Hypoeutectoid Steel Research Articles

Contribution to the Study of the Chemical and Microstructural Quality of Reinforcing Bars in DR. Congo “Case of the City of Lubumbashi”

This research highlights the contribution to the study of the microstructural quality of reinforcing bars on the Lubumbashi market including bars imported from South Africa (FA), Zambia (FZ) and those produced locally ( FC) by the only steel industry, in the former province of Katanga, the iron processing company SOTRAFER, in acronym. The samples of the locally produced reinforcement bars (FC) were collected at SOTRAFER at the end of production, while the samples of the FA and FZ reinforcement bars were taken randomly on the Lushois market in a hardware store specializing in sales to avoid errors.Microstructural analysis of all bars revealed a similar microstructure consisting of a ferrite (light areas) and pearlite (dark areas) matrix and a ferrito-pearlitic structure. This microstructure, as predicted by the Fe-C equilibrium diagram, could be justified by the carbon content (lower than the eutectoid point content); the results prove that all bars are hypoeutectoid steels (%C ≤ 0.77). They can also be assimilated to the category of mild steels (0.1 ≤ %C ≤ 0.25). This also shows that all the bars have not undergone a particular heat treatment and have been cooled very slowly in order to allow the diffusion of atoms and reach equilibrium conditions. The results obtained from the variance analyses of different materials of dimensions of 10, 12 and 16 mm revealed that at the level of chemical and mechanical analyses, there was no significant difference on all the parameters studied and that all the reinforcement bars could perfectly be used in construction. Finally, the survey reveals that FA is reputed to be of better quality among consumers, for several reasons including the psychological one although its price is lower than that produced locally.

Optimization of Zn-Cr proportions for hybrid multilayer electroplating of AISI 1045 steel – towards super-hydrophobicity and anti-corrosive nature

Localized corrosion in steel heat exchangers is a major concern, occurring mostly during operational shutdowns. Due to the absence of flow and moisture content availability, corrosion reduces operation efficiency. A study aimed to optimize the proportions of Zinc and Chromium (Zn-Cr) for developing a hybrid multi-layer electroplating method to inhibit corrosion in hypo-eutectoid carbon steel commonly used in heat exchangers. The Zn-Cr coating was performed using the amperometric method in an electrochemical workstation, producing coatings of varying thicknesses by varying the potential and time. The surface morphology was characterized using Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD) analysis. The average thickness of the Zn-Cr coating was found to be 2.5 µm using the scanning probe microscope analysis. The corrosive action of a standard 3.5% NaCl solution was investigated on both coated and uncoated specimens through open circuit potential measurement and electrochemical impedance spectroscopy. The results indicated that the Zn-Cr coating effectively resisted corrosion. Contact angle measurement of water particles with the Zn-Cr coated specimen revealed a near-super hydrophobic nature with the highest contact angle of 146.8°.

The Brittle Fracture of Iron and Steel and the Sharp Upper Yield Point Are Caused by Cementite Grain Boundary Walls

Brittle fractures of iron and steel above twinning temperatures are caused by cementite grain boundary wall cracks. These were revealed by an Atomic Force Microscope (AFM). At temperatures below the ductile–brittle transition (DBT), cracks must propagate longitudinally within cementite walls until the stress is sufficiently high for the cracks to propagate across ferrite grains. Calculations using these concepts correctly predict the stress and temperature at the DBT required for fractures to occur. At temperatures above the DBT for hypoeutectoid ferritic steels, dislocations must be emitted across the walls transversely for plastic deformation to continue. This is responsible for the upper yield point at the elastic limit in these steels followed by a large drop in stress to the lower yield point. Here, the walls completely surround all of the grains. Where the walls are segmented, such as in iron, dislocations can pass around the walls, resulting in a gradual change from elastic to plastic deformation. The Cottrell atmosphere theory of yielding is not supported experimentally. It was the best available until later experiments, including those using the AFM, were performed. Methods are presented here giving yield strength versus temperature and also the parameters for the Hall–Petch and Griffith equations.

