جوشکاری فلزات دیرگداز (نیوبیوم، تانتالیوم، مولیبدن و تنگستن) و لحیم کاری گرم آن ها

The welding and brazing of the refractory metals

niobium, tantalum, molybdenum and tungsten 

مروری بر جوشکاری و لحیم کاری گرم فلزات دیرگداز 

نیوبیوم، تانتالیوم، مولیبدن و تنگستن

ABSTRACT

This review covers the present and future problems in the development and welding of niobium, tantalum, molybdenum and tungsten and their alloys. Their metallurgical characteristics are discussed together with the influence of impurities and alloying elements on mechanical properties, alloy development and weldability. Other factors briefly discussed are availability, high temperature strength, ductility and oxidation resistance.

 

مقاومت به فرسایش جرم های ریختنی دیرگداز

What Affects Erosion Resistance Of Refractory Castables?

چه چیزی مقاومت به فرسایش  جرم های ریختنی دیرگداز را تحت تأثیر قرار می دهد؟

ABSTRACT

The wear of refractory linings is often a critical problem in process units where transfer lines are subjected to high velocity gas stream containing solid particles. Knowledge regarding erosive wear of refractory linings under these conditions is restricted and, as consequence, the technological development is slow and, in general, carried out in an empirical way. The objective of the present work is to review relevant aspects related to erosive wear in industrial applications and to investigate the erosion resistance of refractory castables currently used by the aluminum industry and in petrochemical cracking units. Among the erosion variables tested are: the particle impingement angle, line pressure, particle size and hardness, the microstructure of the refractories, the binding agent content, type of aggregate, thermal history and porosity. It was found that the higher erosion was attained at 90º of impingement and decreased slightly when the angle was close to 30º. The results also pointed out that erosion was proportional to the particle velocity, its hardness and size. In addition, it was observed that the refractory erosion resistance is mostly affected by reducing the porosity.

 

سرامیک­ های فوق ­دمابالا جهت کاربرد در محیط­ های سرسخت

Ultra-High Temperature Ceramic Materials for
Extreme Environment Applications

سرامیک­ های فوق ­دمابالا جهت کاربرد در محیط­ های سرسخت

ABSTRACT

For the purposes of this paper, we will simply define UHTC materials by their usefulness in a real structural (load-bearing) application where the very high temperatures are generated rapidly by burning fuels or friction with the atmosphere (not steady state). This will quickly eliminate most of the materials mentioned above. While oxides are reasonable to consider for use in oxidizing environments, poor thermal shock resistance due to high thermal expansion and low thermal conductivity eliminates them from further discussion. The silicon based refractory compounds (SiC, Si3N4, MoSi2, etc.) possess excellent oxidation resistance up to 1700°C due to the formation of a layer of SiO2 glass that inhibits oxygen diffusion to the parent material.4 This is the primary reason for the popularity of these materials for a wide variety of applications. However, active oxidation (the direct formation ofSiO(g) instead of a protective SiO2 layer) can occur at very high temperatures (> 1350°C, depending on PO2) and reduced system pressures. In addition, decomposition of already-formed SiO2, or the interface reaction between SiC and SiO2 results in SiO(g) formation at high temperatures and reduced pressure environments. Other materials, such as TiB2, TiC, NbB2, NbC, while having high melting temperatures, form oxides with low melting points (TiO2 – Tm = 1840°C and Nb2O5 – Tm = 1485°C). Graphite has the highest melting temperature of any material known, but starts to burn thet 800°C. While it is a most widely used material in high-temperature applications, it must be protected by coatings for long-term use.

اکسیدهای دیرگداز

Refractory Oxides

اکسیدهای دیرگداز

ABSTRACT

Refractory oxides encompass a broad range of unary, binary, and ternary ceramic compounds that can be used in structural, insulating, and other applications. The chemical bonds that provide cohesive energy to the crystalline solids also influence properties such as thermal expansion coefficient, thermal conductivity, elastic modulus, and heat capacity. This chapter provides a historical perspective on the use of refractory oxide materials, reviews applications for refractory oxides, overviews fundamental structure–property relations, describes typical processing routes, andsummarizes the properties of these materials.The term refractory refers to materials that are resistant to the effects of heat. Refractory oxides, therefore, are ceramic materials that can be used at elevated temperatures. These nondescript restrictions allow nearly any oxide to be classified as refractory. For this article, refractory oxides will refer, somewhat arbitrarily, to common crystalline compounds with melting temperatures of at least 1,800°C. These compounds can contain one or more metal or metalloid cations bonded to oxygen. As an introduction to the topic, this section provides a brief historic overview of materials commonly used in the refractories industry, including some lower melting temperature materials. The section also reviews some current trends in the industries that produce and use refractory oxides. The other sections of this chapter focus on phase-pure oxide ceramics that can be used at elevated temperatures.

آلومینا Alumina

Alumina

آلومینا

ABSTRACT

The uses, processing, structure, and properties of alumina are summarized in this article. Various polymorphs of alumina and its phase relations with other oxides are described. The following properties are discussed: mechanical, thermal, thermodynamic, electrical, diffusional, chemical, and optical. Quantitative values for these properties are given in tables. The usefulness of alumina results from its high strength, melting temperature, abrasion resistance, optical transparency, and electrical resistivity. Traditional uses of alumina because of these properties are furnace components, cutting tools, bearings, and gem stones; more recent applications include catalyst substrates, tubes for arc lamps, and laser hosts. Possible new uses of alumina are in electronic circuits, optical components, and biomaterials. Alumina fibers for composites and optics must be pure, defect free, and cheap.

کاربردهای مختلف میکروسیلیس

Silia Fume In Various Applications

کاربردهای مختلف میکروسیلیس

ABSTRACT

Siliea Fume uaagc ia varied and open to 1he imagination of the delligDen. It has become 10 an integralpart oflllllll)' of tho items we use in our daily life. Ill thia section we have chosen  some of tho applicatiau that demoustrate 1ho wrslll:ilily of1ho product. From High Pcrl'OIDilii1Ce  Conaete (HPC) in cODS1ruc:tim projects to:lik in lawn care pruducta ilill Tllther IIIIIUiJg how far  thia "smoke'' by-product bas evolved. Tho earliest applications for hip-slrfilllt!lh aWes. fume conerm. were in columns for Jiish-rise  a1ruc:turea • As CG11Crele stralllh b:mlses coblm size can be nclw.:ed ao4reinforcms steeldeaips in  the columns can be simplified. Smaller cohmms eqaata1 to more ovuaD floor apace available to the  owner of1be structure 1111d a aigail:klllt coat advaotage, particularly in urban aettinaJ.