Refractory recrystallization

PIN, PIP, POST (or prop), printer s bit, RING, SADDLE, SAGGAR, SETTER, SPUR, STILT, thimble see also Fig. 4. As well as fireclay refractory, recrystallized silicon carbide and nickel metal have been used. 

Molybdenum is a high-strength refractory metal, although recrystallizes above 950°C with accompanying reduction in mechanical properties. It is easily fabricated. Its properties are summarized in Table 6.6. CVD is commonly used for the production of molybdenum coatings and free-standing shapes. 

The resistance of ceramically bonded and recrystallized silicon carbide to thermal shock, oxidation, and corrosion is utilized in its use as a refractory construction... 

Sources of stones typically include devitrification, raw material contaminants, refractory materials from erosion of furnace components, and cuUet contaminants. In many cases, stones change crystalline phases when elevated furnace temperatures are encountered. Primary stones have at least a portion of the original inclusions present, and have not substantially changed in terms of phases. Secondary stones have melted and then recrystallized to form a different phase compared to the primary stone (2,3). Since the sources of stones can be anything from a mined contaminant, to a misplaced coffee cup in a recycle container, methods need to be in place to identify the stone in order to determine the source. 

The resistance of ceramically bonded and recrystallized SiC to thermal shock, oxidation and corrosion is utilized in its use as a refractory construction material, for example, in the linings and skid rails for furnaces and hot cyclones, and as a kiln furniture, especially in saggars [164,552-556]. The good electrical conductivity of the material at high temperatures, coupled with its outstanding oxidation resistance, led to its early use in the electric heating industry [557-559], which markets its products... 

The major advantage of the recrystallization method is that alloys can be prepared of the desired composition by arc melting or some other suitable method of melting and then heat treated without changing composition. This is even true of metals with wide differences in vapor pressure. Alloys with a volatile component can be heat treated under pressure or in a sealed refractory metal crucible. McEwen and Touborg (1973) used an overpressure of 4 atmospheres while arc melting Tm alloys to prevent excessive vaporization of Tm. The alloys were then sealed in Ta to prevent loss during heat treatment. They also produced crystals of Y, Sm, Gd, Tb, Dy, Ho, Er, Tm, and many other intra-rare earth alloys. 

Table 3.1-H i- Summary of fatigue data of refractory metals, pretreatment Aw = as worked, Sr = stress relieved, Rxx = recrystallized. RT = room temperature...

Table 3.1-11 r Summary of fatigue data of refractory metals, pretreatment Aw recrystallized, cont. 

Although tantalum is by far the most widely used of the refractory metals to impart corrosion resistance to chemical-process vessels and components, there are some applications where molybdenum has been used with great success. Molybdenum support structures have replaced graphite in the processing of high-purity alcohols. Molybdenum-rhenium alloys, first developed because of their vastly improved ductility at low temperatures and in the recrystallized condition, have been used as vessel lining and piping components for the manufacture of Freon replacements. ... 

Crystar. Tradename. A form of RECRYSTALLISED SILICON CARBIDE (q.v.) in which SiC and electronic grade Si react at over 2300 °C, and the grains recrystallize to form a continuous network of SiC. Crystar is used to produce thermal shock resistant kiln furniture as well as tubes and other refractory shapes. (Norton Co, USA). C-Scan Acoustic Microscopy, C SAM. Focussed transducers from 10 to 100 MHz are coupled to the test piece in a water-immersion tank so that the test-piece is located at the desired depth below the test-piece surface. (The reflections from that surface itself are cut out electronically). Scanning the transducer over the surface produces an image of internal flaws at that depth in the specimen. 

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