Development of Fully Pearlitic Microstructure in a Hypoeutectoid Steel

Development of Fully Pearlitic Microstructure in a Hypoeutectoid Steel

Archeometallurgical Investigation of a Fragment from a Medieval Sword Blade

A fragment from a medieval sword blade was investigated by metallography, Vickers microhardness tests and slag inclusions analysis are to extract technological information about its manufacturing process. Optical microscopy observations and microhardness measurements indicated that the sword blade was forged via hammer welding, combining different steel bars for an optimal balance of hardness and toughness. A steeling technique involved wrapping a steel bar around a composite billet, crafted by enclosing a hypoeutectoid steel bar around a near-eutectoid steel core. Moreover, it was found that the hardness of the cutting edges was increased with a quenching heat treatment. After quenching, the blade exhibited martensitic microstructure with Vickers microhardness ranging from 500 to 640 HV0.3. The compositional data of a large set of nonmetallic inclusions were collected by scanning electron microscopy coupled with X-ray dispersive spectroscopy. Slag inclusion analysis and multivariate statistics confirmed the blade's composite nature and revealed distinct smelting and forging-related SI groups. Liquidus temperatures indicated smelting temperatures of at least 1156°C for the external section and 1031°C for the internal. The forging temperature was estimated at a minimum of 1143°C. These findings provide insight into the blade’s metallurgical history.

Archaeometallurgical Study of a 16th–17th Century “Rapier” Sword Manufactured in Caino (Northern Italy)

AbstractA metallurgical study was performed on a 16th–17th century “rapier” sword manufactured in Caino (northern Italy). Metallographic investigations and Vickers microhardness measurements indicate that the rapier was forged by assembling via hammer-welding different hypoeutectoid and near eutectoid steel bars. The rapier blade was heat treated by slack-quenching to increase its hardness, especially near the blade tip, improving the thrusting performance. The chemical composition of slag inclusions was analyzed by scanning electron microscopy coupled with X-ray dispersive spectroscopy. Compositional data of slag inclusions were analyzed by a multivariate statistical strategy aimed to distinguish and classify slag inclusions on the basis of their origin. It was estimated that the temperature reached during the finery and forging processes was at least 1270 °C and 1160 °C, respectively.

A mathematical model for the prediction of mechanical properties on ASTM A510/A853 cold-drawn hypoeutectoid steel wire after batch annealing

Annealed ASTM A510/A853 hypoeutectoid steel wire is a very useful and versatile material that finds its applications in the construction industry; this is mainly due to its mechanical properties, since this product can reach high ductility. In order to achieve the sought quality and homogeneity in this material, it is necessary to have a suitable control during operation. Important operational control parameters in batch annealing are: heating and cooling rates, holding temperature and cycle time. These parameters have a direct effect on the process, and it is essential to elucidate the way they impact in the final product. This work concerns the development of a mathematical model targeted at the prediction of mechanical properties in terms of the operational parameters, in the outer spirals of American Iron and Steel Institute/Society of Automotive Engineers (AISI/SAE) steel drawn wire coils after they have been subjected to batch annealing. The model addresses non-isothermically the involved phenomena that take place during the annealing process: recovery, recrystallization, and grain growth below and above Ac1. It predicts grain size, tensile strength, and yield strength values with very good accuracy.

Metallurigical Analysis of Third to Fifth Century Swords with Ring Pommels Excavated in the Asan-Cheonan Region

