Pearlite Reaction Research Articles

The Pearlite Reaction in Steels Mechanisms and Crystallography: Part I. From H. C. Sorby to R. F. Mehl

The mechanisms for the formation of pearlite in steels are reviewed with particular reference to the work of R. F. Mehl and colleagues. The role of crystallography in the early theories, and inherent in the mechanisms of Mehl et al., is also discussed. Certain inconsistencies are shown to exist between Mehl’s ideas and more current concepts. A description of Mehl’s pioneering work on quantifying the pearlite reaction in terms of nucleation and growth rates and of interlamellar spacings also is presented.

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

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.

A microscopic investigation of the precipitation phenomena observed during the pearlite reaction in vanadium alloyed carbon steels

During the isothermal pearlitic transformation in vanadium alloyed low and medium carbon steels the formation of proeutectoid ferrite precedes that of pearlitic formation. Within this proeutectoid ferrite interphase precipitation of vanadium carbide (VC) occurs in a form of linear uniformly sized and shaped arrays of particles. When pearlite subsequently forms, two distinctive pearlitic morphologies exist. Lamellar and non-lamellar spheroidised types of pearlite. Within the pearlitic constituent of both of these pearlitic microstructures VC interphase precipitation occurs. However, in the lamellar type of pearlite curved arrays of uniformly sized and shaped VC particles whose direction is perpendicular to the cementite long axis are observed while in the non-lamellar type of pearlite apparently random distributions of VC particles exist. It was consistently found that VC interphase precipitation always occurred on the moving austenite/ferrite interphase boundaries at all temperature of isothermal pearlitic transformation.

Isothermal transformation diagrams for alloyed ductile cast iron

AbstractIsothermal transformation (IT) diagrams which were determined metallographically are presented for a ductile cast iron alloyed with manganese, molybdenum, and copper, following austenitisation at 870 and 920°C. Isothermal transformations were conducted over the temperature range 275–650°C for times between 0·5 and 120 min. The IT diagrams displayed a nose at ~650 and 425°C for the pearlite and upper bainite reactions respectively. The diagrams could be separated into two overlapping diagrams corresponding to transformations in the eutectic cell and intercellular regions. When compared with previous data for an unalloyed iron, the alloying additions were found to delay the transformation, and to a greater extent above 450°C than below this temperature. Increasing the austenitising temperature was observed to delay the transformation. The effect of a 50 K increase was equivalent to increasing the total alloy content in the eutectic cell area by about 0·4 wt-% in this alloy.MST/2040

Influence of microalloying additions on thickness of grain boundary carbides in ferrite–pearlite steels

AbstractFor a series of plain C and microalloyed steels at two levels of Mn, the growth of grain boundary carbides has been monitored after heating to 920°C and cooling at 40 and 150 K min−1 through the austenite–ferrite/pearlite transformation down to room temperature. In pearlite free steels, on cooling to room temperature, all the C in solution in the ferrite is able to precipitate as carbides at the boundaries and the grain boundary carbide thickness is dependent on the number of nucleation sites for precipitation. Increasing the cooling rate increases the number of sites and reduces the carbide thickness. In ferrite–pearlite steels, the grain boundary carbides form the ‘tails’ to the pearlite colonies. The thickness of the grain boundary carbide is related to the pearlite reaction, since the temperature at which this occurs controls both the thickness of the carbide nuclei and the amount of C available for precipitating out on these tails. Increasing the cooling rate and Mn content causes a decrease ...

The interaction between proeutectoid ferrite and austenite during the isothermal transformation of two low-carbon steels — a new model for the decomposition of austenite

Both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) have been employed to examine the austenite to proeutectoid ferrite and ferrite/carbide reactions in two low-carbon (0.04 wt%) steels. It is demonstrated that proeutectoid ferrite (both polygonal and Widmanstatten) can “partition” the prior austenite grains into several smaller units or pools. It is also shown that prior to the initiation of the pearlite reaction, ferrite grain growth can occur. The pools of austenite exert a Zener-like drag force on the migrating ferrite grain boundaries. However, the ferrite boundaries can eventually break away and small pools of austenite become completely embedded in single proeutectoid ferrite grains. Subsequently, these small pools of austenite transform to discrete regions of cementite, together with epitaxial ferrite. Conversely, certain small pools remain in contact with the ferrite grain boundaries and it is considered that transformation of these latter pools will eventually lead to the formation of massive films of cementite at the ferrite grain boundaries. Larger pools of austenite prevent ferrite boundary breakaway, and these latter, austenitic regions eventually transform to pearlite.

