Showing papers on "Pearlite published in 2011"

Effect of austenite microstructure and cooling rate on transformation characteristics in a low carbon Nb–V microalloyed steel

Abstract: Deformation dilatometry has been used to simulate controlled hot rolling followed by cooling of a Nb–V low carbon steel, looking for conditions corresponding to wide austenite grain size distributions prior to transformation. Recrystallization and non-recrystallization deformation schedules were applied, followed by controlled cooling at rates from 0.1 °C/s to about 200 °C/s, and the corresponding continuous cooling transformation (CCT) diagrams were constructed. The resultant microstructures ranged from polygonal ferrite (PF) and pearlite (P) at slow cooling rates to bainitic ferrite (BF) accompanied by martensite (M) for fast cooling rates. Plastic deformation of the parent austenite accelerated both ferrite and bainite transformations, displacing the CCT curve to higher temperatures and shorter times. However, it was found that the accelerating effect of strain on bainite transformation weakened as the cooling rate diminished and the polygonal ferrite formation was enhanced. Moreover, it was found that plastic deformation had different effects on the refinement of the microstructure, depending on the cooling rate. An analysis of the microstructural heterogeneities that can impair toughness behavior has been done.

Phase field modeling of microstructure evolution in steels

Abstract: This article provides an overview on the application of phase field models to describe microstructure evolution in steels. The focus will be on phase field modeling of the austenite–ferrite transformation as this has emerged as a particularly active area of research in the past few years. Phase field models are powerful tools to deal with the complex morphologies, e.g. Widmanstatten ferrite, that may result from these transformations. Even though much progress has been attained there is still significant work to be done in applying these models to processing of advanced steels with complex multi-phase microstructures. In particular, the phase field approach promises to have significant impact on modeling of bainite formation and the microstructure evolution in the heat affected zone of welds.

Austenite Formation in Plain Low-Carbon Steels

Abstract: In this study, austenite formation from hot-rolled (HR) and cold-rolled (CR) ferrite-pearlite structures in a plain low-carbon steel was investigated using dilation data and microstructural analysis. Different stages of microstructural evolution during heating of the HR and CR samples were investigated. These stages include austenite formation from pearlite colonies, ferrite-to-austenite transformation, and final carbide dissolution. In the CR samples, recrystallization of deformed ferrite and spheroidization of pearlite lamellae before transformation were evident at low heating rates. An increase in heating rate resulted in a delay in spheroidization of cementite lamellae and in recrystallization of ferrite grains in the CR steel. Furthermore, a morphological transition is observed during austenitization in both HR and CR samples with increasing heating rate. In HR samples, a change from blocky austenite grains to a fine network of these grains along ferrite grain boundaries occurs. In the CR samples, austenite formation changes from a random spatial distribution to a banded morphology.

Friction stir lap welded advanced high strength steels: Microstructure and mechanical properties

Abstract: Friction stir welding was carried out under different heat input and cooling rates to produce lap joints between high strength martensitic steel sheets. The microstructure of the welds was characterized, and microhardness was evaluated. Joint efficiency was determined by lap shear test. Variation in processing conditions governed total heat input, peak temperature and cooling rate during friction stir welding. Weld nugget microstructure depended principally on cooling rate. The slowest cooling rate promoted ferrite–pearlite and the fastest cooling rate resulted in martensite formation in the weld nugget. The weakest region of all the joints was the heat affected zone, which consists of ferrite with small quantities of pearlite. Fracture during shear testing occurred along the heat affected zone of welded joints. The width and grain size of ferrite in heat affected zone controlled the joint efficiency.

