Strengthening of steel by the method of spraying oxide particles into molten steel stream

A new technique, Spray-Dispersion Method, which produces a steel containing homogeneously dispersed fine oxide particles sprayed from outside into the molten steel stream has been developed. The conditions for the distribution of particles in solid steel, and the mechanical properties of A1₂O₃- or ZrO₂-dispersed steels were studied. The homogeneous distribution of fine oxide particles is obtained by the addition of a certain suitable controlling element which lowers the contact angle of molten steel on various oxides and the interfacial tension at the oxide—molten steel interface. Among others columbium (niobium) was found to be the most effective on decreasing the average particle size of sprayed oxides. Because of fine dispersion of particles of less than 120 nm, the strength of steels increased with their volume fractions; for example, 1.15 vol pct ZrO₂ causes an increment of 79 MN/m² in proof strength and 94 MN/m² in tensile strength. For practical applications, the Spray Dispersion Method makes it possible to produce 18 chromium-8 nickel austenitic stainless steels dispersion-strengthened by A1₂O₃ or ZrO₂ particles, and ThO₂-dispersed nickel with an average particle size of 61.4 nm and a volume fraction of 5.1 pct.     

Oxide dispersion strengthened nickel-base heat resistant alloys by means of the spray-dispersion method

The spray-dispersion method, which produces a metal containing homogeneously dispersed fine oxide particles sprayed from outside into the molten metal stream, was applied to HASTELLOY^* X and Ni−Cr−W alloy. The condition of the dispersion of ZrO₂ particles and the mechanical properties were examined for ZrO₂ dispersed alloys. The addition of a dispersion controlling element, Nb or Ta, was found to be effective for decreasing the average particle diameter of sprayed particles and for increasing the volume fraction of the particles. Ultimate tensile strength and 0.2 pct proof strength were increased by ZrO₂ particle dispersion. And creep strength of ZrO₂ particle dispersed alloys was greater than that of non-dispersed ones. From these results, the production of oxide dispersed nickelbase heat resistant alloys has been established by the spray-dispersion method.     

Reactive sintering in Fe-Co system

Abstract

Low porosity powder metallurgy compacts have been manufactured from treated elemental iron and cobalt powders sintered at 1150°C under an H_2(g) atmosphere. Their microstructures consist of an interconnected mixed oxide network which encapsulates both the iron and cobalt phases. The production technique employed is an innovative process termed reacto-thermitic sintering (RTS), which leads to near full density and near net shape parts utilising conventional uniaxial compaction and mesh belt furnace practices. The RTS technique relies on microscale exothermic reaction between small quantities of added elemental Al and oxides present on the surface of the bulk powder, together with the bulk powder itself. This results in the production of a transient liquid phase which freezes rapidly and consolidates the compact without slumping. In order to generate an interconnected mixed oxide network, experiments were designed such that the Al powder reacts with the cobalt and the surface of the iron powder which is artificially doped with Fe and Cr oxides.

Differential thermal analysis (DTA) and energy balance calculations revealed that the Al and the oxide coating reaction does not proceed directly. Instead the main contribution to the exothermic process is the reaction between Al and Co/Fe. The system does not exhibit true RTS behaviour and the interconnected network of mixed Al, Cr, and Fe oxides is created by subsequent reaction of Co-Al and Fe-Al intermetallics with the artificial Fe-Cr oxide coating on the Fe. The microstructure obtained exhibits negligible porosity with the metallic particles on the whole fully encapsulated by the oxide.     

Effect of the method of introduction of Y2O3 into NiAl-based powder alloys on their structure: I. Agitation in a ball mill

The effect of the sintering temperature (1100–1400°C) of NiAl alloy samples with oxide Y₂O₃ produced by hydrostatic pressing on their structure and phase composition and the distribution of oxide particles in a NiAl-based intermetallic matrix alloyed with ～0.5 at % Fe is considered. It is found that dispersed oxide particles in the compact material prepared from a mixture of oxide Y₂O₃ powder and a NiAl alloy (produced by calcium hydride reduction of a mixture of nickel and aluminum oxides) powder in a standard ball mill are nonuniformly distributed in the volume. The morphology of oxides changes during sintering: sintered samples contain rounded particles, which differ strongly from the clearly faceted angular particles of oxide Y₂O₃ added to a mixture (they represent conglomerates of single crystals). In the sintered samples, large aggregates of oxides are revealed along grain boundaries. Mass transfer is possible at the NiAl/Y₂O₃ interface in the system: it leads to partial substitution of aluminum and/or iron atoms for yttrium atoms in the Y₂O₃ lattice and to the formation of submicroscopic particles of (Fe,Al)₅Y₃O₁₂-type oxides.     

