Proeutectoid Cementite Research Articles

Machine Learning Approaches for Classification of Ultra High Carbon Steel Micrographs

The aim of this investigation is to analyze the performance of several supervised machine learning algorithms for solving the automatic classification problem of steel image microstructures. We conducted an experiment using a public-domain dataset of Ultra High Carbon Steel Micrographs (UHCSM). This image database consists of a collection of scanning electron micrographs (SEM) taken from samples of a commercial roll-mill casting with a nominal carbon of 2%. Heat treatments such as annealing, water quenching, air and furnace cooling were performed on steel samples so primary microconstituents could be found in micrographs. Each of these microconstituents defines each of the categories of classification to be accomplished by machine learning algorithms. The heat treatments brought about 4 usable classes (sets of images) of primary microconstituents: pearlite, spheroidite, proeutectoid cementite network, pearlite containing spheroidite. All labeled images are prepared to improve models' accuracy in a preprocessing stage so that the image dataset is ready for feature extraction. In order to develop classification models, we put to the test distinct machine learning approaches by working with Matlab's classification learner application where we perform automated training to search for the best classification model type, including Decision Trees, Support Vector Machines (SVM), Discriminant Analysis, Nearest Neighbors, Naive Bayes, Ensemble k-NN, and Neural Network classification. For obtaining the features of the images (feature extraction) we choose the method of Bag-of-features with 400 words for the first experiment, and 327 words by removing less important features for a second experiment. The experimented models reached very different accuracy values on training, with SVM as the best classifier which gets 91.6% accuracy. We can conclude that classic machine learning algorithms solve the classification, but an accuracy improvement can be reached by investigating deep learning techniques.

Improvement of Mechanical Properties of 20CrMnTi Steel through Microstructure Modification

Microstructure modification is an effective approach to improve the mechanical properties of materials. In the present study, expanded graphite (EG) was added in a 20CrMnTi matrix to form pearlite microstructure and proeutectoid cementite after sintering. Mechanical alloying was used to obtain a fine milled mixture with a ball/powder weight ratio of 20:1. After mechanical alloying for 10 h, the milled mixtures with different EG contents were pressurelessly sintered at 1250 °C for 10 min and then hot-pressed at 1150 °C under 30 MPa for 30 min in Ar to obtain dense and modified 20CrMnTi materials. The content of EG has a profound influence on the microstructure and mechanical properties of 20CrMnTi. A high tensile strength of 1088 MPa and a high Vickers hardness of 4.7 GPa were achieved in the 1% EG-modified 20CrMnTi, which were increased by 105% and 28%, respectively, compared with the pure dense 20CrMnTi. The formed fine pearlite and small proeutectoid cementite in the modified microstructure contributed to the improved mechanical properties of 20CrMnTi.

Continuous Cooling Transformation Behavior of High Carbon Pearlitic Steel

The continuous cooling transformation behavior of high-carbon pearlitic steel was studied by employing optical microscopy, scanning electron microscopy, and the Vickers hardness test. The results show that the microstructure of the test steel is composed of proeutectoid cementite and lamellar pearlite in the cooling rate range of 0.05–2 °C/s and lamellar pearlite in the range of 2–5 °C/s. Further, martensite appears at 10 °C/s. With the increase in the cooling rate, the Vickers hardness of the test steel first decreases and then increases. In the industrial production of high-carbon pearlite steel, the formation of proeutectoid cementite at a low cooling rate needs to be avoided, and at the same time, the formation of martensite and other brittle-phase at a high cooling rate needs to be avoided.

Effect of Al on Pearlite Transformation and Spheroidization of High Carbon Steel

Normalizing and isothermal spheroidizing annealing tests were carried out on two types of high-carbon steel specimens with different aluminum (Al) contents. The effects of Al content on the phase transformation and spheroidization of high-carbon steel were analyzed. It was found that addition of Al element can increase the pearlitic eutectoid transition temperature of high-carbon steel, making the eutectoid point move toward the high-carbon direction, and inhibiting the precipitation of proeutectoid cementite during pearlitic transformation. The higher the Al content, the greater the undercooling degree during pearlitic transformation and the finer the pearlite lamellar spacing would be after transformation. In addition, the finer pearlite lamellar spacing allowed the test steel to achieve more phase interfaces in the same crystal range and more dislocations and other defects on the interface, thus providing more nucleation centers for lamellar cementite during isothermal spheroidizing annealing. In addition, more defects provided more diffusion channels for carbon during spheroidizing annealing and smaller interlamellar spacing to shortening the diffusion distance of carbon atoms, thus accelerating carbon diffusion rate and spheroidizing the lamellar cementite into granular one. The addition Al element can improve the machinability of steel and facilitate the commercial application of pearlitic steel.

