Phase relationships in the Fe-Cr-C system at solidification temperatures

The phase relationships between the liquid phase and the primary solid phases were investigated in the iron-rich corner of the Fe-Cr-C system. The investigation consisted of measurements of tie-lines and the liquidus surface of the liquid-delta (bcc) and liquid-gamma (fcc) equilibria in the Gibbs triangle, bounded by 0 to 1.4 wt pct C and 0 to 25 wt pct Cr (bal. Fe). The peritectic surface of the three-phase equilibrium was also measured. The temperature ranged from 1811 to about 1750 K. The tie-lines were obtained from liquid-solid equilibrium couples, and the liquidus and peritectic surfaces, by differential thermal analysis (DTA). A statistical procedure was applied to determine from the experimental results the parameters required for a thermodynamic model of the system. Calculations by the model are in good agreement with the experimental results. As a consequence the model can be used to interpolate and extrapolate properties and compositions of phases in equilibrium in the system within the composition and temperature field investigated. D.M. KUNDRAT, formerly Research Fellow at Massachusetts Institute of Technology M. CHOCHOL, formerly Research Assistant, Massachusetts Institute of Technology     

Phase relationships in the fe-cr-mn-ni-c system at solidification temperatures

The phase relationships between the liquid phase and the primary solid phases were investigated in the iron-rich corner of the quinary system Fe-Cr-Mn-Ni-C. Of the five quaternary systems that comprise the quinary system, this study was limited to the three quaternary systems which contain both carbon and iron as two of the components;viz.: Fe-Cr-Mn-C, Fe-Cr-Ni-C, and Fe-Mn-Ni-C, as well as all of the binary and ternary subsystems that have iron as a component. This paper discusses the modeling efforts for these systems, with particular emphasis on the ternary systems Fe-Cr-Mn and Fe-Mn-Ni and the quaternary systems Fe-Cr-Mn-C and Fe-Mn-Ni-C. The experimental investigation consisted of measurements of tie-lines for the liquid-delta (bcc) and the liquid-gamma (fcc) equilibria in the iron-rich corner of the Gibbs simplex bounded by 0 to 25 wt pct Cr, 0 to 12 wt pct Mn, 0 to 25 wt pct Ni, and 0 to 1.2 wt pct C (bal. Fe). The temperature ranged from 1811 to about 1750 K. Compositions for the tie-lines were obtained from liquid-solid equilibrium couples, and the temperatures for the equilibrium by differential thermal analysis (DTA). Parameters were selected in a thermodynamic model of the alloy system to minimize the square of the difference between experimentally and calculated tie-lines, the latter being implicitly a function of the derived parameters in the model. Binary and higher-order parameters were generally required. Ternary parameters were obtained on ironcarbon base alloys Fe-Cr-C, Fe-Mn-C, and Fe-Ni-C, and for the Fe-Cr-Ni system, but not for the Fe-Cr-Mn and Fe-Mn-Ni systems. Of the quaternary systems investigated, quaternary parameters were required only for theL/δ equilibrium in the Fe-Cr-Ni-C system; the Fe-Cr-Mn-C and Fe-Mn-Ni-C systems were found to be represented adequately by employing only binary and ternary parameters.     

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.     

Phase relationships in the iron-rich Fe-Cr-Ni-C system at solidification temperatures

