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

Introduction

304H and 321H are two austenitic stainless-steel grades commonly specified for high-temperature and corrosion-resistant applications. Engineers and procurement professionals often weigh the trade-offs between corrosion resistance, high-temperature performance, weldability, and cost when selecting between them. Typical decision contexts include pressure-vessel and furnace components, heat-exchanger tubing, and piping in petrochemical or power-generation plants.

The principal technical distinction is their behavior at elevated temperature: 321H is stabilized with titanium to resist chromium carbide precipitation and attendant grain-boundary corrosion and oxidation during prolonged exposure to high temperatures, whereas 304H relies on higher carbon to retain strength at temperature but is more prone to sensitization unless processed carefully. Because of this, the two grades are routinely compared for service at intermediate and high temperatures where oxidation, creep resistance, and post-weld corrosion are concerns.

1. Standards and Designations

• Common standards and specifications:
• ASTM/ASME: A240/A312 (sheet, plate, and tubing for stainless steels); A358/A213 for some high-temperature applications.
• EN: EN 10088 series (stainless steels).
• JIS: JIS G4303/G4305 equivalents exist for 300-series.
• GB: GB/T standards for stainless steels (PRC national standards).
• Classification:
• Both 304H and 321H are stainless steels (austenitic).
• They are not carbon steels, tool steels, or HSLA grades.

2. Chemical Composition and Alloying Strategy

Table: typical composition ranges (weight %) — ranges reflect common specification bands; exact limits depend on the standard and product form.

Element	304H (typical ranges)	321H (typical ranges)
C	0.04 – 0.10	0.04 – 0.10
Mn	≤ 2.0	≤ 2.0
Si	≤ 1.0	≤ 1.0
P	≤ 0.045	≤ 0.045
S	≤ 0.03	≤ 0.03
Cr	17.5 – 19.5	17.0 – 19.0
Ni	8.0 – 11.0	9.0 – 12.0
Mo	Usually ≤ 0.6 (often absent)	Usually ≤ 0.6 (often absent)
V	—	—
Nb	—	—
Ti	≤ 0.7 (typically low to none)	5 × C (min) to ≈ 0.7
B	Trace/controlled	Trace/controlled
N	Trace; small amounts	Trace; small amounts

Notes: - 304H is a higher-carbon variant of 304 designed to retain tensile strength at elevated temperatures; stabilized elements are not intentionally added. - 321H is titanium-stabilized: titanium ties up carbon as titanium carbides/ carbonitrides during thermal exposure, preventing chromium carbide formation at grain boundaries. - The presence of Ti in 321H distinguishes its alloying strategy: stabilization for high-temperature service and resistance to intergranular corrosion after sensitizing cycles.

How alloying affects properties: - Carbon increases high-temperature strength but raises sensitization risk (chromium carbide precipitation) that can lead to intergranular corrosion. - Chromium provides corrosion resistance and high-temperature oxidation resistance by forming a protective oxide scale. - Nickel stabilizes the austenitic phase, improving toughness and ductility. - Titanium (in 321H) scavenges carbon and reduces chromium carbide formation, improving resistance to grain boundary attack during high-temperature exposure.

3. Microstructure and Heat Treatment Response

• Typical microstructure (annealed): both grades are fully austenitic (face-centered cubic) in the annealed condition. Carbide or Ti-carbide dispersions may appear depending on thermal history.
• 304H: higher carbon content produces more tendency for carbide precipitation (Cr23C6) after exposure in the sensitization range (~450–850 °C). Such precipitation occurs at grain boundaries and can lead to local chromium depletion and intergranular corrosion. Without stabilization, microstructure after exposure may show continuous carbide networks at boundaries.
• 321H: titanium forms stable titanium carbides/nitrides preferentially over chromium carbides. This results in less chromium depletion at boundaries and a microstructure more resistant to high-temperature sensitization and intergranular attack.

