Showing papers on "Pearlite published in 2012"

Influence of bainite morphology on impact toughness of continuously cooled cementite free bainitic steels

Abstract: The influence of bainite morphology on the impact toughness behaviour of continuously cooled cementite free low carbon bainitic steels has been examined. In these steels, bainitic microstructures formed mainly by lath-like upper bainite, consisting of thin and long parallel ferrite laths, were shown to exhibit higher impact toughness values than those with a granular bainite, consisting of equiaxed ferrite structure and discrete island of martensite/austenite constituent. Results suggest that the mechanism of brittle fracture of cementite free bainitic steels involves the nucleation of microcracks in martensite/austenite islands but is controlled by the bainite packet size.

Iron and Steel

Abstract: Steels Steels are iron-base alloys usually containing carbon. Figure 7.1 shows the iron-carbon phase diagram. Below 911°C and between 1410°C and the melting point, iron has a bcc crystal structure called ferrite . Between 1410°C and 911°C it has an fcc crystal structure called austenite . Austenite dissolves much more carbon interstitially than ferrite. On slow cooling below 727°C, the austenite transforms by a eutectoid reaction into ferrite and iron carbide or cementite (which contains 6.7%C). The ferrite and cementite form alternating platelets called the eutectoid temperature. The resulting microstucture is called pearlite (see Figure 7.2). Pearlite Formation When a steel containing less than 0.77%C ( hypo-eutectoid steel ) is slowly cooled, some ferrite forms before any pearlite. A steel containing more than 0.77%C ( hyper-eutectoid steel ) will form some cementite before any pearlite. The formation of pearlite from austenite on cooling requires diffusion of carbon from ahead of the advancing ferrite platelets to the advancing carbide platelets as indicated in Figure 7.3. Because diffusion takes time, pearlite formation is not instantaneous. The rate at which pearlite forms depends on how much the temperature is below 717°C. The rate of diffusion increases with temperature, but the driving force for the transformation increases as the temperature is lowered. The result is that the rate of transformation is fastest between 500 and 600°C, as indicated schematically in Figure 7.4.

Synthesis of syntactic steel foam using mechanical pressure infiltration

Abstract: Steel foam offers potential advantages over aluminum foams and other metallic foams. The inherent strength of steel combined with the reduced density of foam presents an attractive material with greater strength and modulus than light alloy foams, greatly reduced density relative to solid steel, and efficient energy absorption. However, thus far there have been few reports describing efforts to produce steel foam, and these have relied on powder metallurgical approaches as opposed to molten state processing. This study demonstrates a feasible synthesis method to consistently produce lab-scale foam samples with uniform distributions of microspheres and negligible unintended porosity using a simple liquid state method of infiltration. To accomplish this, the effects of process parameters were investigated. The preheatment temperature of the microspheres must be close to the melting temperature of steel and a minimum pressure must be exerted to produce the steel infiltration into the microspheres. Syntactic foams with relative density of 0.54 were achieved. The resultant syntactic foams were characterized by chemical analysis, microstructural analysis and hardness measurement. The basic mechanical properties of two different steel compositions were studied under compression loading, one with a ferrite microstructure and the other with a pearlite microstructure. The pearlite foam has greater compression strength and energy absorption capacity than the ferrite foam. The properties of the steel syntactic foams were compared to those of steel foams reported elsewhere. Prospects and challenges for achieving higher energy absorption capacity are discussed.

Investigations on Microstructures and Two-body Abrasive Wear Behavior of Fe–B Cast Alloy

Abstract: The microstructures of Fe–B alloys containing different carbon and boron concentrations have been investigated. The solidification microstructures of Fe–B alloy consist of the eutectic boride, pearlite, and ferrite. Borides precipitate along the grain boundary during the formation of eutectic. After heat treatment, the phases in Fe–B alloy are composed of the boride and martensite. With increase of carbon and boron concentrations, the Rockwell hardness of Fe–B alloy becomes larger. Meanwhile, by using a pin-on-disk abrasion tester, the effects of carbon and boron concentrations on the wear behaviors including ploughing depth, roughness, and wear weight loss under different loads have been studied. The results show that the wear resistance of Fe–B alloy with higher carbon and boron concentrations is comparable with the high chromium white cast iron.

