Desilicating Zircon with Plasma Heating and Phase Equilibrium Analyses

Desilicated zirconia has a great variety of applications. The maximum SiO₂ content in desilicated zirconia is <7 mass%. The present techniques to produce zirconia are always accompanied by many drawbacks. So the authors have developed a new plasma process. Desilicated zirconia (88.6 ~ 96.9 mass% ZrO₂) and magnesia‐stabilized zirconia (> 91.54 mass% ZrO₂, < 5.39 mass% MgO) were produced successfully from zircon using a 150 kW plasma rotating furnace. The effects of time and carbon content on the desilication degree were investigated. The mixture treated in the plasma furnace included condensed phases Zr, ZrC, ZrN, ZrO₂, C, Si, SiC, SiO₂ and gaseous phases SiO, O₂, N₂, CO. The established phase equilibrium diagrams of the Si‐C‐N‐O and Zr‐C‐N‐O systems suggest that the formation of Si₃N₄ is thermodynamically impossible, and the formation of ZrC and ZrN is thermodynamically possible in the central high‐temperature region of the plasma furnace. Experimental results supported the analyses.     

Fatigue and Structural Changes of High Interstitial Stainless Austenitic Steels

Steels with 18 to 19 mass% Cr and Mn each were studied in the as‐cast condition containing 0.85 mass% C + N and in the elektro‐slag‐remelted and hot worked condition containing 0.96 mass% C + N after final solution annealing. The latter was also tested after 20% prestraining. The results of tensile tests were compared to those of rotating bending and push/pull loading. The higher C + N content raised the 0.2% proof strength to about 600 MPa of which 70% were retained as fatigue limit of rotating bending at 10⁷ cycles and a failure probability of 50%. Prestraining further improved this limit but lowered it in relation to the proof strength. The structural components of cold work hardening under unidirectional loading and cyclic loading were similar (planar slip, dislocation, twins and ε‐martensite) except for precipitates in the latter. Nitrides appeared in the austenite and carbides in the ε‐plates.     

High‐Interstitial Stainless Austenitic Steel Castings

Interstitial atoms are most effective in strengthening austenitic steels. In stainless grades, chromium strongly reduces the solubility limit of carbon. High‐nitrogen contents require costly pressure or powder metallurgy to dissolve N in the melt. The combination of both elements comes with a high‐interstitial solubility at normal pressure of air. Sand casting with 18 mass% Cr and Mn each and 0.85 mass% (C + N) were industrially produced. The investigation revealed: proof strength R_p0.2 = 457 [MPa], true fracture strength R = 1714 [MPa], fracture elongation A = 44%, notch impact toughness KV = 290 J combined with a DBTT of ?94°C, an impact wear resistance comparable to Hadfield steel X120Mn12 but combined with a good corrosion resistance. Deep freezing and cold working does not effect the low relative magnetic permeability. This unique combination of properties offers advantages in application.     

The Microstructure and Mechanical Properties of Ultrafine Grained Plain C‐Mn Steels

An ultrafine microstructure was produced in plain C‐Mn steels with different carbon contents (0.15 ‐ 0.3 mass% C) by heavy warm deformation. The rolling was simulated by the plane strain compression test with a simulated post rolling coiling. The final microstructure consists of an ultrafine grained ferrite matrix with the average grain size of 1.1 ‐ 1.4 μm and spheroidized cementite particles of two different size groups. The fraction of high‐angle grain boundaries maintained in the range of 60% to 65%. With the increase of C content from 0.15 mass% to 0.3 mass% the strength increases by about 100 MPa, while the total elongation of 23% hardly changes. The (specific) upper shelf energy decreases from 320 J/cm² to 236 J/cm² but a rather low ductile‐to‐brittle transition temperature (DBTT) of about 206 K does not rise with increasing C content. The ultrafine steel with higher C content (0.3 mass%) exhibits a superior strength‐toughness combination.     

