Showing papers on "Pearlite published in 2005"

The effect of microstructural characteristics of pearlite on the mechanical properties of hypereutectoid steel

Abstract: The relationship between mechanical properties and microstructural characteristics of pearlite was investigated using various heat treatments on a hypereutectoid steel. The materials were reheated between 900 and 1200 °C and these microstructures were then subjected to isothermal transformation at temperatures of 550, 580 and 620 °C. For the hypereutectoid steel, the mean value of the interlamellar spacing was observed to increase with increasing reheat and transformation temperatures. Examination of the mechanical properties of the resulting pearlitic microstructures indicated that the strength was related primarily to the interlamellar spacing by a Hall–Petch type relationship, while the ductility was dependent also on the prior-austenite grain size and pearlite colony size.

The influence of microstructure on the corrosion rate of various carbon steels

Abstract: This paper presents a fundamental study of the influence of carbon steel microstructure on the corrosion rate Subsequently, the corrosion performance of various grades of carbon steels were evaluated in stirred autoclaves under elevated carbon dioxide and temperature conditions Corrosion and penetration rates were determined via mass loss and optical microscopy, respectively It was found that the corrosion rate of carbon steel line pipe is influenced by microstructure More specifically, a relationship between localized corrosion susceptibility and the presence of pearlite bands in the steel microstructure was found However, no correlation was evident between minor elemental concentrations (ie, Ni, Cr, Mo) and corrosion resistance It has been proposed that the corrosion stability of the various microstructures may arise from variations in the distribution of carbon bearing phases within the steel In the banded ferrite/pearlite structure, the carbon-bearing phase (pearlite) is distributed in layers whereas in the other structures the carbon-bearing phases are much more evenly distributed This study reports on the corrosion resistance of carbon steels in relation to their chemical and physical properties

Ultrafine grained steels processed by equal channel angular pressing

Abstract: Recent development of ultrafine grained (UFG) low carbon steels by using equal channel angular pressing (ECAP) and their room temperature tensile properties are reviewed, focusing on the strategies overcoming their inherent mechanical drawbacks In addition to ferrite grain refinement, when proper post heat treatments are imposed, carbon atom dissolution from pearlitic cementite during ECAP can be utilized for microstructural modification such as uniform distribution of nano-sized cementite particles or microalloying element carbides inside UFG ferrite grains and fabrication of UFG ferrite/martensite dual phase steel The utilization of nano-sized particles is effective on improving thermal stability of UFG low carbon ferrite/pearlite steel but less effective on improving its tensile properties By contrast, UFG ferrite/martensite dual phase steel exhibits an excellent combination of ultrahigh strength, large uniform elongation and extensive strain hardenability

Influential microstructural changes on rolling contact fatigue crack initiation in pearlitic rail steels

Abstract: Rail life is controlled by the balance between wear and fatigue damage due to in service loading. To model and optimise rail life, knowledge of the fatigue crack initiation mechanism is required. This paper reports the effect of in service loading on microstructural changes in the subsurface layer of pearlitic rail steels and observations of early stage (10–50 μm length) fatigue crack formation. Micro and nanohardness measurements are reported, along with microstructural observations, showing differential work hardening in the proeutectoid ferrite and pearlite phases. It is proposed that the differential straining results in ductility exhaustion in the proeutectoid ferrite and therefore fatigue crack initiation and initial growth in the proeutectoid ferrite phase. Observations of short (<50 μm) cracks in rails taken out of service containing significant amounts of proeutectoid ferrite (≈20%) confirm the proposed mechanism.

Analysis of the γ → α transformation in a C-Mn steel by phase-field modeling

Abstract: This article deals with the austenite (γ) decomposition to ferrite (α) during cooling of a 0.10 wt pct C-0.49 wt pct Mn steel. A phase-field model is used to simulate this transformation. The model provides qualitative information on the microstructure that develops on cooling and quantitative data on both the ferrite fraction formed and the carbon concentration profile in the remaining austenite. The initial austenitic microstructure and the ferrite nucleation data, derived by metallographic examination and dilatometry, are set as input data of the model. The interface mobility is used as a fitting parameter to optimize the agreement between the simulated and experimental ferrite-fraction curve derived by dilatometry. A good agreement between the simulated α-γ microstructure and the actual α-pearlite microstructure observed after cooling is obtained. The derived carbon distribution in austenite during transformation provides comprehension of the nature of the transformation with respect to the interface-controlled or diffusion-controlled mode. It is found that, at the initial stage, the transformation is predominantly interface-controlled, but, gradually, a shift toward diffusion control takes place to a degree that depends on cooling rate.