Precipitation of Nb in Ferrite After Austenite Conditioning. Part II: Strengthening Contribution in High-Strength Low-Alloy (HSLA) Steels

Abstract: Often, Nb contributes to the strength of a microalloyed steel beyond the expected level because of the grain size strengthening resulting from thermomechanical processing Two different mechanisms are behind this phenomenon, and both of them have to do with the amount of Nb remaining in solution after hot rolling The first of them is the increase of the hardenability of the steel as a result of Nb, and the second one is the fine precipitation of NbC in ferrite Three Nb microalloyed steels were thermomechanically processed in the laboratory and coiled at different temperatures to investigate the effect of Nb content on the tensile properties The extra strength was linearly related to the Nb remaining in solution after the hot working The maximum contribution from Nb was reached for a coiling temperature of 873 K (600 °C)

Divorced pearlite in steels

Abstract: Steels containing large carbon concentrations are used particularly when a high hardness is required, for example, in the manufacture of components such as bearings. This, however, makes it difficult to shape or machine the alloys during the process of component manufacture unless they are first heat-treated into a softened condition. One method of achieving this economically is to generate a microstructure known as divorced pearlite, in which ferrite and cementite grow from the austenite in a non-cooperative manner, leading to a final microstructure that consists of coarse, spherical particles of cementite dispersed in a matrix of ferrite. This is in contrast to the harder lamellar pearlite which normally develops when high-carbon steels are cooled. The theoretical framework governing the transition from the divorced to the lamellar form is developed and validated experimentally.

Microstructural modifications and changes in mechanical properties during cyclic heat treatment of 0.16% carbon steel

Abstract: In this work an annealed 0.16 wt% carbon steel was subjected to cyclic heat treatment process that consisted of repeated short-duration (6 min) holding at 910 °C (above Ac 3 temperature) followed by forced air cooling. A typical microstructural development, not so common for low carbon hypoeutectoid steel, was observed. The main features involved were: (i) a substantial grain refinement (mainly at initial stage), (ii) generation of dislocations (at initial stage) and its annihilation (at later stage) and (iii) generation of grain boundary network of cementite and cementite cluster owing to divorced eutectoidal reaction. In low carbon steel, the presence of large proportion of grain boundary areas promoted grain boundary diffusion of carbon during short-duration holding. This phenomenon finally led to the generation of grain boundary cementite network and cluster through divorced eutectoidal reaction. Unlike high carbon steel, the contribution of divorced eutectoidal reaction to spheroidization was not so significant. For higher cycles (5–8 cycles) substantial presence of grain boundary cementite and cementite cluster deteriorated the strength property. However, quite a high strength (UTS = 455 MPa) was achieved for this low carbon steel with two cycles of heat treatment due to fine ferrite grain size, high dislocation density and adequate proportion of fine lamellar pearlite in the microstructure.

Development of high strength ductile hypereutectoid steel by cyclic heat treatment process

Abstract: In this work a homogenizing annealed 1.24 wt% carbon steel was subjected to cyclic heat treatment process that consisted of repeated short-duration (6 min) holding at 894 °C (above Acm temperature) followed by forced air cooling. During short-duration holding at 894 °C, fragmentation (owing to partial dissolution) and thickening of cementite lamella occurred at the preferred high energy sites of lamellar faults. These undissolved fragmented cementite acted as pre-existing nuclei promoting the divorced eutectoidal reaction (DET) during forced air cooling that generated non lamellar region (NLR) in the microstructure apart from the presence of lamellar pearlite region (LPR) and cementite network (CN). The CN was also fragmented with repeated holding and cooling cycles. Besides, new lamellar fault sites were generated during forced air cooling to promote lamellar fragmentation. Accordingly, as the number of heat treatment cycles increases, more uniformly dispersed finer cementite particles (pre-existing nuclei) in austenite matrix was obtained at 894 °C. Therefore, the proportion of NLR increased and the proportions of LPR and CN decreased with the progress of cyclic heat treatment. After 8 cycles of heat treatment the microstructure mostly consisted of a combination of spheroids (0.55 ± 0.24 μm in size) and non-spheroids (mainly the plate shaped particles of 0.22 ± 0.10 μm in size) of cementite in 1:5 proportion by area% dispersed in ferrite matrix. The generation of cementite plates occurred through the motion of transformation disconnections. This provided an excellent combination of strength (UTS = 1086 MPa) and ductility (%elongation = 13) in this 1.24 wt% C steel. The property obtained had similarity to what was reported for a thermo-mechanically processed steel of much higher carbon content (1.8 wt%) where the microstructure consisted of solely the cementite spheroids of 0.65 μm average size dispersed in ferrite matrix.

