Showing papers on "Pearlite published in 2013"

Constitutive relationships of hot stamping boron steel B1500HS based on the modified Arrhenius and Johnson–Cook model

Abstract: Constitutive relationship of boron steel is one of the most necessary mathematical models in the numerical simulation of hot stamping; it describes the relationship of the flow stress with strain, strain rate and temperature. In order to attain the constitutive relationship of boron steel B1500HS, four types of samples with microstructure of austenite, ferrite+pearlite, bainite or martensite are prepared by the Gleeble 1500D thermo-mechanical simulator. Isothermal uniaxial tension testings for these specimens are performed at 20–900 °C at the strain rates of 0.01 s –1 , 0.1 s –1 , 1.0 s –1 and 10 s –1 by Gleeble 1500D, and the true stress–strain curves at the relative conditions are gained. The experimental results show that, the flow stress of samples with relative microstructure rises with the decrease of the deformation temperature, and with the increase of the strain rate. The modified Arrhenius model is used to describe the hot deformation of samples with austenite microstructure, and the modified Johnson–Cook model is used to describe the deformation process of samples with ferrite+pearlite, bainite or martensite microstructure. The constitutive equations depending on the strain, strain rate and temperature are attained by the regression analysis for the experimental data of flow stress, strain, strain rate, temperature, etc. The comparison of the computational data and the experimental results shows that, the computational data using the constitutive relationships are well consistent with the experimental data.

Influence of ferrite and pearlite content on mechanical properties of ductile cast irons

Abstract: The present work is to study the influence of ferrite and pearlite content on mechanical properties of ductile cast irons. Three casts with different microstructures were produced. The chemical composition of the casts was also considered. The pearlite content is varied from 0 to about 70 percent without using heat treatments. This paper also shows that is possible to obtain different microstructures with good mechanical properties through an adequate balance of alloying elements. The study of the tensile properties indicated that yield and tensile strengths are increased with increasing pearlite content. This analysis also shows that graphite shapes and the second phase particles found to have influence on these parameters. With this purpose an extensive study was also done on mechanical properties to achieve good results. Specimens were under different mechanical tests such as fracture toughness and tensile strength tests. The Charpy impact tests were done at different temperatures.

Effect of microstructure on the impact toughness of Nb-microalloyed steel: Generalisation of existing relations from ferrite–pearlite to high strength microstructures

Abstract: The present work relies on the production of selected microstructures through the application of thermal and thermomechanical laboratory tests, followed by mechanical testing and microstructural characterisation. The relations that link the microstructure parameters and the tensile properties have already been discussed and extended from ferrite–pearlite to high strength microstructures in a previous work. Using these results as a starting point, the present work goes a step forward and develops a methodology to consistently incorporate the effect of mesotexture (EBSD) into the existing relations that link the Charpy impact toughness to the microstructure. The result is an extension of the existing equation for the FATT from ferrite–pearlite to high strength microstructures (bainite, tempered martensite). The upper shelf energy for the Nb-microalloyed steel under consideration correlates linearly with the sum of the different terms of the FATT equation (solutes, grain boundary carbides, pearlite and precipitation/dislocations strengthening) excepting the grain size.

Effect of Destabilizing Heat Treatment on Solid-State Phase Transformation in High-Chromium Cast Irons

Abstract: This work describes the influence of secondary carbide precipitation at destabilizing heat treatment on kinetics of austenite phase transformation at a subcritical range of temperatures in high-Cr cast irons, alloyed with 4 to 6 wt pct of Mn or by complex Mn-Ni-Mo (Mn-Cu-Mo). The samples were soaked at 1073 K to 1373 K (800 °C to 1100 °C) (destabilization) or at 573 K to 973 K (300 °C to 700 °C) (subcritical treatment); the combination of destabilization and subcritical treatment was also used. The investigation was carried out with application of optical and electron microscopy and bulk hardness measurement. Time-temperature-transformation (TTT) curves of secondary carbide precipitation and pearlite transformation for as-cast austenite and destabilized austenite were built in this work. It was determined that the secondary carbide precipitation significantly inhibited the pearlite transformation rate at 823 K to 973 K (550 °C to 700 °C). The inhibition effect is more evident in cast irons alloyed with complex Mn-Ni-Mo or Mn-Cu-Mo. The possible reasons for transformation decelerating could be austenite chemical composition change (enriching by Ni, Si, and Cu, and depleting by Cr) and stresses induced by secondary carbide precipitation.

