Showing papers on "Pearlite published in 1998"

Direct evidence of cementite dissolution in drawn pearlitic steels observed by tomographic atom probe

Abstract: The dissolution of cementite during cold-drawing of pearlitic steels has been directly observed with the tomographic atom probe (TAP). Because of the nanometric interlamellar spacing and size of cementite lamellae, the TAP is shown to be extremely well suited for such a study. Analysis conditions and mass spectra peak deconvolution are shown to lead to quantitative analysis of both ferrite and cementite. It is also shown that specimen preparation always aligns the cementite lamellae habit plane with the analysis direction so that the analysed area is always perpendicular to their habit plane. Local magnification effects are also shown not to affect the carbon concentration measurement in the cementite. The first direct atomic scale quantitative concentration data across a few nanometer-thick cementite lamellae are given, and confirm the dissolution of cementite after cold-drawing. The derived compositions of ferrite, cementite and interfacial areas are obtained, giving information on the cementite dissolution mechanism as well as on its extent.

The divorced eutectoid transformation in steel

Abstract: Experiments are presented which show that the eutectoid transformation in steel can occur by two different modes for temperatures just slightly below A 1. In the normal mode, the transformation product is lamellar pearlite. The second mode occurs if the austenite contains cementite particles or nuclei with a spacing on the order of a few microns or less. In this case, the transformation product consists of spheroidal cementite particles in a ferrite matrix. This second mode is here called the divorced eutectoid transformation (DET), after recent work by Sherby and co-workers. A literature survey shows that the faster kinetics of the DET over lamellar pearlite in the presence of inhomogeneous austenite was established before 1940, but has received little attention. The inhomogeneities are generally small cementite particles. Experiments show that the DET does not occur by a shell of one phase (ferrite) forming around the other phase of the eutectoid (cementite), as is the case in divorced eutectic growth. Rather, a fairly planar austenite/ferrite front simply advances into the austenite, with no apparent effect on its shape being produced by the cementite particles. A first-order kinetic model is presented for the growth velocity as a function of undercooling below A 1 and is compared to the velocity vs undercooling for lamellar pearlite. The simple model indicates that the velocity of the divorced mode should be faster than the lamellar mode at low undercooling for cementite nuclei distributed in the austenite with spacings less than a few microns. This result is consistent with the experimental data.

Strengthening mechanisms in vanadium microalloyed steels intended for long products

Abstract: The present work has concentrated on the roles of vanadium, nitrogen and carbon in controlling the microstructures and strength of steels designed for hot rolled long products. Effects of cooling rate and additional microalloying with titanium have also been included. The degree of precipitation strengthening of ferrite at a given vanadium content depends on the available quantities of carbon and nitrogen. The nitrogen content of the ferrite is approximately the same as that of the austenite from which it forms, i.e. the total nitrogen content in steel. It was confirmed that nitrogen is a very reliable alloying element, increasing the yield strength of V-microalloyed steels by some 5 MPa for every 0.001% N, essentially independent of processing conditions. Carbon content, on the other hand, has usually been considered not relevant to precipitation strengthening when the precipitation occurs in ferrite because of the very small carbon content in solution in ferrite at equilibrium. We demonstrate that the effective carbon for precipitation in ferrite may be much greater than this during the period of phase transformation, which in turn has a great effect on precipitation strengthening. Such behaviour is explained on the basis that the activity of carbon in ferrite is abnormally high in the presence of under-cooled austenite and before cementite nucleation so that profuse nucleation of vanadium carbonitride is encouraged. This new mechanism for precipitation is particularly significant for medium carbon steels typically used for hot rolled bars and sections. The total carbon content of the steel also contributes to the yield strength by increasing the volume fraction of pearlite. It is shown that the contribution from pearlite is stronger than generally recognised.

