Directional solidification of white cast iron

Several studies of the ledeburite eutectic (Fe-Fe₃C), in pure Fe-C alloys have shown that it has a lamellar morphology under plane front growth conditions. The structure of ledeburite in white cast irons, Fe-C-Si, consists of a rod morphology. It is generally not possible to produce plane front growth of Fe-C-Si eutectic alloys in the Fe-Fe₃C form, because at the slow growth rates required for plane front growth, the Fe₃C phase is replaced by graphite. By using small additions of Te, the growth of graphite was suppressed, and the plane front growth of the ledeburite eutectic in Fe-C-Si alloys was carried out with Si levels up to 1 wt pct. It was found that the growth morphology became a faceted rod morphology at 1 wt pct Si, but in contrast to the usual rod morphology of white cast irons, the rod phase was Fe₃C rather than iron. It was shown that the usual rod morphology only forms at the sides of the two-phase cellular or dendritic growth fronts in Fe-C-Si alloys. Possible reasons for the inability of plane front directional solidification to produce the usual rod morphology in Fe-C-Si alloys are discussed. Also, data are presented on the spacing of the lamellar eutectic in pure Fe-C ledeburite, which indicates that this system does not follow the usual λ² V = constant relation of regular eutectics. Formerly Graduate Student, Department of Materials Science and Engineering, Iowa State University.     

The Fe-rich corner of the metastable C-Cr-Fe liquidus surface

The liquidus surface of the C-Cr-Fe system has been experimentally determined in the Fe-rich region —C ≤6 wt pct, Cr ≤40 wt pct —using a sensitive differential thermal analysis technique, along with optical and scanning electron microscopy and X-ray diffraction. Previous liquidus surfaces for this system have differed on the extent of the (Cr,Fe)₂₃C₆ liquidus field, with one version reporting its existence at ∼20 wt pet Cr, and others finding that it did not occur at Cr levels of less than ∼60 wt pct. The present investigation provides evidence in favor of the second contention, with the (Cr,Fe)₂₃C₆ field not being detected at Cr ≤40 wt pct. Changes are proposed to the accepted liquidus surface in respect of the compositions of the invariant reactions—L + αδFe ⇌γFe + (Cr,Fe)₇C₃ andL + (Cr,Fe)₇C₃ ⇌γFe + (Fe,Cr)₃C —and of the monovariant eutectic valley—L⇌ γFe + (Cr,Fe)₇C₃.     

Microstructural characterization

Rapidly solidified microstructures of Fe-Cr-W-C quaternary alloy deposited on low-carbon steel by laser cladding were investigated. The clad-coating alloy, a powder mixture of Fe, Cr, W, and C with a weight ratio of 10:5:1:1, was processed with a high-power continuous wave CO₂ laser. The developed clad coatings possessed fine microstructures, uniform distributions of al- loying elements, and high microhardness. Analytical electron microscopy and energy dispersive X-ray spectroscopy were used to characterize the crystal structures and microchemistries of the various phases in the clad coatings. The laser processed microstructure comprised fine primary dendrites of a face-centered cubic (fcc) austenitic y phase and interdendritic eutectic consisting of a network of pseudohexagonal M₇C₃ carbides rich in Cr in an fcc austenitic γ phase. The interlamellar spacing in the eutectic matrix was about 20 nm. The relatively high microhardness, about 900 kg_f/mm², of such fine microstructures is attributed to the formation of complex ter- nary carbides uniformly distributed in the eutectic matrix. In situ transmission electron micros- copy (TEM) of thermally treated clad coatings revealed that transformation of the primary γ phase to body-centered cubic (bcc) ferrite (α phase) commenced after heating at 843 K for about 7 minutes. The transformation initiated at the interface of the primary dendrites and the eutectic and propagated gradually into the primary phase. Phase change of the interdendritic γ austenite to a bcc α ferrite occurred after about 30 minutes of hold period at 843 K. Transformation of the M₇C₃ carbides did not occur even after heating at 843 K for about 3.2 hours. The growth of a thin M₂O₃ (M = Fe, Cr) oxide scale was detected after heating at 843 K for approximately 24 minutes. After cooling gradually to room temperature, the softened (723 kgy/mm²) micro- structure consisted of primary dendrites with a bcc α ferrite crystal structure and interdendritic ternary eutectic of untransformed M₇C₃ carbides in α ferrite.     