Phase Transformation Study in Nb-Mo Microalloyed Steels Using Dilatometry and EBSD Quantification

Abstract: A complete microstructural characterization and phase transformation analysis has been performed for several Nb and Nb-Mo microalloyed low-carbon steels using electron backscattered diffraction (EBSD) and dilatometry tests. Compression thermomechanical schedules were designed resulting in the undeformed and deformed austenite structures before final transformation. The effects of microalloying additions and accumulated deformation were analyzed after CCT diagram development and microstructural quantification. The resulting microstructures ranged from polygonal ferrite and pearlite at slow cooling ranges, to a combination of quasipolygonal ferrite and granular ferrite for intermediate cooling rates, and finally, to bainitic ferrite with martensite for fast cooling rates. The addition of Mo promotes a shift in the CCT diagrams to lower transformation start temperatures. When the amount of Nb is increased, CCT diagrams show little variations for transformations from the undeformed austenite and higher initial transformation temperatures in the transformations from the deformed austenite. This different behavior is due to the effect of niobium on strain accumulation in austenite and its subsequent acceleration of transformation kinetics. This article shows the complex interactions between chemical composition, deformation, and the phases formed, as well as their effect on microstructural unit sizes and homogeneity.

Precipitation behavior of nanoscale cementite in hypoeutectoid steels during ultra fast cooling (UFC) and their strengthening effects

Abstract: We describe here the unique effects of ultra fast cooling (UFC) on strengthening induced by nanoscale cementite precipitation in hypoeutectoid steels containing 0.17 and 0.33 wt% of carbon. In the absence of nanoscale cementite, fine ferrite grain size, refinement of pearlite, and reduced lamellar spacing contributed to strengthening of 0.04 wt% C and 0.5 wt% C steels, respectively. The contribution of nanoscale cementite precipitates of size ∼20–30 nm to yield strength was ∼100 MPa in 0.17C and 0.33C steels. Carbon content and the degree of undercooling were the primary factors that govern nanoscale cementite precipitation. The transformation driving force for the undercooled austenite was calculated using the Kaufman–Radcliffe–Cohen (KRC) model to thermodynamically analyze the possibility of precipitation of nanoscale cementite. It was also observed that dislocations could be “frozen” in austenite because of UFC, which provide channels for diffusion of carbon atoms and consequent nucleation of cementite.

A multiscale approach for the deformation mechanism in pearlite microstructure: Numerical evaluation of elasto-plastic deformation in fine lamellar structures

Abstract: Elasto-plastic deformations in the microstructures of pearlite are studied by finite-element analyses. Various models for the lamellar structure are made and the material properties of cementite and ferrite are established. Deformation of a bare specimen of cementite is unstable immediately after the yield point, while cementite lamellae show some stability when they are layered with ferrite. When higher values of yield stress and strain hardening are used for ferrite phase, cementite deforms well beyond the elastic range and the distribution of plastic strain is not concentrated. These results show that not only the layered structure but also the improved mechanical property of fine lamellae of ferrite contribute largely to stable deformation in the pearlite microstructure.

Influence of morphology and structural size on the fracture behavior of a nanostructured pearlitic steel

Abstract: Subjecting pearlitic steels to severe plastic deformation is known to transform the original colony structure to a nanostructured pearlite with the microstructural constituents aligned parallel to the deformation direction. Besides a huge increase in strength due to an enormous reduction of the interlamellar distance, other mechanical properties such as fracture toughness are deteriorated especially when the crack propagation is parallel to the shear deformed structure. Post-deformation heat-treatments up to 600 °C were applied to modify the oriented nanostructured pearlite after applying high pressure torsion to a micro-duplex structure of ultrafine grained ferrite with spherical cementite dispersoids. Already moderate annealing led to a pronounced increase of the fracture toughness accompanied only by a slight drop in strength. Nevertheless, at those low annealing temperatures the deformation structure was partially present which crucially influenced the crack propagation behavior and thereby the mechanical anisotropy. By raising the annealing temperature it was possible to produce a fully spheroidized microstructure and in further consequence mechanical isotropy was achieved. The decrease in strength due to microstructural coarsening is balanced by a remarkable gain in fracture toughness.

Mechanical properties and fracture characteristics of high carbon steel after equal channel angular pressing

Abstract: The ultra-microduplex structure was fabricated in a fully pearlitic Fe–0.8 wt% C steel after equal channel angular pressing (ECAP) at 923 K via the Bc route. The microstructures and mechanical properties, before and after deformation, were investigated using scanning electron microscopy and mini-tensile tests. The cementite lamellae are gradually spheroidized by increasing the number of ECAP passes. After four passes, the cementite lamellae are fully spheroidized. Microhardness and the ultimate tensile strength of pearlite increase with the strain, up to a peak value (after two passes) and then decrease significantly. The yield strength, elongation and percentage of reduction in area increase with the number of ECAP passes. The tensile fracture morphology changes gradually from brittle cleavage to typical ductile fracture after four passes.

