Showing papers on "Pearlite published in 1996"

Pearlite in ultrahigh carbon steels: Heat treatments and mechanical properties

Abstract: Two ultrahigh carbon steel (UHCS) alloys containing 1.5 and 1.8 wt pct carbon, respectively, were studied. These materials were processed into fully spheroidized microstructures and were then given heat treatments to form pearlite. The mechanical properties of the heat-treated materials were evaluated by tension tests at room temperature. Use of the hypereutectoid austenite-cementite to pearlite transformation enabled achievement of pearlitic microstructures with various interlamellar spacings. The yield strengths of the pearlitic steels are found to correlate with a predictive relation based on interlamellar spacing and pearlite colony size. Decreasing the pearlite interlamellar spacing increases the yield strength and the ultimate strength and decreases the tensile ductility. It is shown that solid solution alloying strongly influences the strength of pearlitic steels.

Transformation behaviour of the high temperature martensitic steels with 8 – 14% chromium

Abstract: Comprehensive development work on martensitic steels belonging to the so-called 12% Cr steel group were performed at the Institute for Materials Research (IMF) of Forschungszentrum Karlsruhe on martensitic steels, the so-called 12% Cr steel group, in order to meet the various requirements in nuclear and conventional energy technology. The transformation characteristics of 29 different grades of steel and 38 heats have been determined and continuous cooling transformation (CCT) diagrams have been prepared. The diagrams are first described by groups of subjects in a chronological order because the change in the chemical composition cannot be correlated in all cases with the change in transformation behaviour. The quenching hardness can be satisfactorily described as a function of the C+N content if, taking into account the Nb, Zr, Ta, Hf fractions, a common effectiveness factor is calculated. This effectiveness factor is also integrated in the calculation of the Ms point by modifying accordingly the equation proposed by Steven and Haynes for low-alloy steels and supplemented by the summands for V and W. An equation is introduced for the calculation of the critical cooling rate for pearlite transformation which takes into account the special influence exerted by elements Cr, V, Mo, W, Ge. The comparison between calculated and measured values for Ms and Vcrit is satisfactory, except for some steels.

Modeling of ferrite growth in nodular cast iron

Abstract: In nodular cast iron, ferrite forms around the graphite nodules and growth proceeds until pearlite nucleates and consumes the remaining austenite. In order to simulate the structure, it is therefore necessary to have accurate models for the ferrite growth. Some investigators have proposed that the growth is completely governed by carbon diffusion through the ferrite shell. In the present work, it is shown that the ferrite growth in nodular cast iron can be divided into three different stages where the growth initially is governed by carbon diffusion in the austenite until the graphite nodule is entirely enveloped by a ferrite shell. During the second stage, it is proposed that the growth is controlled by the incorporation rate of carbon atoms on the graphite nodule. During the later stages of the transformation, the diffusion distance has increased considerably, and therefore, the diffusion of carbon through the ferrite shell will determine the growth rate.

Analysis of phase transformation in Fe-C alloys using differential scanning calorimetry

Abstract: Differential scanning calorimetry (DSC) is a well suited technique for studying phase transformation kinetics provided the transformation involves some kind of heat effect. The diffusional transformation in Fe-C alloys and low alloy steels upon cooling from the austenitic state involve both heat of transformation and changes in heat capacity. In principle, it can therefore be analysed using DSC techniques. Particular complications in the analysis of the heat effects occurring on this transformation are the strong temperature dependence of the heat capacity of the ferrite and the strong temperature dependence of the enthalpy of formation of ferrite from austenite. A simple mathematical method is presented, which allows for the non-linear temperature dependence of heat capacities and enthalpy of formation. As a consequence, the method can be used to analyse the simultaneous formation of pearlite. Using this method, the fractions of both pro-eutectoid ferrite and pearlite as a function of the temperature are determined from heat capacity measurements for several Fe-C alloys. Data for the enthalpy of formation of pearlite are presented. The method presented is generally applicable for analysing reactions or transformations occurring over a wide temperature interval where the heat effects are temperature dependent.

Modeling of Transformation Behavior and Compositional Partitioning in TRIP Steel

Abstract: Transformation behavior and compositional partitioning in TRIP (Transformation Induced Plasticity) steel was investigated by means of microstructural observation and computer modeling. Studies were made on each of three stages of the continuous annealing process applied to TRIP steel. Ortho-equilibrium partitioning of alloying elements of Si and Mn was attained even in short intercritical annealing time. A transformation model, in which transformation is controlled by carbon diffusion, well described the volume fractional change of ferrite and pearlite during the cooling to austempering temperature. Slower cooling rates significantly increased carbon concentration enriched in untransformed austenite and caused pearlite transformation. Ultimate bainite volume fraction obtained by austempering increased with austempering temperature. Analysis with computer modeling revealed that transformation kinetics above 350°C followed a model based on the diffusional mechanism, while it complied with a model based on the displacive mechanism below 350°C.

