THERMOWELLS

 

Guide for the choice of the protective sheath material

 

Type
Description
Carbon steel
Low cost materials with low corrosion resistance. Used in low temperature and stress applications where the measured medium protects it from corrosion, oils, petroleum, tars etc.
Maximum temperature: 530°C.
Chrome steel
High strength steels used in pressure vessels and industrial boiler plant. Resistant to chloroform, cleaning agents, food products and carbon disulphide.
Maximum temperature: 600°C
304SS
Low cost corrosion resistant material, used extensively in food, beverage and chemical processing where good corrosion resistance is required. A low carbon grade, 304L is available which can be welded without impairing it's corrosion resistance. Maximum temperature: 900°C.
321SS
Similar properties to 304SS above, except that this grade is titanium stabilized to prevent intergranular corrosion when welded.
316SS
Best corrosion resistance of the austenitic stainless steels due to the addition of molybdenum, widely used in chemical processing, offers useful resistance to H2S. As with 304, a low carbon grade, 316L is available for welded applications.
Maximum temperature: 900°C.
310SS
Heat resistant material, which can be used up to 1150°C with useful resistance in sulphur bearing atmospheres.
Corrosion resistance is slightly better than 304SS, but not as good as 316SS. Can be welded with caution.
446SS
Ferritic stainless steel with excellent resistance to sulphurous atmospheres at high temperature, however due to its low strength at high temperature, thermowells made from this material should be mounted vertically.
Used in heat treatment processes, iron and steel furnaces, gas production plant and it has some useful resistance to molten lead. Good corrosion resistance to nitric acid, sulphuric acid and most alkalis gives it some limited use in chemical plant. Maximum temperature: 1150°C.
INCONEL 600
A very widely used nickel-chromium-ion alloy with excellent high temperature strength and oxidation resistance, however it is very vulnerable to attack in sulphurous atmospheres above 500oC. Good resistance to chloride-ion stress corrosion cracking and nitriding environments. Use extensively in chemical industries for its strength and corrosion resistance.
Easily welded, can normally be used without post weld heat treatment.
Maximum temperature: 1212°C.

 

 

 

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