Solidification and solid-state transformation mechanisms in Si alloyed high-chromium white cast irons

Chromium white cast irons are widely used in environments where severe abrasion resistance is a dominant requirement. To improve the wear resistance of these commercially important irons, the United States Bureau of Mines and CSIRO Australia are studying their solidification and solid-state transformation kinetics. A ternary Fe-Cr-C iron with 17.8 wt pct (pct) Cr and 3.0 pct C was compared with commercially available irons of similar Cr and C contents with Si contents between 1.6 and 2.2 pct. The irons were solidified and cooled at rates of 0.03 and 0.17 K · s^-1 to 873 K. Differential thermal analysis (DTA) showed that Si depresses the eutectic reaction temperature and suggests that is has no effect upon the volume of eutectic carbides formed during solidification. Microprobe analysis revealed that austenite dendrites within the Si alloyed irons cooled at 0.03 and 0.17 K·s^-1 had C and Cr contents that were lower than those of dendrites within the ternary alloy cooled at the same cooling rate and a Si alloyed iron that was water quenched from the eutectic temperature. These lower values were shown by image analysis to be the result of both solid-state growth (coarsening) of the eutectic carbides and some secondary carbide formation. Hardness measurements in the as-cast condition and after soaking in liquid nitrogen suggest an increase in the martensite start temperature as the Si content was increased. It is concluded that Si’s effect on increasing the size and volume fraction of eutectic carbides and increasing the matrix hardness should lead to improved wear resistance over regular high-chromium white cast irons.     

The Transformation of Carbides during Austenization and Its Effect on the Wear Resistance of High Speed Steel Rolls

The transformation of carbides with austenization time of a high speed steel (HSS) roll material, manufactured by a centrifugal casting method, has been studied. The correlation between wear resistance and the type, morphology, volume fraction, and distribution of the carbides has also been investigated. Microstructural observations, X-ray diffraction (XRD) analysis, hardness measurements, and energy dispersive spectroscopy (EDS) have been used to characterize the carbides. The type and volume fraction of carbides were found to change with austenizing time. During austenization, the transformation of the M₃C carbides can be postulated as M₃C + γ-Fe → M₂C, with much finer nodular and rodlike MC carbides also forming through a solid-state transformation. The M₂C carbide decomposes as M₂C + γ-Fe → MC + M₇C₃ + M₆C. The decomposed carbide substantially maintains a platelike shape until the end of decomposition. The most important finding of this study is that austenization results in changes in the type, morphology, volume fraction, and distribution of carbides and that it can be controlled to produced a homogeneous distribution of hard carbides, resulting in an improvement in the wear resistance of HSS rolls. This finding may be of great use for the industrial production of HSS rolls.     

Constitution of the Lead-Tin-Strontium System up to 36 Atomic Percent Sr

The constitution of the Pb-Sn-Sr system from the Pb-Sn binary up to 36 at. pct Sr was determined by differential thermal analysis, metallography, microprobe analysis, and X-ray diffraction. Pb₃Sr forms a continuous series of solid solutions with Sn₃Sr, and is referred to here as the8 phase. Sn₄Sr was the only other intermetallic phase found and is designated here as γ. A eutectic-like trough is formed between (Pb) and δ. It originates at 1.0 at. pct Sr and 324.5 °C (the (Pb)/Pb₃Sr eutectic) and falls monotonically to ~75 at. pct Pb, 24.5 at. pct Sn, and 0.45 at. pct Sr at 283 °C. At 283 °C, a Class II, four-phase reaction occurs: L + δ→ (Pb) + γ. A eutectic-like trough between (Pb) and γ falls from the four-phase plane at 283 °C to the ternary eutectic at ~26 at. pct Pb, ~74 at. pct Sn and <0.3 at. pct Sr at 182 °C. The ternary eutectic reaction is L → (Pb) + (Sn) + γ.     

