Globular Cementite Research Articles

Magnetic domain wall dynamics studied by in-situ Lorentz microscopy with aid of custom-made Hall-effect sensor holder

We built a custom-made holder with a Hall-effect sensor to measure the single point magnetic flux density inside a transmission electron microscope (TEM, JEM-F200, JEOL). The measurement point is at the same place as the sample inside the TEM. We utilized information collected with the Hall-effect sensor holder to study magnetic domain wall (DW) dynamics by in-situ Lorentz microscopy. We generated an external magnetic field to the sample using the objective lens (OL) of the TEM. Based on our measurements with the Hall-effect sensor holder, the OL has nearly linear response, and when it is switched off, the strength of the magnetic field in the sample region is very close to 0 mT.A ferritic-pearlitic sample studied has globular and lamellar cementite (Fe3C) carbides in the ferrite matrix. Based on the in-situ Lorentz microscopy experiments, DWs in the ferritic matrix perpendicular to the lamellar carbides start to move first at ∼10 mT. At 160 mT, DWs inside the globular carbide start to disappear, and the saturation occurs at ∼210 mT. At 288 mT, the DWs parallel to the lamellar carbides still exist. Thus, these lamellar carbides are very strong pinning sites for DWs. We also run dynamical micromagnetic simulations to reproduce the DW disappearance in the globular carbide. As in the in-situ experiments, the DWs stay stable until the external field reaches the magnitude of 160 mT, and the DWs disappear before the field is 214 mT. In general, the micromagnetic simulations supported very well the interpretation of the experimental findings.

Microstructural characterization of near-surface microstructures on rail wheels in service - an insight into "stratified surface layers".

Background: To decrease maintenance costs and improve safety in rail transportation, the understanding of rail and wheel defects is vital. Studies on "white etching layers" (WEL) on rails and wheels, prone to fatigue crack initiation, have been extensively studied. Recently, a relative named "brown etching layer" (BEL) and its combination, the so-called "stratified surface layer" (SSL), are observed in the field. This study presents an investigation on a rail wheel affected by mechanical and thermal loadings from service with focus on the different evolved layers in the near-surface region. Methods: Optical microscopy is performed on etched cross-sectional cuts to identify different evolved microstructures (WEL, BEL, SSL), further, specific regions are investigated in detail by scanning electron microscopy to evaluate the microstructural characteristics. To analyze the change in mechanical properties, low-load Vickers hardness investigations are executed in distinctive zones. Results: This study highlights the broad variety of evolved microstructures, however, a rough classification of WEL (fine mesh-like microstructure, 900 - 1200 HV0.01) and BEL (globular cementite particles, 400 - 600 HV0.01) is given. Further, results indicate that the BEL is commonly accompanied by a WEL, representing an SSL. Conclusions: The complex loading situation in a wheel-rail contact can lead to the formation of WEL, BEL and SSL. The observation of numerous initiated fatigue cracks within these regions demonstrates the relevance of in-depth studies on evolved microstructures in wheel-rail contacts.

Microstructural characterization of near-surface microstructures on rail wheels in service – an insight into “stratified surface layers”

Background: To decrease maintenance costs and improve safety in rail transportation, the understanding of rail and wheel defects is vital. Studies on “white etching layers” (WEL) on rails and wheels, prone to fatigue crack initiation, have been extensively studied. Recently, a relative named “brown etching layer” (BEL) and its combination, the so-called “stratified surface layer” (SSL), are observed in the field. This study presents an investigation on a rail wheel affected by mechanical and thermal loadings from service with focus on the different evolved layers in the near-surface region. Methods: Optical microscopy is performed on etched cross-sectional cuts to identify different evolved microstructures (WEL, BEL, SSL), further, specific regions are investigated in detail by scanning electron microscopy to evaluate the microstructural characteristics. To analyze the change in mechanical properties, low-load Vickers hardness investigations are executed in distinctive zones. Results: This study highlights the broad variety of evolved microstructures, however, a rough classification of WEL (fine mesh-like microstructure, 900 – 1200 HV0.0.1) and BEL (globular cementite particles, 400 – 600 HV0.01) is given. Further, results indicate that the BEL is commonly accompanied by a WEL, representing an SSL. Conclusions: The complex loading situation in a wheel-rail contact can lead to the formation of WEL, BEL and SSL. The observation of numerous initiated fatigue cracks within these regions demonstrates the relevance of in-depth studies on evolved microstructures in wheel-rail contacts.

