Showing papers on "Pearlite published in 2000"

Mechanical properities of an HSLA bainitic steel subjected to controlled rolling with accelerated cooling

Abstract: Controlled rolling followed by accelerated cooling was utilised in laboratory simulations to study the microstructure and mechanical properties of an HSLA low carbon bainitic steel. The effects of processing parameters, such as cooling start temperature and cooling rates, on the final microstructure and mechanical properties were studied. Optical microscopy and transmission electron microscopy were used to evaluate the complex microstructures consisting of polygonal ferrite, pearlite, bainite and martensite/retained austenite constituent. The use of the multiple regression analysis allowed establishment of the relationships between mechanical properties and accelerated cooling variables: cooling rates and cooling start temperatures.

Tensile stress-strain analysis of single-structure steels

Abstract: In an effort to establish a universal model to predict the mechanical properties from processing conditions, tensile tests have been conducted of four single-structure steels, namely, ferrite, pearlite, bainite, and martensite; the data obtained were analyzed in terms of the Ludwik, Hollomon, and Swift equations to characterize their work-hardening behavior It was found that the differential Crussard-Jaoul (C-J) analysis, based on the Ludwik equation, can describe the work-hardening behavior of these steels fairly well The differential C-J analysis has shown that the ferrite and pearlite steels deform with two stages of work hardening, each stage associated with a distinctive value of the work-hardening exponent n Martensitic steels exhibit single-stage work hardening In bainite, the behavior was found to be dependent on transformation temperature; upper and lower bainite exhibit a behavior similar to pearlitic steels and quenched martensite, respectively This can be well understood in terms of the similarity of the corresponding microstructures On the basis of these results, the work-hardening behavior of single-structure steels falls into four categories, according to the n value This classification may serve as a useful guide to predict the flow behavior of steels with a known microstructure or to judge the microstructure merely by stress-strain curves, without microstructural observations

The role of the divorced eutectoid transformation in the spheroidization of 52100 steel

Abstract: Austenite containing fine particles of carbides may undergo the eutectoid transformation by either the pearlite reaction or the divorced eutectoid transformation (DET) reaction. The latter reaction automatically produces a spheroidized structure, while the pearlite reaction produces a structure that resists spheroidization. This study has evaluated the heat treatment conditions that promote the DET in 52100 steel. The results show that the DET is dominant over the pearlite reaction for austenitization at temperatures of 830 °C and below and for cooling rates of 500 °C/h and below. This information should be useful in the design of more energy efficient heat treatment processes for spheroidization annealing than the very long time processes currently recommended in the literature.

Cleavage initiation in Ti-V-N and V-N microalloyed ferritic-pearlitic forging steels

Abstract: The effects of silicon (053 and 105 wt%) and titanium (<0002 and 0022 wt%) on microstructure and mechanical properties of vanadium microalloyed medium carbon steels heat treated after rolling to simulate the thermal cycle of hot forging have been determined using room temperature tensile tests, impact tests, optical microscopy and scanning electron microscopy (SEM) Silicon was found to increase strength values whilst titanium had a strong refining action on prior austenite grain size Room temperature Charpy ‘U’ notch impact energies were all on the lower shelf; ductile–brittle transition temperatures, determined from fracture appearance in Hounsfield impact tests, ranged from 100 to 145°C, scaling with material strength Initiation in the Charpy tests was by microcracking of coarse (Ti,V)(C,N)-containing single or multi-phase inclusions except in the low strength, titanium-free case when the absence of a completely continuous grain boundary ferrite layer allowed matrix microstructure initiation by interfacing pearlite colonies to occur

Micromechanical modeling of ferrite-pearlite steels using finite element unit cell models

Abstract: An axisymmetric unit cell model based on a regular array of second-phase particles arranged on a BCC lattice is used to study deformation mechanisms of ferrite-pearlite structural steels. Microstructural characteristics of the steels were parameterized by the pearlite volume fraction, the aspect ratio of the pearlite particles, and the neighboring factor, which represents the ratio of interparticle spacing in the longitudinal direction to that in the transverse direction. FE analyses were carried out to investigate the macroscopic and microscopic response of unit cells with morphological features based on idealizations of the microstructures of the actual steels. Tensile properties of each constituent phase were obtained experimentally and used in the analyses. As compared to traditional axisymmetric models, the BCC cell model appears to be able to capture more realistically the behavior of the materials, and it accurately estimates the tensile behavior of the ferrite-pearlite steels even with a relatively large volume fraction of the pearlite phase. The effects of volume fraction and morphology of the second-phase particles on deformation behavior were also investigated.

