Thermodynamic prediction of the eutectoid transformation temperatures of low-alloy steels

The experimental eutectoid transformation temperatures (A ₁) of low-alloy steels, as reported in the USS Atlas of I-T diagrams, have been compared to the thermodynamic predictions of a model proposed by Kirkaldy and Venugopalan. The analysis is consistent with the model prediction that Cr atoms are almost fully partitioned, while Ni and Mo atoms are scarcely partitioned, during the eutectoid transformation. This study also shows that Mn atoms are partitioned fully or partly in C-Mn, Cr-Mn, and Mo-Mn steels, while they are scarcely partitioned in Ni-Mn steels. The difference (ΔT) between the orthoequilibrium (OE) eutectoid temperature (A _e1) and the paraequilibrium (PE) eutectoid temperature (A _p1) has been investigated as a function of the content of each substitutional alloying element. The slope of ΔT increases with substitutions of Mo, Ni, Mn, Si, and Cr, with Mo having the least effect, Ni the next-greatest effect, and so on. Considering both Mn partitioning and the slope of ΔT, the equation for the prediction of A ₁ temperatures of low-alloy steels proposed by Kirkaldy and Venugopalan is modified. This new equation is in better agreement with the experimental A ₁ temperatures.     

Phase transformations in reactive sintering of tungsten

Abstract

It has been demonstrated recently that tungsten (T _m = 3410±20°C) can be sintered by reactive sintering in a reductive atmosphere such as hydrogen. This alternative technique to the conventional sintering (T _s>2000°C) makes use of a small amount of aluminium addition which acts as a sintering aid and hence lowers the sintering temperature significantly (T _s1200°C). This study explores the phase transformations that take place during reactive sintering of tungsten in view of the mechanisms involved. DSC, SEM and TEM have been used for a fundamental understanding of this system.     

Effect of prestrain and deformation temperature on the recrystallization behavior of steels microalloyed with niobium

The evolution of microstructure during the hot working of steels microalloyed with Nb is governed by the recrystallization kinetics of austenite and the recrystallization-precipitation interaction. The present study focuses on the effects of prestrain and deformation temperature on the rectrystallization behavior in these steels. The extent of recrystallization is characterized by a softening parameter calculated from a series of interrupted plane strain compression tests carried out at different deformation temperatures and strain levels. The results indicate that at low temperatures, softening is caused by static recovery, while at higher temperatures, static recrystallization is the predominant mechanism. The recrystallization-stop temperature (T _5pct) and the recrystallization-limit temperature (T _95pct), marking the beginning and end of recrystallization, respectively, are determined as a function of strain. In order to achieve a homogeneous microstructure, finish rolling should be carried out outside the window of partial recrystallization (T _5pct<T<T _95pct), as determined in this study. The Nb(CN) precipitation kinetics have been calculated using a model proposed in an earlier work, and these results are used to estimate the precipitate pinning force under the given processing conditions. Based on these estimations, a criterion has been proposed to predict the onset of recrystallization. The predicted results are found to be in reasonably good agreement with the experimental measurements.     

Effects of nitrogen on the mechanical behavior of hydrogenated V,Nb, and Ta

The effects of hydrogen, nitrogen, and combined nitrogen and hydrogen on the strength and ductility of V, Nb, and Ta have been investigated for temperatures from 295 to 78 K. The influence of nitrogen and hydrogen on the yield stress appeared to be additive. The effect of nitrogen on ductility was minimal, while hydrogen induced brittle behavior. The combined effect of nitrogen and hydrogen on ductility was generally similar to that of hydrogen alone. However, the addition of nitrogen decreased the temperature where hydrides were observed to form. In all cases studied the ductile-brittle transition temperature (DBTT) occurred above the temperature where hydride precipitation (T _s) was observed. However, there was no apparent correlation between DBTT andT _s when the results for V, Nb, and Ta were taken as a group. Current models relating the strength and ductility in these metals to hydride precipitates appear inadequate for explaining the results showing that the DBTT can occur at temperatures significantly aboveT _s (up to 140 K) and that the strengthening may not occur until temperatures are significantly belowT _s (up to 70 K). Formerly with the Ames Laboratory     