Precipitation Kinetics of Cr2N in High Nitrogen Austenitic Stainless Steel

The precipitation behavior of Cr2 N during isothermal aging in the temperature range from 700 ℃ to 950 ℃ in Fe-18Cr-12Mn-0.48N （in mass percent） high nitrogen austenitic stainless steel, including morphology and content of precipitate, was investigated using optical microscopy, scanning electron microscopy, and transmission electron microscopy. The isothermal precipitation kinetics curve of Cr2 N and the corresponding precipitation activation energy were obtained. The results show that Cr2N phase precipitates in a cellular way and its morphology is transformed from initial granular precipitates to lamellar ones in the cell with increasing aging time. The nose temperature of Cr2 N precipitation is about 800 ℃, with a corresponding incubation period of 30 min, and the ceiling temperature of Cr2N precipitation is 950℃. The diffusionactivation energy of Cr2 N precipitation is 296 kJ/mol.     

Effect of interaction between the structural components of W-Co-Sn pseudo-alloys on the kinetics of their compaction during liquid-phase sintering

Compaction kinetics during liquid-phase sintering of W-Co-Sn powder composites containing 90 mass% refractory component and 10 mass% readily-melting component is studied. It is established that compaction kinetics depends markedly on cobalt content in the melt. Specimens with a cobalt content up to 3 mass% at 1200 °C (in the nonisothermal heating stage) undergo an increase in volume, and then they are compacted at a rate typical for liquid-phase sintering. The nonuniform nature of compaction is observed with an increase in cobalt in the test composites. Specimens with a cobalt mass fraction of more than 2% (cobalt content with three-phase equilibrium) experience considerable additional growth due to formation of the intermetallic compound W₆Co₇ whose decomposition temperature exceeds the liquid-phase sintering temperature.     

Development and Application of a Stainless CrNbMoC Tool Steel

Steels with 15 mass% Cr, 2 mass% Mo and varying contents of C, V, Nb and Ti were investigated by thermodynamic calculations to find a stainless grade promising a wear resistance equal to that of the standard cold work tool steel X153CrMoV12. It was shown that Nb is most suited to form MC carbides for wear resistance thus reducing the content of M₇C₃, which in turn raised the Cr content dissolved in the matrix to a passivating level. Small melts in the vicinity of steel X140CrNbMoTi15‐5‐2 confirmed this concept in respect to hardenability, wear resistance and corrosion resistance. Industrial manufacturing and application of the new grade RN15X® will be discussed.     

Effects of dispersal and mechanical boron alloying on the thermal stability of hydride phases in the Ti-B-H system

Methods have been applied from scanning electron microscopy, hydrogen thermal desorption, XRD, and differential thermal analysis on the effects of grain size and alloying with boron as regards the thermal stability and decomposition temperatures of hydride phases in mechanical alloys in the Ti-B-H system. The alloys were prepared by high-energy processing for 50 h in a planetary ball mill with mixtures of TiH_1.9 + 9 mass% B + 13 mass% Ti and also with TiH_1.9 + 50 mass% TiB₂ at speeds of 1000 rpm, in addition to mixtures of TiH_1.9 + 40 mass% B and TiH_1.9 + 50 mass% TiB₂, which were treated for 20 min at speeds of 1680 rpm. The dispersal on mechanical treatment and the addition of boron to the titanium hydride powder have substantial effects on the thermal stability. The processing of the mixture TiH_1.9 + 9 mass% B + 13 mass% Ti lasting 50 h in argon gave temperatures for the dissociation of the Ti(B, H)_x hydride phase in the mechanical alloy lower by 300 deg than the decomposition temperature for the initial titanium hydride TiH_1.9. The mechanisms have been identified for the effects of the dispersal and boron alloying on the thermal stability of the titanium hydride.     

High Strength Stainless Austenitic CrMnCN Steels – Part I: Alloy Design and Properties

Generally the strength of stainless austenitic steels does not live up to their good corrosion resistance. Solid solution hardening by interstitial elements is a means of raising the strength, but is used only moderately because of poor weldability, which, however, is not required in various applications. The solubility of nitrogen is high in stainless austenite of steels with 18 mass% of Cr and Mn each, but low in the melt. Carbon reveals the opposite behaviour. Instead of producing high nitrogen steels by pressure metallurgy, about 1 mass% of C+N is dissolved in the melt at ambient pressure. The new cost‐effective C+N steel reaches a yield strength of 600 MPa, a true fracture strength above 2500 MPa and an elongation above 70 %. Conduction electron spin resonance revealed a high concentration of free electrons. Thus, the ductile metallic character of the C+N steel is enhanced, explaining the high product of strength times toughness. The high interstitial content requires rapid quenching to avoid an embrittling precipitation and respective intercrystalline corrosion.     