Studies of Carbides in a Rapidly Solidified High-Speed Steel

Rapid solidification by electron beam surface melting of a Mo-base high-speed steel (M7) has produced microstructural features different from those observed in the conventionally processed material. As a result of rapid solidification, the volume percent of the carbide phases formed has decreased sharply and has resulted in the formation of M₂C and M₂₃C₆ carbide phases, while in the conventionally processed material, M₆C and MC carbides were present. Microanalysis of the extracted carbides formed by electron beam melting has yielded an intriguing finding. M₂₃C₆ is found to be unusually rich in molybdenum, tungsten, and vanadium; the concentration of (Mo + W), for instance, is approximately 60 wt pct. The corresponding values for Fe and Cr are surprisingly low (6 wt pct Cr and 1 wt pct Fe). This is in marked contrast with carbides encountered in the conventionally processed high-speed steel, where Cr and Fe are the major constituents. The shift in composition of the carbide phases could be attributed to the accelerated evaporation of chromium during surface melting as compared to the evaporation of Mo, W, and V. formerly Research Associate, University of Connecticut     

Thermodynamics of the carbides in the system Fe-Cr-C

Phase relations in the Fe-Cr-C system in the temperature range 900 to 1150°C have been studied using metallographic and X-ray methods and the electron microprobe. An isothermal section of the phase diagram at 1000°C is shown. Lattice dimensions of the three carbides were determined for several values of the ratio Cr:(Fe +Cr). The solubilities of the carbides at each temperature were determined by metallographic study of quenched specimens. The distribution of Cr between austenite (γ) and the several carbides was determined by use of the electron microprobe. Data of Wada et al on the activity of carbon were used to calculate activities at the y-phase boundary and the free energy of the several carbides as a function of their chromium content. The data are treated thermodynamically on the basis of assumed random mixing of Cr and Fe atoms in each carbide. While this randomness was not definitely proved, the assumption was shown to be reasonable and the results useful. Extrapolation to 0 and 100 pct Cr gives values for the standard free energy of Cr₇C₃ and the hypothetical carbides Cr₃₆C, and Fe₇C₃. Formerly with the Research Staff, Massachusetts Institute Technology     

Overall kinetics and morphology of the products of austenite decomposition in a Fe-0.46 Pct C-5.2 Pct Cr alloy transformed isothermally above the bay

The overall transformation kinetics of austenite isothermal decomposition above the bay of the time-temperature-transformation (TTT) curve and the eutectoid morphology of the resulting products have been studied in a Fe-0.46 pct C-5.2 pct Cr alloy. Classical lamellar pearlite was formed at high temperatures while complex ferrite plus carbide morphologies, sometimes described as spiky pearlite, arborescent structures, or nonclassical decompositions products of austenite in Fe-Cr-C alloys, formed at low temperatures. While X-ray diffraction of extracted carbides and selected area diffraction-transmission electron microscopy (TEM) showed evidence for a mixture of M₃C and M₇C₃ carbides, thermodynamic calculations results obtained only M₇C₃ as the equilibrium carbide at the temperatures studied. A tentative explanation for the arborescent morphology is presented, based on the hypothesis of the existence of a drag force or free energy dissipation term that is locally relaxed by the partition of Cr into the carbides at the reaction front, consequently removing Cr from the interface. This article is based on a presentation made in the “Hillert Symposium on Thermodynamics & Kinetics of Migrating Interfaces in Steels and Other Complex Alloys,” December 2–3, 2004, organized by The Royal Institute of Technology in Stockholm, Sweden.     

