Direct observations of the <Emphasis Type="Italic">α</Emphasis> → <Emphasis Type="Italic">γ</Emphasis> transformation at different input powers in the heat-affected zone of 1045 C-Mn steel arc welds observed by spatially resolved X-ray diffraction

Spatially resolved X-ray diffraction (SRXRD) experiments have been performed during gas tungstenarc (GTA) welding of AISI 1045 C-Mn steel at input powers ranging from 1000 to 3750 W. In-situ diffraction patterns taken at discreet locations across the width of the heat-affected zone (HAZ) near the peak of the heating cycle in each weld show regions containing austenite (γ), ferrite and austenite (α+γ), and ferrite (α). Changes in input power have a demonstrated effect on the resulting sizes of these regions. The largest effect is on the γ phase region, which nearly triples in width with increasing input power, while the width of the surrounding two-phase α+γ region remains relatively constant. An analysis of the diffraction patterns obtained across this range of locations allows the formation of austenite from the base-metal microstructure to be monitored. After the completion of the α → γ transformation, a splitting of the austenite peaks is observed at temperatures between approximately 860 °C and 1290 °C. This splitting in the austenite peaks results from the dissolution of cementite laths originally present in the base-metal pearlite, which remain after the completion of the α → γ transformation, and represents the formation of a second more highly alloyed austenite constituent. With increasing temperatures, carbon, originally present in the cementite laths, diffuses from the second newly formed austenite constituent to the original austenite constituent. Eventually, a homogeneous austenitic microstructure is produced at temperatures of approximately 1300 °C and above, depending on the weld input power.     

High temperature plastic-flow behavior of mixtures of austenite,cementite, ferrite,and pearlite in plain-carbon steels

The plastic-flow behavior of ferrite + pearlite, pearlite + cementite, and austenite + cementite mixtures in plain carbon steels has been examined over the temperature range 500 to 1050 °C, strain-rate range 6 x l0⁻⁶ to 2 x l0⁻² s⁻¹, and carbon range 0.005C to 1.89C. Up to the eutectoid temperature the strength of the ferrite + pearlite mixture more than doubles as the carbon content increases from 0.005C to 0.7C, so that whereas in low-carbon steels the ferrite is weaker than the higher temperature austenite phase, in eutectoid steels the fully pearlitic structure is stronger than the fully austenitic structure. Manganese and silicon strengthen ferrite more effectively than they do austenite. A 0.17 pct phosphorus addition strengthens the ferrite + pearlite mixture across the range of microstructures from fully ferritic to fully pearlitic. Beyond the eutectoid composition, the amount of proeutectoid cementite does not significantly affect the strength of the pearlite, but above the eutectoid temperature it appreciably strengthens the austenite and cementite mixture at the strain rate of 2 X 10^-2 s^-1.     

Formation of austenite in 1.5 pct Mn steels

The formation of austenite from different microstructural conditions has been studied in a series of 1.5 pct Mn steels that had been heated in and above the intercritical (α+ γ) region of the phase diagram. The influence of variables such as cementite morphology, initial structural state of the ferrite and the carbon content has been assessed in terms of their respective effects on the kinetics of austenite formation and final microstructure. Austenite was found to form preferentially on ferrite-ferrite grain boundaries for all initial structures. The results of this study have shown that the 1.5 pct Mn has lowered both the AC₃ and AC₁, lines causing large amounts of austenite to form in low carbon steel. The kinetics of austenite formation at 725 °C were not only very slow but also were approximately independent of the amount formed. Austenite appeared to form slightly more rapidly from cold rolled ferrite than from recrystallized ferrite or ferrite-pearlite structures.     