Heat treatment and processing routes: - Annealing: Typical anneal for austenitic stainless steels (including 304H and 321H) is around 1010–1120 °C followed by rapid cooling to retain a homogeneous austenitic structure. Rapid cooling reduces carbide precipitation. - Normalizing is not standard for these austenitic grades because they do not exhibit the ferrite-pearlite transformation typical of carbon steels. - Quenching & tempering: Not applicable in the carbon-steel sense; austenitic stainless steels are not hardened by martensitic transformation. - Thermo-mechanical treatments: Cold work increases strength via strain hardening for both grades; final anneal may be applied depending on required properties. - For service that cycles through sensitization temperatures or requires long-term elevated-temperature stability, 321H requires less post-weld heat control to avoid sensitization than 304H.

4. Mechanical Properties

Table: Typical annealed-room-temperature property ranges (indicative; depend on product form, thickness, and exact specification)

Property	304H (annealed, typical)	321H (annealed, typical)
Tensile Strength (MPa)	480 – 700	480 – 700
Yield Strength, 0.2% (MPa)	190 – 310	190 – 310
Elongation (%)	40 – 60	40 – 60
Impact Toughness (Charpy V, room temp)	High; good ductility	High; good ductility
Hardness (HB or HRB, annealed)	Low–moderate (soft)	Low–moderate (soft)

Interpretation: - At room temperature and in the annealed condition, 304H and 321H have very similar tensile, yield, and ductility characteristics because both are austenitic stainless steels with comparable Cr and Ni contents. - Differences become more pronounced during prolonged high-temperature exposure: 304H may lose localized corrosion resistance and ductility at grain boundaries if sensitized; 321H retains more stable grain-boundary chemistry and therefore maintains toughness and corrosion resistance better in those regimes. - Mechanical property selection must account for the product form (sheet, plate, tubing), cold work, and whether the component will be used in creep-critical high-temperature service.

5. Weldability

Weldability considerations include carbon content (higher carbon raises hardenability and sensitization risk), presence of stabilizing elements, and heat input control.

Common weldability indices: - Carbon Equivalent (IIW): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - Pitting resistance equivalent number (when relevant for pitting corrosion evaluation): $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation: - Both 304H and 321H are readily welded by standard processes (TIG, MIG, SMAW, etc.). Because they are austenitic, they do not harden martensitically and are not susceptible to cold cracking. - Higher carbon in 304H increases risk of sensitization after welding if slow cooling occurs; this can lead to intergranular corrosion in the heat-affected zone (HAZ). Post-weld solution annealing or rapid cooling practices can mitigate this. - 321H, with titanium stabilization, is less prone to post-weld sensitization; Ti ties up carbon during heating/welding and forms stable Ti(C,N) precipitates, reducing chromium depletion at grain boundaries. This makes 321H a preferred choice for weldments that will see extended exposure in the sensitization range. - For both grades, good welding practice—control of heat input, interpass temperature, and use of appropriate filler metals—ensures acceptable joint performance. When corrosion resistance in the HAZ is critical, choose a stabilized grade or low-carbon L-grade alternatives (e.g., 304L).

6. Corrosion and Surface Protection

• For stainless (both 304H and 321H): passive chromium oxide layer provides general corrosion resistance. Neither grade contains significant molybdenum, so localized pitting and crevice corrosion resistance in chloride environments is limited compared with Mo-bearing grades (e.g., 316).
• PREN (for pitting resistance equivalence where Mo and N matter): $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ Because Mo is typically absent or low in 304H/321H and N is low, PREN values will be modest; PREN is more applicable to duplex and Mo-bearing austenitics.
• Elevated-temperature corrosion / oxidation:
• 321H demonstrates improved resistance to sensitization-related intergranular corrosion and scale adherence during prolonged exposure to the 500–800 °C range due to titanium stabilization.
• 304H, while designed to maintain tensile strength at elevated temperatures, can form chromium carbides at grain boundaries leading to local depletion of chromium and reduced intergranular corrosion resistance unless heat input and cooling are controlled.
• Non-stainless materials (not applicable here): where non-stainless steels are used, protection options include galvanizing, paint systems, or high-temperature coatings; for high-temperature stainless steels, protective oxide scale characteristics and alloy choice dominate.

