Role of Silicon Carbide in Phase-Evolution and Oxidation Behaviors of Pulse Electrodeposited Nickel-Tungsten Coating

Silicon carbide (SiC) was reinforced in the pulse electrodeposited nickel-tungsten (Ni-W) coatings deposited on the steel substrate, and isothermal oxidation test was performed at 1273 K (1000 °C) for 24 hours. Addition of just 2 vol pct of SiC showed 26 pct increase in the relative oxidation resistance of Ni-W coating. The increased oxidation resistance was attributed to the phase evolution (SiO₂, Cr₂O₃, CrSi₂, Ni₂SiO₄, Cr₇C₃, Cr₃C₂, and Cr₃Si), which suppressed the spallation of the oxide scale in Ni-W-2 vol pct SiC. The presence of Fe₂O₃ phase in the oxidized Ni-W coating was mainly responsible for the major multiple spallations at the interface and in the bulk, which resulted in the degradation of oxidation resistance.

     

Evolution of Ti-Based Nonmetallic Inclusions During Solution Treatment of Maraging 250 Steel: Thermodynamic Calculations and Experimental Verification

The evolution of Ti-based nonmetallic inclusions in Maraging 250 steel, namely Ti(C_xN_1–x) and Ti₄C₂S₂, was investigated experimentally. Their stability in austenite also was analyzed by a thermodynamic analysis of the Fe-Ni-Ti-C-N-S system. It was established that the total concentration of the inclusions decreases from 0.024 pct to 0.008 pct after treatment at 1453 K (1180 °C) for 3 hours. The Ti₄C₂S₂ inclusions completely dissolve in austenite at 1523 K (1250 °C) during 1 hour of treatment. The composition of the carbonitride inclusions is shifted toward higher TiN contents when they dissolve in austenite. Nitrogen-enriched titanium carbonitride inclusions are stable in austenite and their fraction may be reduced only by controlling nitrogen content in the steel. The experimental observations are in good agreement with the results of the thermodynamic analysis.     

Formation of Fe3N, Fe4N and Fe16N2 on the surface of iron

In order to clarify the phenomenon of nitride formation on the surface of iron, highly polished specimens of well refined and coarsened iron grains have nitrided in flowing H2 + NH3 gas. The morphology and the conditions for formation of Fe₃N are clarified; it forms only on the surface of {lll}_α or near {lll}_α grains and grows in {112}_α directions during nitriding treatment at temperatures between 450 and 550°C. Fe₁₆N₂ and Fe₄N are also formed preferentially on the surfaces of {00l}_α and {210}_α grains, respectively. It is suggested that these iron surfaces are those satisfying the coherency relationships between nitrides and iron matrices. The morphologies and the formation temperature regions of Fe₁₆N₂ and Fe₄N on the surface of iron are quite different to those observed in iron. In particular, Fe₁₆N₂, which has been generally accepted as metastable in bulk iron below 200°C, can exist even at temperatures from 450 to 500°C when it is formed on the surface of iron.     

The Kinetics of the Nitriding of Ternary Fe-2 at pct Cr-2 at pct Ti Alloy

The mechanism and the kinetics of growth of the nitrided zone of ternary Fe-2 at pct Cr-2 at pct Ti alloy was investigated by performing gaseous nitriding experiments at temperatures of 833 K and 853 K (560 °C and 580 °C) and at nitriding potentials r _N = 0.004 atm^−1/2 and 0.054 atm^−1/2. The microstructure of the nitrided zone was investigated by transmission electron microscopy and the elemental compositional variation with depth was determined by employing electron probe microanalysis. Fine platelet-type mixed Cr_{1 – x} Ti _x N nitride precipitates developed in the nitrided zone. To describe the evolution of the nitrogen concentration depth profile, a numerical model was developed with the following parameters: the surface nitrogen content, the solubility product(s) of the alloying elements and dissolved nitrogen in the ferrite matrix, and a parameter defining the composition of the inner nitride precipitates. These parameters were determined by fitting model-calculated nitrogen depth profiles to the corresponding experimental data. The results obtained demonstrate that the type of nitride formation (i.e., whether Cr and Ti precipitate separately, as CrN and TiN, or jointly, as mixed Cr_{1 – x} Ti _x N) as well as the amounts of mobile and immobile excess nitrogen taken up by the specimen considerably influence the shape and extent of the nitrogen concentration profiles.     

