Heat-Resistant Steel Grades: A Complete Guide
Jul 22, 2026
Introduction
Heat-resistant steels are the backbone of industrial thermal processing equipment - from furnace linings and heat exchangers to automotive exhaust systems and petrochemical reformer tubes. These specialized steel grades are engineered to maintain mechanical strength, resist oxidation and scaling, and resist microstructural degradation at temperatures where ordinary carbon steels and standard stainless steels fail completely.
For engineers and procurement professionals sourcing materials for high-temperature service, navigating the complex landscape of heat-resistant steel grades - with their varying chromium, nickel, silicon, and aluminum contents - can be challenging. This guide provides a comprehensive overview of the most important heat-resistant steel grades, their compositions, properties, and selection criteria.
Product Overview
Heat-resistant steels fall into two main metallurgical categories:
Austenitic grades (containing high chromium and nickel) dominate high-temperature applications due to their superior creep strength, oxidation resistance, and microstructural stability. The most commonly specified austenitic grades include:
- 309S/309H (UNS S30908/S30909): 22% Cr, 12% Ni - General-purpose heat-resistant steel
- 310S/310H (UNS S31008/S31009): 25% Cr, 20% Ni - Premium oxidation resistance up to 1100°C
- 314 (UNS S31400): 25% Cr, 20% Ni, 2% Si - Enhanced silicon for carburization resistance
- 330 (UNS N08330): 18% Cr, 35% Ni - Exceptional thermal shock and carburization resistance
Ferritic grades (containing chromium with little or no nickel) offer a cost-effective alternative for less demanding applications:
- 446 (UNS S44600): 25% Cr - Oxidation resistance up to 1100°C without nickel
- 409 (UNS S40900): 11% Cr - Automotive exhaust systems
- 439 (UNS S43035): 18% Cr - Improved weldability over 409
The choice between austenitic and ferritic grades depends primarily on the required temperature, the need for creep strength (favors austenitic), and budget constraints (ferritic grades are significantly cheaper).
Comparison Table
Chemical composition of key heat-resistant steel grades:
| Element | 309S | 310S | 314 | 330 | 446 |
|---|---|---|---|---|---|
| C | ≤ 0.08 | ≤ 0.08 | ≤ 0.25 | ≤ 0.08 | ≤ 0.20 |
| Cr | 22.0–24.0 | 24.0–26.0 | 23.0–26.0 | 17.0–20.0 | 23.0–27.0 |
| Ni | 12.0–15.0 | 19.0–22.0 | 19.0–22.0 | 34.0–37.0 | ≤ 0.50 |
| Si | ≤ 1.00 | ≤ 1.50 | 1.5–3.0 | ≤ 1.00 | ≤ 1.00 |
| Mn | ≤ 2.00 | ≤ 2.00 | ≤ 2.00 | ≤ 2.00 | ≤ 1.50 |
| P | ≤ 0.045 | ≤ 0.045 | ≤ 0.045 | ≤ 0.040 | ≤ 0.040 |
| S | ≤ 0.030 | ≤ 0.030 | ≤ 0.030 | ≤ 0.030 | ≤ 0.030 |
| N | - | - | - | - | ≤ 0.25 |
Physical properties:
| Property | 309S | 310S | 314 | 330 | 446 |
|---|---|---|---|---|---|
| Density (g/cm³) | 7.98 | 7.98 | 7.98 | 7.94 | 7.58 |
| Melting Range (°C) | 1370–1450 | 1370–1450 | 1370–1450 | 1340–1430 | 1426–1510 |
| Max Oxidation Temp (°C, continuous) | 980 | 1100 | 1150 | 1150 | 1100 |
| Thermal Conductivity (W/m·K, 100°C) | 15.6 | 14.2 | 13.8 | 12.5 | 20.0 |
| CTE (μm/m·K, 0–100°C) | 14.9 | 15.9 | 15.6 | 14.5 | 10.4 |
| Magnetic Properties | Non-magnetic | Non-magnetic | Non-magnetic | Non-magnetic | Magnetic |
Mechanical properties (annealed condition):
| Property | 309S | 310S | 314 | 330 | 446 |
|---|---|---|---|---|---|
| Tensile Strength (MPa) | 620 | 655 | 690 | 586 | 550 |
| Yield Strength (MPa) | 310 | 310 | 345 | 290 | 310 |
| Elongation (%) | 45 | 45 | 40 | 45 | 20 |
| Hardness (HRB) | 85 | 85 | 90 | 75 | 90 |
Performance Analysis
309S/309H - The General-Purpose Choice
Type 309S is a 22Cr-12Ni austenitic stainless steel that provides good oxidation resistance up to 980°C in continuous service. It is the most economical heat-resistant austenitic grade, making it the default choice for moderately high-temperature applications where 310S performance is not required.
