Observations concerning transformation interfaces in steels

The transformation interfaces of pearlite, allotriomorphic cementite, M₂₃C₆, and Widmanstätten cementite plates in high-Mn high-C alloy steels have been studied by TEM. Linear striations in the interface have been analysed and related to intersections with stacking faults in the parent austenite phase. Emphasis is given to the pearlite interface where it is found that the striations at the interface increased as a result of thermomechanical treatment of the austenite prior to isothermal transformation, consistent with an increased density of planar defects. The effect of heat treatment, and Si alloying additions, are also considered. Both conventional and in situ TEM of the pearlite interface showed that the linear defects stretched across both ferrite and cementite phases at the pearlite interface, apparently without any deviation or change in image contrast. The results are compared with similar ones made of the static γ/α interphase boundaries in duplex stainless steel. The effect of prior deformation structure in the parent austenite on the growth and interface structure of Widmanstätten cementite plates has also been considered.     

Morphology of bainite and widmanstätten ferrite

Morphology of bainite and Widmanstätten ferrite in various steels has been investigated by means of microstructural and surface relief observations. It was shown that upper and lower bainite should be classified by ferrite morphology,i.e., lathlike or platelike, and that the morphology of cementite precipitation cannot be the index for the classification. Widmanstätten ferrite formed in the upper C-nose where ferrite grain-boundary allotriomorphs nucleate exhibits quite similar appearance with bainitic ferrite that forms in the lower C-nose of bainitic reaction. The only difference between them exists in the fact that Widmanstätten ferrite laths grow in the temperature range where primary ferrite forms and often terminate at a grain boundary ferrite but that bainitic ferrite has its own C-curve at temperatures belowB _s and nucleates directly at an austenite grain boundary. The mechanisms for their formations are discussed.     

Transformation Characteristics of Ferrite/Carbide Aggregate in Continuously Cooled, Low Carbon-Manganese Steels

Transformation characteristics and morphological features of ferrite/carbide aggregate (FCA) in low carbon-manganese steels have been investigated. Work shows that FCA has neither the lamellae structure of pearlite nor the lath structure of bainite and martensite. It consists of a fine dispersion of cementite particles in a smooth ferrite matrix. Carbide morphologies range from arrays of globular particles or short fibers to extended, branched, and densely interconnected fibers. Work demonstrates that FCA forms over similar cooling rate ranges to Widmanstätten ferrite. Rapid transformation of both phases occurs at temperatures between 798 K and 973 K (525 °C and 700 °C). FCA reaction is not simultaneous with Widmanstätten ferrite but occurs at temperatures intermediate between Widmanstätten ferrite and bainite. Austenite carbon content calculations verify that cementite precipitation is thermodynamically possible at FCA reaction temperatures without bainite formation. The pattern of precipitation is confirmed to be discontinuous. CCT diagrams have been constructed that incorporate FCA. At low steel manganese content, Widmanstätten ferrite and bainite bay sizes are significantly reduced so that large amounts of FCA are formed over a wide range of cooling rates.     

Effects of Widmanstätten ferrite on the mechanical properties of a 0.2 pct C-0.7 pct Mn steel

Laboratory melted and rolled C-Mn steel plates were austenitized at either 925 °C or 1150 °C to produce nominal austenite grain sizes of 60 and 200 μm, resspectively. The plates were then cooled at rates in the range of about 2 °C/min to 400 °C/min to produce mixed polygonal ferrite/Widmanst?tten ferrite/pearlite microstructures. The percentage of Widmanst?tten structure (a Widmanst?tten ferrite/pearlite aggregate) increases with increasing prior austenite grain size and cooling rate. Both yield strength and impact toughness increase with decreasing austenite grain size and increasing cooling rate. This simultaneous improvement in strength and toughness is attributed to overall refinement of both the polygonal ferrite and Widmanst?tten structure. Both yield and tensile strength increase with an increase in the volume fraction of Widmanst?tten ferrite and a reduction in ferrite grain size. In contrast, the toughness level achieved in these polygonal ferrite/Widmanst?tten ferrite/pearlite microstructures depends largely on the ferrite grain size; the finer the grain size, the better the toughness.     

