Exposure to high temperatures - where liquid water is absent - degrades metals in three primary ways:
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Corrosion or scaling - Leads to material loss and reduced functionality
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Loss of strength - Includes immediate strength reduction and, above ~500°C, long-term creep.
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Microstructural changes - Phase transformations or the formation of harmful precipitates, which can cause embrittlement and increased corrosion risk when later exposed to water. These effects tend to be more pronounced in ferritic structures than in austenitic ones.
Austentic stainless steels have the broadest service temperature range, from cryogenic conditions to high-hundreds of degrees Celsius. Beyond this range, high nickel alloys become dominant.
Mechanical properties
High-temperature strength is often enhanced by precipitates that block grain movement. This can be achieved through:
- Carbides (carbon content: 0.04%–0.10%)
- Nitrides (Nitrogen content: 0.10%–0.25%)
- Grain size control (smaller numbers = larger grain size):
- Stainless steel: ≤7
- Nickel alloys: ≤5
At medium-high temperatures, high-carbon stainless steels like 304H (0.04%–0.1% carbon) are commonly used, provided that wet corrosion resistance is not required—since chromium will be bound in carbides rather than forming protective oxides.
For applications in the mid-hundreds degrees Celsius range, 321 stainless steel (stabilised with titanium) and 347 stainless steel (stabilised with niobium) are widely used. These alloys offer improved high-temperature strength while maintaining corrosion resistance. Both are readily weldable, though 347 consumables are recommended for welding both grades due to titanium’s high melting point.
Up to ~480°C, strength decreases steadily, but 0.2% proof strength and elongation values remain useful for design. Above this, materials experience:
- Initial distortion, followed by,
- Slow, linear creep, defined by the stress required either for 1% deformation over 10,000 hours or,
- Creep rupture strength, which is the higher stress level causing fracture within the same timeframe.
Beyond 540°C, austenitic stainless steels and nickel alloys exhibit much better strength retention than hardenable carbon and low-alloy steels.
High-temperature corrosion
The most common high-temperature corrosive agent is oxygen, forming chromium oxides that are significantly thicker than the passive films present at room temperature. However, at extreme temperatures, oxide layers can grow so thick that they impede chromium diffusion, allowing iron to migrate into the scale and weaken its integrity.
Increasing chromium content raises the maximum continuous operating temperature:
- ~900°C – 301 stainless steel
- ~925°C – 304, 321, 347 stainless steels
- ~1095°C – 309 stainless steel
- ~1150°C – 310 and 330 stainless steels
Low-carbon variants of 309 and 310 offer slightly lower high-temperature strength but better aqueous corrosion resistance.
Enhancing oxidation resistance
Adding silicon improves scale adhesion and reduces oxide growth, as seen in:
- 302B stainless steel
- 308 stainless steel
- Proprietary grades with specific alloy additions, commonly under grade designation UNS S30815.
For intermittent high-temperature exposure, thermal expansion mismatches between the oxide layer and base metal reduce service temperature limits by up to 100°C. The Pilling-Bedworth (PB) ratio quantifies this mismatch. Alloying with cerium, yttrium, lanthanum, and other reactive elements enhances scale adhesion and also mitigates sulphur-induced degradation.
Nickel alloys for extreme temperatures
When stainless steels reach their oxidation limits, high-nickel alloys are preferred. However, in sulphur-rich environments, even austenitic stainless steels can form low-melting nickel sulphides, leading to cracking at high temperatures.
In reducing conditions (e.g., environments with water vapour - even at <1% concentrations), suggested service temperatures should be reduced by 40 –60°C from the standard limits.
Ferritic stainless steels for exhaust systems
Low-alloy ferritic stainless steels, such as 12% chromium grades, are commonly used in exhaust ducting and combustion-fuelled vehicle exhausts. While these alloys offer good weldability, they have relatively poor aqueous corrosion resistance and require strict composition control to prevent sensitisation cracking—particularly in welded joints and repairs.
This article was featured in Australian Stainless Magazine 82.