Interphase Precipitation of VC and Resultant Hardening in V-added Medium Carbon Steels

Abstract: Interphase precipitation of vanadium carbide (VC) accompanying ferrite and pearlite transformations and its effect on hardness have been examined by using medium carbon steels containing 0.1, 0.3 and 0.5 mass%V. Specimens transformed in a temperature range between 873 and 973 K consist of pearlite and small amount of proeutectoid ferrite. Ferrite fraction increases with raising transformation temperature or with increasing the V content. In addition to proeutectoid ferrite and pearlite, bainite is formed below 853 K, whose fraction is increased by the V addition. Hardening is significant in the V-added alloy between 873 K and 973 K and becomes larger by increasing V content in this temperature range. Meanwhile the alloying effect of V on the hardness remarkably decreases at 823 K where bainite transformation takes place partly. TEM characterization has revealed that VC are precipitated in both of proeutectoid and pearlitic ferrite with holding Baker-Nutting (B-N) orientation relationship with ferrite in the manner of fine rows parallel to the austenite / ferrite interphase boundary. Single variant of VC, whose habit plane is closer to ferrite / austenite boundary than the other two B-N variants, tends to be formed. The size of VC decreases and its number density increases by lowering transformation temperature, corresponding to the larger hardness increase.

Phase-Field Modeling of Austenite Formation from a Ferrite plus Pearlite Microstructure during Annealing of Cold-Rolled Dual-Phase Steel

Abstract: The prediction of microstructure and, consequently, of mechanical properties, as a function of chemical composition and heat treatment parameters, is currently a major topic of scientific investigations There has been considerable effort to investigate the phase transformation during cooling and its modeling In comparison, the metallurgical processes of the ferrite-austenite transformation during heating and annealing have not been intensively studied However, the austenite formation during heating and annealing has a great influence on the final microstructure, as it is one of the first steps in the complex microstructure development of modern high-strength strip steels In this study, the simulation of austenite formation from a ferrite plus pearlite microstructure during heating is realized by means of a multiphase-field approach For the description of pearlite dissolution, a simple pseudo-phase approximation is used, which neglects the substitutional elements Instead, the modeling of austenite formation from ferrite is performed as a second step, taking into account the substitutional elements under LE negligible partitioning (LENP) conditions, using direct coupling to a commercial thermodynamic database The results of these two-dimensional (2-D) simulations will be presented and compared with experimental findings

Microstructure and mechanical properties of high boron white cast iron with about 4 wt% chromium

Abstract: The microstructure and mechanical properties of high boron white cast irons with about 4 wt% chromium before and after treating with rare earth magnesium alloy were studied in this article. The experimental results indicate that the cast irons comprise a dendritic matrix and interdendritic eutectic borides M2B and M′0.9Cr1.1B0.9 that distributed in the form of continuous network in as-cast condition. The matrix is made up of fine pearlite in the alloys with and without modification, but the grain size of the matrix is decreased greatly after modification. After water quenching at 1,303 K and tempering at 473 K, the matrix of the alloy mostly changes to lath-type martensite. For the alloy without modification the boride morphology remains almost unchanged after heat treatment. And a secondary precipitation of M23(C,B)6 compound appears in the central region of dentritic matrix grains. The morphology of the eutectic borides is changed to the form of isolated blocks after heat treatment and there is only little intragranular M23(B,C)6 particles in the matrix are found in the alloy modified with rare earth magnesium alloy. The modification by rare earth magnesium alloy can refine the primary austenite and the eutectic borides. Combined with a high austenitizing temperature the modification can improve the morphology of the borides which results in the improvement of toughness and tensile strength.

Effect of Composition and Deformation on Coarse-Grained Austenite Transformation in Nb-Mo Microalloyed Steels