Multiscale prediction of mechanical behavior of ferrite–pearlite steel with numerical material testing

Abstract: SUMMARY Multiscale mechanical behaviors of ferrite–pearlite steel were predicted using numerical material testing (NMT) based on the finite element method. The microstructure of ferrite–pearlite steel is regarded as a two-component aggregate of ferrite crystal grains and pearlite colonies. In NMT, the macroscopic stress–strain curve and the deformation state of the microstructure were examined by means of a two-scale finite element analysis method based on the framework of the mathematical homogenization theory. The microstructure of ferrite–pearlite steel was modeled with finite elements, and constitutive models for ferrite crystal grains and pearlite colonies were prepared to describe their anisotropic mechanical behavior at the microscale level. While the anisotropic linear elasticity and the single crystal plasticity based on representative characteristic length have been employed for the ferrite crystal grains, the constitutive model of a pearlite colony was newly developed in this study. For that reason, the constitutive behavior of the pearlite colony was investigated using NMT on a smaller scale than the scale of the ferrite–pearlite microstructure, with the microstructure of the pearlite colony modeled as a lamellar structure of ferrite and cementite phases with finite elements. On the basis of the numerical results, the anisotropic constitutive model of the pearlite colony was formulated based on the normal vector of the lamella. The components of the anisotropic elasticity were estimated with NMT based on the finite element method, where the elasticity of the cementite phase was numerically evaluated with a first-principles calculation. Also, an anisotropic plastic constitutive model for the pearlite colony was formulated with two-surface plasticity consisting of yield functions for the interlamellar shear mode and yielding of the overall lamellar structure. After addressing the microscopic modeling of ferrite–pearlite steel, NMT was performed with the finite element models of the ferrite–pearlite microstructure and with the microscopic constitutive models for each of the components. Finally, the results were compared with the corresponding experimental results on both the macroscopic response and the microscopic deformation state to ascertain the validity of the numerical modeling. Copyright © 2011 John Wiley & Sons, Ltd.

Microstructures and mechanical properties evolution during friction stir welding of SK4 high carbon steel alloy

Abstract: Stir-in-plate welds were produced on 2 mm thick plates of SK4 high carbon steel alloy (0.95% C) using a load controlled friction stir welding (FSW) machine. The welding was carried out at a constant welding speed of 100 mm/min and different rotation speeds varied between100 and 400 rpm. The microstructure and mechanical properties of the weld metals were investigated. When FSW was carried out at 100 rpm, a duplex structure of spheroidal cementite and fine ferrite was formed and homogenously distributed in the entire stir zone. On the other hand, when the FSW was carried out at a rotation speed higher than 100 rpm, very fine pearlite and martensite structures were formed in the upper part of the stir zone and increased with the increasing rotation speed. Thin film of retained austenite was found in the microstructure of the weld metal stirred at 200 rpm and its volume fraction increased with further increasing in the rotation speed. Hardness values measured in the stir zone formed at 100 rpm were slightly higher than that of the base metal and homogenously distributed throughout the entire stir zone. Increasing rotation speed more than 200 rpm led to a sharp increase in the hardness values of the stir zone and maximum values around 800–820 Hv were attained at 400 rpm below the tool shoulder. Yield and ultimate tensile strengths of the weld metals increased with the increasing rotation speed, while elongation decreased. Fracture surface of the weld metal formed at 100 was similar to the base metal (BM) and exhibited only a microvoid coalescence dimple fracture, while that formed at 200–400 rpm showed a mixed mode of quasi-cleavage transgranular fracture and ductile dimple fracture mode.

The effects of alloying elements and microstructure on the impact toughness of powder metal steels

Abstract: The impact toughness of powder metal steels is typically deteriorated when the strength is increased by adding specific alloying elements and modifying the microstructure. An earlier study by the authors demonstrated that the tensile strength and hardness of Ni-containing PM steels were much enhanced by the additions of Cr and Mo. To develop high-strength PM steels with high toughness, the effects of alloying elements and microstructure on the toughness were further investigated. The results showed that the toughness of binary Fe–C alloys was decreased by Mo and increased by Ni and Cr, though these three alloying elements improved the tensile strength. Mo also impaired the toughness of Fe–Ni–C alloys by the formation of low-toughness pearlite and bainite. However, the addition of Cr in the Fe–Ni–C alloys much improved the tensile strength without sacrificing toughness. This improvement was attributed to the formation of a large amount of tough Ni-rich martensite by adding Cr, which promoted the homogenization of Ni and C. As a result, a superior combination of tensile strength and impact toughness could be achieved, as demonstrated in the Fe–1.5Cr–0.2Mo–4Ni–0.5C steel compact.