Annealing behavior of submicrocrystalline oxide-bearing iron produced by mechanical alloying

The structural changes upon annealing of an ultrafine-grained iron containing dispersed oxides were studied. The starting material was subjected to mechanical milling followed by consolidating bar rolling. The fine-grained microstructures were essentially stable against discontinuous grain growth and/or primary recrystallization during annealing at temperatures up to 800 °C, where the specimens maintain a very fine ferrite grain size. The annealing behavior is mainly characterized by normal grain growth accompanied by recovery. The grain-growth kinetics correlates with the oxide coarsening. Both the grain growth and the oxide coarsening can be enhanced by an increase in the temperature. The static restoration/recrystallization processes operating in the ultrafine-grained matrices, as well as the effect of dispersed oxides particles, are discussed in some detail.     

OBSERVATIONS ON THE EFFECT OF VACUUM HEAT-TREATMENT (UP TO 4000°F) ON DISPERSIONS OF VARIOUS OXIDES,NITRIDES, AND BORIDES IN MOLYBDENUM

Abstract

The compatibility between molybdenum and several refractory compounds present as dispersed phases in the 3000-4000°F (1650–2210°C) temperature range has been evaluated. Compounds included in the study were BN, ZrN, HfN, LaB₆, ZrB₂, TaB₂, Y₂O₃, MgO, ZrO₂, and ThO₂. Hot-pressed alloys, consisting of nominally 2.5 vol.%, of the refractory phase in a molybdenum matrix, were vacuum-annealed to determine elevated-temperature stability. The evaluation was based on changes in grain size, dispersed-phase morphology, hardness, and gross macroscopic deterioration. The materials HfN, ZrO₂, and ThO₂ showed promise for dispersion-strengthening and were recommended for further development.     

High-temperature oxidation of composite AlN-ZrB2-ZrSi2 ceramics

The high-temperature (to 1450°C) oxidation of AlN-ZrB₂-ZrSi₂ powders and compact ceramic materials with different contents of ZrB₂-ZrSi₂ solid solution in air is examined using scanning electron microscopy and differential thermal, thermogravimetric, x-ray phase, and x-ray spectrum microanalyses. It is established that the hot-pressed (practically porousless) ceramic materials have high corrosion resistance up to 1350–1400°C when scale components (individual oxides) interact with each other to form solid solutions based on aluminum oxide, zirconium oxide, and aluminum borate. These phases become sintered in the presence of the liquid B₂O₃ phase, self-reinforced scale being formed. __________ Translated from Poroshkovaya Metallurgiya, Vol. 47, No. 1–2 (459), pp. 196–203, 2008.     

Oxidation of Aluminium Alloy Melts and Inoculation by Oxide Particles

One of the main concerns in recycling aluminium alloy scrap is the removal of oxide inclusions. Understanding the nature and behaviour of oxide films in the alloy melts is an important step for developing efficient recycling technologies. In this work, characterisation of oxides formed in pure Al and Al?CMg alloy melts was carried out. In commercially pure Al melt, ??-Al₂O₃ platelets and ??-Al₂O₃ particles were found to form at 750 and 920?°C, respectively. The oxides were in the form of liquid-like films consisting of numerous individual particles. The addition of 0.49 and 0.70?wt% Mg resulted in the formation of MgAl₂O₄, and the MgAl₂O₄ particles were {1 1 1} faceted and had a cube-on-cube orientation relationship with ??-Al. The MgAl₂O₄ films were also liquid-like in which large numbers of the particles were held by the melt. Grain refinement was achieved by intensive shearing of the melts prior to solidification. It is believed that intensive melt shearing broke up the oxide films and dispersed the potent oxide particles which in turn enhanced the heterogeneous nucleation, resulting in grain refinement. The potency of the oxide particles and the mechanism of the inoculation by the oxides were discussed on the basis of the TEM results and theoretical analysis of the lattice misfits at the interfaces along specific orientation relationships.     