A Novel Bonding Method of Pure Aluminum and SUS304 Stainless Steel Using Barrel Nitriding

A great deal of research is being carried out on welding or bonding methods between iron and aluminum. However, it is not so easy to make Fe-Al bonding materials with both high strength and light weight. Recently, a new nitriding process has been proposed to produce aluminum nitride on an aluminum surface using a barrel. This study proposes a new concept in the production of a multilayer which has an AlN and Fe-Al intermetallic compound layer between the aluminum and steel using a barrel nitriding process. The bonding process was carried out from 893 K to 913 K (620 °C to 640 °C) for 18, 25.2, and 36 ks with Al₂O₃ powder and Al-Mg alloy powder. After the process, an aluminum nitride (AlN) layer and a Fe-Al intermetallic compound (Fe₂Al_5.4) layer were formed at the interface between the pure aluminum and SUS304 austenitic stainless steel. The thicknesses of the AlN layer and the intermetallic compound layer increased with increasing treatment temperature and time. The maximum hardnesses of the AlN layer and Fe₂Al_5.4 layers were found to be 377HV and 910HV, respectively, after barrel nitriding at 893 K (620 °C) for 18 ks.     

Effect of Cr Additions on Toughness,Strength, and Oxidation Resistance of an Nb-4Si-20Ti-6Hf Alloy at Room and/or High Temperatures

The mechanical properties and oxidation resistance of Nb-4Si-20Ti-6Hf-yCr alloys (where y = 6, 10, 14 at. pct) with an Nb_SS-dominant Nb_SS/Nb₃Si/Nb₅Si₃ microstructure at room and/or high temperatures were primarily studied. It was found that at room temperature, fracture toughness was decreased by Cr additions, the fracture mode of the Nb_SS phase was intergranularly cleavage, and the silicides (Nb₃Si and Nb₅Si₃) fractured in a brittle manner. It is interesting that strength as a function of Cr content is dependent on temperature. Up to 1423 K (1150 °C), the strength was improved by Cr additions, whereas above that temperature, the strength decreased. As the Cr content increased from 6 to 14 at. pct, for example, the yield strength σ _0.2 increased from 198 to 258 MPa at 1423 K (1150 °C) but decreased from 109 to 83 MPa at 1623K (1350 °C). Cr additions cannot improve the oxidation resistance of this Nb_SS-dominant microstructure considerably; the weight gain at 100 hours of air exposure at 1523 K (1250 °C) was 255 mg/cm² at a nominal Cr content of 6 at. pct and 220 mg/cm² at a Cr content of 14 at. pct.     

Deterioration of electromotive force of chromel-alumel thermocouples in reducing atmospheres at high temperatures

The deterioration of electromotive force (emf) of Chromel-Alumel (CA) thermocouples in 80 pct H₂ + 15 pct CO + 5 pct CO₂ has been analyzed in terms of the corrosion behavior of Chromel. Emf of the CA thermocouple deteriorated drastically in 80 pct H₂ + 15 pct CO + 5 pct CO₂. After exposure for about 1000 hours at 900 °C, the decrease of emf was about 16 mV. The deterioration process could be separated into three terms. The first term, which has the smallest time constant of about 20 hours, was attributed to carbon deposition on the Chromel surface in the temperature range of 600 to 700 °C. The second term, which has a time constant of about 100 hours, was attributed to the severe internal oxidation of chromium in the temperature range of 500 to 800 °C. The third term, having the largest time constant of several thousand hours, might be attributed to the moderate and gradual preferential oxidation of chromium in Chromel in the range 800 to 900 °C. Boron nitride (BN) coating on CA thermocouples could reduce this deterioration of emf; the decrease of emf was improved to about 3 °C during 700 hours test at 900 °C.     