Effect of Al2O3 and TiO2 on Viscosity,Surface Tension,and Density of Blast Furnace Slag with CaO/SiO2 = 1.13

The influence of Al₂O₃ in the range of 10–20 mass% and TiO₂ in the range of 0.55–5 mass% on the flow behavior, viscosity, density, and surface tension of molten industrial blast furnace slag with CaO/SiO₂ = 1.13 is investigated using a high-temperature microscope, a rotating viscometer, and the maximum bubble pressure method. The measurement results show that Al₂O₃ acts as a network former in the studied CaO–SiO₂–MgO–Al₂O₃–TiO₂ slags. With an increase in the Al₂O₃ content from 10 to 20 mass%, the viscosity and surface tension of the slags increase and the density decreases. In contrast to Al₂O₃, the TiO₂ acts as a surfactant and network breaker in the range of up to 15 mass%. The addition of TiO₂ up to 15 mass% results in a decrease in the viscosity in the liquid-dominated region and a decrease in the surface tension of the studied slags. Therefore, the density increases with the addition of TiO₂ due to increasing molar volume. The behavior of the breakpoint temperature on all the viscosity curves is in complete agreement with the behavior of the flow point temperature and crystallization temperatures of melilite and perovskite.     

Interaction of Boron Carbide with Nickel Oxide

Thermodynamic methods and x-ray analysis have been applied to the chemical interactions in 0.9625 B₄C + 0.05 NiO + 0.0375 C, B₄C + NiO, B₄C + 4 NiO + 3 C mixtures; stages occur in the processes there that correspond to various temperature ranges. It is found that NiB always contains small amounts of lower borides, whose quantity does not exceed 2 mass% when the synthesis conditions are optimal.     

Interaction of Boron Carbide with Nickel Oxide

Thermodynamic methods and x-ray analysis have been applied to the chemical interactions in 0.9625 B₄C + 0.05 NiO + 0.0375 C, B₄C + NiO, B₄C + 4 NiO + 3 C mixtures; stages occur in the processes there that correspond to various temperature ranges. It is found that NiB always contains small amounts of lower borides, whose quantity does not exceed 2 mass% when the synthesis conditions are optimal.

     

Research on preparation optimization of nano CeB6 powder and its high temperature stability

High temperature self-propagating synthesis (SHS) process is very rapid, the reaction process becomes un-controlled after the SHS reaction is ignited. So the initial reaction conditions will have great effects on phase compositions and microstructures of reaction products. In this paper, the effects of the proportioning amount of Mg on the yield ratio and particle sizes of CeB6 were studied. The SHS reaction products and leached products were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results indicated that the SHS products consisted of MgO, CeB6 and little Mg3B2O6. The single CeB6 phase was contained after the SHS reaction products were leached. The purity of CeB6 was higher than 99.0 mass%, and the minimum particle sizes of CeB6 were within 30-70 nm. When the proportioning amount of Mg was 25% more than the theoretic amount, the yield of CeB?6 was 68.68%. The antioxidant ability of CeB6 was rather stronger, which was oxidized step by step, and the initial oxidation temperature was 750 oC, which indicated that it had good high temperature stability. The apparent activation energies of oxidation reactions of CeB6 were 200.09 and 312.10 kJ/mol, respectively, and reaction orders were 0.69 and 0.40, respectively.     

Structure and Certain Properties of Hot-Pressed Boron Carbide-Based Ceramic with Calcium-Silicon Additive

The influence which the composition of powder mixtures, the treatment conditions which the mixtures are subjected to, and the conditions under which the hot-pressed composite materials B₄C – (5-10 mass%) calcium-silicon are fabricated exert on the structure, nature of failure, and mechanical properties of these materials is investigated. Optimum properties are possessed by material containing 10 mass% of addition. It is shown that the structure, morphology, and dispersivity, as well as the nature of the distribution of the components that are added to the composite material (secondary phase) vary as the temperature of hot pressing changes. Maximal mechanical characteristics of the composite material (σ_bend = 560 MPa, K _1c = 4.7 MPa·m^1/2, HV = 37 GPa) are attained at hot-pressing temperatures in the range 2000-2100°C.     