The phase relationships between the liquid phase and the primary solid phases were investigated in the iron-rich comer of the Fe-Cr-Ni-C system as part of a larger study of the Fe-Cr-Mn-Ni-C system. The investigation consisted of measurements of tie-lines for the liquid-delta (bcc) and the liquid-gamma (fcc) equilibria in the iron-rich corner of the Gibbs tetrahedron bounded by 0 to 25 wt Pct Cr, 0 to 25 wt Pct Ni, and 1.2 wt Pct C (bal. Fe). The temperature ranged from 1811 to 1750 K. Compositions for the tie-lines were obtained from liquid-solid equilibrium couples and the temperatures of the equilibrium, by differential thermal analysis (DTA). A mathematical procedure was employed on the experimental data to obtain parameters for a thermodynamic model of the alloy system. This involved minimization of an error function. The details of this analysis are discussed fully in this paper. Calculations by the model employing the “best-set” parameters are in good agreement with the experimental results. The usefulness of the model is demonstrated by calculation of the three-phase equilibrium in the quaternary system as a function of temperature. Formerly Research Fellow, Massachusetts Institute of Technology, is Senior Research Engineer, Armco Inc., Middletown, OH 45043     

Grain refinement in aluminum alloyed with titanium and boron

The aluminum corner of the ternary Al-B-Ti diagram was explored. A eutectic: Liq — Al + TiAl₃ + (Al, Ti)B₂ was found at approximately 0.05 wt pct Ti, 0.01 wt pct B; 659.5‡C. TiB₂ and A1B₂ form a continuous series of solid solutions, but no distinct ternary phase was found. The addition of boron to aluminum-titanium alloys expands the field of primary crystallization of TiAl₃ toward lower titanium contents and steepens the liquidus. In equilibrium conditions, pronounced grain refinement is found only in alloys in which TiAl₃ is primary and nucleates the aluminum solid solution before any other impurity can act. The peritectic reaction facilitates this priority but it is not necessary for grain refinement. Because of the low diffusivity of titanium and boron in aluminum, equilibrium is seldom attained and in commercial practice grain refinement by TiAl₃ is found also outside its equilibrium field of primary crystallization.     

A pressure-composition-temperature study of Zr-Nb-H system

The solubility of hydrogen was determined in the (Zr + 5 wt pct Nb)-H₂, (Zr + 10 wt pct Nb)-H₂, and (Zr + 20 wt pct Nb)-H₂ systems as a function of composition, temperature (700° to 950°C) and hydrogen equilibrium pressure (0.5 to 760 mm Hg). The position of boundariesβ - (β + δ) and(β + δ)-δ were determined in each of the above three systems. Niobium significantly reduces the solubility of hydrogen in theβ andδ phases and increases the equilibrium hydrogen pressure for any fixed concentration. The equilibrium pressure-temperature relations in the two phase region (β + δ) were derived and the heat of formation ofδ-hydride from saturatedβ-Zr, ΔH _β → δ, were determined. The value of ΔH _β → δ increases up to about 5 wt pct Nb after which the effect of niobium seems to be insignificant. The maximum hydrogen pick-up of zirconium at room temperature decreases with increasing niobium content of the alloy.     

Determination of the Fe-rich portion of the Fe-Ni-C phase diagram

The iron-rich portion of the Fe-Ni-C phase diagram has been determined in the composi-tion range from 0 to 20 wt pct Ni and 0 to 6.67 wt pct C for four temperatures, 773, 873, 923 and 1003 K. Long term heat treatments were used to grow the ferrite plus austenite assemblages, while slow cooling heat treatments (25 K/h) were used to grow the metal plus carbide assemblages. Other types of heat treatments produced metal plus graphite. The two phase tie-lines and three phase tie-triangles were measured using electron mi-croprobe techniques. In samples where bulk equilibration had not been achieved, tie-lines were obtained by using extrapolated interface compositions, on the assumption of local equilibrium at the interface. The tie-lines lie at higher Ni contents than the equilibrium tie-line through the bulk composition. The tie-line shift was required to produce match-ing growth rates of Ni and C for the carbides. The addition of Ni slightly reduces the solubility of carbon in austenite and decreases the stability of the carbide phase. In addi-tion, the carbide is always Ni-poor relative to the coexisting metal phase(s).     