Study on the Effect of Elastic Stress and Microstructure of Low Carbon Steels on Barkhausen Noise

Abstract: The present study investigates the effect of elastic stress and microstructure on Barkhausen noise in low carbon steels subjected to different heat treatments. Barkhausen noise in an as-received test piece and a test piece heated at 450 ∘C for 1.5 hours was found to increase with increasing elastic stress. However, in a test piece heated at 700 ∘C for 10 hours, Barkhausen noise was observed to saturate with increasing elastic stress following an initial increase. To clarify the reason for this saturation behavior, magnetization measurements were carried out and the microstructure and texture of the test pieces were evaluated using microscopy and electron backscatter diffraction. The results indicated that for the test piece heated at 700 ∘C for 10 hours, a drastic change in the microstructure occurred compared to that for the other test pieces. From the experimental and analytical results, it was concluded that for the former test piece, Barkhausen noise saturated under a low elastic stress due to the globularization of pearlite, which caused 90∘ domain walls to become 180∘ domain walls when a low elastic stress is applied.

Effects of alloying elements on the kinetics of austenitization from pearlite in Fe–C–M alloys

Abstract: The effects of alloying elements on the kinetics of austenitization from pearlite structure were studied by computer simulation in Fe–C–M ternary alloys, where M is Mn, Cr, Si or Ni, assuming local equilibrium conditions at all transformation interfaces. A thin austenite film was assumed to nucleate at ferrite/cementite interfaces and grow in one dimension. The existence of a partition to no-partition transition temperature (PNTT) was rationalized. Above the PNTT, the growth rate of austenite is governed by the difference in carbon activity between austenite/cementite and ferrite/austenite interfaces; a substitutional element influences the reaction rate by affecting carbon activity. Below the PNTT, redistribution of M is necessary. The PNTT increases with the concentration of all alloy elements except Ni, which has a large segregation tendency in austenite from both ferrite and cementite, as well as repulsive interaction with carbon. The amount of overheating at PNTT from Ae1 increases in the order Si (∼...

Derivation of anisotropic flow curves of ferrite–pearlite pipeline steel via a two-level homogenisation scheme

Abstract: In order to calculate the effective macroscopic stress–strain curves of a pearlitic–ferritic pipeline steel, a multi-scale approach based on the homogenisation method is presented. Starting from an experimental material characterisation after the cooling process, material models involving microstructural features (e.g. grain size, carbon content) are derived for each phase. Since pearlite is a eutectoid phase mixture, embedded in a ferrite matrix, three length scales are introduced: the nano-scale of a ferrite–cementite bi-lamella of pearlite, the mesoscopic pearlite/ferrite microstructure and the macro-scale of the component. Firstly homogenisation techniques are applied to a bi-lamella Representative Volume Element (RVE) of pearlite. Initial predictions based on the assumption of a hard, elastic cementite phase are further improved by considering its yield behaviour. The present study outlines that cementite is not only anisotropic in elasticity but also in plasticity. Due to uncertainties about the yield behaviour of cementite, a sensitivity analysis has been performed. Secondly pearlite is treated at the micro-scale as an effective phase in an elasto-plastic ferrite matrix. Virtual tensile and shear tests are performed in order to derive the effective flow curves of the pearlite/ferrite microstructures. Comparisons with experimental stress–strain curves in rolling and transverse directions illustrate the accuracy and entitlement of the two-level homogenisation scheme.

Microstructure and texture evolution in fully pearlitic steel during wire drawing

Abstract: The evolution of morphology of pearlite and crystallographic texture of ferrite matrix in fully pearlitic steels during wire drawing were quantitatively investigated. The study revealed that a fiber structure of the pearlite morphology and a fiber texture of the ferrite matrix begin to take shape and develop gradually with increasing strain. The growth rates of the fiber structure and the texture are different in different regions within the wires with increasing drawing strain. There is a close relationship between the pearlite morphology and the crystalline texture during wire drawing. The pearlite interlamellar spacing (ILS) and thickness of cementite lamellae (Tθ) decrease gradually both in longitudinal and transverse sections. The definition of pearlite colony should be reconsidered for describing microstructure of the wire drawing deformed pearlitic steels.