Abstract: Thermomechanical processing of microalloyed steels containing niobium can be performed to obtain deformed austenite prior to transformation. Accelerated cooling can be employed to refine the final microstructure and, consequently, to improve both strength and toughness. This general rule is fulfilled if the transformation occurs on a quite homogeneous austenite microstructure. Nevertheless, the presence of coarse austenite grains before transformation in different industrial processes is a usual source of concern, and regarding toughness, the coarsest high-angle boundary units would determine its final value. Sets of deformation dilatometry tests were carried out using three 0.06 pct Nb microalloyed steels to evaluate the effect of Mo alloying additions (0, 0.16, and 0.31 pct Mo) on final transformation from both recrystallized and unrecrystallized coarse-grained austenite. Continuous cooling transformation (CCT) diagrams were created, and detailed microstructural characterization was achieved through the use of optical microscopy (OM), field emission gun scanning electron microscopy (FEGSEM), and electron backscattered diffraction (EBSD). The resultant microstructures ranged from polygonal ferrite (PF) and pearlite (P) at slow cooling ranges to bainitic ferrite (BF) accompanied by martensite (M) for fast cooling rates. Plastic deformation of the parent austenite accelerated both ferrite and bainite transformation, moving the CCT curves to higher temperatures and shorter times. However, an increase in the final heterogeneity was observed when BF packets were formed, creating coarse high-angle grain boundary units.

Effects of Chromium Addition on Microstructure and Abrasion Resistance of Fe–B Cast Alloy

Abstract: This research work studies the effects of chromium on microstructure and abrasion resistance of Fe–B cast alloy. The results show that eutectic boride changes from continuous network to less continuous and matrix changes from pearlite to martensite with the increase in chromium content in the alloy. Meanwhile, an increase in chromium addition in the alloy leads to an increase in the chromium content in M2B-type boride because chromium can enter boride by substituting for iron in Fe2B. Under two-body wear, Fe–B cast alloy exhibits excellent wear resistance. When alloys are tested against soft abrasive, chromium can markedly improve the wear resistance of Fe–B cast alloy, whereas excessive chromium can reduce the wear resistance. The wear resistance of Fe–B cast alloy increases first and then decreases with the increase in chromium. But when tested against hard abrasive, since the hardness of SiC is much higher than that of M2B boride, an increase in chromium content marginally increases the wear resistance. Weight losses of Fe–B cast alloy increase with the increase in the load and exhibit the linear relationship.

Hydrogen embrittlement behavior of HSLA line pipe steel under cathodic protection

Abstract: This paper deals with hydrogen embrittlement (HE) under cathodic protection of steel used or recently proposed for buried pipelines of the oil and gas industry. High strength low alloy steel with different chemical composition, microstructure and ultimate tensile strength ranging between 530 and 860 MPa were considered to compare new steel with traditional grades. Their behavior was assessed through constant load tests, fracture mechanics tests on modified wedge-opening load specimens, slow strain rate tensile tests and interrupted slow strain rate tensile tests in solutions simulating soil moisture and sea water, at room temperature and cathodic potentials between -2 and -0.8 V vs. saturated calomel electrode reference. All the steel was found immune under static load and HE phenomena were observed only during slow strain rate tests at potentials more negative than a critical value, depending on steel susceptibility and strain rate. Quenched and tempered steel with high tempered martensite and very fine precipitation distribution showed better behavior than the rolled steels with banded microstructure, exhibiting the highest HE resistance. The resistance of steel characterized by banded microstructures increased with ultimate tensile strength until ∼700 MPa and then decreased. The increase of ultimate tensile strength occurred by changing the steel microstructure, from hot rolling with coarse ferrite/pearlite to fine ferrite/pearlite microstructure, and very fine acicular ferrite, yields the resistance of steel to HE. A further increase of ultimate tensile strength related to untempered martensite inside the acicular ferrite based microstructure produces a worsening of resistance.