Role of carbon and alloying elements in the formation of bainitic ferrite

One approach to the prediction of the carbon content of austenite, remaining after the precipitation of bainitic ferrite, is based on the assumption that bainitic ferrite during growth inherits the carbon content of the parent austenite. An alternative approach is based on the assumption that bainitic ferrite grows with a low carbon content and there is no major difference between Widmanstätten ferrite and bainitic ferrite. The two approaches are now compared using information from alloyed steels with considerable amounts of Si, where the formation of cementite is retarded. The former approach does not account for the effect of Mn and fails severely at low alloy contents. The latter approach seems more promising but is not without difficulties. In particular, in order to explain the effects of Cr and Mo, it seems necessary to introduce a kinetic effect, presumably caused by solute drag.     

Copper precipitation during continuous cooling and isothermal aging of a710-type steels

Precipitation in copper-containing A710 (also referred to as HSLA-80) and modified-A710 steels was investigated by transmission electron microscopy. Isothermal aging of as-quenched specimens at 675 °C produced e-copper precipitates located primarily at α-iron matrix dislocations. The precipitates exhibited multiple variants of an orientation relationship (OR) consistent with that reported by Kurdjumov and Sachs, fine fault formation, and associated streaking in electron diffraction patterns. For reaustenitized and continuously cooled specimens, the primary precipitation event was associated with interphase precipitation of copper at ferrite/austenite interfaces. Interphase precipitates frequently displayed ORs other than that reported by Kurdjumov and Sachs, although a unique crystallographic variant was observed within any one region of interphase precipitation, faults were observed infrequently, and streaking was not observed in diffraction patterns. At high temperatures during cooling, precipitate-free ferrite formed, whereas at lower temperatures, nucleation of copper precipitates occurred at ferrite/austenite interfaces for crystals of polygonal ferrite and Widmanstätten ferrite. This latter feature precludes the formation of Widmanstätten ferritevia a displacive mechanism. Interphase precipitation was not observed for granular ferrite or acicular ferrite. Less-common precipitation events during continuous cooling included the formation of AIN and CuS.     

Widmanstätten ferrite plate formation in low-carbon steels

The mechanism by which Widmanstätten ferrite plates nucleate and grow in low-carbon steels has been studied. In-situ laser scanning confocal microscopy (LSCM) observations, optical microscopy, and electron backscattered diffraction (EBSD) techniques have been used to characterize the relationship between grain boundary allotriomorphs and Widmanstätten ferrite plates. The issue of where Widmanstätten ferrite plates nucleate is one of some debate, with theories including morphological instability and sympathetic nucleation. Evidence has been found that supports the theory of a sympathetic nucleation mechanism being responsible for the formation of Widmanstätten ferrite plates. The EBSD measurements have shown that low-angle misorientations of between 5 and 10 deg exist between ferrite allotriomorphs and Widmanstätten ferrite plates.     

Morphology of cementite decomposition in an fe-cr-c alloy

Dissolution of spheroidal cementite in austenite at 910 °C has been studied by means of scanning and transmission electron microscopy. Three different morphologies of transformation have been observed. Most of the particles undergo a more or less homogeneous shrinkage. However, some of the particles transform to austenite by a Widmanstätten type of reaction, whereby austenitic plates form inside the cementite. It is also observed that some of the cementite transforms to M₇C₃ carbide and austenite by a eutectoid reaction. The results can be understood by considering the phase diagram and assuming that local equilibrium prevails at the phase interface during the reaction.     