Solidification of M2 high speed steel

The freezing process in AISI type M2 high speed tool steel (6 pct W, 5 pct Mo, 4 pct Cr, 2 pct V, 0.8 pct C) was studied by metallographic and thermal analysis techniques. Unidirectional solidification of small laboratory melts in a modified crystal growing apparatus was employed to provide metallographic sections of known macroscopic growth direction. Also cooling curves were obtained on 40 g specimens solidified in thimble crucibles. X-ray microradiography, electron probe scanning techniques, and quantitative microanalysis of dendrites and interdendritic carbides were extensively used to supplement conventional metallography. Carbon and vanadium contents of M2 were varied in order to observe the effect of an austenite and ferrite stabilizer on the thermal analysis curves and microstructure. The nonequilibrium freezing process in M2 includes three major liquid-solid reactions: 1) Liquid → Ferrite, 1435°C; 2) Liquid + Ferrite → Austenite, 1330°C; 3) Liquid → Austenite + M₆C + MC, 1240°C. These reactions account for the as-cast structure of the commercial alloy. The addition of carbon depresses the liquidus (1) and solidus temperatures (3) and narrows the gap between the liquidus (1) and peritectic transformation (2). This gap is eliminated at > 1.39 wt pct C, where the initial freezing reaction is the crystallization of austenite. The accompanying microstructural change is the elimination of σ eutectoid dendrite cores. The addition of vanadium promotes ferrite formation by strongly depressing the peritectic reaction and thus widening the gap between the liquidus and the peritectic.     

Reaction Scheme and Liquidus Surface of the Ternary System Aluminum-Chromium-Titanium

The constitution of the ternary system Al-Cr-Ti is investigated over the entire composition range using X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), differential thermal analysis (DTA) up to 1500 °C, and metallography. Solid-state phase equilibria at 900 °C are determined for alloys containing ≤75 at. pct aluminum and at 600 °C for alloys containing >75 at. pct Al. A reaction scheme linking these solid-state equilibria with the liquidus surface is presented. The liquidus surface for ≤50 at. pct aluminum is dominated by the primary crystallization field of bcc β(Ti,Cr,Al). In the region >50 at. pct Al, the ternary L1₂-type phase τ forms in a peritectic reaction p _max at 1393 °C from L + TiAl. Furthermore, with the addition of chromium, the binary peritectic L + α(Ti,Al) = TiAl changes into an eutectic L = α(Ti,Al) + TiAl. This eutectic trough descends monotonously through a series of transition reactions and ternary peritectics to end in the binary eutectic L = Cr₇Al₄₅ + (Al).     

Melting equilibria in the iron-rich corner of the Fe-Ni-Cr system

Isothermal holding tests were carried out on the Fe-Ni-Cr system in the two-phase region between the melt and crystal in order to determine the γ and δ liquidus surfaces in the iron-rich corner, to establish the position of the peritectic line running between them, and to compare the results with those contained in relevant literature. The initial contents of the 13 test series were staggered in roughly equal intervals from 29.6% Ni and 12.9% Cr to 18.7% Ni and 28.4% Cr. They were held between the temperatures of 1450 and 1530 °C at intervals of 10 °C for 40 minutes in each case, so that 93 liquidus concentrations and temperatures were determined. Both liquidus surfaces can be described very accurately with corresponding mathematical equations and mathematically intersected in order to determine the course of the peritectic line. The results enable sufficiently accurate development of the melting equilibria of the entire ternary system, building on the basis provided by the known γ and δ liquidus lines of the three binary systems.     

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.     