Steel material having high ductility and high strength and process for production thereof

Abstract: A process for the production of a steel material comprising rolling a steel material having a structure mainly comprising ferrite or ferrite plus pearlite or ferrite plus cementite at a percentage reduction of area of at least 20 % in a ferrite recrystallization temperature region to achieve such characteristics as a crystal particle diameter of not greater than 3 νm, preferably not greater than 1 νm, an elongation of at least 20 %, a value of tensile strength (TS: MPa) x elongation (El: %) of at least 10,000 or a percent ductile fracture of at least 95 %, preferably 100 %, in an actual pipe Charpy impact test at -100 °C. Particularly, this process yields a steel material containing 0.05 to 0.30 wt.% of C, 0.01 to 3.0 wt.% of Si, 0.01 to 2.0 wt.% of Mn and 0.001 to 0.10 wt.% of Al and having a structure comprising ferrite alone or ferrite and a second phase, wherein the ferrite particle diameter is not greater than 3 νm and the areal ratio of the second phase is not greater than 30 %. An untreated steel pipe having the composition described above is heated to (Acl + 50 °C) to 400 °C and subjected to stretch reduction at a cumulative diameter reduction ratio of at least 20 % in a rolling temperature range of (Acl + 50 °C) to 400 °C. In this case, the rolling process preferably contains at least one rolling pass having a diameter reduction ratio of at least 6 % in the stretch reduction. When the contents of C, Si, Mn and other alloy elements are kept at low levels and stretch reduction is carried out in the temperature range described above, a steel pipe having high ductility and strength and improved toughness and stress corrosion crack resistance can be manufactured and the resulting pipe can be used as a line pipe. The fatigue resistance can be improved, too.

The mechanical and physical properties of Compacted Graphite Iron

Abstract: The mechanical and physical properties of compacted graphite iron (CGI) have been studied. While it is known that CGI provides at least 70 % higher tensile strength, 35 % higher elastic modulus and 80 % higher fatigue limits than conventional grey cast iron, the objective of this paper was to determine how the properties change with varying nodularity (graphite shape) and pearlite content. Tensile properties, hardness, damping capacity and thermal conductivity were determined over the range 0-90% nodularity and 25-100 % pearlite. Compressive properties, wear resistance and fatigue behaviour of CGI are also reported upon. Of greatest importance is the influence of patches of flake graphite in predominantly CGI microstructures. As soon as flake patches appear, which can occur with a loss of as little as 0.001 % active magnesium, the mechanical properties of CGI abruptly decrease by 25-40%. Proper foundry control technology and quality procedures are therefore required to ensure the reliable production of this improved material.

High strength wire rod excellent in delayed fracture resistance, its production, and high strength bolt

Abstract: PROBLEM TO BE SOLVED: To provide a high strength wire rod having excellent delayed fracture resistance as well as ≥1200 N/mm 2 tensile strength, a useful method for obtaining a high strength wire rod like that, and a high strength bolt having the above characteristics. SOLUTION: This wire rod is composed of a steel containing 0.5-1.0% C, and the area ratio of a pearlitic structure having a pearlite nodule size of No.7 or above by grain size number is regulated to ≥80% by inhibiting the formation of structures of one or ≥2 kinds among pro-eutectoid ferrite, pro-eutectoid cementite, bainite, and martensite and also strength is regulated to ≥1200 N/mm 2 by means of heavy wire drawing. Moreover, the steel is heated to 800-1000°C, cooled rapidly down to 520-650°C, and isothermally held at the temperature, by which the formation of structures of one or ≥2 kinds among pro- eutectoid ferrite, pro-eutectoid cementite, bainite, and martensite is inhibited to regulate the area ratio of a pearlitic structure having a pearlite nodule size of No.7 or above by grain size number to ≥80%. Then, strength is regulated to ≥1200 N/mm 2 by means of heavy wire drawing. COPYRIGHT: (C)1999,JPO

Experimental Investigation on the Significance of the Conventional Endurance Limit of a Spheroidal Graphite Cast Iron

Abstract: Fatigue tests were performed on a spheroidal graphite cast iron in four point plane bending under constant stress amplitude and block loading conditions. The microstructure of this material has a ‘bull’s eyes’ appearance, i.e. the spheroids of graphite are surrounded by ferrite and these nodules and ferrite zones are included in a pearlitic matrix. Scanning electronic microscope observations were carried out at different fractions of life for constant stress amplitude loadings above and below the conventional endurance limit. Non-propagating micro-cracks were observed at a stress level equal to the conventional endurance limit. These observations showed that another limit can be defined below the conventional endurance one, i.e. one below which micro-cracks were not observed to initiate in the matrix. These cracks were found to arrest at the ferrite/pearlite interface when the material was tested below this new limit. This concept was used to rationalize fatigue results from tests with loading in blocks above and below the conventional endurance limit.