Austenite decomposition during continuous cooling of an HSLA-80 plate steel

Decomposition of fine-grained austenite (10-μm grain size) during continuous cooling of an HSLA-80 plate steel (containing 0.05C, 0.50Mn, 1.12Cu, 0.88Ni, 0.71Cr, and 0.20Mo) was evaluated by dilatometric measurements, light microscopy, scanning electron microscopy, transmission electron microscopy, and microhardness testing. Between 750 °C and 600 °C, austenite transforms primarily to polygonal ferrite over a wide range of cooling rates, and Widmanst?tten ferrite sideplates frequently evolve from these crystals. Carbon-enriched islands of austenite transform to a complex mixture of granular ferrite, acicular ferrite, and martensite (all with some degree of retained austenite) at cooling rates greater than approximately 5 °C/s. Granular and acicular ferrite form at temperatures slightly below those at which polygonal and Widmanst?tten ferrite form. At cooling rates less than approximately 5 °C/s, regions of carbon-enriched austenite transform to a complex mixture of upper bainite, lower bainite, and martensite (plus retained austenite) at temperatures which are over 100 °C lower than those at which polygonal and Widmanst?tten ferrite form. Interphase precipitates of copper form only in association with polygonal and Widmanst?tten ferrite. Kinetic and microstruc-tural differences between Widmanst?tten ferrite, acicular ferrite, and bainite (both upper and lower) suggest different origins and/or mechanisms of formation for these morphologically similar austenite transformation products. Formerly Graduate Student, Department of Metallurgical and Materials Engineering, Colorado School of Mines. This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled “Atomistic Mechanisms of Nucleation and Growth in Solids,” organized in honor of H.I. Aaronson’s 70th Anniversary and given October 3–5, 1994, in Rosemont, Illinois.     

The effect of nitrogen on precipitation and transformation kinetics in vanadium steels

The isothermal decomposition of austenite has been studied in a series of vanadium steels containing varying amounts of carbon and nitrogen, (in approximately stoichio-metric proportions), in the temperature range 700 to 850°C. In the basic alloy, Fe-0.27V–0.05C (composition in wt pct), below 810°C the austenite to polygonal ferrite trans-formation is accompanied by interphase precipitation of vanadium carbide, the finer dis-persions being associated with the lower transformation temperatures. However, below 760°C there is an additional precipitation reaction where dislocation precipitation of vanadium carbide predominates; this is shown to occur in association with Widmanstätten ferrite. Above 810° C, a proeutectoid ferrite reaction results, the ferrite being void of precipitates; evidence is provided to show that partitioning of vanadium from ferrite to austenite occurs during the transformation. In the two steels containing nitrogen, namely Fe-0.26V-0.022N-0.020C and Fe-0.29V-0.032 N the basic interphase precipitation re-action is unchanged, but the resultant precipitate dispersions are finer at a given trans-formation temperature. The temperature range over which interphase precipitation oc-curs is expanded by the presence of nitrogen, since the Widmanstätten start tempera-ture is depressed and the proeutectoid ferrite reaction is inhibited. Precipitation in austenite prior to transformation and twin formation during transformation are both en-couraged by the presence of nitrogen.     

Microstructural Study of the Phase Transformations Upon Cooling to Room Temperature of High Mn Steels

The phase transformations of high Mn steels during cooling have been characterized in this study. Widmanstätten plates occur in the austenite matrix upon cooling the steels from 1373 K (1100 °C). The Widmanstätten plates are composed of not only the hexagonal close-packed ε-martensite but also the face-centered cubic (FCC) micro-twins. The formation mechanism of the Widmanstätten phases is probably various stacking faults induced from Shockley partial dislocations in the austenite. The ε-martensitic plates, along with the κ-carbides, were observed in a Mn-Al steel at 873 K (600 °C). As most of the FCC matrix has transformed to κ-carbides, the partial dislocations neighboring ε-martensitic plates could not glide. The ε-martensite retained in the transformed matrix is the strongest evidence to support the above mechanism.     

Effects of austenite grain size and cooling rate on Widmanstätten ferrite formation in low-alloy steels

Deformation dilatometry is used to simulate the hot rolling of 0.20 pct C-1.10 pct Mn steels over a product thickness range of 6 to 170 mm. In addition to a base steel, steels with additions of 0.02 pct Ti, 0.06 pct V, or 0.02 pct Nb are included in the study. The transformation behavior of each steel is explored for three different austenite grain sizes, nominally 30, 55, and 100 μm. In general, the volume fraction of Widmanst?tten ferrite increases in all four steels with increasing austenite grain size and cooling rate, with austenite grain size having the more significant effect. The Nb steel has the lowest transformation temperature range and the greatest propensity for Widmanst?tten ferrite formation, while the amount of Widmanst?tten ferrite is minimized in the Ti steel (as a result of intragranular nucleation of polygonal ferrite on coarse TiN particles). The data emphasize the importance of a refined austenite grain size in minimizing the formation of a coarse Widmanst?tten structure. With a sufficiently fine prior austenite grain size (e.g., ≤30 μm), significant amounts of Widmanst?tten structure can be avoided, even in a Nb-alloyed steel.     