Thermodynamics of the Fe-Cr-C system at 985 K

Carbon solubility and the composition of carbides in the Fe-Cr-C system up to 8.07 pct Cr were measured at 985 K by CH₄/H₂ gas equilibration. An iron-carbon binary alloy was included in the equilibration as a reference material. The chromium-carbon interaction inα-phase was analyzed by the central atoms model. The Wagner interaction coefficient was determined asε _C ^Cr = -72 ± 2, a significantly higher negative value than in austenitic alloy. The activity coefficient of carbon inα-Fe was determined as logϕ _C ^o = 3.617, which supports the values reported by Swartz and by Chipman. The carbide phase was analyzed as a regular solution of two component carbides, FeC _x and CrC _x . M₇C₃ carbide was present in the carbon activity range up toa _c = 0.8, while M₃C carbide was present at higher carbon activities. Partitioning of chromium betweenα and the carbide phases was measured. The standard Gibbs energies of formation of the two component carbides and the interaction energy parameters were determined for both M₇C₃ and M₃C carbides.     

A melting and solidification study of alloy 625

The melting and solidification behavior of Alloy 625 has been investigated with differential thermal analysis (DTA) and electron microscopy. A two-level full-factorial set of chemistries involving the elements Nb, C, and Si was studied. DTA results revealed that all alloying additions decreased the liquidus and solidus temperatures and also increased the melting temperature range. Terminal solidification reactions were observed in the Nb-bearing alloys. Solidification microstructures in gastungsten-arc welds were characterized with transmission electron microscopy (TEM) techniques. All alloys solidified to an austenitic (γ) matrix. The Nb-bearing alloys terminated solidification by forming various combinations of γ/MC(NbC), γ/Laves, and γ/M₆C eutectic-like constituents. Carbon additions (0.035 wt pct) promoted the formation of the γ/MC(NbC) constituent at the expense of the γ/Laves constituent. Silicon (0.4 wt pct) increased the formation of the yJLaves constituent and promoted formation of the γ/M₆C carbide constituent at low levels (<0.01 wt pct) of carbon. When both Si (0.4 wt pct) and C (0.035 wt pct) were present, the γ/MC(NbC) and γ/Laves constituents were observed. Regression analysis was used to develop equations for the liquidus and solidus temperatures as functions of alloy composition. Partial derivatives of these equations taken with respect to the alloying variables (Nb, C, Si) yielded the liquidus and solidus slopes t(m _L , m _S ) for these elements in the multicomponent system. Ratios of these liquidus to solidus slopes gave estimates of the distribution coefficients (k) for these same elements in Alloy 625.     

Correlation of microstructure with the wear resistance and fracture toughness of duocast materials composed of high-chromium white cast iron and low-chromium steel

The objective of this study is to investigate the correlation of microstructure with wear resistance and fracture toughness in duocast materials that consisted of a high-chromium white cast iron and a low-chromium steel as the wear-resistant and ductile parts, respectively. Different shapes, sizes, volume fractions, and distributions of M₇C₃ carbides were employed in the wear-resistant part by changing the amount of chromium and molybdenum. In the alloys containing a large amount of chromium, a number of large hexagonal-shaped primary carbides and fine eutectic carbides were formed. These large primary carbides were so hard and brittle that they easily fractured or fell off from the matrix, thereby deteriorating the wear resistance and fracture toughness. In the alloys containing a smaller amount of chromium, however, a network structure of eutectic carbides having a lower hardness than the primary carbides was developed well along solidification cell boundaries and led to the improvement of both wear resistance and toughness. The addition of molybdenum also helped enhance the wear resistance by forming additional M₂C carbides without losing the fracture toughness. Under the duocasting conditions used in the present study, the appropriate compositions for wear resistance and fracture toughness were 17 to 18 pct chromium and 2 to 3 pct molybdenum.     

Effect of Silicon on Carbide Precipitation after Tempering of H11 Hot Work Steels

The formation of secondary carbides during tempering of H11 hot work steels at 898 K (625 °C) was studied by transmission electron microscopy (TEM) and related to the previously established effects of Si content on mechanical properties. Lower Si contents (0.05 and 0.3 pct Si) and higher Si contents (1.0 and 2.0 pct Si) were observed to yield different carbide phases and different particle distributions. Cementite particles stabilized by Cr, Mo, and V in the lower Si steels were found to be responsible for similar precipitation hardening effects in comparison to the M₂C alloy carbides in the higher Si steels. The much higher toughness of the lower Si steels was suggested to be due to a finer and more homogeneous distribution of Cr-rich M₇C₃ carbides in the interlath and interpackage regions of the quenched and tempered martensite microstructure. The present effects of Si content on the formation of alloy carbides in H11 hot work steels were found to be the result of the retarding effect of Si on the initial formation of cementite, well known from the early tempering stages in low alloy steels.     