A multiscale mean field model for elastic properties of hypereutectoid pearlitic steels with different microstructures

Abstract Multiscale modeling of macroscopic elastic properties of pearlitic hypereutectoid steel using the Eshelby matrix–inclusion approach is possible. The model works through successive homogenization steps, based on the elastic properties of cementite and ferrite. Globular pearlite is homogenized using α Mori–Tanaka approach. Lamellar pearlite and pearlite colonies with fragmented proeutectoid cementite are homogenized by α classical self-consistent scheme. In the case of pearlite colonies surrounded by α continuous cementite film, α generalized self-consistent scheme is used. The influence of microstructural parameters such as the pearlite colony size or the thickness of the proeutectoid cementite on Young’s and shear moduli and on coefficients of the stiffness tensor is simulated. Proof of concept is obtained by comparison between predicted elastic behavior and experimental results from the literature.

Sintered Fe-Cr-Mo-Si-C alloys produced from pre-alloyed Fe-Cr based powders admixed with 4% SiC for high performance applications

The use of pre-alloyed Fe-Cr based powders has been steadily increasing in the automotive industry. Mechanical properties can be tailored due to phase transformations controlled by alloying elements and cooling rates. Employing silicon carbide (SiC) as a source of alloying elements has been proved to be possible. In this study, three different pre-alloyed Fe-Cr based powders, namely Astaloy CrA, Astaloy CrL, and Astaloy CrM, were alloyed with 4 wt. % SiC powder. The powders were mixed and pressed into tensile test bars. The green compacts were sintered at 1250 °C for 45 minutes under the cracked ammonia gas and slowly cooled (<0.1°C/s) to room temperature. All experimental sintered alloys manifested a common feature resembling those of full pearlitic ductile irons, i.e., the feature consisted of black nodular particles enveloped with hyper-eutectoid steel matrix (grain boundary or pro-eutectoid cementite plus pearlite). The tensile testing indicated that Alloy-3 (Astaloy CrM + 4%SiC) was observed to have better mechanical properties in comparison with Alloy-1 (Astaloy CrA + 4%SiC) and Alloy-2 (Astaloy CrL + 4%SiC), apart from slightly lower elongation than Alloy-2. Regardless of SiC addition, the presence of chromium (Cr) content was observed to substantially influence mechanical properties and microstructure. According to the investigation of fracture surfaces by SEM, mixtures of shallow dimples and cleavage fractures were examined in the sintered alloys.

Metallurgical Failure Investigation of Replaceable Tooth Flank Used in Gear Rim Applications

Tooth flanks used in gear rim applications of iron ore pelletizing plant get failed frequently, and the failures are also likely to take place in other suspected flanks. The objective of the study is to identify crack through nondestructive inspection and ascertain the probable cause and causes of failure to occur in the part based on examination and by studying its chemical, metallurgical, and mechanical properties. Ultrasonic inspection has been carried out on many tooth flanks fitted in the gear rim at plant site and found the presence of a subsurface crack in the core region at 40 mm depth from the side surface of the tooth flank. The inspected and identified tooth flank was used for failure analysis. Liquid penetrant test was carried out and it was found that a longitudinal centerline crack existed which propagated from core to the surface toward the trailing edge. Samples identified near the cracked region and surface were cut by machining, and metallographic characterization was carried out through optical, field emission scanning electron microscope, and electron probe microanalyzer. The microstructural features show the presence of many isolated micro-cracks and their orientation and inclusions. The presence of the martensitic phase at subsurfaces and pro-eutectoid cementite at isolated regions with the presence of high carbon concentration, amount to an increase in hardness in comparison with tempered martensite phase at core region. It was observed that micro-crack nucleated at nonmetallic inclusions. The nonmetallic inclusions, i.e., MnS, the difference in microstructures, a variation of hardness and nucleation of the void, void growth, and void coalescence, were found in the grain interface leading to propagation of crack and caused the rupture of the part during service.

Research and control of network carbide in GCr15 bearing steel

Due to the small amount of transformation during the phase transformation process, it is difficult to determine the phase transition temperature of the proeutectoid cementite in the bearing steel by the expansion method. In this paper, the Gleeble3800 thermal simulator was used to study the phase transformation behavior of GCr15 bearing steel at different cooling rates. Firstly, the phase transformation temperature range of pearlite is about 550 °C ∼ 700 °C measured by expansion method. Based on this, the transformation temperature range of proeutectoid cementite is about 700 °C ∼ 900 °C studied by metallographic method; The precipitation curve of proeutectoid cementite was supplemented, and the complete continuous cooling phase transition curve (CCT curve) of bearing steel was drawn. On this basis, the scheme of inhibiting the formation of proeutectoid cementite by two-stage cooling process was proposed and discussed, and the finishing cooling temperature was studied, which provides a basis for controlling network carbide in field production.

