Proeutectoid Cementite Research Articles

Magnetic-field-induced microstructural features in a high carbon steel during diffusional phase transformation

In this work, a high purity, high carbon steel was heat treated without and with a 12-T magnetic field. The microstructural features induced by magnetic field during its diffusion-controlled austenite decomposition were investigated by means of optical microscopy and SEM/EBSD. It is found that the magnetic field increases the amount of the abnormal structure, which is composed of proeutectoid cementite along the prior austenite boundaries and ferrite around it, because magnetic field increases the austenite grain size and promotes the transformation of carbon-depleted austenite to ferrite. No specific orientation relationship between abnormal ferrite and cementite has been found in the non-field- or the field-treated specimens. Magnetic field evidently promotes the spheroidization of pearlite, due to its effect of enhancing carbon diffusion through raising the transformation temperature and its effect of increasing the relative ferrite/cementite interface energy. As magnetic field favors the nucleation of the high magnetization phase-pearlitic ferrite, the occurrence of the P-P2 OR that corresponds to the situation that ferrite nucleates prior to cementite during pearlitic transformation is enhanced by the magnetic field.

Microstructure Evolution in Deformed Ultrahigh Carbon Low Alloy (Wootz) Steel

The evolution of microstructure with degree of deformation in a deformed wedge shaped hypereutectoid steel implement, containing 1.84 C, 0.06 Si, 0.01 S, 0.14 P, 0.11 Cu, 0.017 Zr and 0.05 Ce, has been characterized using optical and scanning electron microscopy (SEM) and electron back scattered diffraction (EBSD). Microstructures consisted of bulky carbides dispersed in a matrix of pearlitic, with grain boundary proeutectoid cementite, side plate Widmanstatten cementite and intragranular cementite were noted in region of low strain. The cast structure was broken down with increasing degree of deformation. The mechanism for the formation of spheroidization of cementite in hypereutectoid steel has been proposed. Microstructural analysis of high strain region consists of fine equiaxed ferrite grains surrounded by high angle boundaries.

Influence of the Microstructure of Eutectoid Steel on the Cyclic Crack Propagation: Pearlite and Spheroidite

In this paper the influence of microstructure on fatigue crack growth was analysed in steel with slightly hypereutectoid composition. A material constituted of pearlite colonies and a thin layer of proeutectoid cementite (pearlitic steel) was studied in its initial condition (as received). In addition, the same material was analyzed after undergoing a spheroidization treatment obtained by an isothermal treatment on pearlitic steel at 700oC and heating time of 10h. Results indicate that fatigue crack propagation curve in the Paris region is not modified by the spheroidization process. Fractometallographic analysis showed a change in the micromechanism of fatigue, evolving from transcollonial (trying to break pearlite lamellae) in the pearlitic steel to intergranular in the spheroidized steel, where cracking takes place through the layer of proeutectoid cementite.

Direct Spheroidization of High Carbon Steels: Effect of Thermomechanical Processing

The eutectoid transformation of austenite can occur cooperatively (pearlite transformation) or by means of a non-cooperative mode (Divorced Eutectoid Transformation). In the cooperative mode, ferrite and cementite grow together, leading to the typical lamellar microstructure of pearlite. In the non-cooperative mode, spheroidal cementite particles grow directly from the undissolved carbides in the austenite phase. The transformation product is a fully spheroidized microstructure. In this study, the parameters promoting the occurrence of DET in a hypereutectoid steel (austenitization temperature, cooling rate, presence of proeutectoid cementite in the initial microstructure) were investigated. It is shown that low undercooling levels and a homogenous distribution of fine carbides in the austenite promote the DET over the lamellar transformation mode. The spheroidized microstructures produced by DET lead to larger ductilities comparing to those obtained by the lamellar transformation mode.

Correlation between laboratory ball-on-disk and full-scale rail performance tests

Abstract Railroads in the United States spend approximately $2.5 billion a year on rail replacement and repairs, making rail the most valuable asset for the railroad industry in North America. The Transportation Technology Centre, Inc. (TTCI) continuously conducts full-scale rail performance tests using the newest generations of premium rails. This in turn allows better understanding of the rail characteristics that require improvements and is used to extend rail's life. Recent research has focused on methods to streamline the developments of prototype rail steels using laboratory tribotests. The results of the tribotests indicate that sliding ball-on-disk experiments can be used to qualitatively approximate rail wear and using scanning electron microscopy (SEM) it is possible to observe different tribological behaviour between rails (mild to severe wear). For instance, SEM micrograph analysis can lead to the determination of the propensity of rail crack formation under pure sliding and the detrimental effects of pro-eutectoid cementite and hard inclusions (e.g., Al 2 O 3 ) on crack formation and delamination.