Observations of Surface Relief of Proeutectoid Widmanstätten Cementite Plates in a Hypereutectoid Carbon Steel

Proeutectoid Widmanstätten cementite in a hypereutectoid carbon steel was found to be associated with a surface relief effect. A hot-stage microscope was used for heat treatment and in situ observation. Widmanstätten cementite plates were obtained near the surface of the specimen. The surface relief effect of Widmanstätten cementite plates was quantitatively characterized by atomic force microscopy. It was found that the relief had either a typical tent shape or apex-lost tent shape. The relief tilt angles were of considerable dispersion, ranging from 20 deg to 50 deg.     

Thermodynamics and kinetics of the formation of widmanstätten ferrite plates in ferrous alloys

Experimental data on the formation of Widmanstätten/bainitic ferrite in ferrous alloys(i.e., the Widmanstätten start temperature, partition of alloying elements, incomplete transformation, lengthening kinetics,etc.) are examined on the basis of thermodynamic calculations and kinetic analyses. A morphological change of ferrite from grain-boundary allotriomorph to Widmanstätten plate occurs well above theT ₀ temperature, except in high Mn and Ni alloys, but does so in the regime of carbon diffusion control in all alloys. Under the assumption that the plate tip consists of a pair of ledges of the height equal to the tip radius, the reported lengthening kinetics of ferrite plates can be accounted for very well by the diffusion-controlled motion of these ledges in a wide range of carbon supersaturation. It is also shown that the transformation stasis (incomplete transformation) observed below the kinetically definedB _s in some iron alloys cannot be unequivocally attributed to either the completion of the precipitation of no-partitioned ferrite or the loss of the driving force for subsequent shear transformation.     

The determination of orientation relationships from the relative orientations of variants of a product phase

A method is given for determining orientation relationships from measurements of the orientations of variants of the product phase. The symmetry rotations relating the different variants are determined and the symmetry axes used to define the orientation of the parent crystal. Results are given for the orientation relationship between {111} annealing twins in a copper crystal and between Widmanstätten ferrite and austenite in an iron carbon alloy.     

Austenitization during intercritical annealing of an Fe-C-Si-Mn dual-phase steel

Partial austenitization during the intercritical annealing of an Fe-2.2 pct Si-1.8 pct Mn-0.04 pct C steel has been investigated on four kinds of starting microstructures. It has been found that austenite formation during the annealing can be interpreted in terms of a carbon diffusion-limited growth process. The preferential growth of austenite along the ferrite grain boundaries was explained by the rapid carbon supply from the dissolving carbide particles to the growing fronts of austenite particles along the newly formed austenite grain boundaries on the prior ferrite grain boundaries. The preferential austenitization along the grain boundaries proceeded rapidly, but the austenite growth became slowed down after the ferrite grain boundaries were site-saturated with austenite particles. When the ferrite grain boundaries were site-saturated with austenite particles in a coarse-grained structure, the austenite particles grew by the mode of Widmanstätten side plate rather than by the normal growth mode of planar interface displacement.     

New insights into the widmanstätten proeutectoid ferrite transformation: Integration of crystallographic and three-dimensional morphological observations