The eutectic carbides and creep rupture strength of 25Cr20Ni heat-resistant steel tubes centrifugally cast with different solidification conditions

Abstract: The eutectic carbides and creep rupture strength of 25Cr20Ni heat-resistant steel tubes centrifugally cast with different solidification conditions were investigated in detail. The results reveal that the eutectic carbides precipitated primarily at the dendrite and grain boundaries show various morphologies from the outer wall to the inner wall along radial direction of the cast tubes, consisting of the thin film-like carbides, the blocky carbides, the lamellar carbide clusters resembling the pearlite and the skeleton-like carbides. The initial solidification conditions have significant influences on the grain morphologies and the distribution of the eutectic carbides in the cast tubes. Increasing the cooling rate markedly promotes the development of the columnar grains and restrains the precipitation of the eutectic carbides, while an applied electromagnetic field during the centrifugal solidification induces a notable grain refining and a marked change of the precipitation zones of the eutectic carbides from the dendrite boundaries to the grain boundaries. The applied electromagnetic markedly improves the creep rupture strength of the centrifugal cast 25Cr20Ni heat-resistant steel tubes. The relationships between the initial solidification conditions and the solidification processes as well as the creep rupture strength were discussed at length.

Specific dislocation multiplication mechanisms and mechanical properties in nanoscaled multilayers: The example of pearlite

Abstract: Dislocation mechanisms responsible for mechanical properties of nanoscaled multilayers are studied using the example of heavily drawn pearlitic steel wires. Two specific types of dislocation source are observed during transmission electron microscopy in situ straining experiments: one dislocation source operates at wide interlamellar spacings by dislocation propagation in ferrite around cementite islands; the other consists of dislocations bulging in ferrite from interfaces, and is more likely to operate at narrow interlamellar spacings. The constitutive law of the material is derived from a balance between dislocation multiplication and annihilation rates, and predicts a linear dependence of the flow stress with the reciprocal interlamellar spacing, and a stress saturation for very small spacings.

Aging behaviour of 25Cr-17Mn high nitrogen duplex stainless steel

Abstract: The precipitation behaviour of a nickel free stainless steel containing 25% chromium, 17% manganese and 0.54% nitrogen, with duplex ferritic‐austenitic microstructure, was studied using several complementary techniques of microstructural analysis after aging heat treatments between 600 and 1 000°C for periods of time between 15 and 6 000 min. During aging heat treatments, ferrite was decomposed into sigma phase and austenite by a eutectoid reaction, like in the Fe‐Cr‐Ni duplex stainless steel. Chromium nitride precipitation occurred in austenite, which had a high nitrogen supersaturation. Some peculiar aspects were observed in this austenite during its phase transformations. Chromium nitride precipitation occurred discontinuously in a lamellar morphology, such as pearlite in carbon steels. This kind of precipitation is not an ordinary observation in duplex stainless steels and the high levels of nitrogen in austenite can induce this type of precipitation, which has not been previously reported in duplex stainless steels. After chromium nitride precipitation in austenite, it was also observed sigma phase formation near the cells or colonies of discontinuously precipitated chromium nitride. Sigma phase formation was made possible by the depletion of nitrogen in those regions. Time‐temperature‐transformation (precipitation) diagrams were determined.

Effects of titanium addition and section size on microstructure and mechanical properties of compacted graphite cast iron

Abstract: Titanium is an anti-spheroidizing element and also carbide former in ductile iron. On the other hand, increasing the casting size essentially lowers the cooling rate that opposes the chilling tendency of titanium. This research was to study the combined effects of titanium and section size on their promotion of compacted graphite (CG) formation, and at the same time how matrix constituents were altered in heavy-wall castings. It was found that at the increasing casting thickness of 30 mm, 65 mm and 80 mm, the percentage of CG increased while that of pearlite decreased either with or without titanium addition. However, titanium (added in an amount of 0.15 wt%) effectively promoted the formation of CG by over 10% and at the same time increased the pearlite content in the matrix. This was especially true in the thinner 30 mm casting. Irons with titanium addition exhibited a bit lower Brinell hardness, elongation, and impact toughness due probably to the higher CG percentage that facilitated easier crack propagation. However, comparing to the un-alloyed iron, fracture toughness increased along with tensile strength for iron with titanium addition in all casting sizes of 30–80mm. The higher pearlite content in the matrix has overridden the effect of increased CG percentage such that tensile strength and KIC value both increased.