Austenite stabilization from direct cementite conversion in low-alloy steels

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

The morphology,crystallography, and mechanism of carbide precipitation in an Fe-0.12 Pct C-3.28 Pct Ni alloy

Carbide precipitation during the eutectoid decomposition of austenite has been studied in an Fe-0.12 pct C-3.28 pct Ni alloy by transmission electron microscopy (TEM) supplemented by optical microscopy. Nodular bainite which forms during the latter stages of austenite decomposition at 550 °C exhibits two types of carbide arrangement: (a) banded interphase boundary carbides with particle diameters of about 20 to 90 nm and mean band spacings between 180 and 390 nm and (b) more randomly distributed (“nonbanded”) elongated particles exhibiting a wide range of lengths between 33 and 2500 nm, thicknesses of approximately 11 to 50 nm, and mean intercarbide spacings of approximately 140 to 275 nm. Electron diffraction analysis indicated that in both cases, the carbides are cementite, obeying the Pitsch orientation relationship with respect to the bainitic ferrite. The intercarbide spacings of both morphologies are significantly larger than those previously reported for similar microstructures in steels containing alloy carbides other than cementite (e.g., VC, TiC). Both curved and straight cementite bands were observed; in the latter case, the average plane of the interphase boundary precipitate sheets was near {110}_α//{011}_c consistent with cementite precipitation on low-energy {110}_α//{111}_γ ledge terrace planes (where α,β, andc refer to ferrite, austenite, and cementite, respectively). The results also suggest that the first stage in the formation of the nonbanded form of nodular bainite is often the precipitation of cementite rods, or laths, in austenite at the α:γ interfaces of proeutectoid ferrite secondary sideplates formed earlier. Although these cementite rods frequently resemble the “fibrous” microstructures observed by previous investigators in carbide-forming alloy steels, they are typically much shorter than fibrous alloy carbides. The bainitic microstructures observed here are analyzed in terms of a previously developed model centered about the roles of the relative nucleation and growth rates of the product phases in controlling the evolution of eutectoid microstructures.     

Metallography of bainitic transformation in silicon containing steels

The formation of carbide in lower bainite was studied in two silicon containing carbon steels by transmission electron microscopy and diffraction techniques. Epsilon carbide was identified in the low temperature isothermally transformed bainite structure. The crystallographic relationship between epsilon carbide and bainitic ferrite was found to follow the Jack orientation relationship,viz, (0001)ε l l(011)α, (101l)ε l 1(101)α. The cementite observed in lower bainite was in the shape of small platelets and obeyed the Isaichev orientation relationship with the bainitic ferrite,viz, (010) cl 1(1-11)α, (103) cl 1 (011)α. Direct evidence showing the sequence of carbide formation from aus-tenite in bainite has also been obtained. Based on the observations and all the crystallo-graphical features, it is strongly suggested that in silicon containing steels the bainitic carbide precipitated directly from austenite instead of from ferrite at the austenite/fer-rite interface as has been proposed by Kinsman and Aaronson (Ref. 1). The uniformity of the carbide distribution is thus envisaged to be the outcome of precipitation at the aus-tenite-ferrite interphase boundary. DER-HUNG HUANG, formerly with the Department of Materials Science and Mineral Engineering, University of California     

Formation of Austenite During Intercritical Annealing of Dual-Phase Steels

The formation of austenite during intercritical annealing at temperatures between 740 and 900 °C was studied in a series of 1.5 pct manganese steels containing 0.06 to 0.20 pct carbon and with a ferrite-pearlite starting microstructure, typical of most dual-phase steels. Austenite formation was separated into three stages: (1) very rapid growth of austenite into pearlite until pearlite dissolution is complete; (2) slower growth of austenite into ferrite at a rate that is controlled by carbon diffusion in austenite at high temperatures (~85O °C), and by manganese diffusion in ferrite (or along grain boundaries) at low temperatures (~750 °C); and (3) very slow final equilibration of ferrite and austenite at a rate that is controlled by manganese diffusion in austenite. Diffusion models for the various steps were analyzed and compared with experimental results.     

