Ferrite Pearlite Research Articles

Effect of annealing on the mechanical characteristics of steel welded joint

This study examines the mechanical progression and responses of different steel grade, annealed under temperature variation and constant holding time with emphasis on the weld pool and heat affected region. The results show that there is an incremental progression in hardness as the temperature increase from 700oc to 900oc at all phase of carbon content. From the microstructure properties, the annealed microstructure of hypo – eutectoid steels observed contain mixture of ferrite and coarse pearlite.

Feature of stress corrosion cracking of degraded gas pipeline steels

Stress corrosion cracking (SCC) of steels can reduce the structural integrity of gas pipelines. To simulate in-service degradation of pipeline steels in laboratory the method of accelerated degradation consisted in subjecting specimens to electrolytic hydrogenation, to loading up the certain plastic deformation and heating of specimen at 250°C was recently developed. The purpose of this paper was to analyse mechanical and SCC behaviour of in-service and in-laboratory degraded gas pipeline steels and to reveal some fractographic features of SCC. Three pipeline steels of the different strength (17H1S, which is equivalent of API X52, API X60 and API X70) were investigated. The characteristics of the as-received pipeline steels with different strength were compared with the properties of pipeline steels after in-service and in-laboratory degradation. An influence of the NS4 solution on SCC resistance of 17H1S and API X60 steels in the as-received state and after the accelerated degradation, using slow strain rate tension method, was analysed. The noticeable decrease of plasticity for 17H1S and API X60 steels after long-term operation was shown. Deep microdelaminations revealed in the central part of fracture surfaces for the operated steels can be considered as the signs of dissipated damaging in the metal caused by texture and hydrogen absorbed by metal. Comparison of the SCC tests results showed that the characteristics of both steels in the as-received state were insignificantly changed under the influence of the environment. At the same time, the degraded steels were characterized by a high sensitivity to SCC. It was shown fractographically that it associated with cracking along interfaces of ferrite and pearlite grains with secondary deep intergranular cracks formation and also by delamination between ferrite and cementite inside pearlite grains. The similar fracture mechanism at SCC tests was revealed for near the outer surface of the specimens and in the central part of the fracture surfaces of in-laboratory degraded specimens. These results demonstrated the key role of hydrogen during SCC and in-bulk cracking as well.

Influence of the gas hydrates on the corrosion rate of gas gathering pipelines

The regularities of corrosion degradation of the low carbon ferrite pearlite St 20 (0.2C) steel of gas gathering pipeline under the mutual effect of corrosion environment, mechanical stresses and hydrate formation were determined. A change in corrosion mechanism in chloride media at increasing chloride concentration from 2.5 mol/L to higher concentration was found. The revealed more significant influence of mechanical stress on localized corrosion rate than on uniform corrosion one was associated with the formation of local galvanic elements on the metal surface and intensification of the metal dissolution in the tensile zone due to the weakening of interatomic interaction caused by increase the distance between grains. Fatigue tests of the pipe steel specimens after exposure to gas hydrate showed a significant decrease in durability (by 15 – 25% depending on test environment), the reason of which was obviously associated with increasing of corrosion damaging of the specimen surface as a result of the aggressive action of gas hydrates.

Microstructure based flow stress model to predict machinability in ferrite–pearlite steels

A new flow stress model is proposed to describe the behaviour of ferrite–pearlite steels based on microstructure properties, including the effect of high strains, strain rates and temperatures. The model introduces strain hardening as a function of the pearlite ratio, interlamellar spacing and ferrite grain size. A non-linear thermal softening, and the coupling between strain rate and temperature are also introduced. Tested on a 2D ALE model, predicted cutting forces, tool temperatures, chip thickness and tool wear results obtained good agreement when compared to orthogonal cutting tests of four ferrite–pearlite steels, covering a wide range of microstructure variants.

Effects of Heat Treatment on Morphology, Texture, and Mechanical Properties of a MnSiAl Multiphase Steel with TRIP Behavior

The effect that the microstructure exerts on the Transformation-Induced Plasticity (TRIP) phenomenon and on the mechanical properties in a multiphase steel was studied. Samples of an initially cold-rolled ferrite–pearlite steel underwent different intercritical annealing treatments at 750 °C until equal fractions of austenite/ferrite were reached; the intercritical treatment was followed by isothermal bainitic treatments before cooling the samples to room temperature. Samples in the first treatment were heated directly to the intercritical temperature, whereas other samples were heated to either 900 °C or 1100 °C to obtain a fully homogenized, single-phase austenitic microstructure before performing the intercritical treatment. The high-temperature homogenization of austenite resulted in a decrease in its stability, so a considerable austenite fraction transformed into martensite by cooling to room temperature after the bainitic heat treatment. Most of the retained austenite transformed during the tensile tests, and, consequently, the previously homogenized steels showed the highest Ultimate Tensile Strength (UTS). In turn, the steel with a ferritic–pearlitic initial microstructure exhibited higher ductility than the other steels and texture components that favor forming processes.