This study conducts a metallurgical analysis of three swords with ring pommels (Hwandu sword) manufactured during the Proto–Three Kingdoms period and the Baekje period and excavated from the Asan and Cheonan. Specifically, we carried out analyses based on a metallurgical microscope, scanning electron microscopy with energy dispersive spectroscopy. the swords were made of bloomery iron produced through direct smelting and formed by hammering hypoeutectoid steel. It was also confirmed that other heat treatment techniques than shaping were not applied to them. In addition, all of them have a high carbon content and the crystal grains are well refined, so it is believed that the hardness and strength are high. Moreover, the analytic results of nonmetallic inclusions, No.2 swords have glassy slag and the others verified the existence of fayalite and wüstite on the glassy matrix. And a high Ca content overall. The results of comparing the swords with other swords with ring pommels excavated from the corresponding region indicated that heat treatment techniques were not applied to these artifacts despite the existence of such techniques. Thus, we analyze that the study subjects were produced as ceremonial items rather than weapons. Furthermore, the calculation results of ratios, such as CaO/SiO<sub>2</sub> and (Al<sub>2</sub>O<sub>3</sub>/SiO<sub>2</sub>)/(CaO/SiO<sub>2</sub>), indicate that the study subjects were produced based on a similar smelting processing system. Based on the analytic results, this study also suggests a possibility of the addition of slag generation agents or the use of iron ores with a high Ca content.

Similarity and difference in machining at mixed scales using nanostructured coated tools milling hypoeutectoid carbon steel

There are similarities and differences when machining at the microscale and macroscale. One difference is the lack of significant heat generated at the microscale, which means that thermal barrier coatings are largely redundant. However, the mechanisms of wear are similar and different depending on machining parameters. In this study, it is discovered that nanostructured coatings play different roles at mixed scales, which are experimentally determined by measuring tool wear, chemical diffusion, temperature, and forces. The main findings show that macroscale wear is thermal in nature and that microscale wear is primarily tribological. The principal conclusions focus on redesigning microtools and nanostructured coatings to eliminate wear, developing standards that describe the action of microtools, and understanding how to calculate the minimum chip thickness that guarantees the formation of chips rather than burrs.

Evolution of Cementite Substructure of Rails from Hypereutectoid Steel during Operation

Transmission electron microscopy methods were used to analyze the cementite substructure in the head of special-purpose long rails of the DT400IK category, made of hypereutectoid steel, after long-term operation on an experimental track on the Russian Railways ring (the tonnage was 187 million tons). It is noted that the study of various aspects of cementite—its structure, morphology, chemical composition, crystal lattice defects—is relevant. The steel structure is represented by three morphological components at a distance of 10 mm from the sample surface: lamellar perlite, fractured and fragmented perlite. The volume fraction of lamellar perlite in the material is 65%. It is shown that after operation, the cementite plates are bent and separated by ferrite bridges. In the plates of ferrite and cementite, a dislocation substructure is formed, which is of a chaotically distributed and network type in ferrite and of an ordered type in cementite. An increased density of dislocations at the ferrite–cementite interfaces compared to the volume of ferrite plates was noted. Two possible mechanisms of deformation transformation of lamellar perlite grains are indicated: fracture of cementite plates and carbon pulling out from the lattice of the carbide phase. It is indicated that in the dissolution of cementite plates, the interfacial boundaries of “α-phase-cementite” play an important role. The removal of carbon from cementite plates occurs most intensively near defects in ferrite and cementite. The formed nanosized particles of tertiary cementite are unevenly distributed in the ferrite plates; most of them are observed at the locations of ferrite subgrains and interfacial boundaries. This results in non-uniform diffraction contrast in dark-field images of cementite plates. Nanosized particles of cementite can be taken out into the interlamellar space of pearlite colonies in the process of dislocation slip, or they are formed as a result of deformation decomposition, which is less likely. The fragmentation of ferrite and cementite plates is revealed and azimuthal components of total misorientation angles are estimated. The mechanisms of mass transfer of carbon atoms over interstitial sites, deformation vacancies, dislocation tubes, grain boundaries and fragments are considered. According to all the established patterns of the cementite substructure transformation, a comparison with the results for rails made of hypoeutectoid steel was performed.

