Compaction and Sintering Behavior of Niobium Powder for Making Consumable Electrodes

Niobium consumable electrodes of 10?mm dia?×?50?mm length were prepared by cold isostaic pressing with compaction pressure of 125?C250?MPa. The compacts were sintered under vacuum at different temperatures in the range of 1000?C1800?oC. The CIPed and sintered electrodes were characterized with respect to chemical analysis, density, microstructure and bend strength. The results showed that there is a marginal improvement in density at a sintering temperature in the range of 1000?C1400?oC, while a significant improvement in density was observed at sintering temperature in the range of 1600?C1800?oC. The bend strength of sintered electrode was found to increase with increasing sintering temperature and that attains a highest value at a sintering temperature in the range of 1600?C1800?oC. Sintering at a temperature higher than 1400?oC leads to decrease in oxygen content of the electrodes. The oxygen content was decreased from 2000 to 500?ppm when electrode was sintered at 1800?oC.     

Thermal stability and elevated temperature mechanical properties of electroslag remelted Fe-16wt.%Al-(0.14–0.5)wt.%C intermetallic alloys

Addition of carbon in the range of 0,14–0.5 wt.% to the Fe₃Al-based intermetallic Fe-16wt.%Al (Fe-28at.%Al) alloy results in the formation of a thermally stable dispersion of Fe,AIC carbide phase. The volume fraction of these precipitates increases with increase in carbon content. Processing of these alloys through a combination of air induction melting and electroslag remelting leads to enhanced elevated temperature mechanical properties compared to those reported for the low (< 0.01 wt.%) carbon alloys with similar Al contents. Enhancement of up to 30% in elevated temperature yield strength was observed at the test temperatures (600, 700 and 800°C) used. The improvement in mechanical properties may be attributed to the presence of strengthening Fe₃AlC phase as well as the interstitial carbon present in the alloy matrix. The addition of carbon also leads to improved room temperature mechanical properties in contrast with other alloying additions (such as Mo, Ti and Si) used for enhancing elevated temperature properties of Fe₃Al-based intermetallic alloys. It is suggested that carbon may be an important alloying addition to these alloys.     

Effect of cerium addition on microstructures of carbon-alloyed iron aluminides

The effect of Ce addition on the microstructure of carbon-alloyed Fe₃Al-based intermetallic has been studied. Three different alloys of composition, Fe-18.5Al-3.6C, Fe-20.0Al-20C and Fe-19.2Al-3.3C-0.07Ce (in at%), were prepared by electroslag remelting process. Their microstructures were characterized using optical and scanning electron microscopies. Stereological methods were utilized to understand the observed microstructures. All the alloys exhibited a typical two-phase microstructure consisting of Fe₃AlC carbides in an iron aluminide matrix. In the alloy without Ce addition, large bulky carbides were equally distributed throughout the matrix with many smaller precipitates interspersed in between. In the alloy with Ce addition, the carbide grain sizes were finer and uniformly distributed throughout the matrix. The effect of Ce addition on the carbide morphology has been explained based on the known effect of Ce in modifying carbide morphology in cast irons.     

Effect of alloying additions on the structure and mechanical properties of Fe<Subscript>3</Subscript>Al -1C intermetallic alloy

Effect of titanium and nickel on the structure and properties of Fe₃Al intermetallic alloy containing about 1.0wt.% C have been investigated. The composition of the alloying element was substituted for Iron. The alloys were prepared by melting commercial grade raw materials iron, aluminum, titanium or nickel in air induction furnace with flux cover (AIMFC). Further these ingots were refined by electroslag refining (ESR) process. These ingots could be successfully hot-worked using conventional hot-forging and hot-rolling techniques. The hot-worked material was sound and free from cracks. ESR hot-rolled alloys were examined using optical microscopy, X-ray diffraction (XRD), scanning electron micrograph (SEM) to understand the microstructure of these alloys. The electron probe micro analysis (EPMA) studies were carried out to determine the matrix and precipitate compositions and to identify the phases present in the alloys. The base alloy and the alloy containing Ni exhibited a two-phase microstructure of Fe₃AlC_0.5 precipitates in Fe₃Al matrix. The alloy containing Ti exhibits three-phase microstructure, the additional phase being TiC precipitate. Ti addition resulted in no improvement in strength at room temperature and at 873 K whereas Ni addition has resulted in greater improvement in strength at room temperature and at 873 K and also improved the creep life significantly from 66 hrs to 111 hrs.     

