JIS G4311 / G4312 Heat-Resistant Steel Guide: SUH Grade Families Explained

The JIS SUH series — defined in JIS G4311 (bars and shapes) and JIS G4312 (sheets and plates) — covers heat-resistant steels across four structural families: martensitic, ferritic, austenitic, and precipitation-hardening. Each family represents a different compromise between oxidation resistance, high-temperature strength, toughness, and cost. Martensitic SUH grades are the lowest-cost option for valve stems and moderate-temperature applications up to ~650°C but lose strength rapidly above that. Ferritic grades offer superior oxidation resistance and low thermal expansion for furnace components. Austenitic grades provide the highest service temperatures (up to 1100°C for SUH310) and the best high-temperature strength retention. Precipitation-hardening SUH660 sits apart — not a traditional stainless but a Ni-base superalloy analogue in steel form, providing the highest room-temperature and elevated-temperature strength of any JIS heat-resistant grade, at the cost of complex heat treatment. Understanding the family logic is the prerequisite for selecting any specific SUH grade.

1. What “Heat Resistant” Means — Oxidation vs Creep vs Thermal Fatigue

Three distinct failure modes occur at elevated temperature, and SUH grades are not equally effective against all three:

• Oxidation (scaling): Steel reacts with oxygen to form iron oxides (scale). Scale grows continuously, consuming base metal. Prevention: high chromium (≥17% Cr) forms a protective Cr₂O₃ layer. Maximum oxidation-resistant temperatures: 800°C (SUH3/11), 1050°C (SUH309/31), 1100°C (SUH310/330), 1150°C (SUH446/21 — ferritic, low-strength). Chromium is the primary oxidation resistance element; aluminum and silicon provide additional resistance at very high temperatures.
• Creep: Slow plastic deformation under sustained stress at elevated temperature. Creep rate depends on temperature, stress, and microstructure. Austenitic grades (FCC structure) have inherently better creep resistance than ferritic/martensitic (BCC) at equivalent temperature because FCC has more active slip systems that resist dislocation movement. For load-bearing applications above 600°C, austenitic grades are required.
• Thermal fatigue: Cyclic heating and cooling generates repeated thermal strains. Low thermal expansion coefficient and high thermal conductivity reduce thermal stress amplitude. Ferritic steels have lower thermal expansion than austenitic — relevant for applications with rapid thermal cycling (automotive exhaust, furnace parts that are frequently heated and cooled).

2. Martensitic SUH Grades

Martensitic heat-resistant steels are 9–13% Cr alloys, hardenable by quench and temper. They achieve the best combination of strength and toughness at room temperature up to ~550°C, making them suitable for engine valves, pump parts, and moderate-temperature structural applications. Above ~650°C, strength retention falls rapidly as the tempered martensite structure coarsens.

Martensitic grades are the standard material for internal combustion engine valves (both intake and exhaust) in medium-duty applications. The hardenability allows hardening of the valve face (seating surface) to 35–45HRC for wear resistance. For high-performance engines or heavy diesel with exhaust temperatures above 750°C, austenitic grades (SUH35, SUH38) are required.

3. Ferritic SUH Grades

Ferritic heat-resistant steels are high-chromium (11–28% Cr) BCC alloys — they cannot be hardened by heat treatment. Their primary advantage is oxidation resistance: at 25% Cr (SUH446), the Cr₂O₃ scale is stable to over 1100°C. They also have low thermal expansion coefficients (~10–12 × 10⁻⁶/K) compared to austenitic grades (~17–18 × 10⁻⁶/K), reducing thermal stress in cyclic heating applications.

Ferritic grades provide maximum oxidation resistance per unit cost. However, they have poor high-temperature strength — SUH446 at 900°C has tensile strength of ~60–80 MPa. They are appropriate for components that resist oxidation without carrying significant load: furnace liners, radiant tubes, heat shields, exhaust pipes. For load-bearing parts above 600°C, austenitic grades are required.

4. Austenitic SUH Grades

Austenitic heat-resistant steels are the workhorse grades for elevated-temperature service above 700°C where both oxidation resistance and load-bearing capability are required. The FCC austenite matrix inherently creep-resists better than BCC at elevated temperature, and high Cr+Ni content provides good oxidation resistance.

The progression from SUH309 (22Cr-12Ni) to SUH310 (25Cr-20Ni) to SUH330 (35Ni-15Cr) represents increasing temperature capability at increasing cost. SUH309 is the most cost-effective austenitic grade for 900–1050°C. SUH310 extends service to 1100°C. SUH330’s high nickel content provides the best resistance to carburizing atmospheres (in heat treatment furnaces, surface carbon pickup embrittles the metal — high Ni limits this attack).

5. Precipitation-Hardening: SUH660

SUH660 stands apart from all other SUH grades. It is a precipitation-hardening iron-nickel-chromium alloy: 25–27% Ni, 13–15% Cr, with 1.9–2.35% Ti and 0.1–0.5% Al — comparable to the Alloy A-286 (UNS S66286) widely used in aerospace. It is not a stainless steel in the conventional sense but a superalloy with iron as the base.

The hardening mechanism: after solution annealing (980°C), aging at 720°C for 16 hours precipitates γ’ phase (Ni₃(Ti,Al) intermetallic) throughout the austenite matrix. These fine coherent precipitates pin dislocation movement, producing yield strengths of 620–690 MPa and tensile strengths of 900–1000 MPa — far exceeding any other JIS heat-resistant grade in the as-strengthened condition.

SUH660 is used where both high temperature AND high stress coexist below 700°C: gas turbine discs, compressor blades in industrial gas turbines, high-strength fasteners for high-temperature bolted joints (nuclear, aerospace, petrochemical), and turbocharger shafts in heavy-duty diesel engines. Above 700°C, the γ’ precipitates coarsen and dissolve — strength drops rapidly, and true superalloys (Inconel 718 etc.) are required for higher temperatures.

6. Full Grade Table

7. Selection Framework

8. FAQ

Q: Is SUH310 the same as SUS310S?

Nearly identical in composition — both are 25Cr-20Ni with low carbon. SUS310S is defined in JIS G4303 (stainless steel bars/sheets for general corrosion service), while SUH310 is defined in JIS G4311/G4312 (heat-resistant steel). The practical difference: JIS G4311 specifies mechanical properties at elevated temperature (creep rupture data), while JIS G4303 specifies room-temperature properties only. For high-temperature furnace use, SUH310 certification provides verified elevated-temperature data. For corrosion applications at ambient temperature, SUS310S is the standard specification.

Q: Why does SUH660 have a lower maximum service temperature than SUH310?

SUH660’s strength comes from γ’ precipitates (Ni₃(Ti,Al)) that coarsen and dissolve above ~700°C — the precipitation hardening mechanism is temperature-limited. Above 700°C, SUH660 loses its strength advantage and is no longer cost-competitive. SUH310’s strength at 900–1100°C comes from solid-solution strengthening of the austenite matrix, which is thermally stable to much higher temperatures. The two grades serve completely different temperature regimes: SUH660 below 700°C for high strength; SUH310 above 900°C for moderate strength and maximum oxidation resistance.