Heat-Resistant Stainless Steel: 304H, 309S and 310S Compared

Quick Answer

Heat-resistant stainless steels are chosen for oxidation resistance (a stable chromium oxide scale) and strength at temperature (creep), not for wet corrosion resistance. 304H is 18Cr-8Ni with carbon held at 0.04–0.10% and a grain size of ASTM No. 7 or coarser, which give it the creep strength for structural and pressure parts at about 500–800 °C; in dry air it resists scaling to about 870 °C intermittent and 925 °C continuous. 309S (22–24% Cr, 12–15% Ni, 0.08% C max) is rated by different sources at roughly 980–1,095 °C, and its relatively low nickel helps in sulfur-bearing gases. 310S (24–26% Cr, 19–22% Ni, 0.08% C max) carries the highest published limits of the three, about 1,035–1,150 °C, and resists carburisation best. The low-carbon S grades give up creep strength; 304H and 310H keep it. Long exposure at about 590–900 °C can form sigma phase in 309 and 310, which embrittles them at room temperature. The product form decides the specification: SA-240 for plate, SA-479 for bars and shapes, and SA-351 for castings such as CK-20, the cast 25Cr-20Ni grade.

A common pattern in heat-treatment shops: fixtures made from 304 sag and scale heavily after a few months at around 950 °C, so the replacements are ordered in 310S. They last at temperature, but a batch that spent most of its life at 700–800 °C cracks when it is knocked about cold during maintenance. The first failure was oxidation and creep; the second was sigma phase.

Both were selection problems. "Heat-resistant" covers several different properties, and 304H, 309S and 310S each trade them differently. This guide sets out what each grade is for, where the published temperature limits come from, and how to specify the right product form.

What "Heat-Resistant" Means

Three properties decide a high-temperature part, and corrosion resistance in the everyday sense is not one of them:

• Oxidation (scaling) resistance. Chromium forms a protective oxide scale, and more chromium raises the temperature at which it stays protective. Thermal cycling cracks and spalls the scale, so each cycle consumes fresh metal; that is why published intermittent limits for these grades are lower than continuous ones.
• Creep strength. Under sustained load at temperature, steel stretches slowly and eventually ruptures. Carbon and a coarser grain raise creep strength, which is what the H grades are for.
• Resistance to the atmosphere. Carburising, sulfur-bearing and nitrogen-rich gases attack in different ways. Nickel and silicon help against carburisation, while high nickel is a weakness in reducing sulfur gases.

A grade can excel at one and fail at another, so the temperature alone never selects the grade.

The Grades: 304H, 309S and 310S

Plate limits under ASTM A240 / ASME SA-240. Pipe, tube and bar specifications differ slightly.

304H: why the carbon floor. Carbon strengthens austenite at temperature. The H version of each grade has higher carbon for elevated-temperature service, which gives significantly higher creep strength, while 304L and 316L lose strength at high temperature. 304H is used for structural and pressure-containing parts from about 500 °C up to about 800 °C, and it carries a grain-size requirement of ASTM No. 7 or coarser. The trade-off: 304H sensitises between 425 and 860 °C, forming chromium carbides at the grain boundaries, so a part that meets wet corrosive conditions after hot service may need a stabilised grade such as 321 (see our guide to stainless steel plate grades). For boiler and superheater tubing, see ASTM A213 alloy tubes.

309 and 309S. The extra chromium over 304 lifts the scaling limit to around 1,000 °C, and the relatively low nickel gives good resistance to sulfur-bearing atmospheres. Resistance to carburisation is only moderate, and one supplier's datasheet advises against 309 in highly carburising atmospheres. 309S caps carbon at 0.08%; a 309H version exists for creep-loaded parts.

310, 310S and 310H. The most chromium and nickel of the three, and the highest scaling limits. Plain 310 allows up to 0.25% carbon, 310S caps it at 0.08%, and 310H holds it at 0.04–0.10%. One distributor's datasheet calls 310H the preferred grade for high-temperature service, generally from about 800 or 900 °C, and notes that the lower carbon of 310S reduces its high-temperature strength. Plate is often dual-certified 310S/310H; product certified as 310 or 310/310S alone may have carbon below 0.04%, which is not acceptable for some high-temperature duties.