Three-Body Abrasive Wear Behavior of Low Carbon Fe–B Cast Alloy and Its Microstructures Under Different Casting Process

Abstract: This study investigates the effect of semi-solid processing on the microstructures, mechanical properties of low carbon Fe–B cast alloy. The as-cast microstructure of Fe–B cast alloy consists of the eutectic boride, pearlite, and ferrite. Compared with the coarse eutectic borides in the ordinary alloy, the eutectic boride structures in the semi-solid alloy are greatly refined. Moreover, the boride area fraction, Rockwell hardness, impact toughness, etc., before and after heat treatment under different casting methods are also investigated systemically. The wear behaviors of low carbon Fe–B cast alloy are studied by three-body abrasive wear tester. The wear weight loss of semi-solid Fe–B cast alloy is lower than that of the ordinary Fe–B cast alloy because of the lower average boride area for semi-solid specimen. Meanwhile, the wear mechanism of the low carbon Fe–B cast alloy under different casting process is depicted and analyzed by using the physical models.

Microstructure and mechanical properties of V–Ti–N microalloyed steel used for fracture splitting connecting rod

Abstract: A new kind of V–Ti–N high strength microalloyed medium carbon steel has been developed, which is used for fracture splitting connecting rod. In this article, the characteristics of this carbon steel and its production process were studied. The microstructure, precipitated phases and their effects on mechanical properties were investigated by optical microscope, SEM, and TEM. The results showed that the steel was constituted of ferrite and pearlite. By reducing the finish rolling temperature and accelerating the cooling rate after rolling, microstructure with fine grain ferrite and narrow lamellar space pearlite could be obtained in V–Ti–N microalloyed medium carbon, and a large number of precipitated phases distributed over ferrite. These led the tensile strength to be more than 1000 MPa, yield strength (YS) more than 750 MPa. The impact fractograph showed typically brittle fracture characteristic.

An Alternate Approach to Accelerated Spheroidization in Steel by Cyclic Annealing

Abstract: In this work an annealed 0.6 wt.% carbon steel was subjected to cyclic heat treatment process that consisted of repeated short-duration (6 min) holding at 810 °C (above Ac3 temperature) followed by cooling in a flowing air medium (flow rate: 6 m3/h). After 8 cycles (about 1 h and 20 min), the microstructure mostly contains spheroidized cementite and ferrite along with trace amount (3%) of pearlite. In addition to the diffusion within lamella, the disintegration of lamellae through dissolution of cementite at preferred sites of lamellar faults during short-duration holding above Ac3 temperature, and the generation of defects (lamellar faults) during non-equilibrium cooling in a flowing air medium are the main reasons of accelerated spheroidization.

Kinetics of Reverse Transformation from Pearlite to Austenite in an Fe-0.6 Mass pct C Alloy and the Effects of Alloying Elements

Abstract: Substitutional alloying effects on reversion kinetics from pearlite structure at 1073 K (800 °C) in an Fe-0.6 mass pct C binary alloy and Fe-0.6C-1 or 2 mass pct M (M = Mn, Si, Cr) ternary alloys were studied. Reverse transformation in the Fe-0.6C binary alloy at 1073 K (800 °C) was finished after holding for approximately 5.5 seconds. The reversion kinetics was accelerated slightly by the addition of Mn but retarded by the addition of Si or Cr. The difference of acceleration effects by the addition of the 1 and 2 mass pct Mn is small, whereas the retardation effect becomes more significant by increasing the amount of addition of Si or Cr. It is clarified from the thermodynamic viewpoint of carbon diffusion that austenite can grow without partitioning of Mn or Si in the Mn- or Si-added alloys. On the one hand, austenite growth is controlled by the carbon diffusion, whereas the addition of them affects carbon activity gradient, resulting in changes in reversion kinetics. On the other hand, thermodynamic calculation implies that the long-range diffusion of Cr is necessary for austenite growth in the Cr-added alloys. It is proposed that austenite growth from pearlite in the Cr-added alloys is controlled by the diffusion of Cr along austenite/pearlite interface.

