of hexagonal ɛ-carbide is determined as Fe 3C. The effect of particle size and ɛ-carbide/martensite orientation relationship on the ɛ-carbide → cementite transformation is discussed.

Aging and tempering behavior of iron-nickel-carbon and iron-carbon martensite

The aging at room temperature (RT) and the tempering behavior in the temperature range 293 to 973 K of ternary iron-nickel-carbon martensite (containing 14.4 at. pct Ni and 2.35 at. pct C) was investigated principally by using X-ray diffractometry to analyze changes in the crystalline structure and differential scanning calorimetry to determine heats of transformation and activation energies. These techniques also were used in the parallel study performed in this work of the tempering behavior of FeC martensite (containing about 4.4 at. pct C) in the temperature range 298 to 773 K. Analysis of the structural changes revealed that in both FeNiC and FeC the following processes occurred: (1) formation of carbon enrichments and development of a periodic arrangement of planar carbon-rich regions up to 423 K; (2) precipitation of e/η transition carbide and transformation of a part of the austenite into ferrite under simultaneous enrichment with carbon of the remaining austenite (between 423 and 523 K); (3) decomposition of the retained austenite into ferrite and cementite between 523 and 723 K (only partly for FeNiC); (4) precipitation of cementite between 523 and 723 K; and (5) for FeNiC, reformation of austenite from ferrite and cementite above 773 K. A short comparative discussion concerning the first stage of martensite decomposition for FeC, FeNiC, FeN, and FeNiN martensites is given.

Mossbauer studies on aging of deformed Fe-6Mn-2Cr-1C alloy

Mossbauer spectroscopy provides information on an atomic scale and is a sensitive tool to detect local atomic arrangements. Such short range phenomenon as pre-precipitation, clustering and so forth that may be often difficult to resolve by the usual techniques of physical metallurgy can be studied by Mossbauer spectroscopy. In fact, this technique has been used to study structural changes during the initial stages of aging of iron-carbon martensite. The phase composition as well as distribution state of carbon and metallic alloying element before and after cold deformation in Fe-6Mn-2Cr-1C alloy aged at 350 C for 6h were studied by the use of Mossbauer spectroscopy in the present work.

First stage of precipitation in iron-carbon-nitrogen martensites; diffraction analysis using synchrotron radiation

A considerable amount of research has been done on the aging and tempering behavior of iron-carbon and iron-nitrogen martensites. These martensites can be described on the basis of an overall body-centered tetragonal (b.c.t.) lattice of iron atoms containing interstitial atoms which are randomly distributed over the c-type octahedral interstices. The interstitial martensites seem to be similar in many respects; they have, for instance, comparable lattice parameters and M{sub s} temperatures (the M{sub s} temperature is the temperature at which the martensite formation starts on quenching from the austenite-phase field). However, some distinct differences occur between the decomposition behavior of both martensites. On aging at room temperature for example, local enrichments of interstitials develop in both nitrogen and carbon martensites. In FeN martensites these enrichments are considered to possess a fully ordered arrangement of the nitrogen interstitials in c-type octahedral interstices according to the {alpha}{double prime}-Fe{sub 16}N{sub 2} structure. However, in FeC martensites no indications for such an ordering have been observed (yet) and the enrichments are denoted as clusters which are conceived as aggregates of iron and carbon atoms, containing different amounts of carbon interstitials distributed randomly over the c-type octahedral interstices. This paper reports that because of the abovemore » mentioned apparent difference between the aging of FeN and FeC martensites, it is interesting to study the (pre)precipitation behavior of iron-carbon-nitrogen martensitic alloys with different ratios of the amounts of carbon and nitrogen atoms. Only a few results on the tempering of FeCN have been reported until now. In these studies alloys were used which were very rich in nitrogen.« less

Lattice changes of iron-carbon martensite on aging at room temperature

The aging behavior of iron-carbon martensite (5.1 at. pct C = 5.3C/100Fe) at about 297 K was studied by analyzing, in particular, the changes in the {002} and {200} diffraction line profiles obtained by X-ray diffractometry. Martensitic specimens were prepared by gaseous carburizing of pure iron in a mixture of H2 and CO, followed by quenching in brine and subsequently in liquid nitrogen. The aging process can be divided into two stages. First, a redistribution of carbon atoms in the martensite matrix occurs in four ways (aging time about 50 hours) leading to diffraction by the matrix independent of the clusters. Within the range of aging times employed (up to 2 years), the diffraction by the retained austenite did not change.

The tempering of iron- carbon martensite; dilatometric and calorimetric analysis

The aging behavior of iron-carbon martensite (1.13 wt Pct C) between -190 °C and 450 °C was investigated by quantitative analysis of the corresponding changes in volume and enthalpy. A method to determine activation energies of the occurring solid-state transformations by performing non-isothermal measurements of some physical property of the specimen has been described. Martensitic specimens were prepared by carburizing pure iron and quenching in brine and liquid nitrogen. The dilatometric and calorimetric experiments were supplemented with microhardness measurements. At least five different stages of structural change can be distinguished, which are quantitatively analyzed in terms of their effects on volume and enthalpy: (i) transformation of retained austenite into martensite (between −180 and −100 °C); (ii) redistribution of carbon atoms (below 100 °C); (iii) precipitation of transition carbide (between 80 and 200 °C); (iv) decomposition of retained austenite (between 240 and 320 °C); and (v) conversion of transition carbide into cementite (between 260 and 350 °C).