Fatigue and fracture crack paths in spheroidized steel

This paper analyses the fatigue and fracture behavior of a steel with quasi-eutectoid composition (slightly hypereutectoid) and spheroidized microstructure. The fatigue crack surface showed ductile micro-tearings produced by propagating the crack through the thin layer of proeutectoid cementite. The fracture is initiated at the fatigue crack tip in a fibrous zone by microvoid nucleation, which is transformed into a cleavage fractography. Microvoids are observed in the large particles of cementite located in the proeutectoid layer, by decohesion with the matrix or by fracture.

An Investigation of the Crystallography of Pearlites Nucleated on the Proeutectoid Cementite

An Investigation of the Crystallography of Pearlites Nucleated on the Proeutectoid Cementite

Coarsening of Inter- and Intra-granular Proeutectoid Cementite in an Initially Pearlitic 2C-4Cr Ultrahigh Carbon Steel

We have examined spheroidization and coarsening of cementite in an initially pearlitic 2C-4Cr ultrahigh carbon steel containing a cementite network. Coarsening kinetics of spheroidized cementite and growth of denuded zones adjacent to the cementite network were investigated by analyzing particle sizes from digital micrographs of water-quenched steel etched with Nital. Denuded zones grew at a rate proportional to t 1/4–t 1/5. Spheroidization of pearlite was completed within 90 minutes at 1073 K and 1173 K (800 °C and 900 °C), and within 5 minutes at 1243 K (970 °C). Bimodal particle size distributions were identified in most of the samples and were more pronounced at higher temperatures and hold times. Peaks in the distributions were attributed to the coarsening of intragranular and grain boundary particles at different rates. A third, non-coarsening peak of particles was present at 1073 K (800 °C) only and was attributed to particles existing prior to the heat treatment. Particle sizes were plotted vs time to investigate possible coarsening mechanisms. The coarsening exponent for the growth of grain boundary carbides was closest to 4, indicating grain boundary diffusion control. The coarsening exponent was closest to 5 for intragranular carbides, indicating suppression of volumetric diffusion (possibly due to reduced effective diffusivity because of Cr alloying) and control by dislocation diffusion.

Effect of Hot Forging on Microstructural Evolution and Impact Toughness in Ultra-high Carbon Low Alloy Steel

The effect of a hot forging ratio on the microstructural variation and tensile properties of ultra-high carbon low alloy steel was investigated. Scanning electron microscopic analyses depict that with an increase in the hot forging ratio, the thickness of the network and acicular proeutectoid cementite decreased. Moreover, the lamella spacing and thickness of the eutectoid cementite decreased and broke up into particle shapes, which then became spheroidized as the hot forging ratio increased. Furthermore, when the forging ratio exceeded 65%, the network and acicular shape of the as-cast state disappeared. By increasing the hot forging ratio, the tensile strength and elongation remained below 50%, but they increased rapidly with an increase in the forging ratio. Strength and elongation were not affected by the thickness of the proeutectoid and eutectoid cementites, but were greatly affected by the shape of the proeutectoid cementite. Due to the decrease in the austenite grain size, as well as the spheroidization of the cementite, the tensile strength and elongation sharply increased.

Microstructural characterisation of rolling contact fatigue damage in flashbutt welds

The development of premium heat treated rail steels with hardness levels of 400HB and above, and the application of these under high axle load conditions, has provided substantial improvements in rail lives. An additional benefit is the potential to increase the intervals between consecutive grinding cycles, where rail grinding is used to control rolling contact fatigue damage. Increased grinding intervals will, however, place more emphasis on the performance of rail welds which are more sensitive to localised rolling contact fatigue damage than parent rail. The increased sensitivity to damage at welds arises from the inherent variability in material characteristics, and may vary between welding procedures, and for any particular welding process, between rail grades.A detailed microstructural examination of the characteristics associated with flashbutt welds in the new condition and after exposure to service conditions which resulted in gauge corner cracking has been performed in order to correlate the development of rolling contact fatigue damage to the underlying microstructure. Of particular interest were the behaviour of the softened (heat affected) zones and the role of grain boundary networks of pro-eutectoid cementite which may develop during welding of hypereutectoid rail steels. Understanding the effects of these microstructures on the material response to wheel rail contact conditions, and using this information to develop improved welding procedures or modifying rail maintenance procedures to minimise their effects, are necessary in order to maximise the benefits obtained through use of these rail steels.Gauge corner cracking was found to initiate in the softened zone associated with spheroidised microstructure. The role of pro-eutectoid cementite network on crack initiation was difficult to evaluate, however, it seems to have only a minor effect on crack propagation. The proposed options to mitigate gauge corner cracking was to make narrow welds to minimise softened zone or to select rail grades that are more resistant to spheroidisation.