Effect of cold drawing on environmentally assisted cracking of cold-drawn steel

Abstract: The fracture behaviour in air and aggressive environments of two eutectoid steels in the forms of hot-rolled bar and cold drawn wire has been compared to elucidate the consequences of cold drawing on their susceptibility to environmentally assisted cracking (EAC) in aqueous environments. Cold drawing produces a microstructural effect on the material: a preferential orientation of the pearlite lamellae aligned parallel to the cold-drawing direction, resulting in anisotropic properties with regard to fracture behaviour in air and aggressive environments. The main consequence is the change in crackpropagation direction approaching that of the wire axis (cold-drawing direction or main average orientation of the pearlite lamellae) and producing a mixed-mode state. The results reported provide insight into the macro- and micro-mechanical effects of cold drawing on the fracture and EAC-behaviour of eutectoid pearlitic steels.

Dual phase steel for cold rolled sheet or strip - contg. manganese@, aluminium@ and silicon

Abstract: Cold rolled sheet or strip is made from a steel contg. in wt.%: 0.05-0.3 C; 0.8-3 Mn; 0.4-2.5 Al; 0.01-0.2 Si; balance Fe plus usual impurities. The steel is free from pearlite and has a predominantly ferritic structure with inclusions of martensite and opt. bainite and/or residual austenite. -

Correlation of microstructure and fracture toughness in high-chromium white iron hardfacing alloys

Abstract: A correlation is made of microstructure and fracture toughness in hypereutectic high-chromium white iron hardfacing alloys. In order to investigate the matrix effect of these alloys, in particular, four different matrices such as pearlite, austenite, and a mixture of pearlite and austenite were employed by changing the ratio of Mn/Si, while the total volume fraction of carbides was fixed. The hardfacing alloys were deposited twice on a mild steel plate by the self-shielding flux-cored arc-welding method. Fracture toughness was increased by increasing the volume fraction of austenite in the matrix, whereas hardness and abrasion resistance were nearly constant.In situ observation of the fracture process showed that cracks initiated at large primary carbides tended to be blocked at the austenitic matrix. This suggested that fracture toughness was controlled mainly by the amount of austenite in the matrix, thereby yielding the better toughness in the hardfacing alloy having the austenitic matrix. Considering both abrasion resistance and fracture toughness, therefore, the austenitic matrix was preferred for the high-chromium white iron hardfacing alloys.

Process for austempering ductile iron

Abstract: A process for austempering ductile iron includes austenitizing a ductile iron casting of low alloy content followed by quenching the workpiece for a controlled period of time in a quench medium such as water, an aqueous polymer solution or a medium speed quench oil. The workpiece is then austempered in an air tempering furnace, resulting in a ausferrite microstructure essentially free of pearlite and martensite, and with mechanical properties meeting ASTM designation A897-90 "Standard Specification for Austempered Ductile Iron Castings." The process eliminates the need for a molten salt bath for quenching and tempering.

Tic/metal nacrous structures and their fracture toughness increase

Abstract: Multilayers of TiC and a series of metals have been fabricated by ion beam sputtering deposition to simulate nacre. The individual layer thickness varies from 1 to 10 nm, and the total thickness of the multilayers is about 1 μm. Transmission electron microscopy (TEM), low-angle x-ray diffraction (LXRD), and high-resolution electron microscopy (HREM) show their periodicity and lattice images. A particular method is devised to evaluate the relative toughness of this artificial pearlite. It is shown that the toughness of these nanocomposite materials can be tremendously improved. A maximum of toughness appears at a certain modulation. Metals with high plasticity such as Al and Cu can produce a particularly spectacular effect on increasing the toughness of these multilayers.

Computer simulation of solid state transformation in Fe-C-Si ternary spheroidal graphite cast iron

Abstract: A model was developed to simulate the post-solidification process of Fe-C-Si spheroidal graphite (SG) cast iron. The calculated results at the end of solidification, like the nodule count, austenite shell radius, graphite volume fraction, graphite spheroid radius and silicon distribution in the austenite shell were considered to be the initial conditions for the post-solidification model. After the end the solidification, the graphite spheroid continues to grow due to the decrease of carbon solubility in the austenite. This growth of graphite in austenite was controlled by the carbon flux diffusing through graphite/austenite interface. The eutectoid transformation was also simulated. The rate constants and indexes of ferrite and pearlite formation in the Kolmogrov-Johnson-Mehl-Avrami's equation were modified from those which had been used for steel or SG cast iron in the literature. The growth of graphite during the eutectoid transformation was also investigated. The calculated results showed that about 50% of the graphite growth occurred after the end of solidification. The graphite growth that occurrs between eutectic equilibrium temperature and eutectoid equilibrium temperature occupies 40% of the total fraction and the rest 10% occurs during the eutectoid transformation. The calculated fractions of ferrite and pearlite agreed well with experimental result. Also from both the calculated and experimental results, it could be understood that the fraction of ferrite increased with decreasing part thickness.

