SUH310 is the 25Cr-20Ni austenitic heat-resistant steel in JIS G4311 — corresponding to AISI 310S worldwide. At 25% chromium and 20% nickel, it reaches the upper limit of standard heat-resistant steel performance: continuous service to 1100°C with meaningful load-bearing capability, a creep rupture strength at 1000°C that makes it suitable for furnace fixtures, heat treatment baskets, radiant tubes, and kiln furniture carrying moderate loads. Above 1100°C or in carburizing atmospheres above 900°C, SUH310 reaches its design limits — and the wrong choice at these conditions does not degrade gracefully. This guide covers SUH310’s capabilities and, equally importantly, the two specific conditions where it fails and what to use instead.
| Property | SUH310 (JIS G4311) |
|---|---|
| Standard | JIS G4311:2013 |
| ASTM equivalent | AISI 310S / UNS S31008 (A276, A240) |
| EN equivalent | EN 1.4845 / X8CrNi25-21 |
| Composition | 24–26%Cr, 19–22%Ni |
| Max continuous service temp | ~1100°C |
| 100-hr creep rupture stress at 1000°C | ~15–25 MPa |
| Carburizing atmosphere resistance | Moderate (SUH330 required above 950°C in carburizing) |
| Magnetic | No (austenitic) |
- Chemical Composition
- Oxidation Resistance to 1100°C
- Mechanical Properties and Creep
- Sigma Phase in SUH310
- Carburizing Atmosphere Limitation
- Weldability
- SUH309 vs SUH310 vs SUH330: Which Grade
- ASTM and EN Equivalents
- Common Mistakes
- Applications
1. Chemical Composition
| Element | SUH310 (JIS G4311) | AISI 310S (ASTM A276) | EN 1.4845 |
|---|---|---|---|
| C (max) | 0.25% | 0.08% | 0.10% |
| Si (max) | 1.50% | 1.50% | 1.50% |
| Mn (max) | 2.00% | 2.00% | 2.00% |
| Cr | 24.00–26.00% | 24.00–26.00% | 24.0–26.0% |
| Ni | 19.00–22.00% | 19.00–22.00% | 19.0–22.0% |
2. Oxidation Resistance to 1100°C
At 25% Cr, SUH310 forms the same dense, self-healing Cr₂O₃ scale as SUH446. The difference from SUH446: SUH310 has 20% Ni, which stabilizes the austenite (FCC) crystal structure and provides significantly better creep resistance and high-temperature strength — but Ni contributes nothing to oxidation resistance directly. The 25% Cr alone drives the 1100°C capability.
Silicon additions (within the 1.50% max) improve scale adherence. Commercially, SUH310 used for the most demanding furnace applications sometimes specifies Si at 1.0–1.5% to maximize scale stability under thermal cycling (scale spalling during cooling-reheating cycles is the dominant failure mode for furnace fixturing above 1000°C).
3. Mechanical Properties and Creep
| Property | Room Temp | 800°C | 1000°C | 1100°C |
|---|---|---|---|---|
| Tensile strength | ≥ 520 MPa | ~200–250 MPa | ~80–120 MPa | ~40–60 MPa |
| Yield strength (0.2% PS) | ≥ 205 MPa | ~130–160 MPa | ~55–80 MPa | ~25–40 MPa |
| Elongation | ≥ 40% | ~50–60% | ~60–80% | High |
| 100-hr creep rupture stress | — | ~50–70 MPa | ~15–25 MPa | ~5–10 MPa |
The creep rupture stress of 15–25 MPa at 1000°C means that a SUH310 component in sustained service at 1000°C can carry a sustained stress of approximately 15–25 MPa for 100 hours before rupture. For a 50 mm × 50 mm cross-section bar, this corresponds to a load of 37.5–62.5 kN — enough for a heat treatment basket loaded with typical steel parts at temperatures up to 1000°C. Above 1050°C, the permissible load drops significantly; above 1100°C, SUH310 is essentially self-weight only.
4. Sigma Phase in SUH310
Like all high-Cr austenitic steels, SUH310 can develop sigma phase (FeCr intermetallic) during exposure in the 600–900°C range. At 25% Cr, sigma formation is faster than in lower-Cr grades like SUS304 (18% Cr). The consequences:
- Room-temperature embrittlement after extended service in the 600–900°C range
- Reduced room-temperature corrosion resistance (Cr depleted from matrix by sigma)
For furnace fixtures that are cycled to room temperature (e.g., basket-style fixtures that are loaded/unloaded at room temperature before each cycle), sigma formation over time reduces impact toughness. Periodic re-annealing at 1050–1100°C dissolves sigma and restores ductility, but this is rarely done in practice. Design for handling by SUH310 fixtures should account for progressive embrittlement over service life.