Analysis of nonisothermal transformation kinetics; tempering of iron-carbon and iron-nitrogen martensites

A powerful method for the analysis of solid-state transformation kinetics can be based on non-isothermal analysis where the temperature corresponding to a fixed stage of transformation is measured as a function of heating rate. Adoption of a specific kinetic model is not required for valid application of this method. This removes constraints imposed unnecessarily on so-called Kissinger-like procedures. The method was applied to study and to compare the tempering kinetics of iron-carbon martensite and iron-nitrogen martensite (about 1.1 wt pct interstitials). Dilatometry and differential scanning calorimetry were employed. This combination of experimental techniques allowed distinction between preprecipitation processes as segregation and clustering of interstitials. In contrast with iron-carbon martensite, no indications were obtained for clustering of interstitials in iron-nitrogen martensite. The preprecipitation stages have activation energies of 75 to 85 kJ/mole, which are ascribed to volume diffusion of interstitials. The precipitation of the transition carbide/ nitride (first stage of tempering) occurs with an activation energy of 110 to 125 kJ/mole, which is ascribed to pipe diffusion of iron. The precipitation of the equilibrium carbide/nitride (third stage of tempering) is associated with an activation energy of 195 to 205 kJ/mole which is ascribed to combined pipe and volume diffusion of iron.

An NMR and Mössbauer study of iron-carbon martensite

Pulsed NMR of 13C and 57Fe nuclei in iron-carbon martensite have been observed and compared with 57Fe Mössbauer measurements. The hyperfine field distributions were explained by the octahedral-tetrahedral mixed site hypotheses and the formation of the local-ordered arrangement of C. DV-Xα calculations were performed for Fe 6 and Fe 6C clusters to compare with the results.

Mössbauer and NMR studies of iron-carbon martensite

The atomic arrangements in Fe-C martensite, especially the interstitial carbon positions have been discussed utilizing57Fe Mossbauer spectroscopy and13C and57Fe spin-echo NMR measurements. The experimental result is well explained by the model of two interstices occupancy of carbon in the bct martensite. Especially, the spin-echo spectra by13C nuclei showed that two main lines first appear and clear satellite lines gradually rise during the aging at 323K, suggesting the carbon jumps from the tetrahedral to the octahedral interstices and further movements to form a local ordered arrangement.

Structure of Iron-Carbon Martensite in the Transition State from the First to the Third Stage of Tempering Studied by Electron Microscopy and Diffraction

The structure of 1.5 mass%C martensite tempered at 470 K for 180 ks has been investigated. In the matrix α-iron, η-Fe2C and θ′-particles coexist. The η-Fe2C particle is the precipitate at the first stage, while the θ′-particle is the precipitate at the early third stage and made of thin layers of θ-Fe3C and χ-Fe5C2 grown microsyntactically. At the transition from the first to the third stage, the θ′-particle nucleates on the surface of the η-Fe2C particle and then grows at the expense of carbon in the matrix. The η-Fe2C particle disappears after the θ′-particle has grown sufficiently large. The carbides making the θ′-particle have slightly larger lattice parameters than the ideal values. Owing to this good coherency is kept with the matrix. When the θ-Fe3C particle appears at the later third stage, it has the ideal lattice parameters, but the coherency with the matrix is not so good as the θ′-particle. Since the θ′-particle and the θ-particle are different in nature from each other, they should be distinguished in discussing the martensite tempering.

Crystallography and tempering behavior of iron-nitrogen martensite

Iron-nitrogen martensite was prepared by the gaseous nitrogenization of iron in the austenite-phase field, followed by quenching. Low- and high-nitrogen contents (lathvs plate morphology) were investigated. Optical microscopy, transmission electron microscopy and diffraction, and differential thermal analysis were employed to study the crystallography and the tempering behavior of the martensites. The orientation relation between austenite and martensite, the habit plane and types of (post-) transformation twins were determined. Tempering-induced changes in hardness and release of heat were related to structural changes as revealed by electron diffraction. Differences with analogous iron-carbon martensites were discussed. Aging at room temperature leads to nitrogen diffusion-controlled precipitation of coherentα″-Fe16N2 platelets, which process is completed after about one day. Aging at higher temperatures (up to about 475 K) results in incoherentα″particles, which, at temperatures above 460 K, exhibit small deviations (≃18 deg) from the usual {001}α′/α habit plane.