The "H" variant (309H) has controlled carbon content (0.04–0.10%) to optimize creep strength at elevated temperatures, qualifying it for ASME Section I boiler and pressure vessel applications. At temperatures above 650°C, 309S begins to form sigma phase (a hard, brittle Fe-Cr intermetallic), which reduces ductility. For applications with thermal cycling through the 650–900°C range, this sigma phase tendency should be considered.
310S/310H - The Industry Standard
Type 310S (25Cr-20Ni) is the most widely specified heat-resistant steel for continuous service up to 1100°C. Its high chromium content forms a stable, self-healing chromium oxide (Cr₂O₃) scale that provides excellent oxidation resistance, while the 20% nickel content stabilizes the austenitic structure and provides creep strength.
Key limitations of 310S include sigma phase precipitation in the 650–950°C range (which can be reversed by solution annealing at 1100–1150°C) and susceptibility to carburization in carbon-rich atmospheres at high temperatures. For carburizing environments, Type 314 or nickel-based alloys are preferred.
314 - The Silicon-Enhanced Grade
Type 314 is essentially 310S with an elevated silicon content (1.5–3.0%). Silicon promotes the formation of a silica-rich sub-layer beneath the chromium oxide scale, which significantly improves resistance to carburization and fuel-ash corrosion in furnace atmospheres. This makes 314 the preferred grade for:
- Carburizing furnace fixtures and retorts
- Heat treatment baskets exposed to carbon-rich atmospheres
- Refractory anchors in carburizing environments
- Components exposed to molten salts or fuel ash
The trade-off is that the higher silicon content increases the risk of sigma phase formation and slightly reduces ductility and weldability compared to 310S.
330 - The Thermal Shock Specialist
Type 330 (18Cr-35Ni) has the highest nickel content among standard heat-resistant steels. This high nickel content provides several advantages: superior resistance to thermal shock and cycling, excellent carburization resistance (even without silicon addition), and outstanding resistance to chloride stress corrosion cracking.
With a maximum continuous oxidation temperature of 1150°C, 330 is the go-to grade for furnace muffles and retorts subject to thermal cycling, heat treatment fixtures in carburizing and nitriding atmospheres, and components in reducing and alternately oxidizing/reducing environments. The primary drawback is cost - the 35% nickel content makes 330 significantly more expensive than 310S.
446 - The Nickel-Free Option
Type 446 is a high-chromium (25%) ferritic grade that contains virtually no nickel. It offers oxidation resistance comparable to 310S (up to 1100°C) at a fraction of the cost. Its low thermal expansion coefficient (10.4 μm/m·K vs. 15.9 for 310S) makes it resistant to thermal fatigue.
However, ferritic grades have significant limitations: poor creep strength at elevated temperatures, embrittlement at room temperature after high-temperature exposure (475°C embrittlement and sigma phase), and lower ductility (20% elongation vs. 45% for 310S). For these reasons, 446 is typically used for non-load-bearing applications such as furnace linings and burner nozzles rather than structural components.