Effectiveness of Machine-Learning and Deep-Learning Strategies for the Classification of Heat Treatments Applied to Low-Carbon Steels Based on Microstructural Analysis

This work aims to compare the effectiveness of different machine-learning techniques for the image classification of steel microstructures. For this, we use a set of samples of hypoeutectoid steels subjected to three heat treatments: annealing, quenching and quenching with tempering. Logically, the samples contain the typical constituents expected, and these are different for each treatment. Images are obtained by optical microscopy at 400× magnification and from different low-carbon steels to generate the data with some heterogeneity. Learning models are created with an image dataset for classification into three classes based on the respective heat treatments. Likewise, we develop two kinds of models by using, on the one hand, classical machine-learning methods based on the “bag of features” technique and, on the other hand, convolutional neural networks (CNN) with a transfer-learning approach by using GoogLeNet and ResNet50. We demonstrate the superiority of deep-learning techniques (CNN) over classical machine-learning methods.

Wear and rolling contact fatigue competition mechanism of different types of rail steels under various slip ratios

This study aims to explore the wear and rolling contact fatigue (RCF) competition mechanism of the hypoeutectoid, eutectoid rail, and bainitic rail steels under different slip ratios. The results showed a clear competition relationship between wear and RCF of rail steel. With the increase in slip ratio, the wear rate of hypoeutectoid and eutectoid rail steels was increased and then remained stable, and the wear rate of bainitic rail steels was increased. Bainitic rail steels suffered catastrophic wear when the slip ratio exceeded 2%. With the slip ratio increased, the RCF crack propagation rate of the hypoeutectoid rail and eutectoid steels rail increased rapidly at first and then increased slowly. Except for the slip ratio of 5%, the RCF crack propagation rate of the bainitic rail increased with the increase in slip ratio. With the increase in slip ratio, the degree of wear rate increase and crack propagation rate was not consistent, which led to the competition between wear and RCF. The competitive relationship between wear and RCF was reflected in the variation of rail wear form. As the slip ratio increased, the main wear form of the hypoeutectoid and eutectoid rail steels was transformed from adhesive wear to adhesive wear and fatigue wear, whereas for bainitic rail steel, it was changed from adhesive wear to fatigue wear, and finally to adhesive wear. At high slip ratios, the retained austenite of bainite rail steel was seriously broken, so cracks in the bainitic rail steel could easily propagate along the plastic deformation layer.

In-situ micro-cantilever bending studies of a white etching layer thermally induced on rail wheels

A complex and untraceable mechanical and thermal loading situation in rolling-sliding contacts can lead to the formation of white etching layers (WELs), depicting critical crack initiation sites. For our detailed studies, to obtain a holistic view of the microstructural characteristics and micro-mechanical properties, we prepared a WEL on a decommissioned (after 200,000 km service life) rail wheel by laser surface treatments with a defined energy input. This WEL is predominantly martensitic down to a depth of 30–40 μm, after which a transition to the deformed ferritic-pearlitic microstructure of the hypoeutectoid rail wheel steel is present. The martensitic region is with 6.98 ± 0.68 GPa significantly harder than the transition zone (5.17 ± 0.39 GPa) and the deformed ferritic-pearlitic base material (3.30 ± 0.33 GPa). In-situ V-notched micro-cantilever bending experiments of the martensitic (and thus most brittle) region show crack initiation and propagation – mostly along the boundaries of the martensitic grains – besides a plastic behavior. Applying the elastoplastic fracture mechanics allows to derive the local fracture toughness KIQ, which is 16.4 ± 1.2 MPam1/2 for this martensitic region of the WEL. The results outpoint the application of micro-cantilever bending tests in addition to hardness testing as a promising tool to discuss the relationship of microstructural characteristics with its micro-mechanical properties.

Optimization of the rhomboidity of continuously cast billets using linear regression and genetic programming: A real industrial study

During the continuous casting of steel billets, several geometrical, inner and surface defects can occur due to the thermomechanical behavior during solidification. One of them is rhombic distortion (i.e. rhomboidity), which can lead to the occurrence of off-corner cracks and twisting of cast billets during further plastic deformation (i.e. rolling). Based on data of 2088 cast batches (64 different hypoeutectoid steel grades), 109,514 billets, produced from January 2022 to September 2022 in Štore Steel Ltd. (Slovenia), chemical composition (content of C, Si, Mn, S, Cr, Mo, Ni and V), casting parameters (average casting temperature, average difference between input and output cooling water, melt level, average cooling water flow and pressure in the first and second zone of secondary cooling) the linear regression and genetic programming were used in order to predict rhomboidity of continuously cast billets. The rhomboidity, in our case defined as relative diagonal difference, was determined using in-house developed computer vision system for measuring of rhomboidity. Based on the modelling results 9 batches (419 billets) of 42CrMos4 were cast in September 2022 with a 10 % higher water pressure in the first zone of secondary cooling (from 2.41 bar to 2.67 bar). The rhomboidity of continuously cast billets improved by 18.18 % (from 1.43 % to 1.21).