Grain boundary faceting and abnormal grain growth in nickel

A correlation between grain boundary faceting and abnormal grain growth has been observed in recrystallized polycrystalline Ni at varying annealing temperatures, with or without C added. Carburized Ni specimens deformed to 50 pct show faceted grain boundaries and abnormal grain growth when annealed at temperatures below 0.7 T _m, where T _m is the melting point of Ni in absolute scale. When annealed at or above 0.7 T _m, the grain boundaries are smoothly curved and, therefore, have a rough structure, and normal grain growth is observed. In the specimens annealed in vacuum without carburization, all grain boundaries are faceted at 0.55 T _m, and some of them become defaceted at higher temperatures. The specimens annealed in vacuum at temperatures between 0.55 and 0.95 T _m show abnormal grain growth. When the grain boundaries have a rough structure and are, therefore, nearly isotropic, normal grain growth is indeed expected, as shown by the simulation and analytical treatment. When all or a fraction of the grain boundaries are faceted, with the facet planes corresponding to the singular cusp directions in the variation of the boundary energy against the inclination angle, abnormal grain growth can occur either because some grain boundary junctions become immobile due to a torque effect, or the growth occurs by a step mechanism.     

Thermal-Mechanical fatigue of Ti-48Al-2V alloy and its composite

Thermal-mechanical fatigue (TMF) and isothermal fatigue (IF) of a Ti-48Al-2V alloy and its composite, reinforced with TiB₂ particles, were studied. In-phase TMF testing was conducted under the condition of a minimum temperatureT _min = 100 °C and a maximum temperatureT _max, which ranged from 750 °C to 1400 °C. The applied cyclic stress ranges were 2.8 to 28 MPa and 4.2 to 42 MPa. The IF tests were carried out at aT _max. The TMF and IF lives are longer for lowerT _max and for smaller stress ranges in both the matrix alloy and its composite. The IF life at a givenT _max is shorter than the TMF life in the matrix alloy at all temperatures employed and in the composite only at higher temperatures. At lower temperatures, the TMF and IF lives are essentially the same in the composite. The resistance to TMF is similar in the matrix alloy and the composite, but the IF resistance is greater in the composite than in the matrix alloy. The proposed TMF mechanism is nucleation and growth of voids on interlamellar plate, twin, and grain boundaries; their interlamellar, translamellar, and intergranular linkage; intergranular separation; and disintegration of lamellar structure.     

Numerical simulation of fluid flow and solidification in continuous slab casting mould based on inverse heat transfer calculation

Abstract

A mathematical model based on an inverse heat transfer calculation was built to determine the heat flux between the mould and slab based on the measured mould temperatures. With K–? turbulence model, a mathematical model of three-dimensional heat transfer and solidification of molten steel in continuous slab casting mould is developed. Solidification has been taken into consideration, and flow in the mushy zone is modelled according to Darcy’s law as is the case of flow in the porous media. The heat flux prescribed on the boundaries is obtained in the inverse heat conduction calculation; thus, the effect of heat transfer in the mould has been taken into consideration. Results show that the calculated values of mould temperature coincide with the measured ones. Results also reveal that the temperature distribution and shell thickness are affected by the fluid flow and heat transfer of slab which is governed by the heat flux on the mould/slab interface.     

Mould heat transfer and continuously cast billet quality with mould flux lubrication Part 2 Quality issues

