11. METALLIC & NON METALLIC
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GLOSSARY OF TERMS
THERMOWELL MATERIALS
Austenitic
: Refers to the crystal structure of the 300 series stainless steel.
Carbide precipitation
: The process where chromium carbides form and precipitate out in the steel.
Carbon atoms combine with chromium atoms lead to local depletion of
chromium, thereby reducing the available chromium to form a protective
chromium oxide flm. This allows localized inter granular attack from salts
and acids. Carbide precipitation occurs when a 300 series stainless steel is
held in the 800 ºF range.
Carbide stabilized
: In order to reduce the chance of carbon precipitation, certain 300 series
stainless steel are stabilized with small amounts of titanium, tantalum of
columbium which preferentially combine with the carbon leaving the chro-
mium alone. This result is also accomplished by the ‘low carbon’ stainless
steels, which have less carbon to combine with the chromium.
Carburizing atmospheres : Contain carbonaceous vapours (e.g. hydrocarbons). The present carbon
can react with the alloys at high temperatures to produce metal carbides.
This can result in embrittlement. Generally, high nickel content in an alloy
is helpful in resisting carburizing although it will not completely prevent it.
Creep
: At high temperatures the mechanical strength of metals falls off. Over time
and at high temperatures metals will slowly stretch under an applied load
and will fail at stress much smaller than would normally be expected.
Ferritic
: Refers to the crystal structure of the 400 series stainless steel.
Inert atmospheres
: Consist of inert gas such as argon. There is no problem with alloys in such
an atmosphere. A variation of an inert atmosphere is no atmosphere at
all – i.e. a vacuum. This is used increasingly for heat-treatment purposes.
Oxidizing atmospheres
: Contain oxygen and will react with metals at elevated temperatures to pro-
duce oxides on the surface. The good high temperature performance of the
heat-resisting alloys depends on the formation of a stable protective oxide
flm on the surface. The elements chromium and aluminium, when present
in an alloy, form excellent protective flms of chromium oxide and aluminium
oxide.