Thermocouples - Maximum Operating Temperature Guide for Mineral Insulated Thermocouple Sheaths

AISI 304 Stainless Steel
18/8 Cr/Ni

An austenitic stainless steel resistant to rust and suitable for use in air up to approximately 800°C without excessive oxidation or loss in strength. It will not embrittle at sub zero temperatures. Normally non magnetic but may be slightly magnetised after heavy cold work. The carbon content is limited to a maximum of 0.03% and at this level the material is immune to weld decay. Not recommended in sea water conditions due to susceptibility to stress corrosion cracking. Suitable for general purpose applications and used in food service and biological industries.

Grade 321 Stainless Steel
18/8/1 Cr/Ni/Titanium Stabilised.
To BS EN 10088, Werkstoff No : 1.4541.

The addition of titanium (at five times the carbon content) in this austentic stainless steel acts as a stabiliser. Titanium has a high affinity for carbon and so combines preferentially with it and prevents the formation of chromium carbide, which is the cause of weld decay. Very good corrosion resistance throughout the operating temperature range and suited to a wide range of industrial applications. Enjoys high ductility.

Grade 316L Stainless Steel
Cr/Ni/Molybdenum
To BS EN 10088, Werkstoff No : 1.4404

Similar to 304 but the addition of molybdenum and a small percentage of copper greatly improve it's corrosion resistance to sulphuric acid. Stainless steels rely largely on a surface film of chromium oxide for their corrosion resistance, but the presence of chlorides (saline solutions) can destroy this film and lead to rapid rusting. The molybdenum addition makes the surface film more chloride resistant which provides a good high temperature corrosion resistance and a greater resistance to stress corrosion cracking. Used frequently for offshore applications. High oxidation resistance up to 800°C.

Grade 310 Stainless Steel
25/20 Cr/Ni
To BS EN 10088, Werkstoff No : 1.4845

An austenitic stainless steel which retains strength and oxidation resistance up to 1050°C but is otherwise is similar to 304. Entirely a high temperature material and not specified for use in wet, corrosive conditions. Not subject to weld decay because of a higher chromium content and suitable for use in sulphur bearing atmospheres.

Inconel 600^®
Ni/Cr/Fe
To BS EN 10095, Werkstoff No : 2.4816

Exceptionally good oxidation resistance and strength at elevated temperatures up to 1150°C. Unsatisfactory in reducing sulphurous atmospheres above 550°C due to excessive grain boundary attack. Virtually immune to chloride induced stress corrosion cracking. Not recommended for use above 800°C when used with Type R, S or B thermocouples.

Nicrotherm D™
Nickel/Chromium/Silicon/Molybdenum

A high temperature nickel alloy developed specifically to reduce drift in base metal thermocouples due to the control of low vapour pressure elements and a composition more closely matched to that of the thermoelements in particular type N. The maximum temperature in a reducing sulphurs atmosphere is 500°C and in an atmosphere where oxidising sulphurs are present, the temperature is limited to 800ºC.

AISI 446 Stainless Steel
Chrome/Iron ASTM TP446, AISI 446,
To BS EN 10095, DIN X18CrN28, Werkstoff No : 1.4762

A ferrite structured stainless steel containing 30% of chromium but no nickel. Magnetic and embrittled at low temperatures. Can be used under oxidation conditions to 1100°C and has superior resistance to sulphurous atmospheres up to 900°C. Sensor should be mounted vertically at temperatures above 700°C. Much more difficult to fabricate than austenitic steels.