Influence of Gas Atmosphere Dew Point on the Galvannealing of CMnSi TRIP Steel

The Fe-Zn reaction occurring during the galvannealing of a Si-bearing transformation-induced plasticity (TRIP) steel was investigated by field-emission electron probe microanalysis and field-emission transmission electron microscopy. The galvannealing was simulated after hot dipping in a Zn bath containing 0.13 mass pct Al at 733 K (460 °C). The galvannealing temperature was in the range of 813 K to 843 K (540 °C to 570 °C). The kinetics and mechanism of the galvannealing reaction were strongly influenced by the gas atmosphere dew point (DP). After the galvannealing of a panel annealed in a N₂+10 pct H₂ gas atmosphere with low DPs [213 K and 243 K (?60 °C and ?30 °C)], the coating layer consisted of δ (FeZn₁₀) and η (Zn) phase crystals. The Mn-Si compound oxides formed during intercritical annealing were present mostly at the steel/coating interface after the galvannealing. Galvannealing of a panel annealed in higher DP [263 K and 273 K, and 278 K (?10 °C, 0 °C, and +5 °C)] gas atmospheres resulted in a coating layer consisting of δ and Г (Fe₃Zn₁₀) phase crystals, and a thin layer of Г ₁ (Fe₁₁Zn₄₀) phase crystals at the steel/coating interface. The Mn-Si oxides were distributed homogeneously throughout the galvannealed (GA) coating layer. When the surface oxide layer thickness on panels annealed in a high DP gas atmosphere was reduced, the Fe content at the GA coating surface increased. Annealing in a higher DP gas atmosphere improved the coating quality of the GA panels because a thinner layer of oxides was formed. A high DP atmosphere can therefore significantly contribute to the suppression of Zn-alloy coating defects on CMnSi TRIP steel processed in hot dip galvanizing lines.     

Phase Diagrams of the Zr–Si–RE (RE=La and Er) Ternary Systems at 773?K (500?°C)

The isothermal sections of the phase diagram of the Zr–Si–RE (RE=La and Er) systems at 773 K (500 °C) have been investigated using X-ray power diffraction (XRD), scanning electron microscopy (SEM), and optical microscopy (OM) with the aid of metallographic analysis. The existences of 10 binary compounds, namely ZrSi₂, α-ZrSi, α-Zr₅Si₄, Zr₃Si₂, Zr₂Si, RESi₂, RESi_2–x , RESi, RE₅Si₄, and RE₅Si₃ have been confirmed in the Zr–Si–RE (RE=La and Er) systems, respectively. As for the reported binary compound RE₃Si₂, only La₃Si₂ has been observed in the Zr–Si–La system, whereas Er₃Si₂ was not found. No binary compound was found in the Zr–RE binary systems, and no ternary compound was found in the current ternary systems. None of the phases in Zr–Si–La system reveals a remarkable solid solution at 773 K (500 °C). However, the maximum solid solubility of Zr in Er, Er₅Si₃, Er₅Si₄, ErSi, ErSi_1.67, and ErSi₂ is determined to be approximately 12.0 at. pct, 2.4 at. pct, 3.0 at. pct, 3.3 at. pct, 2.2 at. pct, and 1.8 at. pct, respectively. The maximum solid solubility of Er in ErSi₂ is approximately 1.8 at. pct. No remarkable solid solubility of the elements in any of the other phases has been observed.     

Formation of Fe3N,Fe4N and Fe16N2 on the surface of iron

In order to clarify the phenomenon of nitride formation on the surface of iron, highly polished specimens of well refined and coarsened iron grains have nitrided in flowing H2 + NH3 gas. The morphology and the conditions for formation of Fe₃N are clarified; it forms only on the surface of {lll}_α or near {lll}_α grains and grows in {112}_α directions during nitriding treatment at temperatures between 450 and 550°C. Fe₁₆N₂ and Fe₄N are also formed preferentially on the surfaces of {00l}_α and {210}_α grains, respectively. It is suggested that these iron surfaces are those satisfying the coherency relationships between nitrides and iron matrices. The morphologies and the formation temperature regions of Fe₁₆N₂ and Fe₄N on the surface of iron are quite different to those observed in iron. In particular, Fe₁₆N₂, which has been generally accepted as metastable in bulk iron below 200°C, can exist even at temperatures from 450 to 500°C when it is formed on the surface of iron.     

THE NITRIDING OF SILICON POWDER COMPACTS

Abstract

The reaction-bonding process to prepare silicon nitride by nitriding silicon compacts was studied, and an examination made of the influence of raw material and process variables on the properties of the resulting silicon nitride. The silicon powder grain size and the impurities content were considered as powder variables, and the green density and thermal cycles as process parameters. The examination of green-density effects indicates that, under the experimental conditions, the gas permeation of nitrogen through the silicon compacts was the rate-determining step of the reaction-bonding process. Regarding the effect of nitriding temperature, the final conversion, Si to Si₃N₄, is an increasing function of the temperature in the range 1300–1400°C. As to the composition of silicon nitride obtained, α-phase formation is favoured when oxygen is present as an impurity in silicon powder. Finally, physical, chemical, and thermomechanical tests showed that reaction-bonded silicon nitride has good bending strength (21 kgf/mm²) and can be used in very severe conditions up to 1200°C.     