Abstract

With many billet producers adopting mould powder lubrication, there is a need to clarify the gains in quality that can be achieved with this practice. Over the past three decades considerable research has been conducted to establish the relationship between mould behaviour and defect formation for billets continuously cast with oil lubrication, but little has been done to compare oil cast billets with powder cast billets. In this study, conducted at a Canadian minimill, four faces of a copper mould were instrumented with thermocouples and mould temperatures and billet quality were monitored with mould powder lubrication during casting of 208 × 208 mm billets. In the first part of this two part series (in Ironmaking & Steelmaking No. 1 2000), the results of the mould heat transfer analysis and the influence of variables were presented, together with a comparison between oil and powder lubrication. In the present paper, Part 2, billet quality is examined in detail. The difference in turbulence at the meniscus between oil and powder lubrication is established, and the need to tune mould level sensors when switching to mould powders is demonstrated. Previous work has shown that mould level fluctuations have a strong influence on defects such as offsquareness and transverse depressions, both of which are markedly reduced when casting with mould powders. The inherent stability of the meniscus is improved when employing mould powder lubrication and a submerged entry nozzle. Furthermore, the significant reduction in mould heat transfer at the meniscus, when mould powders are employed, particularly for medium carbon steels has been shown to correlate well with the observed reduction in offsquareness. The paper also elucidates the reasons for the reduction, and in most cases, elimination of transverse depressions in B–Ti grades when casting with mould powders. The mechanism of longitudinal depression formation and subsurface cracking observed in many of the powder cast, medium carbon billets has also been established.     

Grain boundary faceting and abnormal grain growth in nickel

A correlation between grain boundary faceting and abnormal grain growth has been observed in recrystallized polycrystalline Ni at varying annealing temperatures, with or without C added. Carburized Ni specimens deformed to 50 pct show faceted grain boundaries and abnormal grain growth when annealed at temperatures below 0.7 T _m , where T _m is the melting point of Ni in absolute scale. When annealed at or above 0.7 T _m , the grain boundaries are smoothly curved and, therefore, have a rough structure, and normal grain growth is observed. In the specimens annealed in vacuum without carburization, all grain boundaries are faceted at 0.55 T _m , and some of them become defaceted at higher temperatures. The specimens annealed in vacuum at temperatures between 0.55 and 0.95 T _m show abnormal grain growth. When the grain boundaries have a rough structure and are, therefore, nearly isotropic, normal grain growth is indeed expected, as shown by the simulation and analytical treatment. When all or a fraction of the grain boundaries are faceted, with the facet planes corresponding to the singular cusp directions in the variation of the boundary energy against the inclination angle, abnormal grain growth can occur either because some grain boundary junctions become immobile due to a torque effect, or the growth occurs by a step mechanism.     

Reaction sequences in the formation of silico-ferrites of calcium and aluminum in iron ore sinter

Complex silico-ferrites of calcium and aluminium (low-Fe form, denoted as SFCA; and high-Fe, low-Si form, denoted as SFCA-I) constitute up to 50 vol pct of the mineral composition of fluxed iron ore sinter. The reaction sequences involved in the formation of these two phases have been determined using an in-situ X-ray diffraction (XRD) technique. Experiments were carried out under partial vacuum over the temperature range of T=22 °C to 1215 °C (alumina-free compositions) and T=22 °C to 1260 °C (compositions containing 1 and 5 wt pct Al₂O₃) using synthetic mixtures of hematite (Fe₂O₃), calcite (CaCO₃), quartz (SiO₂), and gibbsite (Al(OH)₃). The formation of SFCA and SFCA-I is dominated by solid-state reactions, mainly in the system CaO-Fe₂O₃. Initially, hematite reacts with lime (CaO) at low temperatures (T ∼ 750 °C to 780 °C) to form the calcium ferrite phase 2CaO·Fe₂O₃ (C₂F). The C₂F phase then reacts with hematite to produce CaO·Fe₂O₃ (CF). The breakdown temperature of C₂F to produce the higher-Fe₂O₃ CF ferrite increases proportionately with the amount of alumina in the bulk sample. Quartz does not react with CaO and hematite, remaining essentially inert until SFCA and SFCA-I began to form at around T=1050 °C. In contrast to previous studies of SFCA formation, the current results show that both SFCA types form initially via a low-temperature solid-state reaction mechanism. The presence of alumina increases the stability range of both SFCA phase types, lowering the temperature at which they begin to form. Crystallization proceeds more rapidly after the calcium ferrites have melted at temperatures close to T=1200 °C and is also faster in the higher-alumina-containing systems.     