Effects and Mechanisms of RE on Impact Toughness and Fracture Toughness of Clean Heavy Rail Steel

Abstract: Clean high carbon heavy rail steel was prepared by the process of vacuum induction furnace smelting, forging and rolling. Mechanisms of RE on the impact toughness and fracture toughness for clean high carbon steel were investigated. In addition, the appropriate range of RE content for clean high carbon steel was determined. Both the austenite grain size and pearlite lamellar spacing decreased due to small amount of RE, consequently the impact toughness and fracture toughness were improved evidently. When the RE content exceeded a critical value, the pearlite lamellar spacing was increased, because RE was segregated on the austenite grain boundaries, damaged the orientation relationship of pearlite transformation, caused the disorder growth and morphology degenerating of pearlite. With the increasing of RE content, both the impact toughness and fracture toughness of clean high carbon steel were gradually increased at first and then decreased. It was found that when the RE content was between 0. 008 1% and 0.008 8 %, both the impact toughness and fracture toughness of clean high carbon heavy rail steel were the best. The maximum ballistic work was 21.2 J (20 °O and 12. 2 J ( — 20 °O, respectively. The maximum plane-strain fracture toughness was 45.67 MPa • m1/2 (20 °O and 37. 04 MPa • m1/2 (-20 °O, respectively.

Critical points of hypoeutectoid steel - prediction of the pearlite dissolution finish temperature Ac 1f

Abstract: Purpose: of this work is to present possibility of calculation of pearlite dissolution finish temperature Ac1f during heating of hypoeutoctoid steels. Design/methodology/approach: The presented multiple linear regression equations for calculating the Ac1f temperature are based on experimental data set containing chemical composition and values of critical temperatures obtained by use of the dilatometric technique at the own laboratory only. Findings: The elaborated multiple linear regression equations for calculating the critical temperatures are an alternative to dilatometric examinations to obtain data necessary for proper heat treatment conditions planning. Research limitations/implications: All presented equations for calculating pearlite dissolution finish temperature are limited by range of mass concentrations of elements which is a consequence of limited data set used for elaboration of these equations. The obtained relationships do not concern other factors influencing Ac1f temperature such as heating rate, grain size and interlamellar spacing of pearlite. Practical implications: Broadening the knowledge on the chemical composition influence on the critical temperatures, which will help in designing heat treatment conditions, especially of the Dual Phase steels. Originality/value: An attempt was made to find out a multiple linear regression formula between chemical composition and the pearlite dissolution finish temperature of hypoeutectoid steels.

Mixed diffusion-controlled growth of pearlite in binary steel

Abstract: A kinetic theory for the diffusion-controlled growth of pearlite is presented, which accounts simultaneously for diffusion through the austenite and via the transformation front. The simplified method abandons the need for mechanical equilibrium at the phase junctions and yet is able to explain experimental data on the growth rate of pearlite. Furthermore, unlike previous analyses, the deduced value for the activation energy for the interfacial diffusion of carbon is found to be realistic when compared with corresponding data for volume diffusion.

Thermal degradation of pearlitic steels: influence on mechanical properties including fatigue behaviour

Abstract: With the aim to predict the durability of railway wheels, thermomechanical damage was studied for two steels with different alloying levels of silicon and manganese in the temperature range of 500–725uC. Softening caused by cementite spheroidisation in pearlite leads to changes in the mechanical behaviour and an accompanying decrease in fatigue lifetimes. It was found that higher contents of Si and Mn lead to better resistance to softening of both virgin and plastically deformed material. Correspondingly, the high Si–Mn alloyed steel loses much less in fatigue lifetime than the lower alloyed steel.

Study on dislocation slips in ferrite and deformation of cementite in cold drawn pearlitic steel wires from medium to high strain

Abstract: The deformation of cementite was studied via optical microscopy and SEM in the longitudinal sections of pearlitic steel wires from medium to high strain. The cementite shows good deformability, the angle between the deformation direction of cementite and drawing direction decreases with increasing strain, and finally the deformation directions of cementite turn to the drawing axis at high strains. The deformation of the cementite is strongly related to plastic deformation in the ferrite, with coarse slip steps, S-bands and cracks across cementite observed parallel to either {110}α-Fe or {112}α-Fe plane traces determined by the largest Schmid factors.