Modelling of the Eutectoid Reaction in Spheroidal Graphite Fe-C-Si Alloys

Abstract: The so-called eutectoid reaction of spheroidal graphite cast irons (SGI) )proceeds by competitive nucleation and growth of ferrite and pearlite. In the present study are first reviewed the physical models of the ferritic reaction in SGI previously described in the literature. Then, a new model is presented that uses a recent description of the conditions to be fulfilled for the ferritic and pearlitic reactions to start. This description is based upon the knowledge of the relevant phase diagram. Growth of the ferrite halos during the ferritic reaction is described as controlled by carbon transfer from the austenite/ferrite interface to the graphite nodules and by an interfacial reaction at the ferrite/graphite interface. Modelling of the pearlitic reaction accounts for nucleation of pearlite colonies, and their growth law is expressed according to experimental data available in the literature. It appeared also necessary to describe the diffusion of carbon in austenite before the beginning of the decomposition of this phase. Predictions are compared to experimental transformation kinetics obtained by means of differential thermal analysis on spheroidal graphite Fe-C-Si alloys, and could be easily extended to alloys with low level additions of pearlite promoter elements.

Simulations of pro-eutectoid ferrite formation using a mixed control growth model

Abstract: A simple numerical model is proposed to describe the formation of pro-eutectoid ferrite during the decomposition of austenite. The growth kinetics are analysed using a recently developed mixed control growth model. This model describes the movement of an interface in terms of an interaction between carbon volume diffusion and the intrinsic mobility of the α/γ-interface. Nucleation phenomena are treated using classical nucleation kinetics modified for compatibility with the mixed control growth model. A simple geometry is assumed for the ferrite nuclei. Isothermal transformation (IT) diagrams, as well as continuous cooling transformation (CCT) diagrams, are constructed from this model. The model yields C-shaped curves for pro-eutectoid ferrite formation during both isothermal and continuous cooling conditions. At deeper undercoolings, the transformation kinetics are determined primarily by the intrinsic interface mobility. An overlap between pro-eutectoid ferrite formation and pearlite formation is predicted. The fraction of pro-eutectoid ferrite is predicted to decrease with increasing undercooling or cooling rate. The effects of various model parameters on the transformation kinetics are investigated and discussed.

The effect of Mo in Si-Mn Nb bearing TRIP steels

Abstract: Experimental studies were performed to determine the effect of Mo in Si-Mn TRIP steels, as well as its potential for reducing the levels of Si. Three compositions were investigated. Bainite transformation conditions were investigated on the final mechanical properties. Results revealed that Mo has an important retardation effect on the formation of both ferrite and pearlite. The Mo and reduced Si level steel generated excellent mechanical properties (as high as: UTS=1269, T.El.=36%) in the range observed by previous investigators.

Effects of Rare Earth Element on Isothermal and Martensitic Transformations in Low Carbon Steels