冶炼钢铁过程中多种固体废物的鉴别

在钢铁的冶炼过程中,主要产生炉渣、除尘灰和氧化皮等固体废物,其中氧化皮是国家规定可以进口的固体废物,炉渣和除尘灰属于不能进口的固体废物。实验针对冶炼钢铁过程中产生的固体粉末进行鉴别,首先利用肉眼和扫描电镜(SEM)对样品进行初步判断,例如炉渣的外观不是正常天然矿物的块状或粉状,而除尘灰颗粒较细、较轻,氧化皮呈鳞片状、有金属光泽。再利用X射线荧光光谱(XRF)对制得粉末中的元素进行分析,炉渣的主要元素为钙、硅、镁和铝,铁的含量极低;而除尘灰中铁的含量很高,同时含有锌和钙;氧化皮的主要元素也是铁。最后利用X射线衍射(XRD)技术对粉末中存在的物相进行分析,从而推断出固体粉末的属性,炉渣中的主要物质是CaO-MgO-Al₂O₃-SiO₂形式存在的配合物;除尘灰中的主要物质是铁的氧化物以及一些氧化锌;氧化皮的主要物质也是铁的氧化物,其中氧化亚铁的含量高。通过实验建立了这3种固体废物的鉴别方法,对进口固体废物的监管提供指导。     

MECHANISMS OF THE REDUCTION OF ZINC FERRITES IN H2/N2 GAS MIXTURES

The rates of reduction of dense pure zinc ferrite and zinc ferrite containing 1 wt% of CaO, MgO, MnO, and Al₂O₃ in solid solution reduced with H₂/N₂ gas mixtures have been investigated at temperatures between 500 and 1100°C. The rate measurements obtained in these studies were complemented with the structural characterization of the partially reduced zinc ferrite samples. The presence of impurity oxides in ZnFe₂O₄ solid solution influenced the conditions for formation of the porous iron morphology. Where porous iron-type microstructures were formed during the reduction reaction, the pore structure of the reduced samples was found to be dependent on reduction temperature, hydrogen partial pressure, and type of impurity oxide added into the solid solution. The reduction plots of zinc ferrite solid solutions exhibited an initial linear rate followed by a gradual decrease in rate with increased percentage reduction. The changes in growth mechanisms and reduction kinetics that occur with changing reduction conditions are discussed.     

Thermal stability of finely dispersed ferromagnetic powders

The oxidation mechanism and kinetics have been examined for finely dispersed powders of iron and compounds containing it and noble and platinum metals; these powders were made by a thermochemical method and examined at 20–400°C. The powders do not change in composition and magnetic characteristics up to 110°C because the surfaces of the particles are protected by iron oxides and carbide. This allows the powders to be used to make materials for medical purposes, since they can be sterilized at that temperature. Translated from Poroshkovaya Metallurgiya, Nos. 5–6(413), pp. 1–4, May–June, 2000.     

The Influence of calcia and magnesia in wustite on the kinetics of metallization and iron whisker formation

Calcium oxide andJor magnesium oxide are introduced in different ways to high purity wustite powders, followed by metallization in mixtures of CO, CO₂, and N₂ at 750, 900, and 1100 °C. A new technique is developed to study the nucleation and growth of iron where wustite plates are coated with thin films of CaO of MgO prior to metallization. Calcia, in solid solution with wustite, influences the kinetics of metallization and the morphology of iron. It is reasonably confirmed that nucleation and outward growth of iron, to form iron whiskers, occur at sites higher in CaO concentration on the wustite surface. MgO, on the other hand, has almost no noticeable effect on the iron whisker formation.     

The investigation of the aluminothermic reduction of dissolved ilmenite in molten cryolite

In this research, aluminothermic reduction of dissolved oxides in cryolite-based molten salts is investigated. It isacheaperand most effective method ofthe production ofaluminium-based alloys in comparison to the other common methods. Two sets of experiments were conducted. In the first set of experiments, ilmenite was dissolved in cryolite at 770 and 1030°C. A saturated solution of ilmenite in cryolite was made. Pure aluminium was added to the saturated solution in order to reduce the dissolved ilmenite. The experiments were conducted in an ambient atmosphere. An aluminiumbased alloy containing iron and titanium aluminides phases was produced. In the second set of experiments, at first iron oxide content of ilmenite was reduced and separated of the solution. Then pure aluminium was added to the remaining solution in order to reduce the dissolved TiO₂.Finally a master alloy of aluminium titanium with 21.1 wt-%, Ti was produced.     