Corrosion behavior of a Kh30N45YuT alloy and 20Kh23N18 steel in a lithium and potassium carbonate melt during anode polarization

The effect of temperature, the gas phase composition, and the addition of sodium peroxide on the corrosion behavior of a Kh30N45YuT alloy and 20Kh23N18 steel in a eutectic Li₂CO₃-K₂CO₃ melt is studied by measuring the corrosion potential and during steplike anode polarization.     

Improvement in the Mechanical Properties of Silicon Nitride Ceramic in High-Temperature Deformation Processes

Studies have been made on the changes in structure and properties of sintered materials: Si₃N₄ - 5 mass% Y₂O₃ - 2 mass% Al₂O₃, Si₃N₄ - 5 mass% Y₂O₃ - 5 mass% Al₂O₃, and Si₃N₄ - 40 mass% TiN on deformation in a high-pressure chamber of toroid type (pressure 4–5 GPa, temperature 1000–1600 °C), and also by direct extrusion with degrees of reduction of 55 and 72% (temperature 1750–1850 °C, pressure on the plunger 20–30 MPa). After pressure-chamber treatment, the materials have elevated mechanical characteristics: HV₁₀ ≈ 16.7 GPa, K_Ic up to 8.4 MPa · m^1/2 for the system Si₃N₄ - Y₂O₃ - Al₂O₃; and HV₁₀ ≈ 16.9 GPa, K_Ic up to 9.4 MPa · m^1/2 for Si₃N₄ - TiN. A structure feature is the small size of the coherent-scattering regions: 51 nm for Si₃N₄ and 65 nm for TiN in the system Si₃N₄ - TiN, and 33 nm for specimens in the system Si₃N₄ - Y₂O₃ - Al₂O₃. High-temperature extrusion results in a structure with β-Si₃N₄ grains elongated along the deformation direction. The anisotropic structure has K_Ic values in directions perpendicular to and parallel to the direction of extrusion of 11.5–12.0 MPa · m^1/2 and 7.5–7.8 MPa · m^1/2, respectively. The hardness after extrusion becomes 16.0 GPa.     

High-Temperature Corrosion of AlN-Based Composite Ceramic in Air and in Combustion Products of Commercial Fuel. Part 1. Corrosion of Ceramic Composites in the AlN – SiC System in Air and in Combustion Products of Kerosene and Diesel Fuel

We have established that scale formed upon oxidation of ceramic composites in the AlN – SiC system in air at temperatures up to 1550°C contains mullite 3Al₂O₃·2SiO₂ as the major phase of the outer layer, which provides its high protective properties. The inner layer of the scale contains β-SiO₂, α-Al₂O₃, and a fairly small amount of the oxynitride Al₁₀N₈O₂. In dry air, even at 1500°C with a long holding time (50 h), material of 50 mass% AlN – 50 mass% SiC retains extremely high corrosion resistance. We have shown that upon prolonged (up to 240 h) oxidation of the indicated ceramic in the combustion atmosphere of S- and Na-containing fuels (kerosene and marine diesel fuel) at 1200-1300°C, along with mullite in the scale we see formation (due to reaction of β-cristobalite with Al₂O₃ and gaseous Na₂O and NaOH) of low-viscosity silicate glass Na₂SiO₃ and NaAlSiO₄. Together with impurity Fe₂O₃ and gaseous Na₂SO₄, it partially destroys the protective mullite layer and leads to degradation of the protective properties of the scale.     