Modeling of kinetics of austenite-to-allotriomorphic ferrite transformation in 0.37C-1.45Mn-0.11V microalloyed steel

The present article is concerned with the theoretical and experimental study of the growth kinetics of allotriomorphic ferrite in medium carbon vanadium-titanium microalloyed steel. A theoretical model is presented in this work to calculate the evolution of austenite-to-allotriomorphic ferrite transformation with time at a very wide temperature range. At temperatures above eutectoid temperature, where allotriomorphic ferrite is the only austenite transformation product, the soft-impingement effect should be taken into account in the modeling. In that case, the Gilmour et al. analysis reliably predicts the progress of austenite-to-allotriomorphic ferrite transformation in this steel. By contrast, since pearlite acts as a carbon sink, the carbon enrichment of austenite due to the previous ferrite formation is avoided, and carbon concentration in austenite far from the α/γ interface remains the same as the overall carbon content of the steel. Hence, the soft-impingement effect should be neglected, and allotriomorphic ferrite is considered to grow under a parabolic law. Therefore, assumption of a semi-infinite extent austenite with constant boundary conditions is suitable for the kinetics of the isothermal decomposition of austenite. An excellent agreement (higher than 93 pct in R ²) has been obtained between the experimental and predicted values of the volume fraction of ferrite in all of the ranges of temperature studied.     

Modeling of kinetics of austenite-to-allotriomorphic ferrite transformation in 0.37C-1.45Mn-0.11V microalloyed steel

The present article is concerened with the theoretical and experimental study of the growth kinetics of allotriomorphic ferrite in medium carbon vanadium-titanium microalloyed steel. A theoretical model is presented in this work to calculate the evolution of austente-to-allotriomorphic ferrite transformation with time at a very wide temperature range. At temperatures above eutectoid temperature, where allotriomorphic ferrite is the only austenite transormation product, thesoft-impingement effect should be taken into account in the modeling. In that case, the Gilmouret al., analysis reliably predicts the progress of austenite-to-allotriomorphic ferrite transformation in this steel. By contrast, since pearlite acts as a carbon sink, the carbon enrichment of austenite due to the previous ferrite formation is avoided, and carbon concentration in austenite far from the α/λ interface remains the same as the overal carbon content of the steel. Hence, the soft-impingement effect should be neglected, and allotriomorphic ferrite is considered to grow under a parabolic law. Therefore, assumption of a semi-infinite extent austenite with constant boundary conditions is suitable for the kinetics of the isothermal decomposition of austenite. An excellent agreement (higher than 93 pct inR ²) has been obtained between the experimental and predicted values of the volume fraction of ferrite in all of the ranges of temperature studied. C. CAPDEVILA, Research Associate, formerly with the Department of Physical Metallurgy, Centro Nacional de Investigaciones Metalurgicas (CENIM), Consejo Superior de Investigaciones Cientificas (CSIC), 28040 Madrid, Spain     

On the Effect of Manganese on Grain Size Stability and Hardenability in Ultrafine-Grained Ferrite/Martensite Dual-Phase Steels

Two plain carbon steels with varying manganese content (0.87 wt pct and 1.63 wt pct) were refined to approximately 1 μm by large strain warm deformation and subsequently subjected to intercritical annealing to produce an ultrafine grained ferrite/martensite dual-phase steel. The influence of the Mn content on microstructure evolution is studied by scanning electron microscopy (SEM). The Mn distribution in ferrite and martensite is analyzed by high-resolution electron backscatter diffraction (EBSD) combined with energy dispersive X-ray spectroscopy (EDX). The experimental findings are supported by the calculated phase diagrams, equilibrium phase compositions, and the estimated diffusion distances using Thermo-Calc (Thermo-Calc Software, McMurray, PA) and Dictra (Thermo-Calc Software). Mn substantially enhances the grain size stability during intercritical annealing and the ability of austenite to undergo martensitic phase transformation. The first observation is explained in terms of the alteration of the phase transformation temperatures and the grain boundary mobility, while the second is a result of the Mn enrichment in cementite during large strain warm deformation, which is inherited by the newly formed austenite and increases its hardenability. The latter is the main reason why the ultrafine-grained material exhibits a hardenability that is comparable with the hardenability of the coarse-grained reference material.     