Phase relations and thermodynamics of the system Fe-Cr-0 in the temperature range of 1600 °C to 1825 °C (1873 to 2098 K) under strongly reducing conditions

Equilibrium relations involving alloy and oxide phases in the system Fe-Cr-O were determined in the temperature range from 1600 °C to 1825 °C (1873 to 2087 K). Compositions of coexisting alloy and spinel phases were established as a function of oxygen pressure by equilibrating liquid Fe-Cr alloys with iron chromite (Fe_3-xCr_xO₄) solid solutions at 1600 °C and 1700 °C. Combinations of these experimental data and thermodynamic calculations were used to construct composition-oxygen pressure diagrams for the system at 1600 °C and 1700 °C. Additional runs for selected mixtures were made at still higher temperatures (1700 °C to 1825 °C), and thermodynamic parameters were derived for spinel-containing phase assemblages at temperatures up to 1865 °C. The spinel phases occurring in the present system are typically in the high-chromium range of the solid-solution series Fe₃O₄-Cr₃O₄,i.e., in the range between stoichiometric iron chromite (FeCr₂O₄) and Cr₃O₄. The activities of the various oxide components of the spinel solid solution at 1600 °C were calculated from experimentally determined parameters for coexisting alloy and spinel phases, as well as by statistical-mechanical modeling of the same spinel solid solution based on crystal-chemical considerations. The agreement between the two sets of results was excellent. Temperature variation of parameters characterizing the univariant equilibria spinel + Cr₂O₃ + alloy and spinel + alloy + liquid oxide was established. The univariant curves were found to display temperature maxima of 1715 °C ± 5 °C and approximately 1865 °C, respectively. In analogy with relations in the Cr-O system, the increase in divalent chromium of the liquid oxide phase with decreasing oxygen potential was identified as the main cause of the sharp decrease in liquidus temperatures of chromites in contact with Fe-Cr alloys of high Cr contents. Formerly Graduate Research Assistant, Department of Metallurgy, The Pennsylvania State University L.S. DARKEN and ARNULF MUAN, formerly Professors of Geochemistry and Materials Science, The Pennsylvania State University, University Park, PA 16802, are deceased.     

On sintering of Ti–Ni (–TiB2) alloys to near full density

Abstract

Using a combination of mixed elemental powders and TiB₂, a series of Ti–Ni and Ti–Ni–B alloys were optimised for sintering by varying the nickel and boron contents, the particle size of the elemental powders and the compaction pressure. The sintering temperature was maintained at 1200°C to limit the costs of a potential commercial sintering operation. For Ti–Ni alloys, a density of 99% was attained in Ti–7Ni made using fine Ti and Ni powders sintered in the solid state, and from liquid phase sintering of Ti–8Ni made using coarser powders. Porosity was almost eliminated from Ti–7Ni–xB alloys made by adding 1–3%TiB₂ to the coarser Ti and Ni powders. The action of TiB₂ as a sintering aid is possibly owing to a combination of the formation of a small amount of liquid at the sintering temperature and the restriction of grain growth owing to the presence of TiB particles.     

Effects of oxygen atmosphere,FeOx and basicity on mineralogical phases of CaO–SiO2–MgO–Al2O3–FetO–P2O5 steelmaking slag

ABSTRACT

The mineralogical phase of slag after crystallisation is essential to utilisation of steelmaking slag. The mineralogical phases of cooled multicomponent CaO–SiO₂–MgO–Al₂O₃–Fe_tO–P₂O₅ slag with different iron oxide contents and basicities (defined as the ratio of mass percentage of CaO to mass percentage of SiO₂ (w(CaO)/w(SiO₂))) in different atmospheres were investigated in the present work by scanning electronic microscopy and energy dispersed spectroscopy analysis and X-ray diffraction. The mineralogical phases in steelmaking slag cooled in argon are mainly nCa₂SiO₄-Ca₃(PO₄)₂ (thereafter nC₂S-C₃P) solid solution, (Fe, Mn, Mg)O (RO) phase. Some CaMgSiO₄ phases could be found in slag with lower basicity. The mineralogical phases in steelmaking slag cooled in air are mainly nC₂S-C₃P solid solution, spinel phase. The overall crystallisation of slag cooled in both argon and air was enhanced with increasing basicity. However, the crystal sizes become smaller in sample with high basicity. The Fe-enriched phases were transformed from non-faceted RO phase in sample cooled in argon to faceted spinel phases in sample cooled in air. The crystallisation of slag cooled in both argon and air was promoted with increasing FeO_x content. The phosphorus content in solid solution was elevated with decreasing basicity and increasing FeO_x content. It was implied by the present work that appropriate basicity and air oxidation would be beneficial to magnetic separation and phosphorus utilisation.     