The crystallography and three-dimensional (3-D) morphology of Widmanstätten proeutectoid ferrite precipitates are examined in an Fe-0.12 wt pct C-3.28 wt pct Ni steel isothermally reacted at 650 °C, 600 °C, and 550 °C. This article integrates new orientation mapping (OM) results with the findings of a companion article to this one on the 3-D morphology of proeutectoid ferrit^[1] and an earlier transmission electron microscopy (TEM) study which is reanalyzed here in light of the new OM and 3-D results. All of these studies were performed for the same alloy and heat treatments. The 3-D morphologies and distributions of proeutectoid ferrite precipitates are now known to often be quite different from those deduced by conventional two-dimensional (2-D) microscopy techniques. The present crystallographic studies indicate that “primary” ferrite (nucleated directly on prior austenite grain boundaries) forms monolithic single crystals and can be approximated as elongated triangular pyramids. “Secondary” ferrite morphologies can be described as laths and plates branching into the austenite from a thick and/or broad allotriomorphic ferrite base. These secondary Widmanstätten branches are composed of many misoriented crystals with ferrite: ferrite boundaries between them and appear to approach a common orientation as they extend into the austenite grain. Implications of the current findings on existing growth and crystallography models are discussed, and a preliminary hypothesis or mechanism of ferrite formation has been proposed to account for the present observations.     

A study of nonisothermal austenite formation and decomposition in Fe-C-Mn alloys

Experiments using a hot-stage confocal scanning laser microscope (CSLM) have been carried out to observe phase transformations in two steels: Si-killed resulfurized Fe-0.38 wt pct C-1.43 wt pct Mn and Al-killed Fe-0.20 wt pct C-0.87 wt pct Mn. Austenite formation during continuous heating was investigated on the surface of samples that were etched to reveal the ferrite and pearlite regions. It was found that the austenite precipitated first at the pearlite colonies and subsequently in the ferrite phase. The measured advance rates of the γ/pearlite front were roughly twice those of the γ/α front and both interfaces were found to be curved. The γ/pearlite migration rate was found to be in qualitative agreement with published rate equations for isokinetic austenite formation where diffusion is the rate-limiting step. Austenite decomposition was studied during cooling. Widmanstätten ferrite laths precipitate as distinct colonies from the existing allotriomorphic ferrite phase but then also at MnS precipitates. The electron backscatter diffraction (EBSD) analysis showed that all of the laths in a particular colony exhibit similar orientation to one another but a slightly different orientation than the parent allotriomorph, supporting a sympathetic nucleation mechanism. The growth rate of the laths was found to vary widely within a range of 1.5 to 11 μm/s. The ferrite formation is finally halted by impingement with other advancing fronts. The results are presented in a phenomological discussion, with some quantitative analysis of the transformation kinetics.     

Three-dimensional analysis and classification of grain-boundary-nucleated proeutectoid ferrite precipitates

The three-dimensional (3-D) shapes and distributions of grain-boundary-nucleated proeutectoid ferrite precipitates have been obtained by computer-aided 3-D reconstruction of serial sections of an Fe-0.12 wt pct C-3.28 wt pct Ni alloy. Isothermal transformation for short times at 650 °C was used to produce a low volume fraction of ferrite, which appeared as both Widmanstätten shapes and more-equiaxed grain-boundary precipitates. While the two-dimensional (2-D) cross sections of these precipitates appeared to fit into the previously accepted categories of precipitate morphologies, 3-D reconstructions revealed important aspects of connectivity and shape that were not observed earlier. A partially revised morphological classification based on these 3-D observations is provided here. Significant differences between the 3-D morphology of proeutectoid ferrite and that of proeutectoid cementite are also discussed, indicating (among other things) that the 3-D morphological classifications of ferrite are generally more complex than those of proeutectoid cementite.     

Diffusion in growth of bainite

Edgewise growth rates for Widmanstätten ferrite and bainite in low alloy steels can be represented with an empirical equation showing proportionality to the square of the supersaturation of the austenite. The proportionality constant has a value in reasonable agreement with the assumption of rate control by carbon diffusion. The growth rates are too low to give a noticeable supersaturation of carbon in the growing ferrite. The experimentalB _s for low alloy steels does not seem to be related to theT _s line, nor doesBs evaluated from the incomplete transformation to bainite for an alloy steel. By assuming rate control by carbon diffusion, the empirical equation can be used to calculate the growth rate under paraequilibrium or no partition, local equilibrium (NPLE) conditions. Experimental growth rates for a similar steel falls in-between. The fact that paraequilibrium does not seem to apply is taken as an indication that the α/γ interface for Widmanstätten ferrite and bainite is not of a purely martensitic type.