Synthesis of new bulk metallic glassy Ti60Al15Cu10W10Ni5 alloy by hot-pressing the mechanically alloyed powders at the supercooled liquid region

The mechanical alloying method has been used to fabricate multicomponent Ti₆₀Al₁₅Cu₁₀W₁₀Ni₅ glassy alloy powders, using a low-energy ball-milling technique. The glassy powders that were obtained after 720 ks of milling have a sphere-like morphology with an average particle size of 0.38 μm in diameter. This new glassy alloy exhibits a glass transition temperature (T _g) at 733 K. It crystallizes at a crystallization temperature (T _x ) of 804 K through a single sharp exothermic peak, with an enthalpy change of crystallization (ΔH _x ) of −5.20 kJ/mol. The supercooled liquid region before crystallization ΔT _x of the obtained glassy powders shows a large value (71 K). The reduced glass transition temperature (ratio betweenT _g and liquidus temperatures,T ₁(T _G /T ₁) was found to be 0.46. The synthetic glassy powders were uniaxial hot-pressed into consolidated round objects with large dimensions (20 mm in diameter × 30 mm in height) in an argon gas atmosphere at several temperatures with a pressure of 936 MPa. The samples that were consolidated at the temperature range of 755 to 775 K (within the ΔT _x region) are fully dense (∼99.85 pct) and maintain the chemically homogeneous glassy structure. These hot-pressed glassy samples exhibit excellent mechanical properties for Ti-base metallic glasses. They have high Vickers microhardness values, in the range between 8.0 and 8.2 GPa. They also show high fracture strength (2.28 GPa) with an extraordinarily high Young’s modulus of 153 GPa. Neither yielding stress nor plastic strain could be detected for this glassy alloy, which shows an elastic strain of 1.39 pct.     

Bulk amorphous metallic alloys: Synthesis by fluxing techniques and properties

Bulk amorphous alloys having dimensions of at least 1 cm in diameter have been prepared in the Pd-Ni-P, Pd-Cu-P, Pd-Cu-Ni-P, and Pd-Ni-Fe-P systems using a fluxing and water-quenching technique. The compositions for bulk glass formation have been determined in these systems. For these bulk metallic glasses, the difference between the crystallization temperature (T _x) and the glass transition temperature (T _g, ΔT=T _x−T _g) ranges from 60 to 110 K. These large values of ΔT open the possibility for the fabrication of amorphous near-net-shaped components using techniques such as injection molding. The thermal, elastic, and magnetic properties of these alloys have been studied, and we have found that bulk amorphous Pd₄₀Ni_22.5Fe_17.5P₂₀ has spin glass behavior for temperatures below 30 K. This article is based on a presentation made in the “Structure and Properties of Bulk Amorphous Alloys” Symposium as part of the 1997 Annual Meeting of TMS at Orlando, Florida, February 10–11, 1997, under the auspices of the TMS-EMPMD/SMD Alloy Phases and MDMD Solidification Committees, the ASM-MSD Thermodynamics and Phase Equilibria, and Atomic Transport Committees, and sponsorship by the Lawrence Livermore National Laboratory and the Los Alamos National Laboratory.     

Investigation on viscosity of mould fluxes during continuous casting of aluminium containing TRIP steels

Abstract

Transformation induced plasticity steel with a high content of Al (Al-TRIP) is an advanced high strength steel. The reaction between Al and SiO₂ would lead to compositional changes in mould fluxes, which results in a decrease in SiO₂ content and an increase in Al₂O₃ content, and influences the physical properties of the molten fluxes. In the present study, the changes of viscosities of a series of mould fluxes with different additives and addition amounts were examined by a rotating viscometer and the crystals in the D series mould fluxes were analysed by scanning electron microscopy and X-ray diffraction. The results show that (a) the Al₂O₃ in mould fluxes containing Li₂O presents amphoteric characteristic; (b) the viscosity of the fluxes with 8% B₂O₃ decreases with the increase in w(Al₂O₃)/w (SiO₂) ratio, the result shows that the Al₂O₃ presents alkaline; (c) the viscosity of flux containing both 4% Li₂O and 5% B₂O₃ changes a little with the increase in w(Al₂O₃)/w(SiO₂) ratio. When the Al₂O₃ content is 30%, the viscosity is 0·204 Pa s and the break temperature is 1130°C; (d) mould fluxes used for Al-TRIP steel precipitate CaF₂ crystals; (e) base on the Riboud model, a suitable viscosity mathematical model for the mould fluxes with high Al₂O₃ content is established.     