7. Fabrication, Machinability, and Formability

• Machinability: Both grades are typical of austenitic stainless steels—work-hardening and gummy behavior require rigid tooling, adequate speeds, and sharp inserts. Machinability is moderate and similar for 304H and 321H; 321H may be marginally more difficult due to the presence of Ti carbides affecting tool wear.
• Formability: Both grades are highly ductile and formable in the annealed condition. Cold working increases strength via strain hardening but reduces ductility.
• Surface finishing: Both take common stainless finishing methods (grinding, polishing, electropolishing) and respond similarly, although Ti-containing inclusions in 321H can impact microetching behavior.
• Welding and post-weld operations: As noted, 321H reduces the need for post-weld solution anneal when service includes prolonged high-temperature exposure; 304H may require more care to avoid sensitization.

8. Typical Applications

304H — Typical Uses	321H — Typical Uses
Pressure vessels and piping for elevated-temperature steam systems where higher carbon strength is required but sensitization can be controlled	Aircraft exhaust components, furnace parts, and heat-exchanger tubing exposed to cyclical high temperatures where stabilization is needed
Heat-exchanger tubes and headers in boilers where higher-temperature tensile strength is needed	Chemical and petrochemical process equipment exposed to sensitizing temperature ranges or with repetitive thermal cycling
General high-temperature structural components and fittings where 304 family corrosion resistance is acceptable	Exhaust stacks, catalytic converter housings, and furnace muffles requiring stable grain-boundary chemistry at temperature

Selection rationale: - Choose 304H when room-temperature toughness and somewhat higher elevated-temperature tensile strength are primary needs and when welding and cooling practices can be controlled to limit sensitization. - Choose 321H when service involves prolonged exposure in the sensitization temperature range, repeated thermal cycling, or when oxidation/crevice behavior in the HAZ is a concern.

9. Cost and Availability

• Cost: 321H is typically modestly more expensive than 304H due to the addition of titanium and its niche high-temperature use. Market prices fluctuate with Ni and alloying-element markets.
• Availability: 304H is widely available in plate, sheet, tube, and bar. 321H is also available in common product forms but may have longer lead times for some specialty sizes or finishes depending on region.
• Procurement tip: Specify product form, required heat treatment, and any post-weld anneal requirement explicitly to avoid supply-chain mismatches or unexpected fabrication costs.

10. Summary and Recommendation

Table: Quick comparison

Criterion	304H	321H
Weldability	Good, but HAZ sensitization risk with slow cooling	Very good; Ti stabilization reduces HAZ sensitization risk
Strength–Toughness	Similar at room temp; good elevated-temp tensile strength	Similar at room temp; retains intergranular toughness in high-temp service
Resistance to high-temp sensitization/oxidation	Lower (more prone to carbide precipitation)	Higher (Ti stabilization improves high-temp stability)
Cost	Lower	Higher (modestly)

Conclusion — Choose 304H if: - The component requires higher carbon content for tensile strength at elevated temperature but the service or welding practices minimize time in the 450–850 °C sensitization window. - Cost and broad availability are primary considerations and pitting/corrosion exposure is moderate.

Conclusion — Choose 321H if: - The service involves prolonged exposure to elevated temperatures, repeated thermal cycling, or situations where post-weld sensitization and intergranular corrosion are a concern. - You need a stabilized austenitic alloy that better preserves grain-boundary chromium and oxidation resistance in the HAZ and in long-duration high-temperature service.

Final recommendation: - For general elevated-temperature structural or pressure applications where fabrication can control cooling and corrosion exposure is not extreme, 304H is an economical choice. For components that will experience sustained high temperatures, cyclic heat, or weld-sensitive environments, 321H offers a more robust, lower-risk option despite a modest premium in cost. Validate specific alloy selection with actual service temperature profiles, weld procedures, and corrosion data for the intended environment.