Modelling Upper and Lower Bainite Trasformation in Steels

Abstract: Bainite is of considerable importance in the design of high strength steels. There are two types of morphologies, upper and lower bainite. In upper bainite, cementite forms between adjacent bainitic ferrite plates. In certain steels, however, the cementite reaction is suppressed so that carbon-enriched austenite remains untransformed between bainitic ferrite plates. In lower bainite, cementite also has the opportunity to precipitate within bainitic ferrite plates. In order to model the development of these microstructures, it is necessary to treat the simultaneous formation of both the ferritic and carbide components of the microstructure. A theory has been developed to do exactly this, enabling the estimation of the phase fractions, the cementite particle size and the transition from upper to lower bainite. The results have been compared against experimental data.

Ultra-fine (α+θ) Duplex Structure Formed by Cold Rolling and Annealing of Pearlite

Abstract: The ferrite (α)+cementite (θ) microduplex structure formed by heavy cold rolling and annealing of pearlite was studied in an Fe-1.4Cr-1.0C (mass%) alloy. Cold-rolled pearlite structure is inhomogeneous consisting of three components; (1) irregularly bent lamellae (IBL), (2) coarse lamellae with shear band (CLS) and (3) fine lamellae (FL) as was previously reported by the present authors. Misorientation in α is large in the IBL and near the shear band in the CLS. As rolling reduction increases, the proportion of FL increases. By annealing at 973K after heavy cold rolling, the (α+θ) microduplex structure with α and θ grain sizes less than 0.5 μm is formed. This structure consists of a coarse grain region (dα∼0.4 μm) containing high-angle α boundaries and a fine grain region (dα∼0.2 μm) with low-angle α boundaries by inheriting local orientation distribution in the deformed α structure. The coarse grain region is formed at the deformed region where local misorientation in α is large essentially by recovery under pinning by θ particles. As the annealing is prolonged, the fraction of the coarse grain region increases. The cold-rolled and annealed pearlite exhibits a wide range of strength-ductility balance.

Additivity hypothesis and effects of stress on phase transformations in steel

Abstract: Theoretical analysis shows that the fraction of pearlite formed from nucleation is additive and that from growth is not. A modified additivity model is established with two continuous cooling experiments. Calculations of Ae 3 temperature for proeutectoid ferrite formation under stress and nucleation rate as well as incubation period of ferrite or pearlite transformation under stress are successfully made. Kinetics equations for ferrite and pearlite transformations under stress are expressed from modification of J–M–A equation with addition of a stress item. The acceleration effect of stress on bainite formation is mainly attributed to the increase of diffusivity of solute atoms and even iron. By consideration of the grain size effect, Patel and Cohen’s equation expressing the effect of stress on M s is modified. Calculations of M s for fcc → bcc(bct) and fcc → hcp under stress are introduced. An equation showing the relationship between strain and nucleation of martensite which can well explain the morphology of martensite formed under stress is mentioned. Appearance and mechanism of mechanical stabilization of austenite in martensitic transformation, i.e., the lowering of M s , resulted from the work hardening of austenite, are different from retardation of bainite formation under stress, i.e., after B s raising, occurring the retardation of bainite growth resulted from hindrance by defects.

High strength thin steel sheet having high hydrogen embrittlement resisting property

Abstract: The purpose of the present invention is to provide a high strength thin steel sheet that has high hydrogen embrittlement resisting property. In order to achieve the above purpose, a high strength thin steel sheet having high hydrogen embrittlement resisting property comprises: C: 0.10 to 0.25%; Si: 1.0 to 3.0%; Mn: 1.0 to 3.5%; P: 0.15% or less; S: 0.02% or less; and Al: 1.5% or less (higher than 0%) in terms of percentage by weight, with balance of iron and inevitable impurities; and the metal structure comprises: residual austenite; 1% by area or more in proportion to the entire structure; bainitic ferrite and martensite: 80% or more in total; and ferrite and pearlite: 9% or less (may be 0%) in total, while the mean axis ratio (major axis/minor axis) of said residual austenite grains is 5 or higher, and the steel has tensile strength of 1180 MPa or higher.