The role of solution heat treatment on corrosion and mechanical behaviour of Mg–Zn biodegradable alloys

The mechanical properties and bio-corrosion behaviours of T4 solid solution heat-treated Mg–1.5Zn and Mg–9Zn alloys at 340°C under different heat treatment durations were investigated. In vitro corrosion behaviour of the heat-treated alloys immersed in simulated body fluid (SBF) were measured by electrochemical, hydrogen evolution and mass loss tests. Surface examination and analytical studies were carried out using optical and scanning electron microscopy, EDX, and X-ray diffractometry. The results show that the grains size of both the alloys apparently remained unchanged after T4 treatment. T4 treatment at 340°C for 6?h slightly increased the strength and elongation of Mg–1.5Zn alloy while it significantly improved the strength and elongation of the Mg–9Zn alloy because of the presence of residual Mg₅₁Zn₂₀ and Mg₁₂Zn₁₃ secondary phase at the grain boundary. The results of electrochemical tests show that the corrosion rate of both the alloys decrease with increasing treatment temperature. The result also shows corrosion resistance of both the T4 tread alloys much better than that of as-cast samples. The corrosion mechanism exhibited that the occurrence of galvanic and pitting corrosion, which varied with the alloy composition and treatment time.     

Optimizing the technology for making TiB2-TiC materials

Research has been done on the reduction of a combined charge containing titanium dioxide, boron carbide, and carbon black at 1333-2000°C for various times. Semifinished products reduced for not more than 30 min do not sinter and do not require grinding, but can be sintered with the formation of a material in the TiB₂-TiC system. The grinding rate for incompletely reduced products increases as the reduction temperature is reduced. Mixtures containing 64 mass% TiB₂ and 36 mass % TiC formed by reducing the combined charge are ground more rapidly and with less contamination by iron than a control mixture of 80 mass% TiB₂ and 20 mass% TiC formed from the powders. It is concluded that it is best to prepare the final TiB₂-TiC mixture by reducing a combined charge, which reduces the costs in preparing the mixtures before sintering.Materials Science Institute, Ukrainian National Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 3/4(384), pp. 114–118, March–April, 1996. Original article submitted March 9, 1994.     

Effect of microstructure on the corrosion and deformation behavior of a newly developed 6Mn-5Cr-1.5Cu corrosion-resistant white iron

An experimental study has been made of the effect of heat treatment on the transformation behavior of a 4.8 pct Cr white iron, alloyed with 6 pct Mn and 1.5 pct Cu, by employing optical metallography, X-ray diffractometry, and differential thermal analysis (DTA) techniques, with a view to assess the suitability of the different microstructures in resisting aqueous corrosion. The matrix microstructure in the as-cast condition, comprising pearlite + bainite/martensite, transformed to austenite on heat-treating at all the temperatures between 900 °C and 1050 °C. Increasing the soaking period at each of the heat-treating temperatures led to an increase in the volume fraction and stability of austenite. M₃C was the dominant carbide present in the as-cast condition. On heat-treating, different carbides formed: M₂₃C₆ carbide was present on heat-treating at 900 °C and 950 °C; on heat-treating at 1000 °C, M₇C₃ formed and persisted even on heattreating at 1050 °C. The possible formation of M₅C₂ carbide in the as-cast and heat-treated conditions (900 °C and 950 °C) is also indicated. Dispersed carbides (DC), present in austenite up to 950 °C, mostly comprised M₃C and M₅C₂. On stress relieving of the heat-treated samples, M₇C₃-type DC also formed. The hardness changes were found to be consistent with the micro-structural changes occurring on heat-treating. The as-cast state was characterized by a reasonable resistance to corrosion in 5 pct NaCl solution. On heat-treating, the corrosion resistance improved over that in the as-cast state. After 4 hours soaking, increasing the temperature from 900 °C to 1050 °C led to an improvement in corrosion resistance. However, after 10 hours soaking, corrosion resistance decreased on increasing the temperature from 900 °C to 950 °C and improved thereafter on increasing the heat-treating temperature. Deformation behavior responded to the microstructure on similar lines as the corrosion behavior. Although in an early stage of development, the composition thus developed betters the performance of 22 pct Ni containing Ni-Resist irons as far as strength and freedom from pitting and graphitic corrosion are concerned; however, the corrosion resistance is somewhat lower. In conclusion, the usefulness of the different microstructures in attaining a useful combination of corrosion resistance and deformation behavior has been assessed. The data thus generated provide definite clues to developing new materials with improved performance for resisting aqueous corrosion in marine environments. Formerly Postdoctoral Candidate, University of Roorkee