Analysis of the Low Service Life of a Planing Knife – A Case Study

The presented work deals with an analysis of causes of the very low service life of the new version of hand woodworking planing knife, which has replaced the original version of a planing knife. Based on the performed experimental works, chemical analysis, macroscopic and microscopic observation, SEM, diffractometric analysis, and Vickers hardness test it was concluded that the cause for the very low service life (practically the nonexistent one) of the new version of a planing knife are significant features of overheating. During the heating to the hardening temperature used steel was overheated, the cementite carbides dissolved, and the austenitic grains became coarse. The final microstructure of the used steel after hardening and tempering consists of brittle coarse martensitic needles, without any presence of the fine globular cementite carbides, accompanied by a very high amount of retained austenite. Thus, the chipping of the knife's cutting edge took place and the planer was unusable.

Comparison of wear behavior of GCr15 bearing steel prepared by selective laser melting (SLM) and electron beam melting (EBM)

Both the microstructure and wear behavior of GCr15 bearing steel prepared by selective laser melting (SLM) and electron beam melting (EBM), respectively, are compared for the first time in this study. The results show that the microstructure of SLMed-GCr15 bearing steel is composed of residual austenite and fine martensite and EBMed-GCr15 bearing steel is mainly globular pearlite, the globular cementite precipitated in the ferrite surface is fine (80 ~ 400 nm) and uniformly distributed. The Vickers hardness of the samples prepared by SLM is 750.0±17.1 HV0.2, the coefficient of friction (COF) and wear rate are 0.5082 and 24.6±3.2 μm3N−1m−1, respectively, and the wear mechanism is mainly adhesive wear. The sample prepared by EBM has the lowest COF and wear rate, which is 0.4647 and 18.4±1.7 μm3N−1m−1, respectively, and the wear mechanism is mainly abrasive wear and adhesive wear, which shows excellent friction resistance.

TRANSFORMATION OF CARBIDЕ PHASE IN RAILS AT LONG-TERM OPERATION

Evolution of carbide phase in surface layers of volume (passed gross tonnage 500 and 100 million tons) and differentially hardened rails (passed tonnage – 691.8 million tons) to a depth of 10 mm along the central axis and along the rail head fillet was studied by means of transmission electron diffraction microscopy. The grains of lamellar perlite, ferrite-carbide mixture, structurally free ferrite are analyzed. The flow of two complementary mechanisms of transformation of carbide phase of steel in the surface layers during the rails operation was identified: mechanism of cutting cementite particles and their subsequent­ transfer into the ferrite grains or plates volume (in perlite structure); mechanism of cutting and following dissolution of cementite particles, transition of carbon atoms to dislocations (into the Cottrell­ clouds and the dislocation centers), transfer of carbon atoms within dislocations to the volume of grains (or plates) of ferrite, with the following repeated formation of nanoscale cementite particles. A fragmented dislocation substructure is formed instead of former plates. Fragments boundaries decorate places where cementite-α phase interphase boundaries used to be. The main reason for dissolution of cementite is that it is energetically more preferable for carbon atoms to be on dislocation centers and on subboundaries than in cementite lattice. Binding energy of carbon atom-dislocation is 0.6 eV, for carbon atom-subboundary bond it is 0.8 eV, while in cementite it is held by 0.4 eV. Formation of elastoplastic stress fields is detected, concentrators of which are intra and interphase boundaries between grains of ferrite and perlite, cementite and ferrite plates of perlite colonies, particles of globular cementite and ferrite. The main sources of curvaturetorsion of metal lattice of rails metal are intraand interphase bounda­ries­ of grain separation of ferrite and perlite, cementite and ferrite plates of perlite colonies, particles of globular cementite and ferrite. Approaching to the rolling surface, number of stress concentrators and amplitude of internal fields of longrange stress are increasing.