Processing of high carbon Fe3Al based intermetallic alloy

A melting procedure for air induction melting (AIM) of an Fe₃Al based intermetallic alloy Fe-15.38 wt%Al-1.1 wt%C is described. Use of an appropriate slag cover during AIM results in elimination of hydrogen gas porosity in cast AIM ingots. Criteria for slag selection and slag to metal ratio are discussed. Refining by slag-metal reactions results in significant reduction in impurity levels (S, O, N) during AIM. Consequently, low cost raw materials such as mild steel scrap and commercial aluminium were used for melting the alloy. The AIM ingot exhibited excellent tensile properties. The ductility and hot workability of the ingot may be further improved by subsequent processing through electroslag remelting. It is also argued that the presence of carbon may be necessary to get AIM castings with desirable mechanical properties.     

Effect of cerium addition on the corrosion behaviour of carbon-alloyed iron aluminides

The effect of Ce addition on the microstructure and corrosion behavior of carbon-alloyed iron aluminides Fe-20.0Al-2.0C, Fe-18.5Al-3.6C and Fe-19.2Al-3.3C-0.07Ce (in at.%) has been studied. The potentiodynamic polarization behaviour of the alloys was evaluated in freely aerated 0.25 mol/l H₂SO₄. A 0.05% C steel was used for comparison purposes. All the alloys exhibited active-passive behaviour in the acidic solution. The addition of Ce destroyed passivity as indicated by lower breakdown potentials in polarization studies. This has been related to the finer distribution of the carbides in the microstructure. Corrosion rates were evaluated by immersion testing. The iron aluminide with Ce addition exhibited a lower corrosion rate compared to the aluminides without Ce addition. This has been attributed to modifications in surface film with Ce addition. Scanning electron microscopy of corroded surfaces indicated that the carbon-alloyed intermetallics were susceptible to localized galvanic corrosion due to the presence of carbides in the microstructure.     

Workability Studies on High Al Ferritic Fe–Al–C Alloys

Iron-based alloys with high-carbide (Fe₃AlC_0.5) volume fractions (up to 40%) may be obtained by careful aluminum and carbon additions. These need to be hot worked to obtain a uniform distribution of the carbide. The workability of two alloy compositions (Fe–11 wt.% Al with 0.5 wt.% C and 1.1 wt.% C) was investigated using a strain-induced crack opening (SICO) test in a Gleeble 3800 thermomechanical simulator. SICO tests were conducted in the temperature range of 1,073–1,373 K at strain rates of 0.05–0.1 s^?1. Both alloys exhibited good workability with no tendency for cracking despite their high aluminum and carbon contents. However, refinement of microstructure due to thermomechanical processing could only be observed at 1,373 K for both alloys. At lower temperatures, a slightly aligned and elongated structure was observed. It is proposed that the higher solubility of carbon with an increase in temperature as well as the transformation of matrix from ferrite to austenite may play an important role in determining the optimum working temperature for these alloys.     

Effect of Melting Process on the Microstructure and Mechanical Properties of Fe–7 wt% Al Alloy

This article presents the effect of melting process on chemical composition, microstructure and mechanical properties of Fe–7 wt% Al alloy. The alloy ingot was prepared by air induction melting (AIM), air induction melting with flux cover (AIMFC) and vacuum induction melting (VIM) and cast into 50 mm diameter split cast iron mould. These cast ingots were hot-forged and hot-rolled at 1,373 K to 2 mm thick sheet. Hot-rolled alloys were characterized with respect to chemical composition, microstructure and mechanical properties. Ingots produced by AIM, AIMFC and VIM were free from gas porosity, however AIM ingots exhibited higher concentration of hydrogen as compared to AIMFC and VIM. The recovery of aluminium as well as reduction of oxygen during AIM is very poor as compared to AIMFC and VIM. AIMFC ingots exhibit low level of sulphur as compared to AIM and VIM ingots. The alloys produced by AIMFC and VIM exhibited superior tensile ductility compared to the alloys produced by AIM. The tensile properties of alloys produced by AIMFC are comparable to the alloys produced by VIM.     

Melting,Processing and Characterization of Nb-10W-2.5Zr (Cb-752) Alloy

Niobium based alloys are most promising materials for high temperature aerospace and defence applications at temperature above the use of nickel base superalloys (1100 °C). Cb-752 (Nb-10W-2.5Zr) alloy pancakes were prepared by non-consumable arc melting process under argon atmosphere using thoriated tungsten electrode. In view of the very high melting point of tungsten as compared to niobium, it was found necessary to melt the pancake multiple times to ensure complete dissolution of tungsten in the alloy. The temperature of hot forging as well as method for oxidation protection during hot forging was optimised. It was observed that the alloy could be hot forged at 1300 °C. It was further found that the silicide and aluminide coatings, and evacuated stainless steel jacket protected the alloy from oxidation during hot forging. The forged pancakes were cold rolled to 1.5 mm thick sheets. The room temperature mechanical properties of Cb-752 alloy sheet produced from the pancakes were comparable to the data reported in literature.