Influence of Manganese and Boron Alloying and Processing Conditions on the Microstructure and the Mechanical Properties of 0.4% Carbon Steels

The performance of three steels with 0.4% carbon content was evaluated in the warm forging condition and benchmarked against the as-hot-deformed and hardening and tempering condition. The steels include a plain carbon steel S45C, a carbon-manganese steel 40Mn5 and a carbon-manganese-boron-alloyed steel 15B41. Such lean alloyed steels are preferred by automotive companies for component manufacture. In the as-hot-rolled condition, the steel with a ferrite pearlite microstructure shows, a good strength and lowest impact toughness. In the hardened and tempered condition at 650 °C, a tempered martensite microstructure with a moderate improvement in strength and significant enhancement in impact toughness, than the hot-rolled steel, is obtained. The steel subjected to warm forging at 760 °C, shows a ferrite pearlitic microstructure with a moderate improvement in strength but with the highest impact toughness level. The microstructure evolution at the various processing conditions has been brought out and correlated with the mechanical properties. The relative merits in processing the steels at the different processing conditions show that warm forging is an effective means to achieve the highest strength toughness combination in a ferrite pearlitic matrix.

Decarburization of the Carbon Steel C45 During Annealing in Air

Abstract In production it is necessary to achieve conditions that lead to the minimum decarburization of a steel product’s surfaces. In this study, the hypo-eutectoid carbon steel C45 was annealed in air in the temperature range T a = 600–1100 °C. The annealing times were between t a = ½ h and t a = 2 h. Different decarburizations occurred in different microstructures: ferrite–pearlite (T a = 600 °C and 700 °C, T a < AC1, no visible decarburization); ferrite–austenite (T a = 760 °C, AC1 < T a < AC3, visible decarburization); austenite at the beginning, ferrite– austenite after the incubation period (Ta = 850 °C, AC3 < T a < 912 °C, visible decarburization); and austenite (T a= 950 °C and 1100 °C, T a> 912 °C, visible decarburization and overheating of steel). The edges were more prone to decarburization and to overheating. Stress relieving, normalizing and annealing before quenching of the steel C45 can be carried out in air.

Modeling of lüders elongation and work hardening behaviors of ferrite-pearlite dual phase steels under tension

Because of their good combination of strength and ductility, ferrite-pearlite (F-P) dual phase steels are widely used as structural components for various engineering applications. Several numerical models have been developed to predict the flow behavior of F-P steels with a focus on the strain hardening effect. However, little attention has been put on modeling the Lüders elongation phenomenon, which dominates the plastic behavior of F-P steels at a relatively low-strain range. This research is to establish a stress–strain model capable of predicting both the Lüders elongation and work hardening behaviors of F-P steels subjected to room temperature tension. Representative volume element method in combination with finite element simulation is used for the overall stress–strain relationship prediction. The effects of ferrite grain size, pearlite volume fraction, and pearlite inter-lamellar spacing on flow behaviors are investigated. Model validation is achieved by comparing the simulation results with experimental data from literature.

Conditions for notch strength to be higher than static tensile strength in high–strength ductile cast iron

In this study, notch strength σBnotch was studied for a wide range of testing speed and temperature in comparison with the static tensile strength at room temperature σB, RTsmooth. High–speed tensile tests were conducted on high–Si ductile cast iron, conventional ferrite–pearlite ductile cast iron, and fully pearlitic ductile cast iron at a stroke speed ranging between 8.5 × 10−3 and 2.7 × 102 mm/s (strain rate of 2.1 × 10−4 and 1.8 × 101 s−1) and temperature ranging between −180 °C and 22 °C. Then, σBnotch and σB, RTsmooth were compared in terms of the strain rate–temperature parameter R = T ln (A/ε̇) to evaluate the combined influence of the strain rate ε̇ and temperature T. It was found that the notch strength σBnotch can be expressed as a unique master curve in terms of R parameter for each material. Then, a notch–strengthening threshold criterion R ≧ Rth was proposed to describe the lowest service temperature and highest strain rate that can be applied to the structural components. Under the condition R ≧ Rth, σBnotch is always larger than σB, RTsmooth, and therefore, notched components can be used safely. In other words, if R ≧ Rth, σB, RTsmooth can be used to evaluate the notched components in machine design to prevent instantaneous fractures.