Maximum Service Temperature in Air: Why Sources Disagree

Every datasheet quotes a maximum service temperature, and the numbers differ:

The spread has reasons:

• Test basis. These are scaling limits, set by each source from its own test data and criteria.
• Cycling. Intermittent limits for the austenitic grades are lower because cycling spalls the scale.
• Atmosphere. The figures are for clean, dry air. Sulfur lowers them, and above about 1,093 °C (2000 °F) scale volatilisation becomes significant in fast-flowing gas.
• Load. A scaling limit is not a design temperature. Typical 10,000-hour rupture strengths for 309 fall from about 119 MPa at 650 °C (17.2 ksi at 1200 °F) to 33 MPa at 760 °C and 11 MPa at 870 °C. A part near its scaling limit can carry almost no load.

Use the lower figure unless the part is unloaded, the atmosphere is clean and cycling is rare.

Beyond Oxidation: Sigma, Carburisation and Sulfur

Sigma phase is a hard, brittle intermetallic that forms slowly in high-chromium austenitic steels. Its effect is felt at room temperature: it cuts ductility and impact toughness, so the part cracks when it is handled, straightened or shock-loaded cold. Re-annealing restores ductility, and one datasheet suggests solution treating 310 at 1,040–1,150 °C after each 1,000 hours above 650 °C where ductility matters.

Carburisation occurs when carbon from the gas diffuses into the metal and forms carbides, generally above about 800 °C where the gas's carbon activity is below unity. Nickel and silicon reduce it, and oxygen in the atmosphere helps by keeping a protective scale. Metal dusting, at roughly 350–900 °C where carbon activity exceeds unity, is a separate form of attack. Nitrogen also penetrates deeply above about 1,000 °C in nitrogen-rich atmospheres.

Sulfidation. Nickel and nickel sulfide form a low-melting eutectic, so alloys with about 25% nickel or more suffer in sulfur-bearing gases, especially reducing ones. 309S, with high chromium and relatively low nickel, resists sulfur-bearing atmospheres well; one producer limits 310S to 950 °C where sulfur exceeds 2 g/m³. Results vary with the gas, so pilot tests are the safest guide.

Product Form Decides the Specification

The grade name says nothing about product form; the specification does. Buyers mix these up, and so do some product listings:

So "SA-479 309 plate" pairs a bar specification with a plate product, and "SA-351 CK20 plate" a casting specification with a plate product. When you see either, confirm what is needed: SA-240 309S or 310S plate, SA-479 bar, or castings.

Cast equivalents: CK-20, HH and HK

CK-20 is the cast 25Cr-20Ni grade (up to 0.20% carbon, 23–27% chromium, 19–22% nickel), listed in A351 and A743 and generally treated as the cast counterpart of wrought 310. The heat-resistant casting grades go further on carbon: HK in A297 is a 25Cr-20Ni casting with 0.20–0.60% carbon (HK40, at 0.35–0.45%, is also listed in A351), and HH is the 25Cr-12Ni counterpart, closer to the 309 family, with 0.20–0.50% carbon. Higher carbon raises high-temperature strength in castings as in wrought H grades; it also means repair welding needs procedures written for castings.

Where Each Grade Is Used

• 304H: structural and pressure-containing parts from about 500 to 800 °C, such as boiler and superheater tubing (TP304H), hot piping and vessels
• 309S: furnace and heat-treatment hardware such as annealing boxes, muffles, retorts and radiant tubes; kilns, calciners and incinerators; boiler tube hangers and thermowells
• 310S and 310H: the hottest furnace and kiln parts, heat-treatment fixtures and radiant tubes, where scaling or carburisation limits 309
• All three: refractory anchors, chosen by anchor-tip temperature; burner parts; and flare tips, where high-nickel alloys take over at the most severe duty

Welding

Austenitic heat-resistant grades weld readily, with three cautions:

• Match the filler to the duty. For 304H, a high-carbon 308H-type filler keeps the weld's creep strength; for 309 and 309S, E309 or ER309, with ER309L where low carbon is required; for 310S, E310 or ER310.
• Control heat input on 310S. Fully austenitic 310 weld metal is prone to hot cracking; one producer specifies low heat input, an interpass temperature below 150 °C and no hydrogen or nitrogen in the shielding gas. Lower-alloy fillers with more ferrite reduce cracking but dilute the weld's heat resistance.
• Join stainless to carbon steel with an over-alloyed 309-type filler. The 309 composition absorbs dilution from the carbon steel and stays austenitic, the same reason 309L is the buffer layer in weld overlay.