Steel sheet for press forming with excellent ductility after forming, its forming method and automotive parts using the steel sheet for press forming

Abstract: PROBLEM TO BE SOLVED: To provide a steel sheet for press forming having a high strength of 780 to 1,180 N/mm 2 class and excellent ductility after forming and its forming method and also to provide automotive parts using the steel sheet for press forming. SOLUTION: A steel sheet which has a composition consisting of, by mass, 0.1 to 0.4% C, 0.001 to 3.0% Si, 1.5 to 4.0% Mn, ≤0.1% P, ≤0.05% S, 0.005 to 0.1% Al, ≤0.01% N and the balance Fe with inevitable impurities and also has a microstructure composed of ferrite and pearlite or of ferrite, cementite and pearlite is heated at 1 to 100°C/sec heating rate, held in a temperature region of 700 to 850°C for 10 to 6,000 sec and then press formed in a temperature region of 550 to 700°C. The resultant formed steel sheet has a structure consisting of, by area ratio after cooling, 40 to 90% ferrite as a main phase, 10 to 60% martensite as a second phase and the balance bainite. COPYRIGHT: (C)2007,JPO&INPIT

Ferrite/Pearlite Band Prevention in Dual Phase and TRIP Steels : Model Development

Abstract: A model for predicting the conditions for ferrite/pearlite band prevention in dual phase and TRIP steels has been developed. The competition between processing parameters such as the austenitisation time and temperature, the transformation temperature and microchemical segregation wavelength is explored. The effects of alloy composition in the tendency to form ferrite/pearlite bands are quantified. A simple formula combining processing parameters and compositions for describing band formation is presented. The calculations show that the most prominent factor for preventing banding is the control of the microchemical wavelength. In addition to C and Mn, Al and Si concentrations have shown to play a smaller but significant role in band formation behaviour.

Grain boundary engineering for superplasticity in steels

Abstract: The microstructure with suitable boundary characters for superplasticity is summarized for the steels which consist of two phases, i.e., ferrite (bcc α) + austenite (fcc γ) or ferrite (α) + cementite (orthorhombic θ -Fe3C). In (α + γ) duplex alloys, a conventional thermomechanical processing (solution treatment + heavy cold rolling + aging) produces the (α + γ) duplex structure through the competition of recovery/recrystallization of matrix and precipitation. In Fe-Cr-Ni (α + γ) duplex stainless steels with high γ fractions (40–50%), α matrix undergoes recovery to form α subgrain boundaries and γ phase precipitates on α subgrain boundaries with near Kurdjumov-Sachs relationship during aging. By warm deformation, the transition of α boundary structure from low-angle to high-angle type occurs by dynamic continuous recrystallization of α matrix and, simultaneously, coherency across α/γ boundary is lost. Contrarily, α phase first precipitates in deformed γ matrix in Ni-Cr-Fe based alloy during aging. Subsequently discontinuous recrystallization of γ matrix takes place and the (α + γ) microduplex structure with high-angle γ boundaries is formed. The formation of those high-angle boundaries in (α + γ) microduplex structure induces the high strain rate superplasticity. In an ultra-high carbon steel, when pearlite was austenitized in the (γ + θ) region, quenched and tempered at the temperature below A1, an (α + θ) microduplex structure in which most of α boundaries are of high-angle type is formed through the recovery of the fine (α ′ lath martensite + θ) mixture during tempering. Such (α + θ) microduplex structure with high angle α boundaries exhibits higher superplasticity than that formed by heavy warm rolling or cold rolling and annealing of pearlite which contains higher fraction of low angle boundaries.

