Showing papers on "Pearlite published in 2015"

Hot-rolled steel sheet

Abstract: A hot-rolled steel sheet has a specified chemical composition and has such a steel structure that the area ratio of a ferrite is 5 to 50%, the area ratio of a bainite that is composed of an aggregate of a bainitic ferrite having an average crystal orientation difference of 0.4 to 3° is 50 to 95%, and the total area ratio of a martensite, a pearlite and a retained austenite is 5% or less.

Investigations on Microstructures and Three-Body Abrasive Wear Behaviors of Fe–B Cast Alloy Containing Cerium

Abstract: The effect of the element cerium on the microstructure and wear behavior of Fe–B cast alloy was investigated by scanning electron microscope, transmission electron microscope, X-ray diffraction analysis, Leica digital image analysis, hardness tester and abrasion tester. The microstructures of as-cast Fe–B alloy are composed of the phase ferrite, pearlite and eutectic boride. Moreover, the as-cast eutectic boride structures in Fe–B alloy containing cerium are finer than that in the alloy having no cerium. After heat treatment, the average boride area and wear weight loss of the alloy containing cerium are lower than these of the alloy having no cerium. Before the formation of primary austenite, cerium can combine with oxygen to form Ce2O3. Ce2O3 can act as nuclei of primary austenite, promoting the refinement of austenite and borides during solidification, and improve the wear property of Fe–B alloy.

Phase-Field Modeling for Intercritical Annealing of a Dual-Phase Steel

Abstract: A phase-field model has been developed to describe microstructure evolution during intercritical annealing of a commercial DP600 dual-phase steel. The simulations emphasize the interaction between ferrite recrystallization and austenite formation from a cold-rolled pearlite/ferrite microstructure at high heating rates. The austenite-ferrite transformations are assumed to occur under conditions where only carbon partitions between the phases by long-range diffusion. A solute drag model has been integrated with the phase-field model to describe the effect of substitutional alloying elements on the migration of the ferrite/austenite interface. Experimental results including recrystallization and transformation kinetics as well as austenite morphology have been successfully described by carefully adjusting both the austenite nucleation scenario and the interface mobilities.

Effect of initial microstructure on the work hardening behavior of plain eutectoid steel

Abstract: Work hardening capability and tensile properties of the plain eutectoid steel rods are the key factors in the wire drawing process to fabricate high strength wire rods with a minimum failure. In the present research, the work hardening behavior of eutectoid steel with different initial microstructures of bainite, duplex bainite+pearlite, pearlite, partially spheroidized and spheroidized pearlite was assessed in terms of instantaneous work hardening exponent ( n value) and work hardening rate ( θ ) using room temperature tensile test. The results show an inverse parabolic behavior for variation of instantaneous n value versus true strain, i.e., work hardening exponent initially increases up to a maximum value and then decreases. The bainitic microstructure exhibits the lowest n value, whereas the spheroidized pearlitic one shows the highest. It is shown that the fine pearlitic microstructure containing partially spheroidized regions exhibited the best combination of tensile properties, n value and work hardening rate.

Thermomechanical processing of low carbon Nb–Ti stabilized microalloyed steel: Microstructure and mechanical properties

Abstract: Aim of the present study is to investigate the effect of deformation temperature and cooling rate on microstructural features and mechanical properties of Nb–Ti stabilized microalloyed steel. Rolling schedule in 3 different phase regimes (γ-recrystallization region at Tnr+50 °C, γ-nonrecrystallization region at Tnr−50 °C and (α+γ) region) was designed on the basis of critical temperatures, Ar3 and Ar1 (obtained from dilatometric study, Gleeble-3800), and Tnr (determined from Boratto equation). The combination of high yield strength (YS) and ductility of the forced air-cooled (FAC) and quenched specimens rolled in (α+γ) region is attributed to the high misorientation angles of matrix, formation of subgrain ferrite (~2 µm)+larger ferrite (~35 µm) and precipitation of NbC (<10 nm). Whereas, the good combination of YS, ductility and high impact energy of the forced air cooled samples deformed at 1050 °C is endorsed to the high fraction of acicular ferrite (76%), formation of degenerate pearlite and precipitation of nanosize TiC. TEM investigation substantiated the formation of shear bands and nanosize carbide precipitates; whereas, EBSD analysis confirmed subgrain formation and misorientation angles of matrix grains.