Abstract: Rare earth elements (RE) may segregate at the grain boundaries of austenite, lead to form carbide and refine the austenite grain. In case of no change of grain size and carbon content of austenite, an addition of RE is beneficial to the hardenability of steels. In case of a marked refinement of austenite grain, addition of RE will deteriorate the hardenability. The incubation period of the proeutectoid ferrite can be expressed as a function of grain boundary energy, grain size, activation energy for growth and the driving force for transformation and the calculated results are in good agreement with the experimental data. RE may retard the isothermal pearlitic transformation, because RE diminishes the diffusion coefficient of carbon as well as tends to segregate at Fe3C/α interface, showing a pinning effect on the transformation. RE reduces the lamella spacing of the pearlite owning to lowering the interfacial energy, e.g., from 0.7 to 0.53 J/m2 in 0.27C–1Cr–RE steel. RE tends to segregate at ferrite/island interface in the granular bainite. In grain refined steel, at the earlier stage of bainite formation, the transformation rate is high while at later stage it becomes sluggish. The activation energies of pearlitic and bainitic transformations increase by the addition of RE. The segregation of RE at ferrite/island interface may act as a drag effect. A drag factor α is expressed as a function of transformation fraction and calculated in a 0.27–1Cr–RE steel. RE segregates at the grain boundary of austenite and this kind of distribution will not be changed during the martensitic transformation. It is reasonable to predict that RE will lower the martensite/austenite interface energy, resulting in the formation of a finer lath structure. RE lowers Ms, decreases the amount of the retained austenite and retards the autotempering process. It is emphasized that the amount of the retained austenite, γ, in quenched low carbon steel depends on not only the Ms and the temperature of quenching medium, Tq, but also the influence of alloying elements on the carbon diffusion during quenching. A general equation modified from the Magee's equation is derived as γ=exp{α(C1–C1)-β(Ms–Tq)} where C0 and C1 are carbon concentrations in austenite before and after quenching respectively, α and β are constants. RE decreases Ms but also lowers C1 so as to reduce the amount of the retained austenite.

Steel wire capable of rapid spheroidizing and excellent in cold forgeability, and its manufacture

Abstract: PROBLEM TO BE SOLVED: To provide a steel wire capable of combinedly attaining rapid spheroidizing before cold forging and excellent cold forgeability improved in deformability, and to provide a useful method for manufacturing it. SOLUTION: In a hot rolled steel wire rod or a cold drawn steel wire, containing, by mass, 0.2-0.6% C, <=0.3% Si, and 0.2-1.5% Mn, a structure composed essentially of pro-eutectoid ferrite and pearlite is provided. Moreover, average grain size is regulated to 6-15 μm. Further, the ratio of pro-eutectoid ferrite volume ratio (Vf) to equilibrium pro-eutectoid ferrite volume ratio (Vpf1) represented by equation Vpf1=(0.8-Ceq1)×129, Vf/Vpf1, is regulated to 0.05-0.75. In the equation, Ceq1(%)=[C%]+0.10[Si%]+0.06[Mn%] is satisfied and also [C%], [Si%], and [Mn%] represent respective contents (mass%) of C, Si, and Mn, respectively.

Alternatives to the Ferrite-Pearlite Microstructures

Abstract: Structural steels based on a mixed microstructure of allotriomorphic ferrite and pearlite have a well-established history of cost-effectiveness and reliability. The purpose of this paper is to review the possibility of alternatives to this microstructure for large scale applications. Controlled-rolled bainitic steels, accelerated cooled steels, ultra-low carbon bainitic steels and inoculated steels are discussed in this context.

Anisotropic stress corrosion cracking behaviour of prestressing steel

Abstract: This paper evaluates the anisotropic stress corrosion cracking behaviour of high-strength prestressing steel wires. To this end. two eutectoid steels in the form of hot rolled bar and cold drawn wire were subjected to stress corrosion cracking tests in aqueous environments using a constant strain technique and precracked three point bend specimens to measure the crack growth rate da/dt as a function of the stress intensity factor K I under hydrogen embrittlement environmental conditions (pH = 12.5 E = -1200 mV SCE). While the hot rolled bar presents an isotropic stress corrosion cracking behaviour associated with mode I crack growth, the cold drawn wire exhibits a change in crack propagation direction approaching that of the wire axis (cold drawing direction) and producing mixed mode crack growth. This anisotropic stress corrosion cracking behaviour is a consequence of manufacturing, since cold drawing affects the microstructure of the material and produces a preferential orientation of the pearlite lamellae aligned parallel to the wire axis. The differences of crack growth rate as a function of the crack propagation direction are discussed.