Mechanism of accelerated spheroidization of steel during cyclic heat treatment around the upper critical temperature

Abstract: An investigation has been carried out on the mechanism of accelerated spheroidization in 0.6 wt.% carbon steel under cyclic heat treatment involving repeated short duration holdings above its upper critical temperature (A 3) followed by forced air cooling. Thermal grooving, modified capillarity induced perturbation and lamellar thickening, were found to be the main processes responsible for rapid spheroidization. This is in contrast to the conventional subcritical spheroidization process where the termination migration process dominates the mechanism. The new effect is attributed to the higher atomic mobility above A 3 and the generation of potential diffusion sites of lamellar faults causing rapid breakdown of lamellar pearlite into spheroids of cementite even with short duration holding in each cycle.

Aging behavior and delamination in cold drawn and post-deformation annealed hyper-eutectoid steel wires

Abstract: The effects of drawing strains and post-deformation annealing conditions on the aging behavior and the occurrence of delamination in cold drawn hyper-eutectoid steel wires were studied. At low annealing temperatures the increased tensile strength for a short annealing time would be attributed to age hardening, which came from the decomposition of the unstable cementite in deformed pearlite. The decrease of tensile strength at the high annealing temperature was due to age softening, which would be attributed to the decreased carbon content in lamellar ferrite through the spheroidization or the re-precipitation of cementite, and recovery or recrystallization of ferrite. The extent for the occurrence of delamination during annealing expanded to the high temperature region with the increased amount of deformation in steel wires. At high annealing temperatures, the decreased carbon content dissolved in lamellar ferrite due to age softening would result in the decrease of tensile strength and suppress the occurrence of delamination. The total magnitude of carbon content dissolved in lamellar ferrite either by the partial dissolution of lamellar cementite during wire drawing or by the partial decomposition of lamellar cementite during post-deformation annealing would control the occurrence of delamination in cold drawn steel wires.

Evaluation of Railroad Wheel Steel with Lamellar Duplex Microstructures Using Diffuse Ultrasonic Backscatter

Abstract: The effects of lamellar duplex microstructure within grains that contain alternating phases of cementite and ferrite on ultrasonic scattering in railroad wheel steel are evaluated using a diffuse ultrasonic backscatter technique. A new singly scattered response (SSR) model that considers the lamellar duplex microstructure within grains is developed based on a previous SSR model. The results show that the amplitude of ultrasonic scattering decreases with decreasing lamellar space. Corresponding experiments are performed with 10 MHz and 15 MHz focused transducers by scanning both unquenched and quenched wheels. The experimental results show that the ultrasonic scattering amplitudes drop dramatically near the quenched tread surface, a result which is attributed to the creation of duplex microstructure (pearlite phase) within grains due to the quenching process. The lamellar spacing within grains increases progressively from the tread surface to the deeper locations due to the non-uniform cooling rate. The distribution of lamellar spacing within grains as a function of depth is quantified with the modified SSR model. Good agreement with optical microscopy is observed. The diffuse ultrasonic backscatter technique exhibits strong sensitivity to microstructure changes, an outcome that may be applicable for quality control during manufacturing.

The Role of Silicon in the Solidification of High-Cr Cast Irons

Abstract: This work analyzes the effect of different additions of silicon (0 to 5.0 pct) on the structure of a high-Chromium white cast iron, with chromium content of 16.8 pct and carbon 2.56 pct. The alloys were analyzed in both as-cast and heat-treated conditions. Casting was undertaken in metallic molds that yielded solidification rates faster than in commercial processes. Nevertheless, there was some degree of segregation of silicon; this segregation resulted in a refinement in the microstructure of the alloy. Silicon also generated a greater influence on the structure by destabilizing the austenitic matrix, and promoted greater precipitation of eutectic carbides. Above 3 pct silicon, pearlite formation occurred in preference to martensite. After the destabilization heat treatment, the matrix structure of the irons up to 3 pct Si consisted of secondary carbides in a martensitic matrix with some retained austenite; higher Si additions produced a ferritic matrix. The different as-cast and heat-treated microstructures were correlated with selected mechanical properties such as hardness, matrix microhardness, and fracture toughness. Silicon additions increased matrix microhardness in the as-cast conditions, but the opposite phenomenon occurred in the heat-treated conditions. Microhardness decreased as silicon content was increased. Bulk hardness showed the same behavior. Fracture toughness was observed to increase up to 2 pct Si, and then decreased for higher silicon contents. These results are discussed in terms of the effect of eutectic carbides’ size and the resulting matrix due to the silicon additions.