Worked Example: Expansion Allowance for a 310S Furnace Rail

A 310S hearth rail 2.5 m long is installed at 20 °C and runs at 1,000 °C. Its growth is ΔL = α × L × ΔT, where α is the mean coefficient of thermal expansion over the whole range, not the room-temperature value.

• One producer's mean coefficient for 310S from 20 to 1,000 °C is 18.3 × 10⁻⁶ /K: ΔL = 18.3 × 10⁻⁶ × 2,500 × 980 = 44.8 mm
• Another source gives 18.9 × 10⁻⁶ /K for 310 over the same range: ΔL = 46.3 mm
• Using the 20–200 °C value of 16.5 × 10⁻⁶ /K by mistake gives 40.4 mm, about 4.4 mm (10%) short

Design for the larger figure, 46.3 mm. Fixing the rail at its centre halves the movement at each end to about 23 mm, so slotted holes or sliding supports at the ends need at least that clearance plus fabrication tolerance. The grade matters too: from 0 to 538 °C, typical mean coefficients are 18.4 × 10⁻⁶ /K for 304 and 17.0 × 10⁻⁶ /K for 310, so the same rail in 304 would grow about 8% more over that range.

What if both ends are fixed? The rail would have to absorb a thermal strain of 18.3 × 10⁻⁶ × 980 = 1.79%. At room temperature, 310S reaches its minimum proof stress of 205 MPa at an elastic strain of about 0.1% (205 MPa ÷ 200 GPa), so the restrained strain is about 17.5 times that, and at 1,000 °C the steel is far weaker. A restrained part yields on every heat-up, bows or buckles, and after enough cycles cracks at welds and corners. Design hot parts for free movement, not for strength.

Ordering Checklist

• Product form and specification: SA-240 plate or sheet, SA-479 bar, or SA-351 or A297 castings
• Grade and carbon: 304H or 310H where creep strength matters; say whether dual-certified 310S/310H is acceptable, or require a minimum of 0.04% carbon
• Service data: maximum metal temperature, load, cycling and atmosphere (air, combustion gas, sulfur, carbon, nitrogen)
• Dimensions: thickness, width, length and quantity
• Condition: solution annealed; grain size for H grades
• Welding consumables: 308H, 309, 309L or 310 types to suit the joints
• Documents: EN 10204 3.1 or 3.2 certificates showing carbon and grain size; see our guide to material test certificates

Common Mistakes

1. Buying 304L or 310S for creep-loaded parts. Low carbon cuts high-temperature strength.

   Prevention: Specify 304H or 310H, or a minimum of 0.04% carbon, where the part carries load in the creep range.

2. Treating a maximum service temperature as a design temperature. Scaling limits say nothing about load.

   Prevention: Check creep and rupture strength at the metal temperature, and keep loaded parts well below the scaling limit.

3. Ignoring thermal cycling. Spalling scale consumes metal faster than steady service does.

   Prevention: Use the intermittent limit for cycled parts.

4. Specifying 310S for a reducing sulfur atmosphere without testing. Reducing sulfur gases attack nickel-bearing grades faster, and one producer lowers the 310S limit from 1,050 °C to 950 °C where sulfur exceeds 2 g/m³.

   Prevention: Consider 309S, and test in the actual gas.

5. Leaving 309 or 310 parts in the sigma range with no plan. Parts embrittle and crack when handled cold.

   Prevention: Choose a less sigma-prone grade for long service at 590–900 °C, or schedule re-annealing.

6. Ordering the wrong product form. "SA-479 309 plate" or "SA-351 CK20 plate" mixes specifications.

   Prevention: Order plate to SA-240, bar to SA-479 and castings to SA-351 or A297.

7. Joining stainless to carbon steel with a matching 308L filler. Dilution leaves a low-alloy, crack-prone weld.

   Prevention: Use an over-alloyed 309-type filler such as 309L.

8. Fixing hot parts at both ends. Restrained expansion buckles and cracks them.

   Prevention: Fix at one point and let the rest slide.