Ductile steel with high yield strength and process for manufacturing same

Abstract: Multiphase hot-rolled steel exhibiting transformation induced plasticity has a structure comprising ferrite and bainite or a mixture of bainite and martensite, with retained austenite. It contains by weight: 0.05-0.5% carbon, 0.50-2.5% manganese and 0.30-0.80% silicon. Also claimed is preparation of the steel from an ingot of this formula, which is heated at 1150-1300 degrees C for 135-200 minutes, roughly rolled while cooling to between 900 and 1150 degrees C, then finish rolled while cooling to or below the austenite transformation temperature. The resulting steel band is cooled slowly to just above the pearlite formation temperature, then rapidly to below that temperature. It is wound onto a spool below the temperature of bainite formation but above that of martensite formation, thus forming some bainite in the microstructure. Finally it is quenched to stop the bainite formation and prevent precipitation of iron carbide.

Work hardening of hypereutectoid and eutectoid steels during drawing

Abstract: Work hardening of pearlite and bainite during drawing of high-carbon steel wires was studied by using stress-strain curves of drawn wires and making a detailed observation of their microstructure. The study revealed the following relationship between flow stress and strain in high-carbon steel wires. Flow stress σ is represented by a weighted average of the strengths of ferrite and cementite. The amount of work hardening Δσ is represented by an equation corresponding to exp(Be) where B is a constant that depends on the initial microstructure of the steel wire and e is the strain. When the initial microstructure of steel wire is made up entirely of a pearlite lamellar structure, the value of B is 0.5. When the initial microstructure contains some bainite, the value of B is lower than 0.5. Finally the flow stress was formulated as a function of carbon content, supercooling degree, and e. The calculated maximum flow stress showed good correlation with the measured flow stress of 0.8 % C and 0.2%Cr-0.92%C steel wires during drawing.

Effects of hot deformation on austenite transformation in 10w carbon Mo-Nb and C-Mn steels

Abstract: The 0.15C-1.5Mn and 0.07C-0.3Mo-0.055Nb steels were subjected to hot rolling to determine the effects on transformation to ferrite and to hot torsion to measure strength. After 25 or 50% reduction at 830°C, the acceleration during isothermal transformation in the range 700–600°C is much greater for C-Mn than for Mo-Nb; delays of 5–10 s before cooling reduce the acceleration markedly. In continuous cooling, three passes of 20% reduction raise the temperature of austenite decomposition, most noticeably at the higher rate 9 K S-l. At this rate, for the C-Mn the pearlite is refined and for Mo-Nb the promoted ferrite is refined more and enhanced over bainite than at lower rates. Intercritical rolling below Ar3 enhanced ferrite formation more in C-Mn in the range 780–720°C than in Mo-Nb over 750–690°C. The stronger Mo-Nb in torsion (900–1100°C, 0.1-5 S-l) exhibits an activation energy of 353 kJ mol-l compared with 316 kJ mol-l for C-Mn.

Use of the nanoindentation technique for studying microstructure/crack interactions in the fatigue of 4340 steel

Abstract: The objectives of this research are to study the influence of microstructure on the fatigue crack growth behavior in 4340 steel and to explore the application of the nanoindentation technique for determining the plastic deformation zone at a fatigue crack tip. Two heat treatment conditions were chosen for the steel: annealed and quenched plus tempered. The annealed steel consists of coarse pearlite and proeutectoid ferrite, while the quenched and tempered steel consists of fine tempered martensite. Fatigue crack propagation tests were conducted on disklike compact (DCT) specimens. Subsequently, the nanoindentation technique was applied to quantitatively determine the plastic deformation zone at fatigue crack tips. The plastic deformation zone size determined by the nanoindentation test seems larger than the cyclic deformation zone calculated using the fracture mechanics equation, which involves many assumptions. The fatigue crack growth test results show that the annealed steel has a higher resistance to crack growth than the quenched and tempered steel. The fatigue crack in the annealed steel tends to grow along pearlite domain boundaries, or the cementite/ferrite interfaces within a pearlite domain. In contrast, the fatigue crack in the quenched and tempered steel tends to traverse the fine martensite laths. Consequently, the actual crack path in the annealed steel is rougher than in the quenched and tempered steel and more secondary cracks are observed in the annealed steel.