Mechanical Properties of Hypereutectoid Steel with Microduplex (α+θ) Structure Formed by Hot Deformation of Undercooled Austenite

Microstructure evolution and mechanical properties of hypereutectoid steel with the microduplex (α+θ) structures formed by hot deformation of undercooled austenite were investigated by hot uniaxial compression tests in a Gleeble-1500 simulation test machine, and the effects of subsequent annealing and the addition of Al were analyzed. The results indicated that at the beginning of hot deformation of undercooled austenite the formation of proeutectoid cementite was retrained and only lamellar pearlite was produced. With further strain, dynamic spheroidization of pearlite took place, leading to the formation of microduplex (α+θ) structure consisting of ultrafine ferrite matrix and dispersed cementite particles. In comparison with the normal microstructure consisting of lamellar pearlite and proeutectoid cementite, the microduplex (α+θ) structure presented higher strengths with similar ductility. Subsequent annealing could make the microduplex (α+θ) structure more uniform, which demonstrated better balance between strength and ductility. The addition of Al is disadvantageous to the formation of microduplex (α+θ) structure, but can result in the further refinement. With the addition of Al, the strength of microduplex (α+θ) structure was improved and the ductility was not deteriorated markedly.

The morphology and formation mechanism of pearlite in steels

A number of morphological features of pearlite were revealed through scanning electron microscopy using deeply etched specimens. These include cementite branching, bridging, gaps, holes and curvature. The presence of cementite thin films or networks along the austenite grain boundaries in eutectoid steel and at the interface between pearlite and proeutectoid ferrite in hypoeutectoid steel is another characteristic of pearlite. Furthermore, ferrite thin films surrounding the proeutectoid cementite in hypereutectoid steels are also observed. Hence, it is considered that in hypoeutectoid steels the nucleus for pearlite is a film of cementite rather than the expected proeutectoid ferrite and, similarly, in hypereutectoid steels pearlite forms from a ferrite film rather than from proeutectoid cementite. Convergent beam Kikuchi line diffraction was used to accurately determine the orientation relationships between pearlitic constituents and parent austenite in a Hadfields steel. The results show that neither the pearlitic ferrite nor the cementite is crystallographically related to the austenite grain into which the pearlite was growing and to that into which it was not growing. In addition, a new orientation relationship between pearlitic cementite and ferrite in the Hadfield steel was also observed. It is: ( 1 ¯ 03 ) Z | | ( 1 ¯ 01 ) F [ 010 ] Z || [ 131 ] F [ 311 ] Z 8 .5 ° from [ 1 1 ¯ 1 ] F

The proeutectoid cementite transformation in steels

<title/>A comprehensive, critical, and up to date review is presented for the proeutectoid cementite transformation in steels. It is believed that many of the new findings, features, and concepts presented here for this classic phase transformation in steels serve as a model which may be more broadly applicable to test against many other phase transformations systems as well. There were a number of early investigations of cementite morphology, and this review considers those early results in light of many newer studies that provide critical new insight into cementite morphologies in both two and three dimensions. A number of different orientation relationships (ORs) between proeutectoid cementite and the austenite matrix from which it forms have been reported in the literature, in some cases leading to confusion, and they are critically evaluated here, as are the habit plane, growth direction, and interfacial structure of various morphologies of proeutectoid cementite. Quantitative experimental and theoretical investigations of the growth kinetics of the proeutectoid cementite transformation are considered next, and the nucleation site of proeutectoid cementite in austenite is also discussed in some detail. This review considers all of these issues in a critical way in which differences, commonalities, important features, and redundancies are sorted out, in order to present a unified picture that will add some clarity to this subject. The different features and issues of this transformation that are considered in detail throughout this review are finally brought together in a comprehensive way in the last major section of this paper on ‘Formation mechanism(s) of proeutectoid cementite’, in order to provide a complete, modern view of the formation of proeutectoid cementite from austenite. To the best knowledge of the present authors, before this review a thorough assessment of this classic phase transformation in steels had not been undertaken since 1962, when Professor Hubert I. Aaronson covered this topic in a section of the book entitled ‘The Decomposition of Austenite by Diffusional Processes’.<xref ref-type="bibr" rid="b1">1</xref> In large part due to a number of ground breaking new findings on the proeutectoid cementite transformation since then (particularly in the last decade), it is very timely for a new review on this topic.

612 初析セメンタイト・層状セメンタイトに及ぼす熱処理温度の影響に関する基礎研究 : セメマンタイトを利用したマイクロデバイスについて(材料と組織)

In this study, carbon steels(545C,SK105C)were kept constant austenitizing temperature(1200℃)and the cooling rate from 750℃ to 700℃ was changed. This cooling rate will give us the pro-eutectoid cementite and ferrite of large size. Then, the sample was cut off thinly using the wire-cut electric discharge machine and the surface of sample has been corroded with Kalling's reagent. It was able to get the sample that only the ferrite dissolved in reagent the pro-eutectoid cementite and lamellar cementite remain in sample. The voids made by resolving the pro-eutectaid ferrite have been observed by the use of Scanning Electron Micro-scope(SEM).