Effect of microstructure of low carbon steels on ultrasonic attenuation

Abstract: The ultrasonic attenuation in low carbon steel with 0.04 wt% C to 0.80 wt% C was measured over a frequency range of 5 to 15 MHz, and the effects of the carbon content and normalizing temperature were analyzed. In pure iron, the attenuation is determined from the average grain size, which increases as the normalizing temperature increases; there is a noticeable effect caused by a few large grains. In the case of the hypoeutectoid steels, the proeutectoid ferrite grain, the size of which depends on prior austenite grain size, acts as the main scatterer. The prior austenite grain size increases as the carbon content decreases and the normalizing temperature increases. The colony is responsible for scattering in the eutectoid steel; scattering by pearlite is greater than that by ferrite.

Structure of laser remelted surface of cast irons

Abstract: Grey and white cast irons were surface remelted using a high power CO2 laser with a scanning velocity in the range 0·5–200 mm s-1 and the resulting surface structures were investigated using optical microscopy and X-ray diffraction. The remelted surface of the white iron was composed of austenite dendrites, which decompose to pearlite at low cooling rates and martensite at medium cooling rates. The maximum volume fraction of martensite was obtained at 10 mm s-1. The lattice parameter of the austenite increased with increasing solidification velocity owing to solute trapping of carbon. The remelted surface of the grey iron was composed of cellular ledeburite and X-ray diffraction analyses of the grey specimen indicated the presence of austenite, martensite, graphite, cementite, and ɛ phases. The initial austenite partially transformed to martensite at high cooling rates and to ɛ phase at low cooling rates. The appearance of the ɛ phase is therefore linked to the decomposition of supersaturated aust...

Mössbauer study of the retained austenitic phase in multiphase steels

Abstract: Samples of steels with composition 0.30%C-1.5%Mn-1.5%Si-0.5%Al-0.5%Mo (wt.%) were subjected to different thermomechanical treatments to produce ferrite/pearlite/bainite (FPB), spheroidized (ESF) and martensite (MAR) microstructures. Subsequently they underwent a two stage annealing to obtain a final structure comprising of ferrite, bainite, martensite and austenite. The samples were studied by means of Mossbauer spectroscopy (transmission and conversion electron Mossbauer spectrocopy (CEMS)), X-ray diffraction (XRD), and metallographic analysis. Austenite contents were found to be the same for all samples except for the spheroidized sample annealed at 750°C that showed an increase of the austenite with increasing temperature of the treatment. Mossbauer spectroscopy and quantitative XRD analysis exhibited significant discrepancies ascribed to texture effects. It is shown that the thermal treatment was successful in retaining significant quantities of the austenite phase for steels of this composition.

Atmospheric Corrosion Performance of Quenched-and-Tempered, High-Strength Weathering Steel

Abstract: Eight-year atmospheric corrosion tests of A 588B (UNS K12040) weathering steel were conducted in industrial, marine, and rural environments. The material was tested following two types of heat treatment: quenched-and-tempered to produce a tempered martensite microstructure and normalized to produce a microstructure comprised of ferrite and pearlite. Results of these tests indicated that these heat treatments had no effect on the corrosion resistance and that the performance of weathering steels can be estimated solely on the basis of composition.

Hot dip zinc-plated steel sheet with excellent strength- flangeability and production method for the same

Abstract: PROBLEM TO BE SOLVED: To provide a hot dip zinc-plated steel sheet having excellent strength flange formation. SOLUTION: This high strength hot rolled steel sheet is a hot dip zinc-plated steel sheet containing, by mass, 0.04-0.1% carbon, 0.7% or less silicon, 1.3-2.3% manganese, 0.05% or less aluminum, 0.02-0.05% niobium, 0.07% or less phosphorus, 0.01% or less tin, 0.007% or less nickel and the balance substantially iron, and featuring the structure that pearlite or cementite with grain diameter of 5 μm or less is dispersed within ferrite grain boundary which is composed of a matrix of ferrite grains with the average grain diameter of 20 μm or less.

