Carbonization, Graphitization, and Machining

The temperature is then raised to 760 to 980°C (or up to 1200°C in special cases). This can be done faster than the first temperature step, since most of the vplatiles have by now been removed, the material is already hard, and sagging is no longer a problem. 

Graphitization. During graphitization, the parts are heated up to 3000°C (see Ch. 4, Sec. 3). The temperature cycle is shorter than the carbonization cycle and varies depending on the size of the parts, lasting from as short as a few hours to as long as three weeks. It is usually performed in a resistance furnace (the original Acheson cycle) or in a medium-frequency induction furnace. 

Graphitization increases the resistance of the material to thermal shock and chemical attack. It also increases its thermal and electrical conductivities. 

Puffing. Puffing is an irreversible expansion of molded graphite which occurs during graphitization when volatile species, such as sulfur from the coke, are released. Puffing is detrimental as it causes cracks and other structural defects. It can be eliminated (or at least considerably reduced) 

Purification. For those applications that require high purity such as semiconductor components and some nuclear graphites, the material is heat-treated in a halogen atmosphere. This treatment can remove impurities such as aluminum, boron, calcium, iron, silicon, vanadium and titanium to less than 0.5 ppm.f °l The halogen reacts with the metal to form a volatile halide which diffuses out of the graphite. The duration of the treatment increases with increcising cross section of the graphite part. 

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