Kinetics and phase transformation evaluation of Fe-Zn-Al mechanically alloyed phases

Fixed composition ratios of Fe and Zn corresponding to γ-(Fe₃Zn₁₁₀), Γ₁-(Fe₅Zn₂₁), δ-(FeZn₇), and ζ-(FeZn₁₃) with the addition of 5 pct Al (wt) were ball milled in an argon gas atmosphere to form homogenous alloys. Nonisothermal kinetic analyses of the mechanically alloyed materials, based on differential scanning calorimetry (DSC) measurements, revealed two diffusion-controlled processes during the evolution of the δ+5 pct Al and ζ+5 pct Al compositions with activation energies of 227±2 and 159±1 kJ/mole, respectively. Other endothermic and exothermic reactions detected for these compositions are consistent with the Fe-Zn-Al equilibrium phase systems with respect to the formation of the Fe₃Al, Fe₂Al₅, and δ-FeZn₇ phases Based on the evidence of FeAl₂ formation at 440 °C for the ζ+5 pct Al composition from X-ray diffraction (XRD) and DSC measurements, the revision/re-evaluation of the Fe-Zn-Al equilibrium phase diagrams is proposed. The Γ+5 pct Al and Γ₁+5 pct Al compositions evolved similarly through the same fields, except at 400 °C, where the former consisted of α-Fe + Γ + δ, while the later was without the Γ phase.     

The constitution of the system Al-Ti-B with reference to aluminum-base alloys

Direct and differential thermal analyses have been used to determine the liquidus temperatures and isothermals in the aluminum-base corner of the Al-Ti-B system, and with microscopical information and electron microprobe analyses provide results on which is based a provisional phase diagram. The TiB₂ liquidus rises very steeply from the aluminum corner of the diagram and is bounded on either side by monovariant reactions which almost coincide with the binary Al-B and Al-Ti liquidus curves. If the Ti:B ratio exceeds 7∶3 wt pct (1∶2 at. pct), TiB₂ precipitates on cooling until there occurs a monovariant reaction involving Al₃Ti, while at lower Ti:B ratios there occurs a monovariant reaction which involves AlB₂. The grain refining action of ternary alloys is discussed with reference to the form of the phase diagram.     

Melting equilibria in the iron-rich corner of the Fe-Ni-Mo system

Isothermal holding tests were carried out in the Fe-Ni-Mo system in the two-phase region between the liquidus (melt) and the solidus (crystal). The δ and γ liquidus surfaces in the iron-rich corner of the Fe-Ni-Fe₃Mo₂-NiMo subsystem, as well as the position of the peritectic line situated between them, could be very accurately established by the experimentally determined isotherms. Based on these results and on the known liquidus lines in the three boundary binary systems, as well as on the known concentration fields of the Fe₃Mo₂-NiMo solid solutions, the melting equilibria could be developed with sufficient accuracy in the entire subsystem.     

Kinetics of peritectic reaction and transformation in Fe-C alloys

In situ dynamic observation of the progress of a peritectic reaction and transformation of Fe-(0.14 pct C)- and Fe-(0.42 pct C)-peritectic Fe-C alloys has been successfully made with a combination of a confocal scanning laser microscope and an infrared image furnace. The peritectic reaction is characterized by the formation of the γ-austenite phase at the junction of the liquid and the grain boundary of δ-ferrite crystals and subsequent propagation of the three-phase point, liquid/γ/δ, along the liquid/δ boundary, whereas the peritectic transformation occurs by the thickening of the intervening γ toward both the liquid side and the δ side. The rates of the peritectic reaction for the two peritectic alloys are found to be much faster than the rate that would be controlled by carbon diffusion, suggesting that either massive transformation to γ or solidification as γ controls the rate. This is also the case for the Fe-0.14%C transformation in the hypoperitectic alloy. However, the rate of the peritectic transformation in the Fe-0.42%C alloy is determined by carbon diffusion. This article is based on a presentation made in the “Geoffrey Belton Memorial Symposium,” held in January 2000, in Sydney, Australia, under the joint sponsorship of ISS and TMS.     