Metastable bcc-Cu precipitation behavior and its effect on microstructure and strengthening of as-cast Fe–B alloy containing 3 wt% copper content

The metastable bcc-Cu precipitation behavior, with particular focus on effect of metastable bcc-Cu on microstructural evolution and strengthening is investigated by experiment and calculation simulation in an as-cast Fe–B alloy containing 3 wt% Cu content. The results show that the metastable bcc-Cu precipitates with the size of 3 to 5 nm can precipitate from the α-Fe (pearlite). During the precipitation process, the metastable bcc-Cu can act as the highly effective heterogeneous nuclei of pearlitic ferrite, which results in the formation of abnormal pearlite. Additionally, the existence of bcc-Cu will lead to the refinement of abnormal pearlitie, the dissolution of abnormal pearlitic cementite, and the strengthening of pearlite. The microstructural evolution process can be essentially described as: L (liquid phase) → γ (austenite), L → γ + M2B(boride), γ → P(normal pearlite), γ → P + bcc-Cu1 (the first precipitates), γ → P + P*(abnormal pearlite) + bcc-Cu2 (the second precipitates) + bcc-Cu1, and γ → M (martensite). Moreover, the calculation simulation analyses are in good agreement with the experimental results.

Effect of Ti(C, N) Particle on the Impact Toughness of B-Microalloyed Steel

Simultaneously improving the toughness and strength of B-microalloyed steel by adding microalloying elements (Nb, V, Ti) has been an extensively usedmethod for researchers. However, coarse Ti(C, N) particle will precipitate during solidification with inappropriate Ti content addition, resulting in poor impact toughness. The effect of the size, number density, and location of Ti(C, N) particle on the impact toughness of B-microalloyed steel with various Ti/N ratios was investigated. Coarse Ti(C, N) particles were investigated to act as the cleavage fracture initiation sites, by using scanning electron microscopy (SEM) analysis. When more coarse Ti(C, N) inclusions were located in ferrite instead of pearlite, the impact toughness of steel with ferrite–pearlite microstructure was lower. Meanwhile, when the size or the number density of Ti(C, N) inclusions was larger, the impact toughness was adversely affected. Normalizing treatment helps to improve the impact property of B-microalloyed steel, owing to the location of Ti(C, N) particles being partly changed from ferrite to pearlite. The formation mechanism of coarse Ti(C, N) particles was calculated by the thermodynamic software Factsage 7.1 and Thermo-Calc. The Ti(C, N) particles formed during the solidification of molten steel, and the N-rich Ti(C, N) phase precipitated first and, then, followed by the C-rich Ti(C, N) phase. Decreasing the Ti and N content is an effective way to inhibit the formation of coarse Ti(C, N) inclusions.

Effects of Initial Structure and Reversion Temperature on Austenite Nucleation Site in Pearlite and Ferrite–Pearlite

Austenite nucleation sites were investigated in near-eutectoid 0.8 mass pct C steel and hypoeutectoid 0.4 mass pct C steel samples with full pearlite and ferrite–pearlite initial structures, respectively. In particular, the prior austenite grain size had been coarsened to compare grain boundaries in the hierarchical pearlite structure, i.e., the low-angle pearlite colony and high-angle block boundaries with ferrite/pearlite interfaces in the austenite nucleation ability. When the full pearlite in 0.8 mass pct C steel underwent reversion at a relatively low temperature, austenite grains preferentially formed at pearlite block boundaries. Consequently, when the full pearlite with the coarse block structure underwent reversion just above the eutectoid temperature, the reversion took a long time due to the low nucleation density. However, austenite grains densely formed at the pearlite colony boundaries as well, as the reversion temperature became sufficiently high. On the other hand, when ferrite–pearlite in the 0.4 mass pct C steel underwent reversion to austenite, the ferrite/pearlite interface acted as a more preferential austenite nucleation site than the pearlite block boundary even in the case of low-temperature reversion. From these results, it can be concluded that the preferential austenite nucleation site in carbon steels is in the following order: ferrite/pearlite interface > pearlite block > colony boundaries. In addition, orientation analysis results revealed that ferrite restricts the austenite nucleation more strongly than pearlitic ferrite does, which contributes to the preferential nucleation at ferrite/pearlite interfaces. This suggests that austenite grains formed at a ferrite/pearlite interface tend to have an identical orientation even under high-temperature reversion. Therefore, it is thought that the activation of austenite nucleation within pearlite by increasing the reversion temperature may be effective for rapid austenitization and the grain refinement of austenite structure after the completion of reversion in carbon steels.