5. Carburizing Atmosphere Limitation
6. Weldability
SUH310 (JIS, C up to 0.25%) has limited weldability due to the higher carbon content — Cr₂₃C₆ can sensitize the HAZ. For welded fabrications, specify 310S composition (C ≤ 0.08%) or use 310L filler metal. Low-hydrogen welding processes (TIG/GTAW preferred); avoid stick welding with basic electrodes if possible due to higher heat input.
Matching 310 filler (AWS A5.9 ER310) is standard for TIG welding. For applications where a duplex austenite-ferrite weld bead is acceptable, 309L filler provides better hot-crack resistance. The purely austenitic weld metal of matched 310 filler is susceptible to solidification cracking with high heat input — keep interpass temperature below 150°C and use stringer beads.
7. SUH309 vs SUH310 vs SUH330: Which Grade
| Property | SUH309 (22Cr-12Ni) | SUH310 (25Cr-20Ni) | SUH330 (35Ni-15Cr) |
|---|---|---|---|
| Max oxidation temp | ~1050°C | ~1100°C | ~1100°C |
| 100-hr rupture @ 1000°C | ~10–15 MPa | ~15–25 MPa | ~20–30 MPa |
| Carburizing atmosphere | Poor | Poor | Excellent |
| Sigma phase rate | Slower than 310 | Moderate | Slowest |
| Cost (Ni content driver) | Lower | Medium | High (35%Ni) |
| Best use case | 900–1050°C load-bearing, clean atm. | 1000–1100°C load-bearing, clean/oxidizing atm. | Carburizing/nitriding atm., 900–1050°C |
The selection logic: temperature first, atmosphere second, cost third. If the application is clean/oxidizing, use SUH309 (up to 1050°C) or SUH310 (up to 1100°C) depending on temperature. If the atmosphere is carburizing, use SUH330 regardless of temperature. Cost: SUH309 < SUH310 < SUH330; specify the cheapest grade that meets the temperature and atmosphere requirements.
8. ASTM and EN Equivalents
| JIS | ASTM / UNS | EN | Key Note |
|---|---|---|---|
| SUH310 | AISI 310S / UNS S31008 | EN 1.4845 / X8CrNi25-21 | JIS C ≤ 0.25%; ASTM 310S C ≤ 0.08%; specify low-C for welded assemblies |
| SUH309 | AISI 309S / UNS S30908 | EN 1.4833 / X12CrNi23-13 | Lower Cr and Ni than SUH310; lower cost |
| SUH330 | UNS N08330 / ASTM B536 Alloy 330 | EN 1.4886 / X12NiCrSi35-16 | High-Ni; classified as Ni alloy in ASTM |
9. Common Mistakes
10. Applications
Annealing baskets, hardening fixtures, wire mesh trays for parts in clean or slightly reducing atmospheres (N₂, H₂, N₂-H₂). Service to 1100°C. Must be clean atmosphere — not carburizing. SUH310 mesh baskets for normalizing, solution-annealing, and aging operations.
Electrically-heated radiant tubes; muffle inner liners; retorts. The tube geometry provides structural support; the material resists scale and maintains strength at 900–1100°C. Lower cost than ceramic radiant tubes for temperatures below 1100°C.
Setters, saggers, kiln car decks for firing ceramics, glass, and technical ceramics at 900–1100°C. Replaces expensive Si₃N₄ or SiC kiln furniture in applications where thermal cycling is not too severe and loading is moderate.
Summary
- SUH310 = 25Cr-20Ni austenitic; AISI 310S / EN 1.4845 equivalent (note JIS C limit is higher)
- Max service temp: ~1100°C continuous with load; highest standard JIS heat-resistant grade for load-bearing use
- Critical limitation 1: carburizing atmospheres → carbon pickup causes intergranular cracking; use SUH330 instead
- Critical limitation 2: sigma phase formation in 600–900°C range causes progressive room-temperature embrittlement in service
- For welded assemblies: specify C ≤ 0.08% (310S chemistry); use ER310 TIG filler, stringer beads, low interpass temperature
- Grade selection: SUH309 (<1050°C, lower cost) → SUH310 (1050–1100°C) → SUH330 (carburizing/nitriding atmosphere)
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