Transformation of Carbides in Prolonged Rail Operation

The evolution of the carbide phase in the surface layers of bulk-quenched rails (after the passage of 500 and 1000 million t of traffic) and differentially quenched rails (after the passage of 691.8 million t) to a depth of 10 mm at the central axis of the rail cross section and at the nearby rounded section is studied by transmission electron-diffraction microscopy. The grains of plate pearlite, ferrite–carbide mixture, and structure-free ferrite are analyzed. The carbide phase in the surface layers of the steel changes in two mutually complementary processes during rail operation: (1) cleavage of cementite particles with subsequent entrainment in ferrite grains or plates (in the pearlite structure); (2) cleavage and dissolution of cementite particles, with transfer of carbon atoms to dislocations (in Cottrell atmospheres and in dislocational cores), which transport them to the ferrite grains (or plates), where cementite nanoparticles are formed again. In the previous location of the plates, fragmented dislocational substructure appears. The boundaries of the fragments are found at the positions previously occupied by cementite α-phase boundaries. The solution of cementite is mainly due to the energy of carbon atoms at dislocation cores and subboundaries in comparison with the cementite lattice. The binding energy of the carbon atom and the dislocations is 0.6 eV and the binding energy of the carbon atom and the subboundary is 0.8 eV, as against 0.4 eV for the carbon atom in cementite. Elastoplastic stress fields are formed; their stress concentrators are intra- and interphase boundaries of ferrite and pearlite grains, cementite plates and ferrite of the pearlite colonies, and globular cementite and ferrite particles. Those are also the basic sources of curvature and torsion in the crystal lattice of the rail steel. On approaching the contact surface, the number of stress concentrators increases, and the internal long-range stress fields are of greater amplitude.

Long-term operation surface changes in differentially quenched 100-m rails

By optical microscopy and transmission electron diffraction microscopy, the evolution of the structural and phase states in the surface layers over a depth of 10 mm in the head of differentially quenched rail (category DT350) is studied, as the rail is subjected to passed tonnage of 691.8 million t at the experimental loop of AO VNIIZhT. In the initial state, the following structural components are present in the rail head: plate-pearlite grains (relative content 0.7); mixed ferrite–carbide grains (0.25); and grains of structure-free ferrite. After experiencing a passed tonnage of 691.8 million t, this state only remains beyond a depth of 10 mm. At that depth, a large quantity of bend extinction contours is observed. That indicates elastoplastic distortion of the material’s crystal lattice. The stress concentrators in the steel are intraphase and interphase boundaries of the ferrite and pearlite grains, cementite and ferrite plates in pearlite colonies, and globular cementite and ferrite particles. Structural transformations are observed at the macro level: microcracks appear, running at acute angles from the surface to a depth of 140 μm; and a decarburized layer is formed. At the micro level, elastoplastic stress fields are formed, and the cementite plates in the pearlite colonies break down. The stress concentrators in that case are intraphase and interphase boundaries of the ferrite and pearlite grains, cementite and ferrite plates in pearlite colonies, and globular cementite and ferrite particles. In structure-free ferrite grains, cementite nanoparticles are formed. The results are compared with the evolution of the structural and phase states at the surface of a recess in bulk-quenched rail as the rail is subjected to gross loads of 500 million t: the transformation of the structural and phase states in the surface layers is more pronounced. Plate pearlite is characterized by solution of the cementite plates. That leads to the formation of chains of globular carbide particles at the sites of the cementite plates. This may be associated with transfer of the carbon atoms from the cementite lattice to dislocations.