High-strength hot-dip galvanized steel sheet with excellent formability and impact resistance, and process for producing same

Abstract: Provided is a hot-dip galvanized steel sheet which has a tensile strength TS of 590 MPa or higher and excellent processability and which, even when no strain is introduced thereinto by pressing, highly absorbs energy in a low-strain range up to about 5% and has excellent collision resistance. Also provided is a process for producing the steel sheet. The galvanized steel sheet has a composition which contains, in terms of mass%, 0.04-0.13% C, 0.7-2.3% Si, 0.8-2.0% Mn, up to 0.1% P, up to 0.01% S, and 0.01-0.1% Al, with the remainder comprising iron and incidental impurities, and has a structure which comprises, in terms of areal proportion, at least 75% ferrite phase, at least 1% bainitic ferrite phase, and 1-10% pearlite phase, has a martensite phase content of up to 10% in terms of areal proportion, and satisfies (areal proportion of martensite)/((areal proportion of bainitic ferrite)+(areal proportion of pearlite))<=0.6, and in which the ratio of the Mn concentration in the ferrite phase to the Mn concentration in the second phase is 0.70 or above.

Effect of initial microstructure on mechanical properties in warm caliber rolling of high carbon steel

Abstract: In this study, the effect of initial microstructure on change of mechanical properties was investigated by warm caliber rolling (WCR) of high carbon steel. Experiments were carried out with two different kinds of initial microstructures of pearlite and tempered martensite at the temperature of 500 °C. For comparison, the microstructure of austenite phase obtained from the conventional hot rolling at the temperature of 900 °C up to about 83% of the accumulative reduction in area was assumed to be a reference case. It was found that the WCR provided better mechanical properties in terms of strength and toughness compared to the conventional hot rolling based on experimental results of micro-hardness, tension, and Charpy impact tests. The improvement of strength and toughness was attributed to smaller ferrite grain and dispersed cementite particles with smaller interspacing aligned to the rolling direction after the WCR owing to field emission scanning electron microscopy. The investigated WCR might be useful in obtaining the high strength material with better toughness without adding new alloying elements for industrial applications according to the present investigation.

Effect of Microstructure on Fatigue Crack Propagation and S–N Fatigue Behaviors of TMCP Steels with Yield Strengths of Approximately 450 MPa

Abstract: In the present study, stress (S) – number of cycles to failure (N) (S–N) fatigue and fatigue crack propagation behaviors of three thermomechanical control process steels with different microstructures but similar yield strengths of approximately 450 MPa were investigated. The P + F steel was predominately pearlite plus ferrite, whereas B1 and B2 steels were both bainitic steels with martensite-austenite and pearlitic islands. Despite the significant difference in microstructural features, the resulting fatigue crack propagation rates and near-threshold ΔK values were comparable with each other. The hard phases, such as pearlite colonies in the P + F specimen, tended to affect fatigue crack propagation behavior in a similar manner, and severe crack branching was observed in intermediate and high ΔK regimes. Despite similar fatigue crack propagation rates and near-threshold ΔK values, the resistance to S–N fatigue was substantially different for each steel specimen. Depending on fatigue crack initiators, such as the ferrite/pearlite phase boundaries for the P + F specimens and the cracked martensite-austenite and/or small pearlitic islands for the bainitic specimens, the cycles for crack initiation varied greatly.

Quantitative measurement of cementite dissociation in drawn pearlitic steel

Abstract: Fractional dissociation of cementite was quantified as a function of strain by measuring the volume change of cementite in the pearlitic steel. The amount of carbon dissolved into the ferrite was estimated from the decrease of cementite volume, to correlate with the hardness in different strain level. The hardness showed linear relationship with the carbon dissolved into the ferrite matrix, which is believed to contribute in strengthening the drawn wire. Defects introduced from the deformation were believed to lower the energy barrier of cementite break-ups and to enhance the dissolution of carbon into ferrite.