The Tensile Properties of Pearlite, Bainite, and Spheroidite

Abstract: The tensile properties of four steels have been determined as a quantitative function of the measured dimensions of the aggregate structures pearlite and spheroidite, and of the austenite decomposition temperature for the structures pearlite and bainite. Studies of the recalescence effect have been performed in connection with the measurement of the reaction temperature. The strength indices (stress at corresponding strains, tensile strength, hardness) vary linearly with the reaction temperature and the logarithm of the dimensions of the aggregate. Mixtures of pearlite and bainite are intermediate in strength. The ductility indices are low for mixed structures, coarse pearlite and low temperature bainite; higher for bainite and pearlite in the middle of the reaction temperature range for each. It has been observed that spheroidized eutectoid specimens have a typical mild steel yield point; pearlitic specimens of the same tensile strength do not. The spacing of pearlite is shown to be proportional to the carbon diffusion coefficient in austenite, the logarithm of the spacing plotting as a straight line against the reciprocal of the absolute reaction temperature, with the same slope as a similar plot for the diffusion coefficient. Because of this it is concluded that a measurement of the spacing at one temperature permits its calculation at another, using the measured energy of activation for the diffusion of carbon in the steel. A rule of strength for aggregates is proposed, based on these studies, as follows: The resistance to deformation of a metallic aggregate consisting of a hard phase dispersed in a softer one is proportional to the logarithm of the mean straight path through the continuous phase. The rule works for a comparison of the properties of pearlite and spheroidite, as well as for pearlite alone over a wide range of spacings, and extrapolates to reasonable particle sizes for the finest spheroidites (tempered martensite). A simple explanation of the semilogarithmic character of the relationship is advanced.

Steel for mechanical structure for cold working, and method for manufacturing same

Abstract: Provided are a steel for a mechanical structure for cold working, and a method for manufacturing the same, whereby softening and variations in hardness can be reduced even when a conventional spheroidizing annealing process is performed A steel having a predetermined chemical composition, the total area ratio of pearlite and pro-eutectoid ferrite being at least 90 area% with respect to the total metallographic structure of the steel, the area ratio (A) of pro-eutectoid ferrite satisfying the relationship A > Ae with an Ae value expressed by a predetermined relational expression, the average equivalent circular diameter of bcc-Fe crystal grains being 15-35 µm, and the average of the maximum grain diameter and the second largest grain diameter of the bcc-Fe crystal grains being 50 µm or less in terms of equivalent circular diameter

Effects of Plastic Warm Deformation on Cementite Spheroidization of a Eutectoid Steel

Abstract: The warm compression tests were performed on the eutectoid steel to investigate the evolution of cementite morphology. Several processing parameters, such as temperature, strain rate and reduction, were changed to analyze the effect of each parameter on spheroidization of cementite. The results showed that the warm compression promoted the fragmentize and the spheroidization of lamellar cementites. When the specimen was compressed with reduction of 50% at 700 °C and in the strain rate of 0. 01 s−1, the excellent spheroidized cementite was obtained. The mechanism of fragmentation and spheroidization of lamellar cementites during compression was discussed by using transmission electron microscope. The formation of spheroidized cementite was related to the time of compression process. The fragmentize of lamellar cementites was due to the extension of sub-grain boundary in the cementite. The spheroidization of cementite depended on the diffusion of carbon atoms at the tip of bended and breakup cementite.

Severe tempering of bainite generated at low transformation temperatures

Abstract: The response of a strong bainitic-steel to tempering at high temperatures is investigated, because a softened state is sometimes necessary during the manufacture of engineering components, prior to the final heat-treatment which hardens the material. Thermodynamic calculations conducted to determine the maximum temperature at which the steel could be annealed without forming austenite were found to significantly overestimate the actual temperature at which austenite, ferrite and cementite (γ + α + θ) can coexist in equilibrium. As a consequence, the tempering temperature must be limited to below about 700°C, which unfortunately necessitates many days of tempering in order to reduce the hardness to < 250 HV. An alloy modification which may shorten this tempering time is suggested. One unexpected outcome is that the unintended heat treatment in the three-phase field revealed useful information about the nucleation of pearlite on cementite particles. Colonies of pearlite started inevitably on partic...