Solubility of Nitrogen in High Manganese Steel (HMnS) Melts: Interaction Parameter between Mn and N

Nitrogen solubility in Fe-Mn melts was measured using a N₂ bubbling and sampling method at temperatures from 1823 K to 1923 K (1550 °C to 1650 °C) for manganese content to about 25 mass pct. The effect of temperature on the nitrogen solubility was well described based on the thermodynamic behavior of Fe-Mn system. Furthermore, the interaction parameter between Mn and N was evaluated as a function of temperature. The present results can be used in thermodynamic analyses of the formation of nitride compounds such as AlN or TiN in high manganese steel melts for example, transformation induced plasticity (TRIP) and twin induced plasticity (TWIP) aided steels as well as high Mn-N alloyed stainless steels.     

The tempering of iron-nitrogen martensite; Dilatometric and calorimetric analysis

The aging behavior of iron-nitrogen martensitic alloys (0.8 to 7.0 at. pct N) between -190 °C and 450 °C was investigated by quantitative analysis of the corresponding changes in volume and enthalpy. Martensitic specimens were prepared by gaseous nitriding of pure iron in a mixture of NH₃ and H₂ and subsequent quenching in brine and liquid nitrogen. Both X-ray diffraction analysis and metallography (light microscopical analysis and microhardness measurement) were used for interpretation of the structural changes. Analysis of the transformation kinetics was achieved by employing a range of heating rates. At least five different stages of structural change could be distinguished, which were quantitatively analyzed in terms of their effects on volume and enthalpy: (1) transformation of retained austenite into martensite (between-160 °C and -40 °C); (2) segregation and “ordering” of nitrogen atoms (below 100 °C); (3) precipitation of incoherent α′ nitride (between 100 °C and 220 °C); (4) conversion of α′ nitride into γ′ nitride (between 220 °C and 290 °C); and (5) decomposition of retained austenite (between 240 °C and 350 °C). Differences with the tempering behavior of analogous iron-carbon martensites were discussed.     

Effect of Wavelike Sloping Plate Rheocasting on Microstructures of Hypereutectic Al-18?pct Si-5?pct Fe Alloys

To refine and spheroidize the microstructures of hypereutectic Al-Si-Fe alloys, a novel method of wavelike sloping plate (WSP) rheocasting was proposed, and the effect of the WSP rheocasting on the microstructures of hypereutectic Al-18 pct Si-5 pct Fe alloys was investigated. The results reveal that the morphologies of the primary Si crystal, the Al₁₈Si₁₀Fe₅, and the Al₈Si₂Fe phases can be improved by the WSP rheocasting, and various phases tend to be refined and spheroidized with the decrease of the casting temperature. The alloy ingots with excellent microstructures can be obtained when the casting temperature is between 943 K and 953 K (670 °C and 680 °C). During the WSP rheocasting, the crystal nucleus multiplication, inhibited grain growth, and dendrite break-up take place simultaneously, which leads to grain refinement of the alloys.     

Crystallographic textures in rolled and annealed Fe-Ga and Fe-Al alloys

The feasibility of obtaining [001] preferred texture in polycrystalline Fe₈₅Ga₁₅ and Fe₈₅Al₁₅ magnetostrictive alloys containing 1 mol pct NbC using a low-cost conventional thermomechanical processing approach is shown in this work. Thermomechanical processing conditions examined consisted of a sequence of hot rolling, two-stage warm rolling at 400 °C with intermediate anneal at 900 °C and texture anneal in the temperature range of 900 °C to 1300 °C for time periods up to 24 hours. Textures present prior to and after texture annealing were characterized using orientation imaging microscopy in a scanning electron microscope. In the case of Fe₈₅Ga₁₅ alloy with 1 mol pct NbC, the deformation-induced texture after a two-stage warm rolling consists of mixed {100} 〈110〉 and {111} 〈110〉 type partial textures. Texture annealing of the Fe₈₅Ga₁₅ alloy with 1 mol pct NbC at 1150 °C for 2 hours changes the texture to a predominant texture that is close to {001}〈100〉. On increasing the annealing time to 24 hours, the texture shifts toward {110}〈100〉. While texture anneal at both 1150 °C and 1300 °C produces a [001] or near-[001] preferred orientation along the rolling direction in the Fe₈₅Ga₁₅ alloy with 1 mol pct NbC, 1150 °C-24 hour treatment was found to provide the strongest [001] orientation among the conditions examined. Similar trends are observed for the case of Fe₈₅Al₁₅ alloy with 1 mol pct NbC.     