CHANGES IN STRUCTURE AND PHASE COMPOSITION OF THE SURFACE OF DIFFERENTIALLY HARDENED 100-METER RAILS IN OPERATION

By the methods of optical and transmission electron diffraction microscopy the evolution of structural-phase states of the surface layers in the head of differentially hardened rails of category DT350 was studied to the depth up to 10 mm in the fillet after the passed tonnage of 691.8 million gross tones on the experimental ring of JSC “VNIIZhT”. It is shown that in the initial state the following structural constituents present in the rail head: grains of lamellar perlite (relative content is 0.7), grains of ferrite-carbide mixture (0.25) and grains of structurally free ferrite. After the passed tonnage of 691.8 million gross tones this state is preserved only at a depth of more than 10 mm. A distinctive feature of the structure at this distance is a large number of bending extinction contours, which reveals elastoplastic distortions of the crystal lattice in the material. Stress concentrators of the steel under study are intra- and interphase interfaces of grains of ferrite and perlite, cementite plates and ferrite of perlite colonies, globular cementite particles and ferrite. The transformation of the structure is manifested at the macro level in the formation of microcracks at an acute angle to the surface to a depth up to 140 μm, and in the formation of the decarburized layer. At the micro level, the formation of elastoplastic stress fields and the destruction of cementite plates of perlite colonies have been revealed. It is shown that stress concentrators are intra- and interphase interfaces of ferrite and perlite grains, cementite and ferrite plates of ferrite colonies, globular cementite particles and ferrite. In the grains of structurally free ferrite, the formation of nano-sized particles of cementite was observed. The comparison is made with the results of the evolution of structural-phase states in the fillet of volume-hardened rails after the passed tonnage of 500 million gross tones: the most significant transformations of structural-phase states are observed in the surface layers. The evolution of the structural-phase state of perlite of lamellar morphology consists in the dissolution of cementite plates, which leads to the formation of a chain of particles of the carbide phase with globular shape in place of the cementite plate. This is possible due to the transfer of carbon atoms from the crystalline lattice of cementite to dislocations.

Effect of particle size, fraction and carbide banding on deformation and damage behavior of ferrite–cementite steel under tensile/shear loads

Deformation and damage behavior of ferrite–cementite steel was investigated using microstructure-based representative volume element (RVE) methodology. A series of automatically generated 2D RVEs with ferrite matrix and globular cementite particles were generated as representative of various microstructures. The geometrically necessary dislocation (GND) accumulation at the ferrite–cementite interphase was also studied by introducing an intermediate layer around the cementite particles. Damage mechanisms such as ductile fracture in ferrite matrix, brittle fracture in cementite, and decohesion at ferrite–cementite interphase were considered to study the fracture modes. The relationships between interface strength and particle size were estimated according to the modified Argon criterion and showed satisfactory agreement with related works. The influences of microstructural features, such as particle size, particle fraction and carbide banding, on deformation and damage evolution were investigated under tensile and shear loads. Simulation results indicated that small particle size and particle fraction could postpone the initial decohesion under both tensile and shear loads, while carbide banding can lead to early fracture due to local stress concentration, which has potential to cause the loss of ductility and premature failure. These adverse effects become more severe when more cementite particles remain in the band or the gather density of the cementite particles in the band becomes higher.

Effect of Particle Size and Carbide Band on the Flow Behavior of Ferrite-Cementite Steel

A microstructure-based representative volume element (RVE) method is employed for simulating the mechanical properties of the alloy steel 42CrMo4, a typical ferrite–cementite steel with ferrite (α) and spheroid cementite (θ) phases. A series of 2D RVEs with ferrite matrix and globular cementite particles are generated according to various microstructures, among which the geometrically necessary dislocation (GND) accumulation at the α–θ interphase is introduced as a coating layer around the cementite particles. Satisfactory agreement between the simulation and experimental flow curves can be obtained when the ratio of GND layer thickness to particle diameter is 0.25. On this basis, the influences of some modeling parameters such as element type and boundary conditions, as well as some microstructure features like particle size and carbide band, on the mechanical properties are investigated. The results show that small particle size can increase the strength of the material. Carbide band leads to the microstructure inhomogeneity and early finish of uniform elongation, which is potential to cause the loss of ductility and premature failure. These adverse effects become more severe when more cementite particles remain in band or the cementite particles gather denser in carbide band.

Surface layer structure degradation of rails in prolonged operation

By methods of optical, scanning and transmission electron microscopy and microhardness measurement the transformation regularities of structure-phase states, defect substructure, fracture surface and mechanical properties of rail surface layer up to 10 mm deep in process of long-term operation (passed tonnage of gross weight 1000 mln. tons) were revealed. According to the character of fracture and level of structure imperfection the three layers were detected: surface, transition and boundary ones. It has been shown that the surface layer ~20 μm in thickness has a multiphase, submicro- and nanocrystalline structure and it contains micropores and microcracks. The increased density of bend extinction contours at 2 mm depth from the tread contact surface was noted, and it was shown that the maximum amplitude of stress fields was formed on the interphase boundary the globular cementite particle–matrix. The evaluation of stress fields was done.