The toughening mechanisms of microstructural variation and Ni addition in direct-cooled microalloyed ferrite-pearlite steels

The toughening mechanisms of microstructural variation and Ni addition were investigated in two direct-cooled microalloyed ferrite-pearlite steels by using the U-notched Charpy tests. Results show that the impact toughness of the low Ni steel decreases dramatically from 110 J to 20 J when the prior austenite grain size increases from 10 µm to 37 µm, and then stays unchanged as the prior austenite grain size continues increasing from 37 µm to 90 µm. When the prior austenite grains are finer than 37 µm, the cleavage fracture is much postponed and the area of ductile fracture is much enlarged by the significantly improved ability to perform plastic deformation. When the prior austenite grains are coarse, the energy absorbed by cleavage, which is the main mode of fracture, conforms to the Griffith type equation and is much less compared to that consumed by ductile fracture, leading to the low impact toughness. The cumulative effect of ferrite content and pearlite interlamellar spacing on impact toughness is not obvious, because their influences are contradictory as the cooling rate is raised. There is a steady enhancement of about 20 J in impact toughness of the high Ni steel compared to the low Ni steel, when the austenite grain size exceeds about 37 µm. The area of ductile fracture is enlarged by the increased dislocation mobility in high Ni steel, resulting in the improvement of impact toughness. It is difficult to refine the prior austenite grain size to less than 37 µm of microalloyed ferrite-pearlite steels through optimizing forging and cooling process. However, small amount of Ni addition is beneficial to the impact toughness and does not increase the cost significantly.

On significance of initial microstructure in governing mechanical behavior and fracture of dual-phase steels

Different initial microstructures were obtained through combination of intercritical annealing and cold-rolling. Subsequently, steels with different microstructures of ferrite–pearlite (FP), ferrite–martensite (FM) and complete martensite (M) were intercritically annealed at 780 °C for 5 min and water quenched to obtain ferrite–martensite microstructure. The significance of initial microstructures on ultimate microstructure, mechanical properties, strain-hardening ability and fracture behavior in dual-phase steels has been elucidated. Initial microstructures of FP, FM and M yielded different martensite morphologies, notably chain-like network structure, fine and fibrous martensite structure, respectively. Furthermore, with increasing martensite content in the initial microstructure, the average grain size of ferrite was significantly refined from about 12.3 to 2.1 μm, which results in that the ultimate tensile strength (UTS) and yield strength were increased, total elongation remained unaffected, and uniform elongation (UE) and strain-hardening ability were increased. A comparison of mechanical properties for different initial microstructures suggested that when the initial microstructure was complete martensite, the steel had excellent mechanical properties, with UTS × UE of 122.5 J cm−3, which was 24% greater than the conventional continuously annealed steels with ferrite–pearlite initial microstructure (98.8 J cm−3). The variation in tensile properties, strain-hardening ability and fracture mechanism of steels with different initial microstructures were discussed in relation to the ultimate microstructures.

Investigation of the Microstructure, Micro-Texture and Mechanical Properties of a HSLA Steel, Hot-Rolled and Quenched at Different Cooling Rates

In this research, the role of cooling rate on the microstructure, micro-texture and mechanical properties of hot-rolled high-strength low-alloy steel were studied. A specific composition was casted, hot-rolled and quenched at different cooling rates. Tensile test, hardness test and scanning electron microscope equipped with EBSD detector were used to evaluate the different properties of the samples. The results showed that the increase in cooling rate led to the formation of nonequilibrium structures such as acicular ferrite (AF) and martensite (M) instead of polygonal ferrite. Micro-texture analysis revealed that Cube and Goss components formed in the ferrite–pearlite sample and Rotated Cube and shear components formed in the samples with AF and M phases. Tensile strength increased while elongation decreased due to microstructure variations. Moreover, it was seen that enhancement of cooling rate resulted in grain refinement and increased hardness. The relation between the maximum Schmid factor and the main micro-texture components seems to be another factor governing strength. Fractography also revealed that an increase in cooling rate led to a decrease in average dimple size and dimple surface ratio and, consequently, variation in fracture mechanism from ductile to shear ductile fracture.