Metallurgical Study of Iron Artifacts from Guryong-ri Site in Ungcheon, Boryeong

In the 6<sup>th</sup> and 7<sup>th</sup> centuries, 5 iron artifacts excavated form the Baekje Stone Tomb in Guryong-ri site, Ungcheon, Boryeong, were studied. The sample were metal microscopic observation, SEM-EDS analysis and Raman micro-spectroscopy analysis were conducted to understand the metallurgical characteristics. The microstructure observation showed the presence of ferrite and pearlite throughout, and differences in carbon content existed depending on the direction. Non-metallic inclusions were in the form of long lines, and most of them were wüstite, fayalite. It is indicated that the artifacts were forge welded using hypoeutectoid steel, with signs of carburizing and decarburizing processes. Some crystals with high P2<sub>2</sub>O<sub>5</sub>, TiO<sub>2</sub>, CaO content were identified as sarcopside, ulvöspinel, and perovskite, respectively, through Raman spectroscopy. A comparison of the results with previous studies on the sites of Bujang-ri site in Seosan and Bongseon-ri site in Seocheon, which are adjacent sites in the coastal area, revealed that, while heat treatment technology was available, the artifacts were not heat-treated considering the purpose for use for these artifacts. The chemical composition of the non-metallic inclusions P<sub>2</sub>O<sub>5</sub>, TiO<sub>2</sub>, CaO were plotted in proportions to SiO<sub>2</sub> and compared with adjacent sites. Considering that the P<sub>2</sub>O<sub>5</sub>/SiO<sub>2</sub> ratio was widely distributed, the refining technology was not uniform. In addition, the TiO<sub>2</sub>/SiO<sub>2</sub> ratio was found to be higher than that of other sites, meaning that a titanium-containing ore was used to manufacture the artifacts, unlike in surrounding sites, but it is not detected in all artifacts, so it may have been affected by various factors such as furnace walls in addition to raw materials. Although slag formers were used, considering the CaO/SiO<sub>2</sub> ratio and the (Al<sub>2</sub>O<sub>3</sub>/SiO<sub>2</sub>)/(CaO/SiO<sub>2</sub>) ratio, which appear to be similar to the surrounding sites, but it is possible that CaO containing raw ore was used because it is also affected by the components of raw ore. As a result of the study, it is highly likely that ore different from that of the surrounding sites was used for production, but a more comprehensive comparative study with the surrounding sites is needed in the future.

Microstructure-based fatigue life modeling methodology for ferritic-pearlitic hypo-eutectoid steels

A microstructure-based fatigue life prediction methodology was proposed and verified for 44MnSiVS6 hypo-eutectoid steel. As a first step, a statistically equivalent synthetic microstructure generation algorithm was developed by applying the cellular automata under the multi-level microstructure generation approach, and verified by comparing to the real microstructure. Crystal plasticity-based elasto-viscoplastic constitutive models were developed for the primary ferrite and pearlite, and their constitutive parameters were calibrated. Synthetic microstructure-based crystal plasticity finite element method (CP-FEM) simulations were performed for stress-controlled axial fatigue tests. The Fatemi-Socie (FS) parameter was chosen to calculate a fatigue indicator parameter increment (ΔFIP) per cycle, and its cumulative distribution probability (CDP) curve was obtained from CP-FEM simulations. Three different criteria extracting a representative ΔFIP from the CDP curve were proposed after thoroughly examining the nature of the CDP curves, which showed the dual transition in the upper CDP range. Extracted representative ΔFIP values coupled with the power law-based model gave reasonable fatigue life prediction. In particular, the best prediction was made when the representative ΔFIP values were taken beyond the second transition of the CDP curve, in which the ΔFIP hot spots were concentrated in the primary ferrite.