Banding and the nature of large, irregular pearlite nodules in a hot-rolled low-alloy plate steel: A second report

The microstructure of a hot-rolled low-carbon plate steel has been examined using a combination of light microscopy and scanning electron microscopy. It has been found that in the hot-rolled condition, the microstructure consists of alternate bands of ferrite and pearlite, together with relatively large, irregular pearlite nodules. These large nodules were found to be comprised of pearlite, intragranularly nucleated ferrite (both Widmanstatten and idiomorphic), together with carbide-deficient and/or carbide-free regions. It is argued that the carbide-deficient and carbide-free regions form as a result of the premature initiation of the pearlite reaction, i.e. pearlite forms prior to the body of the austenite grains attaining the eutectoid composition. In order to model the formation of the banded structure, specimens were reaustenitized at 1050 °C for 10 min and furnace cooled. This heat-treatment cycle produced an austenite grain size which was less than the chemical banding wavelength. A model for the decomposition of austenite, under these conditions, is presented.

Growth of grain boundary carbides in C–Mn steels

AbstractThe growth of grain boundary carbides has been followed during continuous cooling from the austenite using cooling rates of 7 and 150 K min−1 and Mn levels of 0·5 and 1 wt-%. The isothermal growth of grain boundary carbides in the temperature range 600–650°C has also been examined and growth was found to obey a (time)1/4 relation which is very slow. The activation energy for growth increased from 241 kJ mol−1 for the lower Mn containing steel to 293 kJ mol−1 for the higher Mn steel. For continuous cooling conditions growth was very rapid. Grain boundary carbides started to form at the time of the pearlite reaction and their initial thickness corresponded to the thickness of lath carbides in the pearlite colonies. The grain boundary carbides are believed to form the tails of the pearlite colonies and probably initiate the pearlite reaction. Increasing the cooling rate or Mn content reduced the lath carbide thickness within the pearlite colonies. Subsequent growth of the grain boundary carbides is b...

Mechanisms of the bainite (non-lamellar eutectoid) reaction and a fundamental distinction between the bainite and pearlite (lamellar eutectoid) reactions

A Hillert analysis of the edgewise growth of pearlite is modified by assuming that α and β, the products of the eutectoid decomposition of a γ phase matrix, grow by means of the ledge mechanism and by temporarily decoupling α and β growth kinetics. When equilibrium proportions of α and β form, the growth rates of the two product phases, G α and G β , are found to be equal only when h α λ α = h β λ β , where h = ledge height and λ = inter-ledge spacing. This result reproduces the Hackney-Shiflet observation that ferrite and cementite plates in pearlite formed in an FeCMn alloy lengthen by means of shared growth ledges. When h α λ α ≠ h β λ β , G α ≠ G β ; hence one product phase, say, α, grows more rapidly than β and usually interrupts contact between β and γ. Thus the repeated re-nucleation of β crystals at α−γ boundaries is required. This result is consistent with the Lee-Aaronson observations that the ledge structure on eutectoid α crystals is different from that on eutectoid TiCr 2 crystals in bainite formed in a hypereutectoid TiCr alloy. This interfacial structure criterion for eutectoid decomposition by the microstructural bainite mechanism provides a basis for explaining the very wide variations in the microstructure of bainite which can develop in different alloy systems and even within a particular alloy. The external morphologies of bainite usually observed are concluded to derive from the morphology of the proeutectoid crystals serving as their “substrate”. The much less frequently found nodular morphology is proposed to be the true or “intrinsic” external morphology of bainite, and to appear only in the absence of appreciable intervention by the proeutectoid phase in the development of the bainite structure.

Effect of austenization conditions on austenite decomposition kinetics and mechanical characteristics of Cr?Ni?Mo steels

1. Austenization of 30KhN4MF-type steels in the intercritical temperature range accelerates the supercooled austenite decomposition under isothermal conditions by the bainite as well as by the pearlite reaction. 2. Heating 30KhN4MF-type steels in the intercritical temperature range (700°C), holding isothermally in the pearlite zone (600°C) and water-cooling, produces a uniform, very fine structure, with a good combination of mechanical characteristics: σult ∮ 1150 MPa, σ0.2 ∮ 830 MPa δ≅18%; ϕ≅50%, a1+20≅1.15 MJ/m2; a1−40≅1.8 MJ/m2, which are similar to the characteristics of steels after toughening.

