310S is the workhorse of the 900-1,000°C oxidizing world - a 25Cr-20Ni austenitic used for furnace fixtures, radiant tubes and heat-treatment hardware. This guide covers its chemistry, its temperature limits, where it fails, and how to specify it correctly.
What 310S is
310S (UNS S31008, EN 1.4845) is a fully austenitic stainless steel with the highest chromium-nickel combination of the common austenitic grades. Its oxidation resistance, retained ductility and moderate cost make it the default choice for components that live above 800°C in oxidizing atmospheres.
The chemistry in one glance
| Element | Limit, % | Role |
| Cr | 24.0 - 26.0 | Forms the protective Cr2O3 scale |
| Ni | 19.0 - 22.0 | Stabilizes austenite, resists cycling |
| C | 0.08 max | The "S": protects welds from sensitization |
| Mn / Si | 2.0 / 1.5 max | Si improves scale adhesion at temperature |
| Fe | Balance | Matrix |
Mechanical properties (ASTM A240)
| Property | Minimum |
| Tensile strength | 515 MPa (75 ksi) |
| Yield strength | 205 MPa (30 ksi) |
| Elongation | 40% |
| Hardness | 95 HRB max |
The 1,000°C rule
310S is rated for about 1,000°C in continuous oxidizing service and about 925°C for intermittent duty. Above 1,000°C the oxidation rate accelerates sharply - metal loss at 1,050°C is roughly three times the loss at 950°C - so the design must add wall or shorten the inspection interval.
310 vs 310S: the carbon difference
310 allows 0.25% carbon maximum; 310S caps it at 0.08%. In welded construction the low carbon prevents chromium carbide sensitization at the heat-affected zone, so 310S is the grade for welded components going straight into service.
310S vs 310H: when to upgrade
310H (UNS S31009) carries carbon at 0.04-0.10%, which adds carbide strengthening and higher code allowable stress above 700°C. The rule: buy 310H for hot pressure parts and tubes, buy 310S for welded non-pressure hardware.
Where 310S works
- Furnace muffles, retorts and radiant tube shields
- Annealing covers and inner hoods in steel mills
- Roller hearth components in continuous furnaces
- Thermocouple protection tubes
- High-temperature heat-exchanger tubes
- Kiln and dryer components
Where 310S fails
- Above 1,000°C steady - scale spalls and metal loss accelerates
- In reducing or sulfidizing atmospheres - Cr2O3 is attacked; use a higher-nickel alloy
- Under sustained load above 700°C - creep, not oxidation, is the limit; use 310H
- After long exposure at 650-950°C - sigma phase embrittles the alloy cold; handle with care
Welding in three lines
Weld with matching 310 filler (AWS E310 / ER310). No preheat needed, but keep heat input moderate and interpass temperature low - the fully austenitic weld pool is crack-sensitive if it runs hot. The low-carbon base keeps the heat-affected zone sound for high-temperature service.
Physical facts
| Property | Value |
| Density | 7.9 g/cm³ |
| Melting range | 1400 - 1450°C |
| Structure | Austenitic, non-magnetic |
| Modulus of elasticity | 200 GPa |
Buying checklist
- Grade and standard: "310S, UNS S31008, ASTM A240" (plate) or "TP310S, ASTM A312" (pipe)
- Duty on the enquiry: maximum metal temperature, continuous or cycling
- Atmosphere: oxidizing, reducing, or sulfur-bearing - the alloy decision depends on it
- Pressure or sustained load at temperature - selects 310S, 310H or higher
- Certificate: EN 10204 3.1 with heat chemistry and tensile results
FAQ
Q: Is 310S usable above 1,000°C?
A: Only for short excursions or lightly loaded components. Above 1,000°C the oxidation rate accelerates sharply, and the design must carry extra wall or accept a shortened inspection interval. For steady duty above 1,000°C, look at the high-nickel grades.
Q: What is the difference between 310 and 310S?
A: Carbon. 310 allows up to 0.25% carbon, 310S caps it at 0.08%. The low carbon prevents chromium carbide sensitization at welds, so 310S is the grade for welded furnace components.
Q: Why did my 310S fixture crack when it was lifted out cold?
A: Likely sigma phase. Long exposure in the 650-950°C band precipitates sigma, which embrittles the alloy at room temperature. It is a service-history condition, not a material defect - and it argues for checking components after years of hot service before cold handling.
Q: Can 310S replace 316L in hot service?
A: At high temperature, decisively yes - 310S carries roughly double the chromium and nickel and holds its scale to about 1,000°C, versus roughly 800°C for 316L. At ambient or mildly corrosive temperatures, 316L is the cheaper and more common choice.
Q: When should I use 310H instead of 310S?
A: When the component carries pressure or sustained load above about 700°C. The higher carbon of 310H (0.04-0.10%) provides carbide strengthening and higher code allowable stress. Welded non-pressure hardware stays with 310S.