Network Carbide Inheritance during Heat Treatment Process of Large Shield Machine Bearing Steel GCr15SiMn

Network carbide inheritance during heat treatment process of large shield machine bearing steel GCr15SiMn was investigated by heat treatment experiments and quantitative metallographic. Samples with the proeutectoid cementite network thickness in the range of 0.19~0.54 μm were obtained by changing austenitizing temperature and soaking time of pearlite transformation. The results show that the network in hot rolled bar can be improved when the pre-heat treatment temperature is 950 °C. When the network thickness is above 0.40 μm, the undissolved cementite networks present in microstructures after quenching and tempering. In a Chinese standard, the network grades are 1.5 and 3.0 degree when the networks thickness are 0.40 μm and 0.54 μm, respectively. The critical network thickness that can be eliminated by heat treatment is 0.29 μm.

Effect of vanadium and chromium on the microstructural features of V-Cr-Mn-Ni spheroidal carbide cast irons

The objective of this investigation is to study the influence of vanadium (5.0wt%–10.0wt%) and chromium (0–9.0wt%) on the microstructure and hardness of Cr-V-Mn-Ni white cast irons with spheroidal vanadium carbides. The alloys’ microstructural features are presented and discussed with regard to the distribution of phase elements. The structural constituents of the alloys are spheroidal VC, proeutectoid cementite, ledeburite eutectic, rosette-shaped carbide eutectic (based on M7C3), pearlite, martensite, and austenite. Their combinations and area fraction (AF) ratios are reported to be influenced by the alloys’ chemical composition. Spheroidized VC particles are found to be sites for the nucleation of carbide eutectics. Cr and V are shown to substitute each other in the VC and M7C3 carbides, respectively. Chromium alloying leads to the formation of a eutectic (γ-Fe + M7C3), preventing the appearance of proeutectoid cementite in the structure. Vanadium and chromium are revealed to increase the total carbide fraction and the amount of austenite in the matrix. Cr is observed to play a key role in controlling the metallic matrix microstructure.

Cementite Formation from Hematite–Graphite Mixture by Simultaneous Thermal–Mechanical Activation

In this study, the effect of simultaneous thermal–mechanical activation (STMA) process on carbothermic reduction of hematite and also on iron carbide formation has been investigated. For this purpose, the STMA process was performed for 3 and 6 hours at 973 K (700 °C) and 1073 K (800 °C) on hematite–graphite powder mixtures (with 22 wt pct C) in argon atmosphere. The XRD patterns showed that by performing this process at 973 K (700 °C), the initial hematite reduction led to the formation of wustite in the presence of graphite. Metallic iron phase was also formed along with wustite phase at 1073 K (800 °C) for 3 hours, and by increasing the process time to 6 hours, in addition to the metallic iron, iron carbide was also formed. The SEM images and EDS analysis obtained at 1073 K (800 °C)were also indicative of the formation of pearlite structure along with proeutectoid cementite phase and free carbon in the form of graphite in the structure of the samples. According to the results of the image analysis, the percentage of the carbon content was more than 2.22 wt pct in this process lasting for 6 hours at 1073 K (800 °C). Also, DTA results showed that the sample hot milled for 6 hours at 1073 K (800 °C) contained more than 2.1 wt pct carbon. The mechanism of metallic iron and cementite formation from hematite was proposed. Therefore, the STMA process led to an increase in the rate of carbothermic reduction of hematite to metallic iron and reduced its starting temperature relative to the non-simultaneous application of each of the thermal and mechanical activation. Finally, this process can be brought up as a new method for the production of iron carbide from iron oxides.

Spheroidisation of hypereutectoid state of nanostructured bainitic steel

Nanostructured bainite is extremely resistant to severe tempering making it difficult to reduce its hardness sufficiently to allow certain processing before heat treatment into its final state. An attempt is therefore made to introduce the divorced pearlite reaction that relies on the presence of pro-eutectoid cementite in the austenite in order to produce a microstructure containing spheroidal cementite particles simply by continuous cooling from the two phase austenite and cementite field. It is demonstrated that dramatically reduced hardness values can be achieved using this method.