Coupled Simulation of Heat Transfer and Phase Transformation in Continuous Casting of Steel

Abstract: A computer software package has been developed to simulate temperature, shell growth and phase transformations in continuous casting of steel The package includes two earlier developed models, a heat transfer model and a thermodynamic–kinetic phase transformation model The phase transformation model takes into account the effect of alloying and cooling on the phase transformation temperatures and on this basis, on important, solidification related thermophysical properties, enthalpy and thermal conductivity, generated by the model Hence, while affecting these properties, the phase transformations have a special influence on the heat transfer in the strand The output of these coupled calculations are temperature distribution, phase distribution (liquid, delta ferrite and austenite, and approximately, proeutectoid ferrite, pearlite, bainite and martensite) and volume distribution through the strand, and also, hardness distribution on the cross-section of the strand at room temperature Characteristics of the coupled model and some results of calculations are presented

Composition dependence of the magnetomechanical effect and magnetostriction

Abstract: Magnetomechanical and magnetostriction measurements were performed on a series of plain carbon steels, with carbon contents ranging from 0.1 to 0.9 wt.%. The microstructures in all specimens were similar, being a mixture of ferrite and pearlite, with the pearlite volume fraction increasing with increasing carbon. The magnetomechanical effect was found to vary with carbon content. Specifically, the change in magnetization with stress was proportional to the remanent magnetization for a given carbon level. Also, the sign of the magnetization change upon removal of compressive stress reversed at 0.6 wt.% carbon. The results were explained by the increased number of domain wall pinning sites produced by the increase in carbon content.

Austenite stabilization from direct cementite conversion in low-alloy steels

Abstract: Transformation induced plasticity (TRIP) effects associated with austenite dispersions in low alloy Fe-Mn-Sl steels can be enhanced by austenite stabilisation. Austenite which forms during conventional intercritical annealing does not possess the required stability in order to exhibit TRIP effects. In this work, thermodynamic calculations indicated that it is feasible to form austenite by a cementite to austenite conversion which occurs under paraequilibrium conditions, i.e with partition of carbon but with no partition of substitutional alloying elements. In this way the austenite inherits the manganese content of cementite and is chemically stabilised. A treatment consisting of a two-step annealing has been examined. In the first step, soft annealing, an Mn-enriched cementite dispersion in ferrite is formed. In the second step, intercritical annealing, austenite nucleates on the cementite particles, which are consumed to form austenite. It was experimentally determined that this austenite has been enriched in manganese and carbon and, therefore, is stabilised. The conversion reaction is followed by the conventional austenite nucleation at ferrite grain boundaries. This austenite is lean in manganese and is not stable. The net effect of the two-step annealing treatment is a significant austenite stabilisation relative to simple intercritical annealing, indicating a potential for enhanced TRIP effects in this class of steels.

Effects of Strengthening Mechanisms on Fatigue Properties of Ferrite-Pearlite Hot-rolled Sheet Steel

Abstract: The purpose of this study is to clarify the effects of strengthening mechanism on the fatigue properties of ferrite plus pearlite sheet steel Ferrite plus pearlite sheet steels having the tensile strength of 400 to 600 MPa grade were prepared with being strengthened by solid solution, precipitation, increased dislocation density, grain-refinement or increased pearlite volume fraction Furthermore, a ferrite plus bainite steel was prepared to investigate the effect of replacement of the second phase pearlite with harder phase, bainite Load-controlled fatigue test and strain-controlled fatigue test were carried out ot examine fatigue limits and obtain cyclic stress response curves, respectively Main results are as follows: (1) The ratio of an increase in fatigue limit to an increase in tensile strength (Δσw/ΔσB) heavily depended on strengthening mechanism The ratio was higher for both solid solution and precipitation strengthenings than bainite or grain-refinement strengthenings, while lower for dislocation and pearlite strengthenings (2) Stress amplitude obtained from cyclic stress response curve related more closely to fatigue limit than to tensile strength (3) The initial fatigue cracks were observed only in the ferrite matrix; but not in the second phase (4) It was concluded from these results that it is of vital importance to strengthen the ferrite matrix itself by solid solution or precipitation in order to obtain higher ratio of Δσw/ΔσB

Multiple phase steel and process for its manufacture

Abstract: Cold rolled sheet or strip is made from a steel contg. in wt.%: 0.05-0.3 C; 0.8-3 Mn; 0.4-2.5 Al; 0.01-0.2 Si; balance Fe plus usual impurities. The steel is free from pearlite and has a predominantly ferritic structure with inclusions of martensite and opt. bainite and/or residual austenite.