Kinetics of austenite-ferrite and austenite-pearlite transformations in a 1025 carbon steel

Isothermal and continuous-cooling transformation kinetics have been measured dilatometrically for the γ → α+ γ′ and γ′→ P reactions in a 1025 steel. The isothermal transformation of austenite for each reaction was found to fit the Avrami equation after the fraction transformed was normalized to unity at the completion of the reaction and a transformation-start time was determined. The transformation kinetics under isothermal conditions therefore were characterized in terms of then andb parameters from the Avrami equation together with the transformation-start times. The parametern was found to be independent of temperature over the range studied (645 to 760 ‡C) and to have values of 0.99 and 1.33 for the ferrite and pearlite reactions, respectively. This indicates that the nucleation condition is essentially constant and site saturation occurs early in the transformation process. The continuous-cooling experiments were conducted at cooling rates of 2 to 150 ‡C per second to determine the transformation-start times for the ferrite and pearlite reactions and the completion time for transformation to pearlite under CCT conditions. Both reactions were found to obey the Additivity Principle for continuous cooling provided that the incubation (pre-transformation) period was not included in the transformation time. The isothermal transformation data and CCT transformation-start times have been incorporated in a mathematical model to predict continuous-cooling transformation kinetics that agree closely with measurements made at three cooling rates.

Very-high spatial resolution analysis in STEM

When analyzing a composition profile in a thin foil using a Scanning Transmission Electron Microscope, the measured profile is a convolution of the electron probe diameter, the actual profile and the degree of beam spreading in the sample. While it is possible to show that fractions of a monolayer of segregant are detectable in foils 100 nm thick, similar calculations indicate that if a probe 0.8 nm in diameter is incident upon a foil 20 nm thick, then broadening is insignificant. For example, fig. 1 is a computation (using the method of Hall, et.al) of the predicted measured profile resulting from a real gaussian distribution of chromium in iron, 2.5 nm wide, measured in the conditions just mentioned. Such results indicate that distributions of solutes well under 10 nm in separation should be readily distinguishable from each other.In some pearlitic steels, alloy elements are incompletely partitioned during the pearlite reaction, and subsequently diffuse from the ferrite into the cementite, where they form very narrow (∿ 2 nm wide) enriched zones either side of the cementite plate, which is itself around 10 nm thick.

Pearlite transformation during continuous cooling and its relation to isothermal transformation.

An attempt was made to predict the cooling transformation over the entire range of pearlite reaction, from the isothermal transformation kinetic data. A kinetic equation of pearlite transformation during cooling was derived by applying an additivity rule to the transformed fractions. A good agreement was observed between the measured and calculated cooling transformation curves although the pearlite reaction would not precisely satisfy any one of the additive conditions. A possible deviation of a cooling transformation kinetics from that estimated with the additivity rule was considered based on the nucleation and growth process. It was found that the transformed fractions of pearlite appeared to be additive firstly because the change in the ratio of the nucleation to the growth rate as a function of supercooling was small, and secondly because the cooling transformation occurred either nearly isokinetically when the cooling rate was slow or almost in the condition of site saturation when the cooling rate was fast.

Quantitative analytical electron microscopy of metals and minerals

For a thin specimen X-ray absorption and fluorescence can, to a first approximation, be ignored and observed characteristic X-ray intensity ratios, I A/I B, can be converted into weight fraction ratios, C A/C B by multiplying by a constant, k AB: C A C B = k ABI A I B . K AB values can be calculated or determined experimentally. X-ray absorption within the sample must be considered when there is a significant difference in the energy of the characteristic X-rays. Fluorescence effects due to continuum X-rays can be ignored in thin specimens and characteristic fluorescence is much less than that observed in bulk specimens. Expressions are given for both the absorption and fluorescence corrections. Beam spreading has been calculated in a mineral at 100 kV using models based on Rutherford-scattering, plural-scattering and Monte-Carlo theory. The calculations are compared with experimental results. The application of AEM to metals is illustrated by the study of partitioning of Si, Mn and Cr in the pearlite reaction. Because most minerals are composed predominantly of light elements, absorption and fluorescence can be ignored for a wide range of thicknesses. Applications in mineralogy are illustrated by examples from particle analysis (e.g., asbestos) and by the study of the precipitation of a cellular intergrowth of pyroxene and spinel in olivine.

Chromium partitioning during isothermal transformation of a eutectoid steel

Partitioning of chromium between ferrite and cementite during the isothermal decomposition of austenite to pearlitic or pearlitic/bainitic decomposition products has been studied in a 1.4 wt pct Cr eutectoid steel using analytical electron microscopy on two-stage extraction replicas. Chromium was observed to segregate preferentially to cementite at the pearlite reaction front for temperatures in the range 730 to 550 °C. Although the extent of partitioning decreased with decreasing reaction temperature, a no-partition temperature could not be identified for the steel. It is clear that previous studies on thin foils have underestimated the temperature range over which partitioning of chromium can occur. At high reaction temperatures measured values of pearlite growth rates were found to be in excellent agreement with those calculated, using the assumption that phase boundary diffusion of chromium was rate controlling. At lower reaction temperatures models based on volume diffusion of carbon and on phase boundary diffusion of chromium both gave reasonable predictions of measured growth rates. However, it seems likely that solute drag effects influence pearlite growth at temperatures in the austenite bay region which the chromium addition produces in the T.T.T. diagram. Measurements made on upper bainite which co-existed with pearlite following transformation at 500 and 550 °C showed that preferential partitioning of chromium to cementite did not occur during this reaction.

