Heat-resistant and heat-strength grades: 310S, 20Х23Н18
Heat resistance and heat strength are different requirements, and they must not be confused.
The composition of heat-strength stainless steel is chosen for the operating regime: heat-resistant steel resists oxidation and scale in air when heated, while heat-strength steel retains strength and does not creep under load in a hot zone. The classic choice for high temperatures is the austenitic grade 310S and its Russian equivalent 20Х23Н18: high chromium holds the scale, high nickel preserves the austenitic structure. Below: the composition, the differences from 304 and 321, temperature guidelines and fields of application.
What heat resistance means
When heated, any steel oxidises. A layer of oxides grows on the surface, eventually flakes off, and the metal thins. Heat resistance is the ability to resist such oxidation and to maintain a dense, firmly adherent oxide film.
The film is formed by chromium. The more chromium in the composition, the higher the temperature at which the steel still holds its scale. Nickel plays a second role: it maintains the austenitic structure, which is more ductile and more stable than ferrite when hot.
Heat strength is a different property. What matters here is not losses from oxidation but the behaviour of the metal under load when heated: creep resistance, long-term strength, retention of the part's shape. A burner, a muffle or a fastener in a furnace operates in two regimes at once — the air oxidises it, and its own weight or a force loads it.
| Requirement | What it means | What is responsible |
|---|---|---|
| Heat resistance | resistance to oxidation and scale | chromium and its oxide film |
| Heat strength | strength and creep when heated | nickel, structure, titanium |
| Resistance to intergranular corrosion | resistance of the weld after heating | titanium or low carbon |
That is why ordinary 304 is unsuitable for a hot zone: its chromium and nickel are enough for household temperatures but too little for furnace regimes.
310S and 20Х23Н18: the composition of heat-strength stainless steel
310S is a highly alloyed austenitic grade. The Russian equivalent is 20Х23Н18, and the designation itself shows the composition: 0.20% carbon, 23% chromium, 18% nickel. In 310S the chromium and nickel contents are similar — around 25% and 20% respectively.
| Grade | Chromium | Nickel | Class | What it gives |
|---|---|---|---|---|
| 304 / 08Х18Н10 | about 18% | 8–10% | austenite | basic resistance, not for furnaces |
| 321 / 12Х18Н10Т | about 18% | 9–11% | austenite + titanium | titanium against weld corrosion, moderate heating |
| 310S / 20Х23Н18 | 23–25% | 18–20% | austenite | scale resistance in a hot zone |
Here chromium works for scale resistance and nickel for preserving austenite when heated. Because of the large amount of nickel, the grade is more expensive than 304 and 321, so it is used only where hot-service life is needed.
Titanium is the third important element in this group. In 321 and 12Х18Н10Т it binds carbon and prevents carbides from precipitating along grain boundaries during heating. This protects the weld seam from intergranular corrosion. In 310S the high nickel itself plays that role, in combination with carbon.
Magnetically, 310S behaves like an austenitic grade: weakly magnetic after annealing, with magnetism increasing in certain zones after deformation and welding. A magnet does not distinguish it from 304.
Working temperatures
The exact limit is set not by the name of the grade but by the combination of environment, load and operating time. The general rule: the higher the temperature and the longer the exposure, the more chromium and nickel are needed.
- 304 and 316 are designed for moderate heating. In the hot zone of a furnace they oxidise and lose strength.
- 321 and 12Х18Н10Т withstand higher temperatures and work in welded structures where the part is heated cyclically.
- 310S and 20Х23Н18 are designed for furnace temperatures — around 1000 °C and above. They are used for the hottest units, where other grades no longer hold their scale.
- Cycling matters more than the absolute value: frequent heating and cooling destroy the oxide layer faster than constant operation at the same temperature.
- The environment changes the picture: in combustion products, in sulphur-bearing environments and in a vacuum, the behaviour of the metal differs from operation in clean air.
That is why, for a specific furnace, the grade is selected by regime rather than by a single temperature figure. If the part is critical, the calculation is based on service life and creep, not on the appearance of the metal.
Where they are used
Heat-resistant and heat-strength grades go where the metal works in a hot zone permanently.
- Furnace equipment: muffles, retorts, hearth plates, rollers, guides.
- Burners and combustion units, elements of thermal plant.
- Fasteners and tooling for the hot zone: hangers, posts, pins, heat treatment baskets.
- Heat exchangers and hot gas pipelines where scale resistance matters.
- Exhaust systems and elements operating under cyclic heating.
- Units in the cement, glass and metallurgical industries where other grades burn away.
What to take into account when ordering:
- Environment. Combustion products, sulphur and ash accelerate oxidation, so a larger chromium margin is taken.
- Welding. A hot unit is almost always welded. For welded structures, look at grades with titanium or with low carbon, and after welding do not forget about cleaning.
- Load. If the part carries weight at temperature, heat resistance alone is not enough — heat strength is calculated.
- Format. For furnace tooling, sheet and round bar are more often used; for flues, pipe. See the product range in the catalogue of stainless steel products.
Frequently asked questions
How heat resistance differs from heat strength
Heat resistance is resistance to oxidation and scale when heated, and it is provided by chromium. Heat strength is the ability to retain strength and not creep under load when hot, and it is provided by nickel and the structure.
How 310S differs from 20Х23Н18
They are close equivalents: the Russian grade 20Х23Н18 reads as 0.20% carbon, 23% chromium, 18% nickel. 310S is the same idea of high-chromium austenite in American notation. See the exact composition of the batch in the certificate.
Up to what temperature does 12Х18Н10Т work
12Х18Н10Т (the equivalent of 321) withstands higher temperatures than 304 thanks to the titanium, which protects the weld from intergranular corrosion. For furnace regimes where scale resistance is needed, 310S and 20Х23Н18 are used.
Can 304 be used in a furnace
Not for the hot zone. 304 oxidises and loses strength, and under cyclic heating the scale flakes off. In furnaces, 321, 310S and 20Х23Н18 do the work, while 304 remains for moderate temperatures.
Is 310S magnetic
Weakly after annealing, like other austenitic grades. After welding and deformation, magnetism increases in certain zones. Checking with a magnet does not determine the grade — more on that in the analysis of the magnetism of stainless steel.
What to weld heat-strength stainless steel with
With grades containing titanium or low carbon, matched to the base metal, with protection of the seam and subsequent cleaning. The general principles are covered in the article on welding stainless steel.
Why 321 is used for exhaust systems
The titanium in the composition protects the weld seam from corrosion during cyclic heating and cooling. This matters for parts that constantly work in a heat-and-cool cycle.
Choose a grade for a hot unit
The catalogue does not include furnace tooling, but it does include the basic grades for hot and ordinary conditions: sheet, pipe, round bar, angle. For a critical hot unit, the grade is selected by regime, and the order fixes the size and the shipment week.
- Catalogue of stainless steel products
- Stainless steel grades — table of AISI and ГОСТ equivalents
- How the elements define a grade — chromium, nickel, molybdenum and titanium