Incommensurate Direction of the Modulated Structure in Iron-Carbon Martensite

Incommensurate Direction of the Modulated Structure in Iron-Carbon Martensite

Opening of the Peter G. Winchell symposium on the tempering of steel

Abstract : The structural changes which occur during the aging and tempering of initially virgin iron-carbon martensites are briefly summarized, in terms of important contributions by the late Peter G. Winchell. (Author)

A study of interstitial atom configuration in fresh and aged iron-carbon martensite by mossbauer spectroscopy

Mössbauer spectra of Fe-1.5 ∼ 7.2 at %C alloys without and with a 5 T longitudinal external magnetic field were measured to clarify the structure of iron carbon martensite. The spectra of freshly quenched martensite are decomposed into three magnetic hyperfine patterns having different fields. The 1st component with a hyperfine field of H ∼- 27 T is identified as the absorption caused by 1st neighboring iron atoms of octahedral carbon atoms. The 2nd component ( H ∼- 31.5 T). the intensity of which rapidly decreases during aging, is due to 1st neighboring iron atoms of tetrahedral carbon or possibly 2nd nearest neighbors of octahedral carbon atoms. The 3rd main component ( H ∼- 34 T) is due to iron atoms not strongly perturbed by more distant carbon neighbors. The appearance of three additional components ( H ∼- 37. 26.5 and 18 T) in the aged martensite indicates the formation of the ordered non-stoichiometric Fe 4C x(x < 1) phase. The Mössbauer spectra of the epsilon carbide formed after aging at 413 K are decomposed into three hyperfine components corresponding to different iron environments in the carbide, which suggests the existence of vacant sites of carbon in the Fe 2C structure.

Modulated structure of iron–carbon martensite studied by electron microscopy and diffraction

Satellite spots appear around every fundamental spot in electron diffraction patterns of iron-carbon martensite (α′) tempered in a temperature range from 273 to 363 K. High-resolution electron microscopic observations show that they are due to the formation of a modulated structure, in which interstitial carbon atom clusters smaller than 10 A are distributed randomly in a plane nearly parallel to (102)α′, with inter-cluster distance of 10–20 A and such carbon-rich planar regions are spaced periodically with intervals of 10–20 A, depending on the carbon content of the martensite. A structure analysis was made by measuring the satellite spot intensity of the electron diffraction patterns, and the displacements of atoms from their average positions in martensite were determined. The result supports the above cluster model for the modulated structure.

A study of the early stages of tempering of iron-carbon martensites by atom probe field ion microscopy

The redistribution of carbon atoms during the early stages of ageing and tempering of iron-carbon martensites has previously been studied only by indirect methods. The computer-controlled atom probe field ion microscope permits the direct, quantitative determination of carbon concentrations at the atomic level, and thus all the stages of the martensite decomposition process become amenable to direct study. Analyses of a low-carbon martensite, Fe-1.0 at. pct C, (Fe-0.21 wt pct C), water quenched and tempered for 10 min at 150 °C, showed a matrix carbon content of only 0.14 at. pct. Analysis of a 2 nm diam area centered on a lath boundary showed a local concentration of 2.01 at. pct C. There is some evidence that this carbon level is associated with the presence of a thin film of retained austenite at the boundary. In the case of a higher carbon martensite, Fe-0.64 at. pct Mn, 3.47 at. pct C, (Fe-0.65 wt pct Mn-0.78 wt pct C) water quenched and aged for approximately 24 h at room temperature, analysis of twinned regions showed a matrix carbon level of 2.7 at. pct and a concentration enrichment to 6.9 at. pct in a region 2 nm diam, centered on the coherent twin interface. Assuming the segregated carbon to be located in a single atomic layer at the twin interface, this result indicates that a carbon concentration of 24 at. pct exists locally at the boundary. These results appear to be the first direct demonstration of the segregation of carbon atoms to lattice defects in carbon martensites. Tempering of the higher carbon martensite for 1 h at 160 °C produced further segregation of carbon to the region of twin interfaces. The matrix carbon content fell to 1.5 at. pct and the average carbon content over a 2 nm diam region at the interface rose to 8.7 at. pct. The width of the carbon segregated regions also increased, which seems to imply that incipient carbide precipitation in the plane of the twin boundaries is occurring at this stage of the tempering process.

Low temperature specific heat of dilute iron-carbon martensite

In order to detect density-of-states variations due to carbon dilution in BCC iron, specific-heat experiments were undertaken between 1.2 to 10 K on water-quenched slabs of Fe -C martensites. The data was corrected for the residual-austenite content of the samples, as estimated from magnetization measurements. The results show a very strong increase (about 0.3 mJ/K 2 mol%) of the electronic specific heat coefficient γ.

ON THE STRUCTURE OF IRON-CARBON MARTENSITE

Spectra of Fe-1.5~7 at % C alloys without and 50 kOe longitudinal external magnetic field were measured to clarify the structure of iron-carbon martensite. The spectra of freshly quenched martensite were decomposed into three magnetic hyperfine splittings having different fields, and three additional components were found in the aged specimen. Derived Mossbauer parameters are discussed in connection with the structure of martensite.

メスバウアー分光法によるFe-Cマルテンサイト構造の研究

メスバウアー分光法によるFe-Cマルテンサイト構造の研究