Kinetics of the dissolution of cobalt, nickel, and iron oxides in sulfuric acid

The modeling of dissolution of cobalt, nickel, and iron oxides in sulfuric acid shows that the rate-determining step of the dissolution is the passage of oxide complexes formed on the surface of oxide particles to a solution. A system analysis of the dissolution curves (??-??) of the oxides is developed to calculate the kinetic parameters ( $n_{H^ + } $ ,E _a). Cobalt oxide Co₃O₄ dissolves in H₂SO₄ more rapidly than nickel and iron oxides (Ni₂O₃ and Fe₂O₃).     

Preparation of zirconium dioxide-based powders with rod-shaped particles

The fabrication technology of powders of the ZrO₂-Y₂O₃ composition with rod-shaped particles by the saturation of viscose fabric with zirconyl and yttrium nitrate solutions followed by drying, annealing at 1400°C, and screening is proposed. The influence of production parameters (the concentration of zirconyl nitrate in the starting solution, saturation time and temperature, and drying temperature) on geometric parameters (height and diameter) of powder particles is investigated. It is established that the diameter of the viscose fiber and the concentration of the starting solution affect the size of the particles that form most substantially. The particle height is determined by the cell size of the sieve on which the roasted viscose fabric is sieved and by the number of trituratings. It was established that the fluidity of powders substantially increases as a result of multiple trituratings through a sieve with a cell size of 315 μm and with an increase in the concentration of saturating solutions. Preparing 1 kg of the ZrO₂-7% Y₂O₃ powder under optimal conditions requires about 10 m² of viscose fabric.     

Spark plasma sintering of a nanocrystalline Al-Cu-Mg-Fe-Ni-Sc alloy

The microstructure and aging behavior of a nanocrystalline Al-Cu-Mg-Fe-Ni-Sc alloy was studied. The nanocrystalline powders were produced by milling at liquid nitrogen temperature and then consolidated using spark plasma sintering (SPS). The microstructure after SPS consisted of a bimodal aluminum grain structure (coarse-grained and fine-grained regions), along with Al₉FeNi and Al₂CuMg particles dispersed throughout. The microstructure observed in the as-consolidated sample is rationalized on the basis of high current densities that are generated during sintering. Solution treatment and aging of the SPS Al-Cu-Mg-Fe-Ni-Sc sample resulted in softening instead of hardening. This observation can be explained by the reduced amount of Cu, Mg, and Si in solid solution available to form S′ Al₂CuMg due to the precipitation of Al₇FeCu₂ and Si-rich particles, and by the fact that rodlike S′ Al₂CuMg particles could only precipitate out in the coarse-grained regions, greatly decreasing their influence on the hardness. This lack of precipitation in the fine-grained region is argued to represent a new physical observation and is rationalized on the basis of physical and thermodynamic effects. The nanocrystalline SPS Al-Cu-Mg-Fe-Ni-Sc sample was also extremely thermally stable, retaining a fine-grained structure even after solution treatment at 530°C for 5 h. The observed thermal stability is rationalized on the basis of solute drag and Zener pinning caused by the impurities introduced during the cryomilling process.     

Thermodynamic and experimental study of role of sintering atmospheres and graphite additions on oxide reduction in Astaloy CrM powder compacts

Abstract

Several atmospheres based on N₂–H₂ gaseous mixtures, with occasional additions of CH₄ were used to study the sintering behaviour of Astaloy CrM at temperatures of 1100 and 1240°C. Theoretical thermodynamic calculations and sintering experiments, with and without admixed graphite additions, were carried out devoting particular attention to the oxide reduction reactions. The role of carbon in the reduction of oxides is discussed with reference to Boudouard's reaction, proposing the indirect carbothermal reduction as the controlling mechanism. The microstructural characterisation of the material included inspection of the powder particles as well as sintered specimens. It was established that the Astaloy CrM particles contain two distinct types of oxides. One associated with the particle surface and another, mainly constituted by Cr, forming a dispersion of internal oxides. These internal oxides were microstructurally characterised, both directly and by carbon extraction replicas. A selection of powder mixtures containing 0 and 0.4%C additions were used for obtaining tensile specimens in order to assess neck development, by the strength and elongation obtained under various atmospheres and temperature combinations.