304H vs 321H – Composition, Heat Treatment, Properties, and Applications

Introduction

304H and 321H are two widely used austenitic stainless steels encountered in pressure-vessel, high-temperature, and general fabrication environments. Engineers and procurement teams frequently weigh corrosion resistance, high-temperature performance, and fabrication cost when selecting between them. The common decision contexts include: service temperature (creep and carburization resistance), susceptibility to sensitization during welding and subsequent intergranular corrosion, and lifetime maintenance considerations.

The principal practical difference is that one alloy is intentionally alloyed with a stabilizing element to control carbide precipitation and preserve corrosion resistance after exposure to intermediate temperature ranges, while the other relies on higher carbon for improved high-temperature strength. Because both are derivatives of the 300-series austenitic family, they are often compared where the trade-offs between high-temperature mechanical strength and long-term resistance to intergranular attack must be balanced.

1. Standards and Designations

• Common international standards and specifications:
• ASTM/ASME: A240/A312 (sheet/plate and tubing for stainless), A182 (for forgings), etc.
• EN: EN 10088 series / EN ISO equivalents.
• JIS: JIS G4303, G4311, etc.
• GB: Chinese National standards for stainless steels.
• Classification:
• 304H — Stainless steel, austenitic stainless (high-carbon variant of 304).
• 321H — Stainless steel, austenitic stainless stabilized with titanium (high-carbon variant of 321 where "H" denotes higher carbon for creep strength).

Note: Exact numerical designation and composition limits can vary by standard; always confirm with the applicable specification and mill certificate.

2. Chemical Composition and Alloying Strategy

Table: Typical composition ranges (wt%). Values are representative ranges commonly used in specifications; consult the controlling standard or mill test report for exact limits.

Element	304H (typical wt%)	321H (typical wt%)
C	0.04 – 0.10	0.04 – 0.10
Mn	≤ 2.0 (typ. 1.0–2.0)	≤ 2.0 (typ. 1.0–2.0)
Si	≤ 0.75	≤ 0.75
P	≤ 0.045	≤ 0.045
S	≤ 0.03	≤ 0.03
Cr	17.0 – 19.0	17.0 – 19.0
Ni	8.0 – 10.5	8.0 – 12.0
Mo	~0 (trace)	~0 (trace)
V	trace	trace
Nb (Cb)	trace/0	trace/0
Ti	0 (trace)	0.15 – 0.7 (stabilizer)
B	trace	trace
N	trace (up to ~0.1)	trace (up to ~0.1)

How alloying affects properties - Carbon (C): Higher carbon in "H" grades increases solution-strengthening and creep strength at elevated temperatures but increases the risk of chromium carbide formation at intermediate temperatures if not stabilized. - Chromium (Cr): Primary element for general corrosion resistance and passivation film formation. - Nickel (Ni): Stabilizes austenite, improves toughness and ductility, and helps corrosion resistance. - Titanium (Ti) in 321H: Acts as a carbide former that preferentially binds carbon to form stable TiC/Ti(C,N) rather than chromium carbides; this reduces sensitization and intergranular corrosion after exposure to sensitizing temperatures. - Other elements (Mn, Si, N): Adjust mechanical properties, deoxidation behavior, and pitting resistance (N).

3. Microstructure and Heat Treatment Response

Microstructures - Both grades are austenitic (face-centered cubic) in the solution-annealed condition. Primary features are austenite matrix with possible fine carbides, nitrides, and stabilizer precipitates depending on chemistry and thermal history. - 304H: With higher carbon, there is increased tendency to form chromium carbides (Cr23C6) along grain boundaries when exposed to the sensitization range (~425–850 °C). If cooled from solution anneal without stabilization, sensitization can occur under certain thermal cycles. - 321H: Titanium preferentially forms TiC/Ti(C,N) precipitates, tying up carbon and reducing or preventing Cr carbide precipitation at grain boundaries.

Heat treatment response - Solution annealing (typical for austenitic stainless): high-temperature anneal followed by rapid cooling restores a homogeneous austenite and dissolves most precipitates. For both grades, solution anneal is the standard way to remove prior sensitization if full solutions are feasible. - Stabilization: 321H’s titanium content does not require special stabilization heat treatment beyond normal solution anneal; stabilization occurs metallurgically through TiC formation. - Cold work and aging: Cold work increases strength but can raise susceptibility to stress corrosion cracking in chloride environments for any austenitic stainless. - Normalizing/quenching & tempering: Not applicable in the traditional sense because these are austenitic, non-transforming stainless grades; they do not respond to quench-and-temper in the same way as ferritic or martensitic steels.

4. Mechanical Properties

Table: Typical mechanical property ranges (room temperature, annealed/solution-annealed conditions). These are representative and strongly depend on product form, temp, and heat treatment.

Property	304H (typical)	321H (typical)
Tensile strength (MPa)	~500 – 700	~480 – 700
Yield strength (0.2% proof, MPa)	~200 – 310	~200 – 310
Elongation (%)	~40 – 60	~40 – 60
Impact toughness (Charpy V, J)	Good at RT; decreases with cold work	Good at RT; decreases with cold work
Hardness (HB/HRB)	Relatively low in annealed condition	Similar to 304H in annealed condition

Explanation - Strength: Both grades have broadly similar tensile and yield properties in the annealed condition. Slight differences can occur due to Ni content and minor differences in carbon/stabilizer state. The "H" carbon boost gives somewhat enhanced high-temperature strength relative to standard 304 at elevated service temperatures. - Toughness/ductility: Austenitic structure confers excellent ductility and toughness at ambient temperature for both. Cold work and embrittling precipitates (e.g., continuous Cr carbides) can reduce toughness. - Elevated temperature: 304H and 321H retain ductility at elevated temperatures; however, because 321H resists carbide precipitation, it is favored where repeated or prolonged exposures in the sensitization range are expected and where corrosion properties after thermal cycling are critical. For long-term creep resistance at high temperatures, consult creep data specific to product/heat.