Friction Stir Welding of Steel: Heat Input, Microstructure, and Mechanical Property Co-relation

Abstract: Friction stir welding was performed to join carbon steel plates at tool rotational rate of 800-1400 rpm. Microstructure and microhardness of welded specimens were evaluated across weld centerline. Torque base index, peak temperature, cooling rate, strain, strain rate, volumetric material flow rate, and width of extruded zone at weld nugget were calculated. Peak temperature at weld nugget was ~1300-1360 K. At this temperature, ferrite transformed to austenite during welding. Austenite was decomposed in to ferrite and bainite at cooling rate of ~4-7.5 K/s. The presence of bainite was endorsed by increment in microhardness with respect to base material. Ferrite grain size at weld nugget was finer in comparison to as-received alloy. With the increment in tool rotational rate strain, strain rate, total heat input, and peak temperature at weld nugget were increased. High temperature at weld nugget promoted increment in ferrite grain size and reduction in area fraction of bainite. Heat-affected zone also experienced phase transformation and exhibited enhancement in ferrite grain size in comparison to base alloy at all welding parameters with marginal drop in microhardness. Maximum joint strength was obtained at the tool rotational rate of 1000 rpm. Increment in tool rational rate reduced the joint efficiency owing to increment in ferrite grain size and reduction in pearlite area fraction at heat-affected zone.

Effect of Continuous and Pulsed Current on the Metallurgical and Mechanical Properties of Gas Tungsten Arc Welded AISI 4340 Aeronautical and AISI 304 L Austenitic Stainless Steel Dissimilar Joints

Abstract: In this research work, the weldability of low alloyed AISI 4340 aeronautical steel and AISI 304L austenitic stainless steel joined by continuous current (CC) and pulsed current (PC) gas tungsten arc welding (GTAW) techniques, using ER309L and ERNiCr-3 filler metals was investigated. The main focus of the study involves the investigation on the effect of continuous and pulsed current mode of GTA welding process on the metallurgical and mechanical properties of these dissimilar weldments. Microstructure studies revealed the formation of different zones across the weldments, vis-a-vis martensite at the HAZ of AISI 4340, vermicular δ - ferrite /ferrite stringers at the HAZ of AISI 304L, pearlite colonies at the parent metal of AISI 4340 and equi-axed cellular and/or columnar dendrites at the weld zone. Tensile results showed that current pulsing accrued better tensile properties. The structure - property relationships of these weldments were established based on the current modes employed by utilizing combined techniques of optical microscopy, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS).

Dry rolling/sliding wear of nanostructured pearlite

Abstract: The dry rolling–sliding wear behaviour of pearlite that has an interlamellar spacing of just 85 nm has been characterised. Its wear resistance is found to be comparable to that of much harder bainitic steels. Microstructural observations indicate that there is substantial plastic deformation of both ferrite and cementite components of pearlite in the vicinity of the wear surface. Plasticity is not expected from Hertzian analysis that assumes a smooth contact surface. Instead, it is likely to be a consequence of exaggerated stresses due to surface roughness. The material remains ductile to shear strains in the order of 4. Diffraction data indicate that the coherent domain size is reduced to about half the interlamellar spacing and that some of the cementite may dissolve and contribute to the expansion of the lattice parameter of ferrite.

Mechanical properties and fatigue behavior of railway wheel steels as influenced by mechanical and thermal loadings

Abstract: In the current work the deterioration of mechanical properties of railway wheel steels (UIC ER7T and ER8T) is in focus. These are medium carbon steels (∼0.55 wt.% C) heat treated to a near pearlitic microstructure with some 5-10% pro-eutectoid ferrite. During operation of trains, high thermal loads are evolved because of recurring acceleration, braking, curving and occasional slippage. It is thus relevant to examine the high temperature performance of wheel material and evaluate the decrease in strength after thermal exposure as well as the degradation of fatigue properties. Samples were extracted from virgin wheels and pre-strained to around 6.5% strain as well as cyclically deformed, to also account for the change in properties that is induced by plastic deformation inherent in the wheel tread surface. Both un-deformed and pre-strained material was heat treated for different times in the temperature range of interest, from 250°C to 600-700°C. Hardening was observed in both conditions around 300°C followed by softening at higher temperatures. Spheroidization of the pearlite started to become visible at 450°C for the un-deformed material and at around 400°C for the pre-strained.