Process for producing steel pipe having high ductility and strength

Abstract: of EP0940476A steel product having a structure composed mainly of ferrite or ferrite plus pearlite or ferrite plus cementite. A steel pipe produced from this steel product by rolling at a ferrite recrystallization temperature such that the reduction of area is greater than 20%. This steel pipe is characterized by grain size not greater than 3 mu m, preferably not greater than 1 mu m, elongation greater than 20%, tensile strength (TS : MPa) and elongation (El : %) whose product is greater than 10000, and percent ductile fracture greater than 95%, preferably 100%, measured by Charpy impact test on an actual pipe at -100 DEG C. The structure is characterized by C : 0.005-0.03%, Si : 0.01-3.0%, Mn : 0.01-2.0%, and Al : 0.001-0.10% on a weight basis, and is composed of ferrite or ferrite and a secondary phase, with ferrite grains being not greater than 3 mu m and the secondary phase having an areal ratio not more than 30%. The steel pipe is produced from a steel pipe stock having the above-mentioned composition by heating at a temperature of (Ac1 + 50 DEG C) to 400 DEG C and subsequently performing reducing on it at a rolling temperature of (Ac1 + 50 DEG C) to 400 DEG C such that the cumulative reduction of diameter is greater than 20%. The reducing is preferably performed such that at least one of rolling passes reduces the diameter by more than 6% per pass. The steel pipe will have high ductility and high strength and will be superior in toughness and stress corrosion cracking resistance, if the content of C, Si, Mn, and other alloying elements is limited low and reducing is performed at the temperature specified above. The resulting steel pipe has good fatigue resistance and is suitable for use as line pipe.

Graphitization in High Carbon Commercial Steels

Abstract: Graphitization kinetics in two commercial high carbon steels, AISI types 1075 and 1095, are studied by conducting a series of isothermal annealing treatments in the temperature range of 560 to 680° C for periods of time ranging from 20 to 500 h. The samples selected were collected along the processing route in a commercial production line dedicated to the fabrication of thin strip. The structures studied were those of hot rolling (consisting of fine pearlite), cold rolling (spheroidized carbides within a deformed ferritic matrix), and subcritical annealing (spheroidized carbides in undeformed ferrite). The samples obtained from hot rolled coils do not graphitize, whereas those cold rolled graphitize at a rate that depends on the type of steel and degree of deformation. No graphite was found in samples from the lower carbon steel, which were subcritically annealed, although they were observed in specimens from the other steel, which were cold rolled to a reduction of 50% prior to the subcritical annealing.

Nonclassical decomposition products of austenite in Fe-C-Cr alloys

Abstract: Unusual austenite decomposition products in two Fe-0.4C alloys containing chromium additions of 3.5 and 10 wt pct have been studied. Detailed transmission electron microscopy (TEM) has been carried out on partially transformed specimens in order to determine the identities and morphologies of the phases and the mode of formation. The most descriptive terms for these novel products are spiky pearlite and acicular ferrite/carbide aggregates. The spiky pearlite is distinguished by its nonnodular transformation front and by the presence of individual segments or units composed of ferritesheathed carbides. The acicular aggregates appear as dark-etching, macroscopic plate-shaped structures that are formed from the successive nucleation of these single ferrite/carbide subunits, which are crystallographically related to the austenite grain in which they grow, with a predominant orientation. The uniqueness of these structures has been reinforced by the detection of customary pearlite in both of the alloys and by the presence of classical upper and lower bainites in the low-chromium alloy. It is proposed that the structures develop as a result of the oriented coupled growth of the individual ferrite/carbide segments identified by the study.

Acicular Ferrite Microstructures and Mechanical Properties in a Medium Carbon Forging Steel

Abstract: A set of different acicular ferrite type microstructures have been developed, with both isothermal treatments between 500°C and 400°C and continuous cooling after austenization at 1250°C, in a medium carbon forging steel. The inclusions responsible for the intragranular nucleation of the ferrite have been studied by means of transmission electron microscopy. The results obtained enabled the identification of a MnS core covered by hexagonal CuS, which seems to make nucleation at inclusions energetically more favourable than nucleation at grain boundaries. A good combination of strength and toughness has been obtained associated with a refined acicular ferrite microstructure, generated by continuous cooling, and with a mixture of acicular ferrite and pearlite, obtained at 500°C by isothermal treatment.