11. METALLIC & NON METALLIC
THERMOWELL MATERIALS
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A wide variety of steels and nickel-based alloys are used to make thermowells. There are no other materials
which will stand up to all of the many service conditions which can be found across the industry.
It is important that the proper metal is used in the fabrication of a thermowell. Obviously an improper choice
will lead to premature failure, while over-specifying leads to higher costs than necessary to do a given job.
The primary metals used in the fabrication of thermowells are: carbon steel, chrome molybdenum steels,
stainless steels (304, 310, 316, 321, 347, 304L, 316L, 446), nickel-based alloys (Inconels, Incoloys,
Hastelloys).
The main responsibility of thermowells is to protect the temperature sensor from corrosion or oxidation
conditions found in the process, as well as mechanical stresses. Each of the previously mentioned materi-
als provides different degrees of protection under various service conditions.
The following pages lists the type of materials with some recommendations for their use. As a general
guide, a high chromium content is desirable for high temperature resistance to oxidation and sulfur attack.
The presence of aluminum (1-2%) is also useful as a very resistant surface: ‘ilm of mixed chromium oxide/
aluminum oxide is formed.
INTRODUCTION