High carbon steel sheet excellent in formability

Abstract: PROBLEM TO BE SOLVED: To produce a high carbon steel sheet excellent in formability. SOLUTION: This high carbon steel sheet excellent in formability has a composition containing, by mass, 0.15 to 0.45% C, ≤0.25% Si, 0.3 to 1.2% Mn, ≤0.02% P, ≤0.02% S, 0.01 to 0.1% Al and ≤0.008% N, containing, at need, one or more kinds of 0.01 to 0.06% Ti and 0.0005 to 0.005% B, and the balance Fe with inevitable impurities, in which the fractional ratio of pearlite + cementite is ≤10%, and also, the average grain size of ferritic grains is 10 to 20 μm, and the steel sheet is suitable for a drive plate or a recliner gear as one kind of automotive parts. COPYRIGHT: (C)2001,JPO

Failure modes and materials performance of railway wheels

Abstract: In this study, the failure modes of cartwheel and mechanical properties of materials have been analyzed. The results show that rim cracking is always initiated from stringer-type alumina cluster and driven by a combination effect of mechanical and thermal load. The strength, toughness, and ductility are mainly determined by the carbon content of wheel steels. The fatigue crack growth resistance is insensitive to composition and microstructure, while the fatigue crack initiation life increases with the decrease of austenite grain size and pearlite colony size. The dynamic fracture toughness, KID, is obviously lower than static fracture toughness, KIC, and has the same trend as KIC. The ratio of KID/σYD is the most reasonable parameter to evaluate the fracture resistance of wheel steels with different composition and yield strength. Decreasing carbon content is beneficial to the performance of cartwheel.

Medium-high carbon steel excellent in cold forgeability

Abstract: PROBLEM TO BE SOLVED: To produce a medium-high carbon steel excellent in cold forgeability even as non-refined and capable of securing strength not by the addition of special alloy components but by the structural control of the steel. SOLUTION: This medium-high carbon steel excellent in cold forgeability is the one contg., by mass, 0.40 to 1.0% C, in which the value of Ceq (=C+Si/24+Mn/6+(Cr+Mo)/5+Ni/40+(Ti+Nb+V)/5+5×B) is also controlled to 0.6 to 1.2%, furthermore, the main structure is formed of pearlite and/or pseudo- pearlite, the volume ratio of the 2nd 6 phases composed of one or more kinds among ferrite, bainite, martensite, and pro-eutectoid cementite is controlled to <=7%, the size of pro-eutectoid ferrite precipitated into old austenite grains is, controlled to <=4 μm, the nodule size of the pearlite is controlled to 4.0 to 20 μm, and the lamellar interval of the pearlite is controlled to <=120 nm.

Modelling of austenite decomposition of hot‐rolled plain carbon steels under complex cooling conditions

Abstract: The purpose of the present work is to develop a mathematical model allowing the simultaneous prediction of both transformation product portions and mean ferrite grain size from the same common principles as a result of austenite decomposition during continuous cooling of plain carbon steels. The transformation products considered specifically are polygonal ferrite and pearlite. The model is based on the classical equations of nucleation-growth theory and also contains some empirical parameters. The chemical driving forces for nucleation and composition of elements at the phase interfaces are derived from thermodynamic analysis. Three modes of ferrite nucleation are taken into account that correspond to the nucleation on the austenite grain corners, edges and faces. The model considers the reduction of the nucleation sites due to the occupation of austenite grain boundary surface by ferrite grains. Pearlite transformation starts at the γ/α interface and suppresses further ferrite grain growth. The parameters related to ferrite reaction were determined on the basis of a series of austenite transformation kinetic curves and grain size measurements for a steel with the composition 0.084%C-0.58%Mn-0.02%Si obtained by dilatometric technique for cooling rates from 0.032 to 2.5 K/s. The parameters related to pearlite reaction were determined on the basis of the data for a steel with 0.66%C. After determination of the model parameters the model was applied to complex cooling conditions of the run-out table of the hot strip mill at Voest-Alpine Stahl Linz GmbH. Predicted ferrite grain size appeared to be 1.2 - 1.3 times smaller than the observed one. With regard to experimental data on grain growth in iron, it was suggested that the underestimation of grain size is due to additional ferrite grain growth occurring after the coiling of the steel sheet. Taking that into account provided satisfactory agreement with observed values.