Effects of the Treating Time on Microstructure and Erosion Corrosion Behavior of Salt-Bath-Nitrided 17-4PH Stainless Steel

The effects of salt-bath nitriding time on the microstructure, microhardness, and erosion-corrosion behavior of nitrided 17-4PH stainless steel at 703 K (430 °C) were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and erosion-corrosion testing. The experimental results revealed that the microstructure and phase constituents of the nitrided surface alloy are highly process condition dependent. When 17-4PH stainless steel was subjected to complex salt-bathing nitriding, the main phase of the nitrided layer was expanded martensite (α`), expanded austenite (S), CrN, Fe₄N, and Fe₂N. The thickness of nitrided layers increased with the treating time. The salt-bath nitriding improves effectively the surface hardness. The maximum values measured from the treated surface are observed to be 1100 HV_0.1 for 40 hours approximately, which is about 3.5 times as hard as the untreated material (309 HV_0.1). Low-temperature nitriding can improve the erosion-corrosion resistance against two-phase flow. The sample nitrided for 4 hours has the best corrosion resistance.     

Oxidative Pressure Leaching of Silver from Flotation Concentrates with Ammonium Thiocyanate Solution

The thermodynamics and technologies of the selective pressure leaching of silver from flotation concentrates were investigated in an ammonium thiocyanate medium. Thermodynamic analyses, which include silver solubility in NH₄SCN solution and Eh-pH diagrams of the Me-MeS-NH₄SCN-H₂O system at 25 °C, were discussed. The effects of several factors, such as temperature, leaching time, oxidant, pH value, flotation concentrates concentration, surfactant concentration, and so on, on the extraction percentages of silver and zinc were investigated. The following optimal leaching conditions were obtained: NH₄SCN concentration 1.5 M, lignin concentration 0.5 g/L, Fe³⁺ concentration 2 g/L, flotation concentrates addition 200 g/L, and oxygen pressure 1.2 MPa at 130 °C for 3 hours. Under these optimum conditions, the average extraction percentage of silver exceeded 94 pct, whereas the average extraction percentage of zinc was less than 3 pct. Only 7 pct of ammonium thiocyanate was consumed after 4 cycles, which indicated that ammonium thiocyanate hardly was oxidized under these oxidative pressure leaching conditions.     

High-Temperature Erosion-Corrosion Performance of High-Velocity Oxy-Fuel Sprayed Ni-20 Cr Coating in Actual Boiler Environment

The high-velocity oxy-fuel (HVOF) spray technique was used to deposit Ni-20Cr coating on a commonly used boiler steel ASTM A213 347H. The specimens with and without coating were exposed to the super heater zone of a thermal power plant boiler at a temperature of 973 K (700 °C) under cyclic conditions to ascertain their erosion-corrosion (E-C) behavior. High-temperature oxidation behavior of the specimens was also evaluated under cyclic thermal loading conditions at an elevated temperature of 1173 K (900 °C). Mass change data and thickness loss were measured to formulate the kinetics of E-C/oxidation for the specimens. The exposed specimens were characterized by X-ray diffraction (XRD) and field emission–scanning electron microscopy/energy dispersive spectroscopy (FE-SEM/EDS). The uncoated steel suffered higher E-C in comparison with its coated counterpart in terms of mass loss as well as thickness loss. It was observed that overall mass loss was reduced by 31 pct and thickness loss by 44 pct after the application of the coating. The possible formation of Cr₂O₃ phase in the coated substrate may be suggested to contribute to better E-C behavior. During air oxidation exposures, the coating was found to be intact with only marginal spallation of its oxide scales, which is an indicator of good adhesion between the coating and substrate steel. The air oxidation mass change data indicated that the coating enhanced the oxidation resistance of the steel by 85 pct.