5. Weldability

Both 304H and 321H are considered weldable by standard processes (SMAW, GMAW/MIG, GTAW/TIG, etc.), but there are important considerations:

• Carbon/hardenability: Higher carbon increases the risk of sensitization or formation of brittle intermetallics in the HAZ. Stabilization (321H) minimizes this risk by tying up carbon.
• Weldability indices: Commonly used empirical formulas to estimate weldability risk include:
• IIW Carbon Equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
• Dearden & Smith (Pcm) index: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
• Interpretation (qualitative): Higher $CE_{IIW}$ or $P_{cm}$ values correlate with greater hardenability and increased risk of HAZ cracking in carbon steels; for austenitic stainless steels these formulas are used cautiously. 304H’s higher carbon can increase sensitivity to HAZ carbide precipitation and require attention to interpass temperature and post-weld treatments. 321H typically shows better resistance to intergranular attack after welding due to stabilization with Ti; this makes 321H preferable in welded high-temperature assemblies where sensitization range exposure occurs.

Practical guidance - Use low-oxygen, low-sulfur consumables and appropriate filler metals (matching or stabilized equivalents). - Control heat input and interpass temperatures to limit grain boundary precipitation. - For critical service where intergranular corrosion is unacceptable, select stabilized grades (321/321H) or apply post-weld solution annealing where feasible.

6. Corrosion and Surface Protection

• Austenitic stainless context: Both are stainless; general corrosion resistance in oxidizing environments is excellent due to chromium passivation. Localized attack (pitting/crevice) depends on chloride levels and is not materially different between the two when composition and surface finish are similar.
• Sensitization and intergranular corrosion: 304H, with elevated carbon, is more likely to form chromium carbides after thermal exposure in the sensitization temperature range, which can lead to intergranular corrosion. 321H’s titanium stabilizer reduces this risk by forming titanium carbides instead.
• PREN (pitting resistance equivalent number) is not very useful for these molybdenum-free 300-series alloys, but the general formula is: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ In these grades, Mo is effectively zero, so PREN differences are minimal and depend mostly on nitrogen content.
• Surface protection for non-stainless scenarios: Not applicable here; however, in highly aggressive environments additional coatings or cathodic protection may be required.

7. Fabrication, Machinability, and Formability

• Formability: Austenitic structure provides excellent formability and deep-draw characteristics for both grades in the annealed condition. Cold working increases strength but reduces ductility.
• Machinability: Typical austenitic stainlessels have poor to moderate machinability compared with carbon steels; higher carbon (304H/321H) does not substantially improve machinability. Use appropriate tool materials (carbide tips), rigid setups, and high positive rake tooling. Expect work-hardening during machining, so chip control and cutting parameters are important.
• Surface finish and polishing: Both polish and finish well; 321H may require slightly different pickling/polishing parameters if TiN/TiC particles are present after fabrication.

8. Typical Applications

Table: Common uses by grade

304H – Typical applications	321H – Typical applications
Furnace parts, high-temperature piping and pressure vessels where increased high-temperature strength is needed and sensitization risk can be managed	Exhaust and turbocharger components, aircraft and aerospace ducts, chemical process piping and heat exchangers exposed to thermal cycles and sensitizing temperatures
Boiler components, superheater/reheater tubes (where carbon boost is specified for creep)	Jet engine and gas-turbine components where stabilization against intergranular attack is critical
General fabrication where elevated temperature strength is required with cost sensitivity	Welded assemblies exposed to intermediate temperature ranges requiring resistance to intergranular corrosion after welding

Selection rationale - Choose 304H when elevated temperature strength is required and the thermal cycle or fabrication route will avoid prolonged sensitization or when post-weld solution anneal is feasible. - Choose 321H when thermal cycling or welding in service-exposed components makes protection against sensitization essential and when long-term intergranular corrosion resistance is a priority.

9. Cost and Availability

• Relative cost: Both grades are commonly produced and broadly available. 321H typically commands a modest premium over 304H due to the addition of titanium and the tight control required for stabilizer levels. Pricing depends on Ni content, market conditions, and form (plate, tube, bar).
• Availability by product form: Both grades are widely available in plate, sheet, tube, and pipe; specialty seamless or high-integrity forgings may have lead times. 304H is commonly specified for pressure-vessel steels; 321H is often stocked for high-temperature and stabilized applications.

10. Summary and Recommendation

Table: Quick comparison (qualitative)

Attribute	304H	321H
Weldability	Good with care; higher C raises sensitization risk	Very good for welded, thermally cycled assemblies (stabilized)
Strength–Toughness (high temp)	Good elevated-temp strength due to higher C	Good elevated-temp strength; stabilization preserves toughness after cycling
Cost	Lower to moderate	Moderate (slightly higher)

Recommendations - Choose 304H if: you need improved high-temperature strength from a high-carbon austenitic stainless in applications where either thermal exposure avoids long times in the sensitization range, or where post-fabrication solution annealing and careful welding practice can be applied. It is suitable when cost sensitivity is a factor and stabilizer benefits are not required. - Choose 321H if: the part will undergo welding, repeated thermal cycling, or long-term service in the sensitization temperature range and resistance to intergranular corrosion is critical. 321H is preferred when minimizing post-weld heat-treatment or when preserving corrosion resistance after fabrication is a primary requirement.

Final note: The choice between 304H and 321H should be made with reference to the specific service temperature, thermal cycle, corrosion environment, and regulatory/standard requirements for the component. Consult mill test certificates, creep/rupture data for intended operating temperatures, and welding procedure qualifications when specifying either grade.