Comparative study and quantification of cementite decomposition in heavily drawn pearlitic steel wires

Abstract: Heavily cold-drawing was performed on a pearlitic steel wire and on an ultra-low carbon (ULC) steel wire in order to highlight and quantify the microstructural changes caused by this type of deformation. Both global techniques (thermoelectric power, electrical resistivity, internal fiction background) and local techniques (Atom Probe Tomography) were combined for this study. It was shown that two distinct stages have to be taken into account during the cold-drawing of pearlitic steels. The first stage (below a true strain of 1.5) was attributed mainly to the lamellar alignment, while the second stage (above a true strain of 1.5) was unambiguously interpreted as being due to a gradual enrichment of the carbon content of ferrite arising from the strain induced cementite decomposition. The carbon content in solid solution in ferrite was assessed as a function of the true strain. All the techniques showed that this carbon content exceeds the solubility limit of carbon in the ferrite above a true strain of 2.2. A correlation between the increase in the carbon content of ferrite and the increase in yield strength was also highlighted. Moreover, a scenario was proposed to explain the microstructural changes caused by drawing. (C) 2015 Elsevier B.V. All rights reserved.

Effects of Molybdenum on the Wear Resistance and Corrosion Resistance of Carbidic Austempered Ductile Iron

Abstract: The effects of molybdenum on the microstructure, wear resistance, and corrosion resistance of carbidic austempered ductile iron were studied. The results show that the microstructures of as-cast samples are mainly composed of nodular graphite, pearlite and carbides, and the nodularization grade is 2–3 and graphite size is 6–7 magnitude in all samples. With the increase of Mo content, the amount of nodular graphite decreases, but the pearlite and carbides increase. The microstructures of all heat-treated samples mainly consist of nodular graphite, acicular ferrite, retained austenite, and a certain number of carbides. As the molybdenum content increases, the quantity of retained austenite and carbides increases, the acicular ferrite becomes finer, and the wear resistance and corrosion resistance increased. When the concentration of molybdenum reaches 0.570 wt%, the average wear weight loss decreased to 0.218 mg/m, the corrosion potential reached to −0.5626 V, and the corrosion current density decreased to 6.361 × 10−8 A/cm2.

Optimization of Cellular Automata Model for the Heating of Dual-Phase Steel by Genetic Algorithm and Genetic Programming

Abstract: This study considers a common metallurgical problem associated with the phase transformation of steel during heating where austenite grain tends to grow in size with time and results in poor mechanical properties in the final stages. This investigation was performed using a Cellular Automata model for dual-phase steel developed in house. Data-driven metamodels for a biobjective optimization problem involving minimizing average austenite grain size along with the maximizing of time of heating were constructed using Evolutionary Neural Network (EvoNN) and Biobjective Genetic Programming (BioGP). The input variables selected for this task were (i) heating rate, (ii) pearlite percentage, (iii) nucleation density of austenite, and (iv) the finish temperature of austenite formation. The analyses of the results led to the fact that heating rate is the most influencing factor and it needs to be large during transformation to obtain a refined microstructure. The comparison of Pareto front between EvoNN and BioGP r...

The effect of electropulsing on the interlamellar spacing and mechanical properties of a hot-rolled 0.14% carbon steel

Abstract: The application of electropulsing treatment to a low carbon ferritic–pearlitic steel is studied The effect of electric current pulses on the interlamellar spacing of pearlite is investigated It is found that the interlamellar spacing increases as the number of pulses increases The mechanism of electropulse-effect was discussed by analysing the change in the free energy and the reduction of electrical resistance due to electropulsing Mechanical properties are also examined for the samples with and without electropulsing treatment It is shown that softening occurs during the treatment which is attributed to the formation of precipitation free zone (PFZ), increase in the value of interlamellar spacing and the spheroidization of the lamellar structure

The Relationship Between Microstructural Evolution and Mechanical Properties of Heavy Plate of Low-Mn Steel During Ultra Fast Cooling

Abstract: We describe here the electron microscopy and mechanical property studies that were conducted in an industrially processed 20- and 40-mm C-Mn thick plates that involved a new approach of ultrafast cooling (UFC) together with significant reduction in Mn-content of the steel by ~0.3 to 0.5 pct, in relation to the conventional C-Mn steels, with the aim of cost-effectiveness. The study demonstrated that nanoscale cementite precipitation occurred during austenite transformation in the matrix of heavy plate during UFC, providing significant precipitation strengthening. With decrease in UFC stop temperature and consequent increase in the degree of undercooling, there was a transition in the morphology of cementite from lamellar to irregular-shaped nanoscale particles in the 20 mm heavy plate. With the increase in plate thickness, nanoscale cementite precipitated in bainitic lath at the surface of 40 mm heavy plate, which significantly increased the strength and decreased the elongation. Simultaneously, microstructural evolution in hot-rolled sheets was studied via simulation experiments using laboratory rolling mill to define the limits of microstructural evolution that can obtained in the UFC process and develop an understanding of the evolved microstructure in terms of process parameters.