Localized corrosion susceptibility of Al-Li-Cu-Mg-Zn alloy AF/C458 due to interrupted quenching from solutionizing temperature

Isothermal time-temperature-localized corrosion-behavior curves were determined for the Al-1.8Li-2.70Cu-0.6Mg-0.3Zn alloy AF/C458, to understand the effect of slow or delayed quenching on localized corrosion susceptibility. Alloy samples were subject to a series of systematic interrupted quenching experiments conducted at temperatures ranging from 480 °C to 230 °C for times ranging from 5 to 1000 seconds. Individual samples were then exposed to an oxidizing aqueous chloride solution consisting of 57 g/L NaCl plus 10 mL/L H₂O₂ to induce localized attack. The localized corrosion mode was characterized by optical microscopy. Additionally, the microstructure of selected samples was characterized by transmission electron microscopy (TEM) to relate the corrosion mode and morphology to microstructural features. Results showed that only pitting attack was exhibited by samples subjected to isothermal treatment at temperatures greater than 430 °C. At temperatures ranging from 280 °C to 430 °C, isothermal treatment tended to induce susceptibility to intergranular attack (IGA) and intersubgranular attack (ISGA) for all treatment times investigated. For isothermal treatments at temperatures lower than 280 °C, only pitting was observed for treatment times less than about 30 seconds, while IGA and ISGA were observed for longer treatment times. Comparisons showed that the time-temperature domains for IGA and ISGA were virtually coincident. Based on this finding and the results from TEM characterization, IGA and ISGA appear to be related to the precipitation of a Zn-modified T ₁ (Al₂(Cu,Zn)Li) precipitate, which can occur both on low-angle and high-angle grain boundaries in this alloy. When the alloy is resistant to IGA and ISGA, the grain boundaries are decorated by θ′ (Al₂Cu), and T _B (Al₇Cu₄Li) phase particles, or subgrain boundaries are populated by a comparatively low density of T ₁ precipitates. It is, therefore, speculated that θ′ and T _B are more corrosion-resistant precipitate phases than T ₁, and that a critical concentration of boundary T ₁ must exist for IGA or ISGA to occur.     

Mould heat transfer and continuously cast billet quality with mould flux lubrication Part 1 Mould heat transfer

Abstract

With the drive to cast higher quality, many minimills are adopting mould powder as a lubricant for the continous casting of steel billets. Over the past three decades considerable experience has been accumulated on the relationship between mould behaviour and billet quality for oil lubrication, but comparatively few studies have been conducted for mould powder lubrication. This study, conducted at a Canadian minimill, involved instrumenting four faces of a copper mould with thermocouples and monitoring mould temperatures during casting of 208 × 208 mm billets with mould flux lubrication. Billet samples were also taken to coincide with periods of measurements. Mould temperatures were monitored for two different mould powder compositions, for different mould oscillation frequencies, two mould cooling water velocities, and a range of steel compositions. An inverse heat conduction model was developed to calculate mould heat transfer from the measured temperatures. In this paper, which is the first part of a two part series, details of the inverse heat conduction model and mould heat transfer data are presented. The results obtained for mould flux lubrication have been compared with those for mould heat transfer for oil lubrication. For peritectic steels, with carbon content in the range 0·12–0·14%, it was found that lubricant type has little influence on the measured mould heat flux distribution at the centreline of a face. The peak mould heat flux was found to be approximately 2500 kW m^-2 . In contrast, for medium carbon steels, mould heat transfer with mould powder was significantly lower than when oil was employed as a lubricant. For instance, at the meniscus, the peak heat flux with mould powder was approximately 2500 kW m^-2 , which was half that recorded with oil as a lubricant. The influence of oscillation frequency, mould cooling water velocity, and mould powder type on mould heat flux has also been presented.     

