Thermowell materials — selection table for temperature sensors
A thermowell is a closed-end metal tube that protects a temperature sensor from process pressure, flow and corrosion. Material choice follows the medium: stainless grades such as EN 1.4404 (316L) and EN 1.4571 (316Ti) cover service up to 800 °C, while heat-resisting grades such as AISI 446 and Inconel 600 cover service above 1,000 °C.
Selection table — metallic thermowell materials
The table lists the metallic materials Kjaerulf Pedersen a/s uses for thermowells, with the maximum temperature and the resistance in three process atmospheres. Both the EN material number and the AISI designation are given, so the grade can be identified from either standard.
- Oxygen-rich (oxidising) atmosphere
- Air and flue gas with excess oxygen — the normal condition in open processes and most plants.
- Reducing atmosphere
- An oxygen-starved atmosphere, typically rich in hydrogen or carbon monoxide — furnaces, heat treatment and gasification.
- Sulphurous atmosphere
- Gases carrying sulphur compounds — flue gas from oil and coal, and some process streams.
| Max temperature | Material | Oxygen-rich | Reducing | Sulphurous | Comment |
|---|---|---|---|---|---|
| 650 °C | Hastelloy C22 | Good | Good | Good | Corrosion alloy — chosen for outstanding resistance to chlorides and to both oxidising and reducing media. |
| 650 °C | Hastelloy C276 | Good | Good | Good | Corrosion alloy — chosen for chloride, caustic and reducing-acid service. |
| 800 °C | EN 1.4301 (304) EN 1.4306 (304L) |
Fair | Good | Bad | The general-purpose grade — reliable in organic acids, salt solutions and alkaline media at moderate temperatures. |
| 800 °C | EN 1.4404 (316L) | Fair | Good | Good | Added molybdenum improves resistance to non-oxidising acids such as acetic, phosphoric and sulphuric acid. |
| 800 °C | EN 1.4541 (321) | Good | Fair | Good | Titanium-stabilised for service in steam, flue gases and mineral-oil products. |
| 800 °C | EN 1.4571 (316Ti) | Good | Fair | Fair | Titanium-stabilised variant of 316 — the stabilisation protects against intergranular corrosion after welding and in prolonged high-temperature service. |
| 1,100 °C | EN 2.4816 (Inconel 600) | Good | Fair | Bad | Nickel alloy that retains its mechanical strength at high temperature — strong in oxygen-rich and carburising atmospheres. |
| 1,150 °C | EN 1.4749 (446) | Good | Very good | Very good | The high-temperature ferritic choice — outstanding in reducing, sulphurous atmospheres. |
| 1,150 °C | EN 1.4841 (314) | Good | Fair | Good | Heat-resisting grade for atmospheres rich in sulphur and carbon. |
| 1,300 °C | Pt–10%Rh (platinum–10% rhodium) |
Very good | Bad | Fair | For the most demanding temperatures — extremely good oxidation resistance. |
Sorted by maximum temperature. Select Max temperature or Material to sort the table.
How to choose — medium, temperature and pressure
Three questions decide the material. What is the medium? The chemistry sets the floor: chlorides, caustics and reducing acids point to the nickel corrosion alloys, while steam and flue gases are handled by the stabilised stainless grades. How hot does it get? The maximum temperature in the table applies to the thermowell material itself; the sensor inside has its own temperature range. What is the pressure? Process pressure and flow load the thermowell mechanically, and a metal keeps less of its strength as the temperature rises — where mechanical load and high temperature meet, the choice moves toward a grade that retains its strength, such as Inconel 600.
High-temperature and corrosive service
Above 800 °C the stainless grades give way to the heat-resisting materials. 446 (EN 1.4749) and 314 (EN 1.4841) reach 1,150 °C, and 446 is the strongest choice in reducing, sulphurous atmospheres. Inconel 600 (EN 2.4816) reaches 1,100 °C and keeps its mechanical strength there, and it performs best in oxygen-rich and carburising conditions. For the highest temperatures, platinum–10% rhodium reaches 1,300 °C with extremely good oxidation resistance.
The Hastelloy alloys sit in a different category. C22 and C276 are chosen for chemistry: they resist chlorides, caustics and reducing acids where the stainless grades fail. Their temperature limit is set by the alloy itself — above roughly 650 °C, prolonged exposure precipitates intermetallic phases that reduce toughness and corrosion resistance. Choose a heat-resisting grade such as 446 or 314 when the temperature is the limiting factor.
Frequently asked questions
Which thermowell material should I use for flue gas?
321 (EN 1.4541) is titanium-stabilised for steam, flue gases and mineral-oil products. In reducing, sulphurous atmospheres, 446 (EN 1.4749) is the stronger choice and reaches 1,150 °C.
Which thermowell material works above 1,100 °C?
446 (EN 1.4749) and 314 (EN 1.4841) reach 1,150 °C. Inconel 600 (EN 2.4816) reaches 1,100 °C. For the highest temperatures, platinum–10% rhodium reaches 1,300 °C.
Up to what temperature can Hastelloy be used?
The limit for Hastelloy C22 and C276 is 650 °C. Above roughly 650 °C, prolonged exposure precipitates intermetallic phases that reduce toughness and corrosion resistance. They are corrosion alloys, selected for chlorides, caustics and reducing acids; where temperature is the limiting factor, a heat-resisting grade such as 446 or 314 is the right choice.
What does a thermowell do?
A thermowell is a closed-end metal tube that protects a temperature sensor from process pressure, flow and corrosion. It also lets the sensor be replaced without opening the process.
Related
- Temperature sensor types and ranges — which sensor fits the temperature the thermowell has to survive
- Thermocouple colour codes — identifying the sensor inside the well
- Thermowells — the Kjaerulf Pedersen a/s thermowell programme
- Knowledge Center — all reference guides and tools