Aging and tempering behavior of iron-nickel-carbon and iron-carbon martensite

The aging at room temperature (RT) and the tempering behavior in the temperature range 293 to 973 K of ternary iron-nickel-carbon martensite (containing 14.4 at. pct Ni and 2.35 at. pct C) was investigated principally by using X-ray diffractometry to analyze changes in the crystalline structure and differential scanning calorimetry to determine heats of transformation and activation energies. These techniques also were used in the parallel study performed in this work of the tempering behavior of FeC martensite (containing about 4.4 at. pct C) in the temperature range 298 to 773 K. Analysis of the structural changes revealed that in both FeNiC and FeC the following processes occurred: (1) formation of carbon enrichments and development of a periodic arrangement of planar carbon-rich regions up to 423 K; (2) precipitation of ε/η transition carbide and transformation of a part of the austenite into ferrite under simultaneous enrichment with carbon of the remaining austenite (between 423 and 523 K); (3) decomposition of the retained austenite into ferrite and cementite between 523 and 723 K (only partly for FeNiC); (4) precipitation of cementite between 523 and 723 K; and (5) for FeNiC, reformation of austenite from ferrite and cementite above 773 K. A short comparative discussion concerning the first stage of martensite decomposition for FeC, FeNiC, FeN, and FeNiN martensites is given.     

Precipitation of carbides in low-carbon Fe-AI-C alloys

High-purity Fe-C alloys containing up to 5.8 pct Al were quenched from 730°C, then aged at temperatures up to 400°C. Precipitation of a carbide transition phase (presumably ε) and of cementite were followed by transmission electron microscopy, coercive force measurements, and changes in yield strength. The presence of aluminum increases the temperature at which cementite begins to precipitate during aging. This effect is similar to that produced by silicon in ferrite, and opposite to that produced by manganese. It is suggested that the effects of Si, Mn, and Al on cementite precipitation from ferrite arise from the effects of these elements on the stability of cementite, as reflected in the equilibrium partition of carbon between cementite and ferrite.     

Influence of Aluminum Alloying and Heating Rate on Austenite Formation in Low Carbon-Manganese Steels

This investigation focuses on the austenite formation process during continuous heating, over a wide range of heating rates (0.05 to 20 K/s), in three low carbon-manganese steels alloyed with different levels of aluminum (0.02, 0.48, and 0.94, wt pct Al). High resolution dilatometry, combined with metallographic observations, was used to determine the starting (Ac ₁) and finishing (Ac ₃) temperatures of this transformation. It is shown that both the aluminum content and the applied heating rate have a strong influence on this process. During fast heating (>1 K/s), the pearlite phase present in the initial microstructure remains almost unaffected up to temperature Ac ₁. On the contrary, during slow heating, cementite lamellas inside pearlite partially dissolve, this dissolution effect being more pronounced for the lower carbon and higher aluminum content steels. The changes in the initial microstructure during slow heating affect the austenite nucleation and growth processes. Furthermore, in the aluminum alloyed steels, slow heating conditions shift the Ac ₃ temperature to higher values. This shift is suggested to be due to aluminum partitioning from austenite to ferrite, which stabilizes ferrite and delays its transformation to higher temperatures. Thermodynamic calculations carried out with MTDATA software seem to support some of the experimental observations carried out under very low heating conditions close to equilibrium (0.05 K/s).     

The divorced eutectoid transformation in steel

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.     

Eutectoid Transformations in 4.12 Mass Pct Cr 0.88 Mass Pct C Steel

The sequence of eutectoid microstructures, obtained by lowering the temperature of the isothermal transformation, was studied in synthetic steel with 4.12 mass pct Cr 0.88 mass pct C. The results were compared with observations on plain carbon steels with 1.65 and 1.67 mass pct C. In both cases, the main features can be explained as an effect of a lowered temperature on the increasing supersaturation of cementite in austenite and an even stronger effect on that of ferrite. One distinction was a continuous change in the pearlite structure toward a more acicular structure. This structure is named acicular pearlite.     