Influence of the Initial Microstructure on the Spheroidization of SAE 52100 Bearing Steel

High carbon Cr‐bearing SAE 52100 is widely used for bearing production. The in‐service microstructure of bearing steel is tempered high C martensite, which has superior mechanical properties in severe rolling fatigue conditions. The bearings are produced from steel wire with a pearlitic microstructure. Spheroidization annealing is used to obtain a soft microstructure prior to cold deformation. Spheroidization results in a ferritic matrix with coarse globular cementite. The spheroidization annealing process is lengthy, involving considerable economic cost and an environmental impact related to CO2 emissions. Avoiding spheroidization annealing altogether or substantially reducing the annealing time is, therefore, of industrial relevance. Spheroidization annealing may be omitted if a very soft, i.e., coarse pearlitic microstructure, can be obtained after hot rolling. This is generally considered to be the case for a coarse pearlitic microstructure with a hardness of approximately 270 HV. In practice, the requirement for a guaranteed homogeneous low hardness results in the spheroidization annealing being often still necessary in many circumstances. The present work reports on a study of the potential for the reduction of the spheroidization annealing time by spheroidization of different initial microstructures. The two main findings are (i) that the martensitic and bainitic microstructures spheroidize more rapidly than pearlite and (ii) that an initial martensitic microstructure can achieve a hardness lower than 240 HV within 10 h of spheroidization at 710 °C.

Extension of Fatigue Cracks in a Low-Alloy Steel After Massive Plastic Deformation

Abstract Fatigue crack extension was studied in a ferritic–pearlitic steel in two different material conditions. The first one was the virgin state with a typical pearlitic microstructure containing comparatively large stacks of cementite lamellae. The second material contained predominantly globular cementite and stacks of very fine cementite lamellae and was found after massive plastic deformation during a thermo-mechanical forming process. These differences in the microstructure lead to significantly different mechanisms in the damage accumulation process under fatigue loading. Stacks of cementite lamellae pearlite were both crack initiation sites and microstructural barriers for small cracks in the virgin material, whereas the fine lamellae in the plastically deformed (cold-worked) material could cause crack deviation, but no crack initiation and no significant crack arrest or crack retardation.

Modification of the structure and phase state of a ferrite-cementite composition by an electron beam

The phase composition and defect substructure formed in carbon steel subjected to a microsecond high-current e-beam have been studied by the thin-foil electron diffraction microscopy. It has been shown that the mechanism and the degree of globular cementite dissolution, phase composition and the morphology of the appearing structure depend on the distance from a melt spot and are determined by the temperature field gradient. As an electron beam approaches the "spot" (the increase in steel temperature), the evolution of the material volume containing cementite particles is accompanied by the formation of structures that alternate in a regular manner.

Dislocation substructures and internal stress fields in bulk- and differentially quenched rails

A layer-by-layer analysis of rails bulk-hardened in oil and differentially hardened in a variety of regimes is performed by means of transmission electron microscopy. Quantitative parameters of dislocation substructures and internal stress fields, and their dependences on the distance from the tread contact surface are established. It is shown that the most dangerous stress concentrators are interfaces between globular cementite matrix particles; such interfaces form predominantly in rails subjected to bulk quenching.