Supply from Kasko Makine

Kasko Makine supplies stainless steel plate for high-temperature and general service:

• SA-240M 304H steel plates — hot-rolled high-temperature stainless steel plates to ASTM / ASME SA-240M Grade 304H, whose higher carbon content gives improved strength at elevated temperatures; listed for industrial equipment and pressure vessels, heat exchangers and high-temperature components
• SA-240M 310S steel plates — hot-rolled stainless steel plates to SA-240M Grade 310S, for applications exposed to high temperatures and demanding industrial environments
• SA-479M 309 steel plates — listed under ASTM / ASME SA-479M Grade 309, with resistance to oxidation and scaling at high temperatures; SA-479 is the bar and shapes specification, so confirm the product form when you enquire
• SA-351M CK20 steel plates — listed under ASTM / ASME SA-351M Grade CK20; SA-351 covers castings, so confirm whether you need castings or wrought 310-type plate
• SA-240M 304 steel plates — hot-rolled 304 plates for general construction and industrial applications in normal and mildly corrosive environments

Engineering support: Send the drawing, maximum metal temperature, atmosphere, load and cycling. We will review grade and product form against your data and reply with pricing, lead time and engineering details.

Send the part drawing, operating temperature, atmosphere, product form and quantity for a grade proposal and quotation — we reply within 24 hours. Reach us at info@kaskomakine.com or WhatsApp +90 (537) 521 1399.

Continue Reading

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• Flare Systems: Types, Design and API 521/537 Selection
• SA-387 Chrome-Moly Plate and 12Cr1MoV
• ASTM A213 Alloy Tubes: T11, T22 and T91

Frequently Asked Questions

Q: What is heat-resistant stainless steel?
A: Heat-resistant stainless steels are grades chosen for oxidation resistance and strength at high temperature rather than for wet corrosion resistance. Chromium forms a protective oxide scale, nickel keeps the structure austenitic, and the extra carbon in H grades adds creep strength. The common austenitic grades are 304H, 309S and 310S, with published scaling limits in air from about 870 degrees Celsius for 304 to about 1,150 degrees Celsius for 310.

Q: What is the difference between 304 and 304H stainless steel?
A: 304H is 304 with carbon held between 0.04 and 0.10 percent, against a maximum of 0.07 percent for 304 plate, and with a grain size of ASTM No. 7 or coarser. The higher carbon gives higher creep strength, so 304H is used for structural and pressure parts from about 500 to 800 degrees Celsius. Their oxidation resistance is similar, and 304H sensitises between 425 and 860 degrees Celsius.

Q: What is the difference between 309S and 310S stainless steel?
A: 309S contains 22 to 24 percent chromium and 12 to 15 percent nickel; 310S contains 24 to 26 percent chromium and 19 to 22 percent nickel. Both cap carbon at 0.08 percent. 310S has higher published scaling limits, about 1,035 to 1,150 degrees Celsius against about 980 to 1,095 for 309, and better carburisation resistance. 309S, with less nickel, resists sulfur-bearing gases well.

Q: What is the maximum temperature for 310S stainless steel?
A: Published limits for 310 and 310S in dry air range from about 1,035 degrees Celsius intermittent to 1,150 degrees Celsius continuous in one distributor handbook, while one producer gives 1,050 degrees Celsius continuous and 1,100 degrees Celsius peak, falling to 950 degrees Celsius where sulfur exceeds 2 grams per cubic metre. These are scaling limits, not design temperatures, so loaded parts must also be checked for creep strength.

Q: What is sigma phase embrittlement in stainless steel?
A: Sigma phase is a hard, brittle intermetallic that forms slowly in high-chromium austenitic stainless steels during long exposure at roughly 590 to 900 degrees Celsius, with sources quoting ranges between about 590 and 1,010 degrees Celsius. It cuts ductility and impact toughness at room temperature, so parts crack when handled or shock-loaded cold. 310 is the most prone, 304 is practically immune, and solution annealing restores ductility.

Q: Is SA-351 CK20 a plate material?
A: No. ASTM A351 and ASME SA-351 cover austenitic steel castings for valves, flanges, fittings and other pressure-containing parts. CK-20 is the cast 25Cr-20Ni grade, with up to 0.20 percent carbon, and is generally treated as the cast counterpart of wrought 310. Plate in the same alloy family is ordered as 310S or 310H to SA-240, and bar to SA-479, so confirm the product form before ordering.

Q: Which filler metal is used to weld 309 and 310 stainless steel?
A: 309 and 309S are welded with E309 electrodes or ER309 wire, or ER309L where low carbon is needed, and 310S with E310 electrodes or ER310 wire. 310 weld metal is fully austenitic and prone to hot cracking, so heat input and interpass temperature are kept low. To join stainless steel to carbon steel, an over-alloyed 309-type filler such as 309L is used because it tolerates dilution.