Activity measurement of the constituents in liquid Cu-Mg and Cu-Ca alloys with mass spectrometry

A mass spectrometer has been used to study activities of the constituents in liquid Cu-Mg and Cu-Ca alloys. Activities and heats of mixing have been determined from a series of measurements of the ratios of the ion current intensity of the solution components according to the Belton-Fruehan treatment. The activities in both systems exhibit negative deviation from ideality and tend to approach an ideal behavior at higher temperatures. The terms of RT ln γ _Cu and RT ln γ _Mg show very small temperature dependence within the present experimental range and agree well with the values reported by Garg et al. for the Cu-Mg system. The terms of RT ln γ _Ca and RT ln γ _Cu are independent of temperature and in excellent agreement with the values assessed by Risold et al. for the Cu-Ca system. Relative partial molar enthalpy and integral heat of mixing in the liquid Cu-Mg and Cu-Ca systems were also estimated. 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.     

Oxidation protective silicide coating on Mo-Si-B alloys

A MoSi₂ coating was successfully formed on a Mo-9Si-18B alloy, consisting of Mo₅SiB₂ (T₂) and Mo solid solution (Mo _ss ) phases, using pack cementation with Si. Isothermal and cyclic oxidation tests of pack-cemented Mo-9Si-18B alloys were performed at 1300 °C and 1500 °C. Steady-state oxidation rates at both temperatures are almost equal to those of pure MoSi₂. The MoSi₂ layer is completely transformed into Mo₅Si₃ (T₁) containing B after oxidation at 1500 °C for 24 hours. Thermal expansion of the T₁ phase is anisotropic, but a [001] texture in the growth direction for the columnar grains in the T₁ layer reduces thermal stresses generated around the phases. Evolution of T₁ layers during oxidation between 1300 °C and 1500 °C was investigated; their growth rate constants and the interdiffusion coefficient of Mo and Si in the Mo-Si-B system have been evaluated and compared with those in the binary Mo-Si system. Furthermore, we have studied phase transformations in a simpler system MoSi₂ vs T₂ using MoSi₂/T₂ diffusion couples. Layers of T₁ and MoB + T₁ were formed in the diffusion zone during oxidation at temperatures between 1400 °C and 1600 °C. This behavior is different from that of the pack-cemented Mo-9Si-18B alloy. Pack-cemented T₂ single crystals show a diffusion structure similar to that of MoSi₂/T₂ diffusion couples, but the ratio of layer thickness is different. Based on these diffusion results, a method for extending the lifetime of the MoSi₂ layer is proposed. This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee.     

Calculation of the titanium-aluminum phase diagram

The Ti-Al phase diagram has been calculated by optimization of Gibbs energies with respect to phase diagram and thermochemical data.T ₀ curves, the locus of compositions and temperatures where the Gibbs energies of the liquid and one of the solid phases are equal, have been calculated over the entire composition range. In order to assure physically reasonable extrapolations of theT ₀ curves of the ordered phases far from their equilibrium stability ranges, the Bragg-Williams approximation was used to provide start values for the empirical optimizations. This approximation led to good convergence of the optimizations, and only small deviations from the Bragg-Williams Gibbs energies were needed to obtain excellent agreement with experimental data.     

Effects of Changes in Chemistry and Testing Temperature on Mechanical Behavior of Al-Based Amorphous Alloy Ribbons

The effects of changes in composition, testing temperature, strain rate, and thermal exposure on the flow behavior of a series of Al-Gd-Ni-X amorphous alloy ribbons have been determined via hot microhardness and tension testing. It is shown that the addition of Fe, Co, and Fe/Co combination into these materials increases the strength, T_g, and T _x1 in addition to the activation energy for crystallization, whereas the window between T_g and T _x1 remains similar. The uniaxial tensile tests show these ribbons exhibit a high strength, around 1 GPa, at room temperature (RT), and the results also show that these ribbons maintain their strength, nearly 45 pct of their RT value, at temperatures near T_g. Scanning electron microscopy images of fracture surfaces obtained from tests conducted near T_g illustrate ductile rupture and homogeneous flow behavior near the fracture tip.