A kinetic model of the γ → α + Gr eutectoid transformation in spheroidal graphite cast irons

The eutectoid transformation of austenite in spheroidal graphite cast iron can follow one of two paths: (a) transformation to a mixture of ferrite and graphite or (b) transformation to pearlite. The extents to which the two reactions occur determine the relative amounts of ferrite and pearlite in the microstructure and, hence, the properties of the iron. In this paper, the kinetics of the γ → α+ Gr reaction is studied, and a model is developed to predict the isothermal transformation rates. The transformation occurs at a rate determined by the rate of carbon diffusion. The diffusion of carbon through ferrite, as well as through austenite, has been considered. The model predicts that the volume fraction of austenite transformed isothermally increases with increasing number density of graphite spheroids. Predictions of the model are compared with data available in literature.     

The effect of additives on the eutectoid transformation of ductile iron

The eutectoid transformation of austenite in cast iron is known to proceed by both the meta-stable γ → α + Fe₃C reaction common in Fe-C alloys of near eutectoid composition, and by the direct γ → α + Graphite reaction, with the graphite phase functioning as a car-bon sink. In addition, the meta-stable cementite constituent of the pearlite can dissolve near the graphite phase (Fe₃C → α + Graphite), producing free ferrite. Isothermal trans-formation studies on a typical ductile iron (nodular cast iron) confirmed that all of these reaction mechanisms are normally operative. The addition of 1.3 pct Mn was found to substantially retard all stages of the transformation by retarding the onset of the eutectoid transformation, decreasing the diffusivity of carbon in ferrite, and stabilizing the cemen-tite. Minor additions of Sb (0.08 pct) or Sn (0.12 pct) were found to inhibit the γ →α + Graphite reaction path, as well as the Fe₃C → α + Graphite dissolution step, but did not significantly affect the meta-stable γ → α + Fe₃C reaction. Scanning Auger microprobe analysis indicated that Sn and Sb adsorb at the nodule/metal interphase boundaries during solidification. This adsorbed layer acts as a barrier to the carbon flow necessary for the direct γ → α + Graphite and Fe₃C → α + Graphite reactions. With the graphite phase dis-abled as a sink for the excess carbon, the metal transforms like a nongraphitic steel. The effects of Mn, Sn, and Sb on the eutectoid transformation of ductile iron were shown to be consistent with their behavior in malleable iron.     

Intragranular ferrite nucleation in medium-carbon vanadium steels

In this study, the mechanism of intragranular ferrite nucleation is investigated. It is found that “intragranular ferrite idiomorphs” nucleate at vanadium nitrides which precipitate at manganese sulfide particles during cooling in the austenite region. It is observed that intragranular ferrite has the Baker-Nutting orientation relationship with vanadium nitride which precipitated at manganese sulfide. According to classical nucleation theory, the proeutectoid ferrite nucleation rate depends on the following factors: (1) the driving free energy for ferrite nucleation, (2) the diffusivity of carbon atoms in austenite, and (3) the increase in the interfacial energy associated with ferrite nucleation. In the Baker-Nutting orientation relationship, the lattice mismatch across the habit planes is likely to be very small. Depleted zones of solute atoms such as vanadium are assumed to be formed in the austenite matrix around precipitates. The effect of the depleted zones on factors (1) and (2) is estimated thermodynamically and it is proved that those effects are negligibly small. Thus, we conclude that the most important factor in nucleation kinetics of intragranular ferrite is the formation of precipitates which can develop coherent, low energy interfaces with ferrite.     

Analysis of lattice parameter changes following deformation of a 0.19C-1.63Si-1.59Mn transformation-induced plasticity sheet steel

Austenite and ferrite lattice parameters were monitored using X-ray diffraction subsequent to deformation in uniaxial and biaxial tension and plane straining of a 0.19C-1.63Si-1.59Mn transformation-induced plasticity (TRIP) sheet steel. Details from peak position results suggest the presence of stacking faults in the austenite phase, especially after deformation in uniaxial tension. The results also indicate residual stress or composition effects (through changes in the average carbon concentration due to selective transformation of lower carbon regions of austenite). Compressive residual stresses in the ferrite matrix were measured, and found to increase with increasing effective strain in specimens tested in biaxial tension and plane strain. Strain partitioning between softer ferrite and harder austenite (and possibly bainite or martensite) may be responsible for these residual compressive stresses in the ferrite, although volume expansion from the γ → α′ transformation and texture gradients through the sheet thickness are also possible contributors.