Volume Expansion of Compacted Graphite Iron Induced by Pearlite Decomposition and the Effect of Oxidation at Elevated Temperature

Abstract: Engine cylinder blocks and heads, made of compacted graphite iron, are subjected to prolonged periods of cyclic heating and cooling. These conditions may give rise to the decomposition of the pearlite matrix accompanied by the formation of lower-density graphite and oxides, which will lead to an increase of material volume. The microstructural instability deteriorates the physical and mechanical properties of CGI and accordingly the thermal fatigue properties. In the present work it was shown that the extent and mechanism of volume change are drastically affected by the presence of an oxide atmosphere. It was found that after annealing under atmospheric conditions internal oxidation largely inhibited the progress of pearlite decomposition and therefore much smaller growth rates were obtained as compared to those observed under vacuum conditions in the dilatometer. After 16 h of annealing time at 700 °C in vacuum, the CGI samples exhibited 6 times faster growth kinetics as compared to annealing in open atmosphere.

Dislocation structure of cementite in granular pearlite after cold plastic deformation

Abstract: Cementite microstructure of the U8 steel with a granular pearlite structure has been investigated by the method of electron microscopy. It has been established that, at the early stages of deformation, carbide particles are deformed through the movement of stacking faults, which are characterized by an α[010] partial shift in the (001) planes of cementite. The Burgers vector, the slip plane [010](001) of the split dislocations forming pileups, and deformation bands have been determined using gb analysis. The stacking fault energy has been estimated in a (001) cementite plane: γsf ∼ 12.8 mJ/m2. With increasing degree of deformation, an additional slip has been shown to occur in cementite by the system [100](011).

Mechanisms of toughness improvement in Charpy impact and fracture toughness tests of non-heat-treating cold-drawn steel bar

Abstract: In this study, toughness properties of a non-heat-treating cold-drawn bar were examined by Charpy impact test and fracture toughness test, and the toughness enhancement mechanisms were clarified in relation with microstructure. As the thickness of pearlite bands decreased after the cold drawing, the Charpy impact energy of the cold-drawn bar was higher than that of the as-rolled bar, which could be reasonably explained by the thin sheet toughening. On the other hand, thin pearlite bands negatively affected the fracture toughness because of the decreased spacing between crack or void initiation sites inside the fracture process zone in front of the pre-fatigued crack tip. The Charpy impact test data could also be correlated with the absorbed energy of the dynamic compressive test specimen whose orientation was matched with the hammer impact direction of the Charpy impact test, although the Charpy impact and dynamic compressive test specimens had a notched body and a smooth body, respectively.

Deformation of dual-structure medium carbon steel in cold drawing

Abstract: In this paper, extremely high strength was obtained in medium carbon steel having a carbon content of 0.35% by weight through cold drawing. Experimental results showed that the tensile strength of the steel increased by nearly three folds from the original value ~615 MPa to 1810 MPa corresponding to drawing strain of 3.0. To reveal the mechanisms that govern the strengthen increase, the microstructural evolution was analyzed during cold drawing, with respect to the change of the deformation resistance (measured by micro-hardness) of micro-constituents (i.e., primary or proeutectoid ferrite and pearlite) in the material. The proeutectoid ferrite became elongated and, at the same time, increasingly hardened while the pearlite maintained equiaxed shape after initial drawing. With the increase of the drawing strain, the pearlite was stretched parallel to drawing direction, accompanied by an increase in the 〈110〉 texture intensity and dislocation density in the ferrite phase. Under heavy drawing, a laminate structure formed, consisting of alternating pro-eutectoid ferrite and pearlite both parallel to the drawing direction. The 〈110〉 texture intensity in the ferrite phase became saturated as e>1.2. High density dislocation zones further spread in the ferrite phase. The interlamellar spacing between ferrite and cementite phases in the pearlite decreased. Based upon these observations, mechanistic models were constructed to provide insight into the deformation and strengthening mechanisms of this steel.

Origin and mechanism of torsion fracture in cold-drawn pearlitic steel wires

Abstract: The structural and mechanical factors that control the torsion fracture behavior of cold-drawn eutectoid steel wires are examined. Two types of the fracture are identified; namely, flat- and cleavage-type. Torsion cracks are found to initiate in ferrite and propagate along the ferrite/cementite interface. The shear stress distribution within the wires is affected not only by the applied torque, but also by the residual stress. The maximum shear stress occurs halfway from center to the surface, where the cracks initiated. The growth of torsion cracks is sensitive to the orientation of cementite lamellas in pearlite grains. The influence of thermal history on the occurrence of cleavage fracture is ascertained, with the assistance of atom probe. It shows that the cleavage fracture results from a decrease in dislocation mobility, caused by thermally activated diffusion of carbon atoms into ferrite.