Application Guide
| Application | Recommended Grade | Max Temp | Rationale |
|---|---|---|---|
| Furnace lining, radiant tubes | 310S | 1100°C | Best balance of oxidation resistance and cost |
| Heat treatment baskets (oxidizing) | 310S, 309S | 980–1100°C | Good oxidation resistance; economical |
| Heat treatment fixtures (carburizing) | 314, 330 | 1150°C | Silicon-enhanced carburization resistance |
| Petrochemical reformer tubes | 310H, 330 | 1050°C | High-temperature creep strength |
| Automotive exhaust manifolds | 409, 439 | 850°C | Cost-effective; adequate for exhaust temps |
| Burner nozzles, furnace linings (non-structural) | 446 | 1100°C | Nickel-free; low thermal expansion |
| Brazing fixtures | 330 | 1150°C | Thermal shock resistance; no sigma phase |
| Thermocouple protection tubes | 310S, 330 | 1100–1150°C | Oxidation and carburization resistance |
| Conveyor belts (sintering, brazing) | 314 | 1150°C | Carburization resistance; high-temp strength |
| Power plant boiler tubes | 309H | 750°C | ASME code-approved; controlled carbon for creep |
FAQ
Q1: What is the difference between 310 and 310S?
The "S" designation indicates low carbon content (≤ 0.08% vs. ≤ 0.25% for standard 310). The lower carbon in 310S prevents carbide precipitation during welding and high-temperature service, improving intergranular corrosion resistance and weldability. For most high-temperature applications, 310S is preferred. The "H" designation (310H) specifies controlled carbon in the 0.04–0.10% range for optimized creep strength.
Q2: At what temperature does sigma phase form in 310S, and is it reversible?
Sigma phase forms in 310S in the 650–950°C range, particularly after prolonged exposure (hundreds to thousands of hours). It causes embrittlement and reduced ductility. The transformation is reversible - solution annealing at 1100–1150°C for 1–2 hours per inch of thickness, followed by rapid cooling, dissolves sigma phase and restores properties. However, in service, repeated formation and dissolution can cause cumulative damage.
Q3: Can I use 446 ferritic steel instead of 310S to save cost?
For non-structural, non-load-bearing applications at temperatures up to 1100°C, 446 is an excellent cost-saving alternative. However, for structural components, pressure vessels, or any application requiring creep strength and ductility, 310S or another austenitic grade is essential. Ferritic grades like 446 have only about 40% of the creep strength of 310S at 800°C.
Q4: Why does 314 contain silicon, and is it always beneficial?
Silicon (1.5–3.0% in 314) forms a protective silica sub-scale beneath the chromium oxide layer, dramatically improving carburization resistance. However, silicon also increases the tendency for sigma phase formation, reduces ductility at room temperature, and can cause problems during welding (hot cracking). For carburizing atmospheres, the benefits outweigh the drawbacks; for purely oxidizing atmospheres, standard 310S (without silicon) is adequate and preferred.
Q5: How do heat-resistant steels compare to nickel-based alloys for furnace applications?
Heat-resistant steels (310S, 314, 330) are adequate for furnace temperatures up to approximately 1100–1150°C. Above 1150°C, or in severely carburizing/nitriding environments, nickel-based alloys such as Inconel 601 (up to 1250°C) or Hastelloy X (up to 1200°C) become necessary. The cost transition point typically falls around 1100°C - below this temperature, heat-resistant steels offer better value; above it, nickel alloys provide the only reliable long-term solution.
Conclusion
Heat-resistant steel grades represent a spectrum of solutions tailored to specific temperature ranges, atmospheric conditions, and structural requirements. For general-purpose high-temperature service up to 980°C, 309S provides an economical solution. For continuous oxidation resistance to 1100°C, 310S is the industry benchmark. For carburizing environments, 314's silicon enhancement delivers critical protection. For thermal cycling and carburization resistance, 330's high nickel content excels. And for cost-sensitive non-structural applications, 446 offers nickel-free oxidation resistance.
The key to successful material selection lies in accurately defining the dominant failure mode - whether oxidation, carburization, thermal fatigue, or creep - and matching it to the grade whose composition provides the specific protection required. By understanding the compositional logic behind each grade, engineers can avoid both costly over-specification and dangerous under-specification.
For temperatures exceeding 1150°C or for severely corrosive environments, nickel-based alloys such as Inconel 601 and Hastelloy X remain the superior choice, as explored in our companion article on high-temperature nickel alloys.