Microstructural features induced by a high magnetic field in a hypereutectoid steel during austenitic decomposition

Abstract A 12-T magnetic field was introduced during the austenitic decomposition of a high carbon steel at various cooling rates. It was found that the magnetic field applied can reduce the amount of proeutectoid cementite, increase the lamellar spacing of pearlite and decrease the frequencies of low angle misorientations in ferrite. The experimental results of this work prove the shift of the eutectoid point to high carbon side and high temperature side by the application of a magnetic field.

Effect of Pearlite Structure on the Mechanical Properties of Microalloyed Hypereutectoid Steels

The relationship between mechanical properties and pearlite microstructure was investigated using various heat treatments on a hypereutectoid steels containing 1% carbon with different levels of vanadium and silicon. Specimens were heat treated at various temperatures ranging from 900 to 1200°C and transferred to salt bath conditions at 550, 580 and 620°C to examine the structural evolution of pearlite. The results show that the thickness of the cementite network increases with increasing reheat temperature. This is likely due to the larger austenite grain size reducing the grain boundary area available for proeutectoid cementite nucleation. It was found that the vanadium and silicon additions increased the strength of hypereutectoid steels through refinement of the microstructure and precipitation strengthening.

Three-dimensional analysis and classification of grain-boundary-nucleated proeutectoid ferrite precipitates

The three-dimensional (3-D) shapes and distributions of grain-boundary-nucleated proeutectoid ferrite precipitates have been obtained by computer-aided 3-D reconstruction of serial sections of an Fe-0.12 wt pct C-3.28 wt pct Ni alloy. Isothermal transformation for short times at 650 °C was used to produce a low volume fraction of ferrite, which appeared as both Widmanstatten shapes and more-equiaxed grain-boundary precipitates. While the two-dimensional (2-D) cross sections of these precipitates appeared to fit into the previously accepted categories of precipitate morphologies, 3-D reconstructions revealed important aspects of connectivity and shape that were not observed earlier. A partially revised morphological classification based on these 3-D observations is provided here. Significant differences between the 3-D morphology of proeutectoid ferrite and that of proeutectoid cementite are also discussed, indicating (among other things) that the 3-D morphological classifications of ferrite are generally more complex than those of proeutectoid cementite.

The effect of Mn partitioning in Fe–Mn–Si alloy investigated with STEM-EDS techniques

The partitioning of Mn in the multiphase structure of a low—alloy 04G4 steel, showing a transformation—induced plasticity effect, was investigated using transmission electron microscopy (TEM) and energy-dispersive X-ray spectrometry (EDS). The application of a soft annealing at 898K for 1–60h between hot rolling and cold rolling activates Mn diffusion in the ferrite matrix and Mn enrichment processes in the cementite. During continuous annealing at 973K in the (α+γ) temperature range new austenite (γ) is formed. A non-uniform distribution of Mn between bainitic and polygonal ferrite (α) matrix and retained austenite was observed after accelerated cooling. An increase of Mn content up to 10wt.% was detected in proeutectoid cementite and in martensite–austenite islands after water quenching from 973K.

Characterization of pearlite grains in plain carbon steel by Barkhausen emission

Barkhausen emission (BE) measurements are made on a series of plain carbon steel samples with different carbon contents. Different heat treatments are used to produce a ferrite–pearlite structure for the samples with hypoeutectoid compositions (C<0.76 wt.%), and a cementite–pearlite structure for those with hypereutectoid compositions (C>0.76 wt.%). It is found that as the carbon content increases, the BE signal increases for the hypoeutectoid steel samples but decreases for the hypereutectoid samples. The maximum BE signal is detected in samples with carbon content closest to the eutectoid composition (0.76 wt.%C) at which the volume fraction of pearlite is largest. Therefore, it is evident that the BE signal increases monotonically with pearlite content for both hypoeutectoid and hypereutectoid samples. However, no trivial or obvious relationship is found between the BE signal and the carbon content. It is also observed that the BE peaks of the hypereutectoid samples tend to shift to the lower field region as the content of carbon increases, such shift is due to the pinning effect of the increasing proeutectoid cementite on the domain walls.