Developing a M6C-Reinforced High-Cr White Iron for Abrasive Wear Application

Fast removal of soft phases (e.g., pearlite and ferrite) in the iron matrix limits the wear life of high-Cr white irons. To address this shortcoming, the authors successfully produced fine networks of M₆C carbide in a high-Cr white iron through extensive thermodynamic calculations. Fishbone-like networks of M₆C carbides were observed with an optical microscope. It was experimentally determined that such carbide networks protected the soft matrix and increased the overall hardness. Additionally, electron backscattered diffraction was conducted, which showed that the alloy contained phases of M₇C₃, M₆C, ferrite, and retained austenite. Solidification sequence was determined by correlating the thermodynamic equilibrium calculation results with the size and distribution of each phase. A dry sand/rubber wheel apparatus following ASTM standard G65 Procedure A was utilized to assess the abrasive wear performance of the developed alloy. Results showed that the volume loss of the developed material was 25 pct less than that of conventional high-Cr white irons. Wear scars were investigated using a scanning electron microscope, and the improved wear resistance was attributed to the “buffer” effect and plastic deformation of the introduced M₆C carbide networks.

     

Solidification and phase equilibria in the Fe-C-Cr-NbC system

A solidification model is developed and experimentally checked for Fe-C-Cr-Nb alloys in the white cast irons range. It is based on a partial quaternary Fe-C-Cr-NbC phase diagram and predicts the possible solidification paths for the alloys containing γ, with (Fe,Cr)₇C₃ and NbC as the microconstituents at room temperature. The dendritic γ to massive (Fe,Cr)₇C₃ transition in experimental alloy microstructures with NbC contents up to 22 pet is explained by this model. Thermal analysis is also used to compare the solidification paths and model approach.     

Carbide reactions (M3C→M7C3→M23C6→M6C) during tempering of rapidly solidified high carbon Cr-W and Cr-Mo steels

Carbide transformations of M₃C → M₇C₃ → M₂₃C₆ → M₆C and crystallographic relationships among these carbides were examined by transmission electron microscopy. Two kinds of high carbon-chromium steels containing tungsten or molybdenum were quenched rapidly from the melts and tempered at temperatures up to 700°C. By tempering at 600°C, M₇C₃ carbides nucleated mostly on cementite/ferrite interfaces and grew inward the cementite byin- situ transformation.In-situ transformations from M₇C₃ to M₂₃C₆ and from M₂₃C₆ to M₆C were also found in these alloy steels during tempering at higher temperatures. Mutual relationships of crystal orientations among M₃C, M₇C₃, M₂₃C₆ and M₆C were decided as follows: {fx739-01}.     

Phase transformations in an Fe-7.8Al-29.5Mn-1.5Si-1.05C alloy

Phase transformations in an Fe-7.8Al-29.5Mn-l.5Si-1.05C alloy have been investigated by means of optical microscopy and transmission electron microscopy. In the as-quenched condition, a high density of fine (Fe,Mn)₃AlC carbides could be observed within the austenite matrix. When the as-quenched alloy was aged at temperatures ranging from 550 °C to 825 °C, aγ → coarse (Fe,Mn)₃AlC carbide + DO₃ reaction occurred by a cellular precipitation on theγ/γ grain boundaries and twin boundaries. Both of the observations are quite different from those observed by other workers in Fe-Al-Mn-C alloys. In their studies, it was found that the as-quenched microstructure was austenite phase(γ), and (Fe,Mn)₃AlC carbides could only be observed within the austenite matrix in the aged alloys. In addition, aγ →α (ferrite) + coarse (Fe,Mn)₃AlC carbide reaction or aγ →α + coarse (Fe,Mn)₃AlC carbide +β-Mn reaction was found to occur on theγ/γ grain boundary in the aged Fe-Al-Mn-C alloys.