Effects of deformation parameters on formation of pro-eutectoid cementite in hypereutectoid steels

Brittle pro-eutectoid cementite that forms along prior-austenite in hypereutectoid steels is deleterious to mechanical properties. The optimum process parameters which suppress the formation of pro-eutectoid cementite in hypereutectoid steels with carbon content in the range of 0.8%–1.3% in mass fraction, were investigated. Pro-eutectoid cementite formation is effectively hindered by increasing the deformation temperature and decreasing the amount of strain. Transformation at lower temperatures close to the nose of the cooling-transformation diagram also reduces the tendency of the formation of pro-eutectoid cementite. Control of prior-austenite grain size and grain boundary conditions is important. Due to larger number of nucleation sites, finer prior-austenite grain size results in the acceleration of transformation to pro-eutectoid cementite. However, large prior-austenite and straight boundaries lead to less nucleation sites of pro-eutectoid cementite. The cooling rate and carbon content should be reduced as much as possible. The transformation temperature below 660 °C and the strain of 0.5 at deformation temperature of 850 °C are suggested.

초 고 탄소 저합금강의 경도와 인성에 미치는 열간단조의 영향

Abstract: This study was carried out to investigate the effect of hot forging on the hardness and impact value of ultra high carbon low alloy steel. With increasing hot forging ratio, thickness of the network and acicular proeutectoid cementite decreased, and than were broken up into particle shapes, when the forging ratio was 80%, the network and acicular shape of the as-cast state disappeared. Interlamellar spacing and the thickness of eutectoid cementite decreased with increasing forging ratio, and were broken up into particle shapes, which then became spheroidized. With increasing hot forging ratio, hardness, tensile strength, elongation and impact value were not changed up 50%, and then hardness rapidly decreased, while impact value rapidly increased. Hardness and impact value was greatly affected by the disappeared of network and acicular shape of proeutectoid cementite, and became particle shape than thickness reduction of proeutectoid and eutectoid cementite. Key Words ：Ultra High Carbon, Hot Forging, Hardness, Impact Value, Proeutectoid and Eutectoid Cementite.

EFFECTS OF AUSTENITIZATION AND COOLING RATESON THE MICROSTRUCTURE IN A HYPER-EUTECTOID STEEL

Cold-drawn pcarlitic steel wires have the highest strength of all steel products.It is a promising way to enhance the mechanical properties by increasing the carbon content.However,the proeutectoid cementite forms easily due to the hypereutectoid composition and deteriorates the mechanical and processing properties of steel wires.It is important for hypereutectoid steel wire drawing to achieve a fine and fully pearlitic microstructure without proeutectoid cementite.The austenitization and following continuous cooling process were simulated in the dilatometer for a hypereutectoid steel. The microstructure was observed with OM and SEM.The transformation temperature,prior-austenite grain size,pearlitic interlamellar spacing and proeutectoid cementite thickness were determined by dilatometric curves or OM/SEM images.The austenite grain size increases rapidly with a higher temperature and almost keeps invariant with a longer austenitization time.Faster cooling rate,higher austenitization temperature or longer austenitization time decrease the starting and finishing temperature of phase transformation,widen the temperature range,refine the pearlitic interlamellar spacing and suppress the proeutectoid cementite precipitation(reduce the thickness or make it discontinuous). However,it is easy to form martensite which is bad for the homogeneity of pearlitic microstructure by increasing the cooling rate or austenitization temperature simply.A fine pearlite in a pseudoeutectoid microstructure is achieved by extending the austenitization time to 60 min and controlling the austenitization temperature and cooling rate.Discontinuous proeutectoid cementite is observed in the samples with the higher austenitization temperature.Higher austenitization temperature and longer time are helpful to weaken the carbon concentration gradient.The homogeneous carbon distribution restrains carbon diffusion for cementite nucleation during the proeutectoid cementite precipitation and pearlite transformation,which decreases the transformation temperature.Compare to pearlite transformation, the proeutectoid cementite precipitation is affected more strongly by the carbon diffusion due to a longer diffusion distance.Therefore,the precipitation amount of proeutectoid cementite is reduced if the carbon diffusion is restrained.The amount of grain corner and grain edge reduce more dramatically than that of grain boundary if austenite grain size increases.The proeutectoid cementite only nucleates in grain corner and grain edge.The pearlite nucleation can form in grain boundary.Therefore,the larger austenite grains result in the sharp reduction in the sites for nucleation of proeutectoid cementite. Then,the amount of proeutectoid cementite are reduced and the morphology becomes discontinuous.