Cu-Be合金における共析変態の第2段階について

The behaviour and kinetics of cell growth in a secondary coarse lamellar pearlite reaction in Cu-22.6 at%Be alloy have been studied principally by metallographic observations. The variations in cell radius, interlamellar spacing and volume fraction of γ phase in pearlite cells with ageing time have been determined by quantitative metallographic measurements of the specimens aged at temperatures from 673 to 823 K.The results obtained are as follows:(1) After initial, fine lamellar pearlite reaction has almost completed, coarse lamellar pearlite reaction occurs wherein the fine pearlite is replaced by the coarse pearlite.(2) Cell growth rate of coarse pearlite decreases, and the interlamellar spacing increases with ageing time.(3) Interlamellar spacing of coarse pearlite is 4∼5 times larger than that of fine pearlite in every ageing time.(4) Be concentration of α phase in fine pearlite is supersaturated at the initial ageing stage, then decreases with ageing time and attains an equilibrium value prior to the occurrence of a coarse pearlite reaction. Then it is considered that there are no chemical free energy changes during the cell growth of coarse pearlite.(5) Grain boundary diffusivities, evaluated from the discontinuous coarsening model proposed by Livingston and Cahn are reasonable. Therefore, it can be concluded that the cell growth of coarse pearlite occurs by the discontinuous coarsening mechanism.

黒鉛核発生に対する前熱処理の役割

A systematic explanation of nucleation behavior shown in the foregoing two papers is attempted. From the view point that void forming process plays an important role in forming graphite nodules, an assumption was introduced that “if there were expanding stresses in the matrix, i.e. the tendency to make voids, at the temperatures possible to graphitize, the nucleation of graphite is significantly enhanced”. Explanation, using this assumption, of the experimental facts, revealed that there are two types of nucleation mechanism in the pre-heattreated specimens: (1) Ac1 mechanisms, which are observed in the specimens pre-quenched from the temperatures below Ac3, utilize the expanding stresses in the austenite region, associated with the pearlite→austenite reaction, and the graphite nucleate in the austenite (martensite) regions. (2) Ar1 mechanisms, which are observed in the specimens cooled rather slowly through the Ar1 temperature (air or funace cooled), utilize the expanding stress in the ferrite, associated with the austenite→pearlite reaction, and the graphite nucleate in the pro-eutectoid ferrite regions. Discussions were made on the growth and the disappearance of “defects”, which are expected to exist in the pre-treated specimens and to start growth as graphite nodules after some incubation time, and also on the role of the “expanding stresses” introduced by pre-treatments.

Kinetics of the pearlite reaction in Fe−C−Cr

Velocity and spacing measurements on the pearlite reaction in three eutectoid alloys of Cr (0.4, 0.9 and 1.8 wt pct) have been recorded and compared with growth and spacing equations based on a variety of thermodynamic and kinetic models. At high temperatures the reaction is controlled by phase boundary diffusion of Cr while at lower temperatures, a local equilibrium no-partition model with kinetic control by carbon diffusion best represents the data. At intermediate temperatures there is a smooth transition rather than a sharp break betwewn the two regimes. There is a discrepancy between theory and experiment on the slope of the spacingvs inverse under cooling plot which can be removed by assuming that the ferrite-cementite epitaxy improves with the addition of chromium.

Thermodynamics and phase equilibria for the Fe−C−Cr system in the vicinity of the eutectoid temperature

The low chromium Fe−C−Cr phase equilibria pertinent to the eutectoid transformation of austenite have been modeled and calculated from published thermodynamic information and verified through a series of selected equilibration experiments. In particular, α-γ, α-cm and γ-cm partition coefficients have been measured within the range 700 to 770°C and compared with austenite and carbide models proposed by Hillert and coworkers. While the theoretical-empirical closure is not perfect, the models are proven to be completely adequate for incorporation within the kinetic description of the corresponding ternary pearlite reaction.

Phase stability of Co<inf>5</inf>Sm

Diffusion couples formed between cobalt and samarium provide useful structures for studying the Co <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</inf> Sm eutectoid decomposition. The diffusion zones consist of Co, Co <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">17</inf> Sm <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> , Co <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</inf> Sm, Co <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</inf> Sm <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> , Co <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sm, and Co <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> Sm when formed in the 820° to 1130°C range. The absence of the Co <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</inf> Sm phase in the 775° couple provides supporting evidence for the eutectoid transformation and locates the eutectoid temperature at about 800°C. Couples formed above 820°C and then heat-treated below 800°C provided information on the eutectoid transformation which does not follow the classic Fe-C pearlite reaction. The possible effects of the decomposition on properties of permanent magnets are discussed.