Crystallographic Analysis of Upper Bainite in Fe-9%Ni–C Alloys

Abstract: Crystallographic Analysis of Upper Bainite in Fe-9%Ni-C Alloys In order to determine the orientation relationships among ferrite, austenite and cementite in addition to the habit plane of bainite laths, bainite in Fe-9 mass%Ni-C alloys has been examined mainly by means of transmission electron microscopy. Since little austenite has been retained in the present alloys, the orientation of austenite was estimated by assuming the Kurdjumov-Sachs ferrite/austenite relationship with fixing the ferrite/cementite orientation relationship and the habit of upper bainite lath. The results obtained are as follows: An upper bainite lath is formed by the coalescence of needlelike ferrite subunits elongating in a α ∥ γ on a {111} γ plane, the resultant habit of a bainite lath being a γ {111} γ . The ferrite subunit needle exhibits parallelogram cross section. One set of the parallel needle surfaces inclines slightly from the {111} γ plane and the other set lies almost parallel to the plane expected as that for lattice invariant shear plane expected by the application of phenomenological theory of martensite. At lower temperatures, cementite platelets precipitate on the latter sets of the ferrite needle surfaces, and the coalescence of them leads to a lathlike bainite containing cementite platelets in a specific orientation. The ferrite/cementite orientation relationship is close to that of Isaichev and cementite can be related to austenite by the Pitsch relationship within the deviation less than one degree. Thus, the three phases, i.e., austenite, ferrite and cementite, are mutually almost coherent in upper bainite.

Production of high strength and high toughness ferrite + pearlite type non-tempered forged article

Abstract: PROBLEM TO BE SOLVED: To provide a method for producing an high strength and high toughness forged article having a balance of high strength and toughness by using an inexpensive ferrite + pearlite type non-tempered steel used for wide application. SOLUTION: An alloy having a compsn. contg., by weight, 0.25 to 0.55% C, ≤1.6% Si and 0.08 to 0.5% V, in which manganese equivalent Mneq expressed by Mneq=Mn+Cr+Mo+Cu+V+Ni/2+10(Nb-0.02) is regulated to 0.8 to 2.0 (where ≤2.0% Mn, ≤2.0% Cr, ≤1.0% Mo, ≤4.0% Ni, ≤1.0% Cu and ≤0.1% Nb), and the balance substantially Fe is once heated at 950 to 1300°C, is thereafter cooled to lower its temp. to 800 to 1050°C, is forged at that temp. range, is subsequently cooled and is transformed into ferrite + pearlite. If required, it is incorporated with 0.02 to 0.1% N, 0.015 to 0.010% Al and 0.010 to 0.040% N, is heated to ≥1100°C at which NbC can enter into solid solution, is once cooled to lower its temp. to 800 to 1100°C, is forged, is subsequently cooled and is transformed into ferrite + pearilite. COPYRIGHT: (C)1998,JPO

Modeling of phase transformation behavior in hot-deformed and continuously cooled C-Mn steels

Abstract: Computer models of phase transformation from austenite to ferrite, austenite to pearlite, and austenite to bainite in hot-deformed carbon-manganese steels during continuous cooling were established on the basis of Cahn’s transformation theory, thermal-dilatometric experiments, and thermodynamic calculations. These models showed good agreement with results measured from pilot hot rolling experiments.

Manufacture of high-strength rail excellent in ductility and toughness

Abstract: PROBLEM TO BE SOLVED: To improve ductility and toughness of rail steel by utilizing γ- transgranular transformation of pearlite. SOLUTION: By cooling a slab heating steel containing, by mass, 0.55-0.85% C, 0.20-1.20% Si, 0.50-1.50% Mn, 0.002-0.035% S and, as necessary, >=1 kinds of Cr, Ni, Mo, Ti, V to >=1300 deg.C, or a slab manufactured from molten steel down to >=900 deg.C at 3 deg.C/sec, executing hot rolling and, after that, rapidly cooling between 700-500 deg.C at 1-5 deg.C/sec, pearlite transformation is directly generated from MnS. This method is effective for the specification of ductility stipulated by Rus-sian GOST and the improvement of elongation value stipulated by Chinese specifications.