A Study of the Optimum Quenching Temperature of Steels with Various Hot Rolling Microstructures after Cold Rolling, Quenching and Partitioning Treatment

Quenching and partitioning (Q&P) processes were applied to a cold-rolled high strength steel (0.19C-1.26Si-2.82Mn-0.92Ni, wt %). The effects of the prior hot-rolled microstructure on the optimum quenching temperature of the studied steels were systematically investigated. The microstructure was analyzed by means of transmission electron microscope (TEM), electron backscatter diffraction (EBSD) and X-ray diffraction (XRD). Compared with the ferrite pearlite mixture matrix, the lower martensite start (Ms) temperature and smaller prior austenite grain size in the cold-rolled martensite matrix are the main reasons for the optimum quenching temperature shifting to a lower temperature in the Q&P steels. We found that an empirical formula that only considers the influence of the alloy composition in the calculation of the Ms temperature will cause a certain interference to the pre-determined optimum quenching temperature of the Q&P steel.

Effects of Controlled Cooling-Induced Ferrite–Pearlite Microstructure on the Cold Forgeability of XC45 Steel

The combination of hot/warm and cold forging with an intermediate controlled cooling process is a promising approach to saving costs in the manufacture of automobile parts. In this work, the effects of the ferrite–pearlite microstructure, which formed after controlled cooling, on the cold forgeability of a medium-carbon steel were investigated. Different specimens for both normal and notched tensile tests were directly heated to high temperature and then cooled down at different cooling rates, producing different ferrite volume fractions, ranging from 6.69 to 40.53%, in the ferrite–pearlite microstructure. The yield strength, ultimate tensile strength, elongation rate, percentage reduction of area, and fracture strain were measured by tensile testing. The yield strength, indicating deformation resistance, and fracture strain, indicating formability, were used to evaluate the cold forgeability. As the ferrite volume fraction increased, the cold forgeability of the dual-phase ferritic–pearlitic steel improved. A quantitatively relationship between the ferrite volume fraction and the evaluation indexes of cold forgeability for XC45 steel was obtained from the test data. To validate the mathematical relationship, different tensile specimens machined from real hot-forged workpieces were tested. There was good agreement between the predicted and measured values. Our predictions from the relationship for cold forgeability had an absolute error less than 5%, which is acceptable for industrial applications and will help to guide the design of combined forging processes.

Decarburization and the Influence of Laser Cutting on Steel Structure

By microscopic analysis of 20, 30, 45, У7A, and У8A steel samples obtained by laser cutting in different conditions, the structure of the surface thermal-influence zone is studied. This zone includes a dendritic layer (samples of У8A steel); a region with martensite structure; and an intermediate ferrite or ferrite–pearlite layer, indicating complete (20, 30, 35, or У7A steel) or partial (У8A steel) decarburization of deep regions in laser cutting. The microhardness distribution from the surface into the depth of the samples is studied, and the extent of the thermal-influence zone after laser cutting is determined. A mathematical model is developed and used to analyze the influence of the laser-cutting parameters, the sheet thickness, and the carbon content on the extent of the thermal-influence zone.

Effect of Reinforcing Bar Microstructure on Passive Film Exposed to Simulated Concrete Pore Solution

Use of thermomechanically treated (TMT) reinforcing bars has become popular in the concrete construction industry after their introduction in the 1980s. This can be attributed to the fact that thermomechanical treatment enhances the strength and ductility of reinforcing bars remarkably. However; limited research is available on the corrosion behavior of TMT reinforcing bars after they are embedded in concrete. To enhance knowledge and insights into this area, the authors investigated the protective properties of two types of steel reinforcement bars (reinforcing bars)-namely tempered martensite (TM) and pearlite-ferrite (PF)-after exposing them to concrete pore solution. The microstructures of both types of reinforcing bars were investigated, and their kinetics of growth and mechanism of nucleation were recorded by employing electrochemical impedance spectroscopy and polarization. Studies reveal that protective film on the surface of steel with TM microstructure develops at a significantly higher rate and is more stable compared to that on steel with PF microstructure. This superior protective nature is attributed to the development of a compact and adherent oxide film on TM steel. A model is proposed to explain the results obtained during the study. Raman spectroscopy of passive films formed on the surface of reinforcing bars exposed to concrete pore solution supports the proposed model.

Effect of steel structure and defects on reliability of parts of impact mechanisms

The paper discusses selection of materials suitable for manufacturing critical parts of impact mechanisms. It is shown that in order to extend life of parts exposed to high dynamic loading, it is expedient to use medium- and high-carbon alloy-treated steels featuring low impurity with nonmetallic inclusions and high hardening characteristics. Application of thermally untreated parts is undesirable as steel having ferrite–pearlite structure possesses low fatigue strength. Aimed to ensure high reliability of parts with a hardness of 42–55 HRC, steel should be reinforced by thermal treatement with the formation of multicomponent martensite–bainite structure. High-quality production should include defectoscopy and incoming material control.