Thermal transformations in mechanically alloyed Fe-Zn-Si materials

The ball milling of elemental powders corresponding to Γ (Fe₃Zn₁₀)+0.12 wt pct Si; Γ₁ (Fe₅Zn₂₁) + 0.12 wt pct Si; δ (FeZn₇)+0.12 wt pct Si; and ζ (FeZn₁₃)+0.12 wt pct Si composition ratios yields crystalline, mechanically alloyed phases. Differential scanning calorimetry (DSC) measurements of these materials show that they evolve differently, with well-defined characteristic stages. The activation energies for processes corresponding to these stages, based on kinetic analyses, are determined and correlated to microstructural evolvements. The processes occurring during the first stage below 250 °C, for all of the materials studied using X-ray diffraction (XRD) analysis, are associated with release of strain, recovery, and limited atomic diffusion. The activation energies for recovery processes are 120 kJ/mole for the Γ+0.12 wt pct Si, 131 kJ/mole for δ+0.12 wt pct Si, and 96 kJ/mole for ζ+0.12 wt pct Si alloys. At higher temperatures, recrystallization and other structural transformations occur with activation energies of 130 and 278 kJ/mole for Γ+0.12 wt % Si; of 161 kJ/mole for Γ₁+0.12 wt pct Si; of 167 and 244 kJ/mole for δ+0.12 wt pct Si; and of 641 kJ/mole for the ζ+0.12 wt pct Si. In addition, a eutectic reaction at 420 °C±3 °C, corresponding to the Zn-Si system, and a melting of Zn in Fe-Zn systems are observed for the ζ+0.12 wt pct Si material. The relation of FeSi formation in the Sandelin process is discussed.     

FeS-MnS phase relationships in the presence of excess iron

The FeS-MnS system is reexamined, both with and without excess iron. When excess iron is present, as is true for sulfide inclusions within steel, the pseudobinary reveals a peritectic rather than the previously assumed eutectic invariant. The maximum solubility limits (997 ± 3°C, or 1270 K) in the two solid phases are: a) 7.5 wt pct MnS in FeS, and b) 73.5 wt pct FeS in MnS. The peritectic liquid contains 66 wt pct Fe, ∼34 wt pct S, and ∼0.4 wt pct Mn. The two solid sulfide phases are nearly stoichiometric in the presence of excess iron; the Fe-richer sulfide is metal-deficient in the absence of a metallic iron phase. Based on this study, it is possible to be more specific than heretofore about the Fe-FeS-MnS-Mn region of the Fe-Mn-S ternary. In addition to the presence of a peritectic, it was concluded that the miscibility gap does not cross the univariant line between primary metal and (Mn,Fe)S phases. The peritectic liquid and the Mn-richer solid sulfide equilibrate with a metal containing ≤ 0.36 wt pct Mn. These data help explain the Mn/s ratios required to avoid hot-shortness in regular and resulfurized plain-carbon steels. G. S. MANN, formerly Graduate Student This is a part of the dissertation submitted by G. S. Mann for his Ph.D. at the University of Michigan     

Modeling of the peritectic reaction and macro-segregation in casting of low carbon steel

Macro-microscopic models have been developed to describe the macrosegregation behavior associated with the peritectic reaction of low carbon steel. The macrosegregation model has been established on the basis of previously published work and experimental data. A microscopic model of a three-phase reaction L+δ→γ has been modeled by using Fredriksson’s approach. Four horizontal and unidirectional solidified experimental groups simulating continuous casting have been performed with a low carbon steel containing 0.13 wt pct carbon. The extent of macrosegregation of carbon was determined by wet chemical analysis of millings. It is confirmed, by comparing calculated results with experimental results, that this model successfully predicts the occurrence of macrosegregation. The results indicate that a peritectic reaction which is associated with a high cooling rate generates high thermal contraction and a high tensile strain rate at the peritectic temperature. Therefore, the macrosegregation, particularly at the ingot surface, is very sensitive to the cooling rate, where extremely high positive segregation was observed in the case of a high cooling rate. However, in the case of slow cooling rate, negative segregation was noted. The mechanism of macrosegregation with peritectic reaction is discussed in detail.