Unconventional Microstructures in Tool Steel Obtained by Semi-Solid Processing and Subsequent Heat Treatment

Mechanical properties of all metals depend predominantly on the type and morphology of their microstructure. Microstructure attributes can be altered by various heat treating and thermomechanical treatment procedures. One of the advanced techniques profoundly affecting the microstructure evolution is semi-solid processing. It can produce unconventional microstructures even in conventional steel types. Moreover, subsequent heat treatment can also deliver a wide range of microstructures and correspondingly varied mechanical properties. In the present experimental programme, the X210Cr12 ledeburitic tool steel was studied. Its initial annealed microstructure consisted of ferritic matrix, chromium carbides and globular cementite particles. The semi-solid processed structure, on the other hand, contained polyhedral austenite grains embedded in carbide-austenite network. The austenite volume fraction exceeded 95 %. This microstructure was then altered by subsequent heat treatment or thermomechanical treatment. The experimental programme comprised three stages. At the first stage, the effects of the rate of cooling from the semi-solid region to the ambient temperature on the nature and morphology of the ledeburitic network and the austenitic grain size were explored. The second stage was aimed at the impact of tensile and compressive deformation applied after transition through semi-solid state on the microstructure evolution and, in particular, on grain size. Once suitable processing conditions and parameters were identified, the treatment led to a recrystallized austenitic microstructure with an average grain size of less than 3 μm. As high volume fractions of austenite were obtained, the third stage involved exploring the effects of thermal exposure. The stability of austenite and the decomposition of austenite into other microstructure constituents were mapped. Metallographic observation revealed a resulting wide range of microstructures from fine pearlite to martensite, depending on the heat treating schedule.

Determination of the easy axes of small ferromagnetic precipitates in a bulk material by combined magnetic force microscopy and electron backscatter diffraction techniques.

A method to determine the magnetic easy axes of micro- and nanoscopic ferromagnetic precipitates embedded in a bulk material is proposed and applied to globular cementite (Fe₃C) embedded in a ferrite matrix. The method combines magnetic force microscopy (MFM) with electron backscattered diffraction (EBSD) measurements. Magnetic domain structures in globular and in lamellar cementite precipitates in unalloyed pearlitic steels were imaged using MFM. The domain structure of the precipitates was analyzed in dependency of their size, shape and crystallographic orientation. It was found that the magnetic moments of the cementite precipitates are highly geared to their crystalline axes. The combined MFM and EBSD studies allow the conclusion that the cementite easy direction of magnetization is the long [010] axis. For fine lamellae cementite the determination of their crystallographic orientations using electron diffraction techniques is very difficult. With the previous knowledge of the behavior of the domain structure in globular cementite, the crystalline orientations of the fine lamellae cementite can be estimated by simply observing the magnetic microstructures and the topographic profiles.

The effect of a non-circular drawing sequence on delamination characteristics of pearlitic steel wire

In this study, a non-circular drawing (NCD) sequence was applied to investigate the effect of deformation behavior, microstructure and texture evolution on delamination characteristics of pearlitic steel wire under torsional deformation mode. The multi-pass NCD sequence was numerically and experimentally applied up to the 12th pass in comparison with conventional wire drawing (WD). For investigation of the deformation characteristics of the drawn wires, three-dimensional finite element and flownet analyses were carried out. These simulation results indicated that the NCD could impose relatively homogeneous plastic deformation on the wire compared to the WD. From the scanning electron microscopy and X-ray diffraction results, globular cementite and cylindrical texture component, which might increase likeliness of delamination fracture, were rarely observed in the NCD drawn wire. In the torsion test, the delamination fracture was observed in the WD drawn wire for the 10th pass while it did not occur for the 12th pass NCD. In addition, the ultimate tensile strength (UTS) of 2300MPa grade wire was manufactured by the NCD and the UTS value was 257MPa higher than the one of the WD. Therefore, it was demonstrated that the multi-pass NCD could impose relatively homogeneous plastic deformation on the wire, resulting in high-torsional ductility with better strength compared to the WD.

Formation of gradients of structure, phase composition, and dislocation substructure in differentially hardened rails

A layer by layer analysis of rails, differentially hardened in various modes, has been carried out using transmission electron microscopy on various scale levels. It has been shown that the differential hardening of rails is accompanied by the formation of a morphologically different structure, which is formed according to the diffusion mechanism of γ-α transformation and consisting of plate perlite grains, free ferrite grains, and grains of a ferrite-carbide mixture. The gradient character of modifications of structure, phase composition, and dislocation substructure parameters along the cross section of rail head has been established. It has been revealed that the interfaces between globular cementite particles and the matrix are the most dangerous stress concentrators.