Three-dimensional observations of proeutectoid cementite precipitates at short isothermal transformation times

Three-dimensional (3D) analysis techniques were used to examine a model Fe–1.34% C–13.0% Mn alloy to reveal connectivities of proeutectoid cementite precipitates whose growth was arrested at an early stage. The present results indicate that grain boundary cementite precipitates nucleate at austenite grain boundary edges and corners, then grow and spread on the grain boundary faces. No cementite precipitates were found to connect solely to austenite twin boundaries, which appear to act as barriers to precipitate growth rather than as potential nucleation sites. Cementite precipitates were all connected to austenite grain boundaries or cementite grain boundary precipitates, confirming 3D observations made earlier on a specimen transformed at a longer isothermal transformation time. Widmanstätten lath precipitates appear to emanate only from grain boundary cementite precipitates. While edges of several Widmanstätten plate precipitates were observed to intersect with areas of ‘clean’ austenite grain boundary, they may or may not nucleate directly on austenite grain boundaries.

The influence of (MnS+VC) complex precipitate on the crystallography of intergranular pearlite transformation in Fe–Mn–C hypereutectoid alloys

Vanadium addition to a hypereutectoid Fe–Mn–C alloy decreases the proportion of specific orientation relationship between ferrite in intergranular pearlite and the adjacent austenite grains because many pearlite grains nucleate at (MnS+VC) or VC precipitates formed at austenite grain boundaries, whereas pearlite grows from the proeutectoid cementite at austenite grain boundaries in the vanadium-free alloy.

Application of orientation imaging microscopy to study phase transformations in steels

In this investigation, the methodology, advantages, and limitations of using Orientation Imaging Microscopy (OIM) to study microstructures in multiphase steels are examined. In particular, proeutectoid ferrite precipitation has been investigated in a hypoeutectoid steel (Fe–0.12%C–3%Ni), while the proeutectoid cementite transformation has been studied in a hypereutectoid steel (Fe–1.3%C–13%Mn). By utilizing misorientation maps and inverse pole figure maps to determine the orientation differences between individual crystals, the internal morphology of the solid-state precipitates has been determined. What appear by other types of imaging to be monolithic single crystals are shown to be aggregates of finer crystals with misorientations between them. The relative quality of the Electron Backscatter Diffraction (EBSD) patterns has been correlated to specific phases and to interphase and grain boundaries using pattern quality maps (also referred to as image quality maps). These techniques have been adapted for quantitative identification of different microconstituents, including volume fraction measurements. By making comparisons among optical micrographs, conventional SEM micrographs, pattern quality maps, inverse pole figure maps, and misorientation maps, the advantages and limitations of different types of OIM mapping for studying different aspects of phase transformations in steels are elucidated.

Transmission electron microscopy of deep etched cementite

A novel transmission electron microscopy (TEM) sample preparation technique was applied during a study of proeutectoid cementite precipitates in a Fe–1.34 wt.% C–13.0 wt.% Mn alloy. In isothermally transformed specimens, the material contains interconnected networks of grain boundary and Widmanstätten cementite precipitates. By immersing conventionally prepared (dimpled and electropolished) TEM specimens in an etching solution, the three-dimensional morphology and internal structure of entire electron transparent cementite precipitate networks were revealed to provide insights that were not previously possible. This paper describes the experimental technique and illustrates the benefits of this technique by comparing present results with observations of thin foil specimens of the same alloy.

Three-dimensional analysis of microstructures

When using traditional metallographic techniques, materials scientists often make assumptions about the shape, distribution, and connectivity of three-dimensional (3D) features that lie buried within the material. Recent work has shown that these assumptions can be incorrect. An approach to 3D analysis of microstructures is demonstrated here in three different alloy steels. Proeutectoid cementite, ferrite, and an entire pearlite colony were characterized using computer-aided visualization of 3D reconstructions from serial section images. The present article describes various experimental techniques, as well as recent results and advances in computer-aided reconstruction and visualization.

Abnormal ferrite in hyper-eutectoid steels

The microstructural characteristics of ultra-high carbon hyper-eutectoid Fe–C and Fe–C–Cu experimental steels have been examined after isothermal transformation in a range just beneath the eutectoid temperature. Particular attention was paid to the formation of so-called “abnormal ferrite”, which refers to coarse ferrite grains which can form, in hyper-eutectoid compositions, on the pro-eutectoid cementite before the pearlite reaction occurs. Thus it is confirmed that the abnormal ferrite is not a result of pearlite coarsening, but of austenite decomposition before the conditions for coupled growth of pearlite are established. The abnormal ferrite formed on both allotriomorphic and Widmanstätten forms of pro-eutectoid cementite, and, significantly, it was observed that the pro-eutectoid cementite continued to grow, despite being enclosed by the abnormal ferrite. Under certain conditions this could lead to the eventual formation of substantially reduced amounts of pearlite. Thus, a model for carbon redistribution that allows the pro-eutectoid cementite to thicken concurrently with the abnormal ferrite is presented. The orientation relationships between the abnormal ferrite and pro-eutectoid cementite were also determined and found to be close to those which have been reported between pearlitic ferrite and pearlitic cementite.