A Mathematical Model for the Prediction of the Scale Layer Formation on ASTM A510/A853 Cold-Drawn Hypoeutectoid Steel Wire After Batch Annealing

Annealed hypoeutectoid ASTM A510/A853 steel wire is widely used in the construction industry due to its mechanical properties, which makes it a very versatile material to work with. This work concerns the prediction of the thickness of the scale layer of the hypoeutectoid steel wire once subjected to a batch annealing process. We propose a mathematical model to approach oxidation occurring during the process. The model treats this phenomenon as a non-isothermic instance; it takes as input a heating curve, a cooling curve, and a holding time that corresponds to the heating and cooling cycles and a heating holding cycle. We assume the temperature of material outer spiral segments as the furnace atmosphere temperature. With the aid of this model, it is possible to predict the oxide scale layer of the outer spirals segments of the annealed drawn wire coils after batch annealing with reasonable accuracy. We did not find visible evidence of decarburization; the steels of the study in this work do not decarburize. As an approximation, we assumed that the temperature of the outside of the drawn wire coils equals the furnace atmosphere.

Features of the phase composition and structure of hypoeutectoid steel, manifested in the behavior of magnetization near magnetic saturation

Investigation of the temperature evolution of magnetization curves near magnetic saturation makes it possible to extract new information on the features of the phase composition and structure of hypoeutectoid steel. It is shown that the main contribution to the magnitude and the temperature behavior of the energy density of the local magnetic anisotropy of hypoeutectoid steel is due to the lamellar structure of pearlite. The peculiarity of the temperature behavior of the energy of the magnetic anisotropy, along with the behavior of the paraprocess, indicates the formation of Mn-substituted cementite in the studied steel sample. The observation of the crossover of power-law regularities in the approximation of magnetization to saturation indicates the formation of two-dimensional nano-inhomogeneities of the local axis of easy magnetization in the plates of alpha iron, which are part of the pearlite. Keywords: magnetization, steel, cementite, iron, approach to saturation magnetization, pearlite

Особенности фазового состава и структуры доэвтектоидной стали, проявляющиеся в поведении намагниченности вблизи магнитного насыщения

Investigation of the temperature evolution of magnetization curves near magnetic saturation makes it possible to extract new information on the features of the phase composition and structure of hypoeutectoid steel. It is shown that the main contribution to the magnitude and the temperature behavior of the energy density of the local magnetic anisotropy of hypoeutectoid steel is due to the lamellar structure of pearlite. The peculiarity of the temperature behavior of the energy of the magnetic anisotropy, along with the behavior of the paraprocess, indicates the formation of Mn-substituted cementite in the studied steel sample. The observation of the crossover of power-law regularities in the approximation of magnetization to saturation indicates the formation of two-dimensional nano-inhomogeneities of the local axis of easy magnetization in the plates of alpha iron, which are part of the pearlite.

Analysis of process parameters of hypoeutectoid steel ultrasonic horns with different heat treatment processes

An ultrasonic horn operated at 28 kHz was simulated by finite element analysis. The horns and samples were formed from hypoeutectoid steel containing 0.178 wt% C and 1.3 wt% Cr. They were austenized at 875 °C with a soaking time of 33 min, then quickly water-quenched and oil-quenched. Tempering was performed at different temperatures (200 °C–500 °C) and soaking times (120–240 min). Over the cross-section, the hardness was consistently lower at the center position than at other positions. The maximum (minimum) hardness was achieved after water and oil quenching, tempering at low (high) temperature, and soaking for a short (long) time. Heat treatment decreased the anti-resonance frequency of the ultrasonic horns but increased the mechanical quality factor and the impedance at the anti-resonance frequency. The mechanical quality factor was proportional to both tempering temperature and soaking time. As shown in microstructural images, high mechanical quality was also related to small grain size.