Structure and mechanical behavior of heavily drawn pearlite and martensite in a high carbon steel

Abstract: Neutron diffraction measurements have revealed that cementite peaks disappear in a pearlite steel with drawing and that the residual intergranular stresses are generated. The diffraction profiles in a heavily drawn specimen suggest the tetoragonality with a small c/a in the ferrite matrix. Although cementite was hardly observed in the heavily drawn specimen, its c/a value determined by neutron diffraction and mechanical behavior are quite different from those of as-quenched martensite. The changes in hardness and c/a with annealing or tempering were also different between heavily drawn pearlite and marteniste. Hence, most of carbon atoms do not exist inside the ferrite lattice in the drawn pearlite and multi-scaled heterogeneous plastic deformation in pearlite seems to affect the asymmetry in the diffraction profile. Fracture behavior and hardness change with tempering is different in the two microstructures.

Computer simulation of the austenitizing process in cast iron with pearlitic matrix

Abstract: Austenitizing as a first stage of the heat treatment of castings to produce the cast iron grades like ADI, ACI, or AGI consists in holding of castings at a temperature co mprised within the range of 800-950 0 C to obtain an austenitic structure of the matrix as a point of departure for ausferritic structure. The temperature and time of austenitizing exert an important effect on the structure parameters and mechanical behaviour of mat erial obtained after the final treatment, which is austempering. A mathematical model of the process has been caharcterized, allowing for the temperature field in cas ting, the field of diffusion in the area of pearlite lamellae, and changes of diffusion coe fficient in function of temperature. A numerical program was developed by means of which the kinetics of austenite growth and cementite and ferrite fading in lamellar pearlite during austenitizing were determined, and a non-stationary field of carbon concentration in the examined system was computed. The temperature field in casting was also verified e xperimentally (a sample of 6x10x15 mm held in salt bath).

Effect of prior-austenite grain size and transformation temperature on nodule size of microalloyed hypereutectoid steels

Abstract: The effect of prior-austenite grain size and transformation temperature on nodule size and colony size of hypereutectoid steels containing 1 pct carbon with different levels of vanadium and silicon was investigated. Specimens of the various steels were thermally processed at various temperatures ranging from 900 °C to 1200 °C and transferred to salt bath conditions at 550 °C, 580 °C, and 620 °C to examine the structural evolution of pearlite. The heat-treatment work showed that for only the hypereutectoid steel without vanadium there was a continuous grain boundary cementite network, the thickness of which increased with increasing reheat temperature. Analysis of the thermally processed hypereutectoid steels also indicated that the prior-austenite grain size and transformation temperature controlled the nodule size, while the colony size was dependent on the latter only.

Sintered valve guide and manufacturing method thereof

Abstract: Disclosed is a sintered valve guide guide formed of a sintered alloy consisting essentially of 3.5 to 5 % copper, 0.3 to 0.6 % tin, 0.04 to 0.15 % phosphorus, 1.5 to 2.5 % carbon and the balance iron, by mass, and as occasion needs, further containing 0.46 to 1.41 % metal oxide, and MnS and/or magnesium silicate. The metallographic structure has: a matrix containing a pearlite phase, a Fe-P-C compound phase and a Cu-Sn alloy phase; pores; and a graphite of 1.2 to 1.7 % by mass of the sintered alloy. In the cross section, the ratio of the pearlite phase to the matrix is 90 area% or more, the ratio of the Fe-P-C compound phase is 0.1 to 3 area% of the cross section, the ratio of the Cu-Sn alloy phase to the cross section is 1 to 3 % by area, and the ratio of a portion of the Fe-P-C compound phase having a thickness of 15 microns or more is 10 area% or less of the whole Fe-P-C compound phase.

High strength thin steel sheet having high hydrogen embrittlement resisting property and high workability

Abstract: The present invention provides a high strength thin steel sheet that has high hydrogen embrittlement resisting property and high workability. The high strength thin steel sheet having high hydrogen embrittlement resisting property has a metallurgical structure after stretch forming process to elongate 3% comprises: (i) 1% or more residual austenite; 80% or more in total of bainitic ferrite and martensite; and 9% or less (may be 0%) in total of ferrite and pearlite in terms of proportion of area to the entire structure, wherein the mean axis ratio (major axis/minor axis) of the residual austenite grains is 5 or higher, or (ii) 1% or more residual austenite in terms of proportion of area to the entire structure; mean axis ratio (major axis/minor axis) of the residual austenite grains is 5 or higher; mean length of minor axes of the residual austenite grains is 1 μm or less; minimum distance between the residual austenite grains is 1 μm or less; and the steel has tensile strength of 1180 MPa or higher.