600-MPa grade vanadium-containing high-strength hot-rolled steel bar and production method thereof

Abstract: The invention relates to a 600-MPa grade vanadium-containing high-strength hot-rolled steel bar and a production method thereof, and belongs to the technical field of metallurgical steel-making and processing; the technical scheme is that the steel bar is prepared by the following components by mass: 0.21-0.25% of C, 0.35-0.60% of Si, 1.35-1.55% of Mn, 0.08-0.12% of V, 0.005-0.04% of N, not more than 0.040% of S, not more than 0.040% of P, and the balance of Fe and inevitable impurities; by adopting vanadium-increasing, nitrogen-increasing, and nitrogen-fixing processes during the smelting process, and by decreasing the initial rolling temperature and the finish rolling temperature during the rolling process, fine-grain, solid-solution and precipitation strengthening are realized at a lowtemperature and a high pressure, so the strength is increased; the hot-rolled steel bar contains a dual-phase structure of ferrite and pearlite, wherein the pearlite accounts for 25-50%; good cooperation of strength and toughness can be reached by controlling ratios, forms and sizes of the phases.

Back‐scattered electron imaging combined with EBSD technique for characterization of pearlitic steels

Abstract: Summary Microstructure of pearlitic steels subjected different heat treatments were characterized combining the usage of backscatteredelectronimagingandelectronbackscatterdiffraction in a scanning electron microscope. The results indicated that the method used in current study enabled the acquisition of pearlite nodule, colony and interlamellar spacing of pearlite structure only through sample preparation of one time. Both the morphology of pearlite lamellae and the crystallographic orientation of ferrite matrix can be released in backscattered electron imaging image and electron backscatter diffraction micrograph acquired at the same region. The definitions of pearlite colony and the low-angle boundaries existed in ferrite matrix were also discussed based on this method.

Effect of ferrite-pearlite microstructure on structural steel properties

Abstract: A comprehensive study is provided for the effect of a ferrite-pearlite microstructure with a pearlite content from 0 to 100% on the main operating properties of structural steel, including fracture toughness and ductility, brittle failure resistance, and crack resistance. It is found that an increase in carbon content is accompanied by rapid ferrite grain refinement. It is confirmed that an increase in pearlite content in a microstructure, especially above 20% has a marked, but not always favorable effect on all steel deformation and failure characteristics.

High-strength wire rod having superior rod drawability, and manufacturing method therefor

Abstract: A high-strength wire rod having a pro-eutectoid ferrite area ratio of 3% or below and a pearlite structure area ratio of 90% or higher, produced by subjecting a hard steel wire rod of specified components to hot rolling and thereafter either direct molten salt patenting or re-austenitizing and subsequent molten salt or lead patenting.

Effects of Rolling and Cooling Conditions on Microstructure and Mechanical Properties of Low Carbon Cold Heading Steel

Abstract: Effects of rolling and cooling conditions on microstructure and mechanical properties of low carbon cold heading steel were investigated on a laboratory hot rolling mill. The results have shown that the mechanical properties of low carbon steels exceed the standard requirements of ML30, ML35, ML40, and ML45 steel, respectively due to thermomechanical controlled processing (TMCP). This is attributed to a significant amount of pearlite and the ferrite-grain refinement. Under the condition of relatively low temperature rolling, the mechanical properties exceed standard requirements of ML45 and ML30 steel after water cooling and air cooling, respectively. Fast cooling which leads to more pearlite and finer ferrite grains is more critical than finish rolling temperatures for low carbon cold heading steel. The specimen at high finish rolling temperature exhibits very good mechanical properties due to fast cooling. This result has great significance not only for energy saving and emission reduction, but also for low-carbon economy, because the goals of the replacement of medium-carbon by low-carbon are achieved with TMCP.

Enhancement of mechanical properties by changing microstructure in the eutectoid steel

Abstract: The different microstructures of eutectoid steel were analyzed with SEM and its corresponding room-temperature tensile tests were carried out. The results show that the ultrafine ( α + θ ) duplex structure consisting of ferrite matrix ( α ) with grain size of about 1 μm and dual-size distributed cementite particles ( θ ) could be formed by hot deformation of undercooled austenite and subsequent annealing. Moreover, the mixed microstructures consisting of polygonal ferrite grains with size of about 2 μm and fine pearlite colonies could be also obtained by this process under different conditions. Although the yield strength and total elongation of ultrafine ( α + θ ) duplex structure increase markedly, its tensile strength decrease obviously comparing to that of lamellar pearlite. The lower work-hardening rate at the beginning of uniform plastic deformation is probably responsible for tensile strength decrease in the ultrafine ( α + θ ) duplex structure. Moreover, the tensile strengths and total elongations of mixed microstructures are both larger than that of ultrafine ( α + θ ) duplex structure and lamellar pearlite due to their well work-hardening capabilities.