304H vs 321H: Carbon Content and Creep Strength for High-Temperature Pressure Vessels

Introduction

When designing pressure vessels for high-temperature service-refinery heaters, boiler superheaters, catalytic crackers, and heat exchangers-engineers repeatedly face one question: should they specify 304H or 321H? Both are austenitic stainless steels carrying the "H" suffix that designates controlled elevated carbon for improved creep performance. Yet they differ in stabilization strategy, weldability, and long-term reliability. This comparison breaks down carbon content, creep strength, and selection criteria in plain language, so specifiers can make confident, code-compliant decisions.

What Are 304H and 321H Stainless Steels?

304H and 321H are both austenitic, chromium-nickel stainless steels in the 18Cr-8Ni family; the "H" suffix signals a controlled carbon range (0.04–0.10%) optimized for high-temperature service. 321H additionally contains titanium for carbide stabilization.

Both grades share the same 17.0–19.0% chromium and 9.0–13.0% nickel base chemistry. The distinction lies in how each handles carbon at high temperature:

304H (UNS S30409) is the high-carbon variant of standard 304. Its controlled carbon content (0.04–0.10%) strengthens the alloy at elevated temperature by raising the solution-strengthening contribution of carbon dissolved in the austenite matrix.

321H (UNS S32109) is the high-carbon variant of 321, which is essentially 304 chemistry plus titanium. Titanium binds preferentially with carbon to form stable titanium carbides (TiC), preventing chromium carbide precipitation at grain boundaries.

Both alloys fall under ASTM A240/A240M (plate, sheet, strip), ASTM A312 (seamless and welded pipe), and ASME Boiler and Pressure Vessel Code (BPVC) Section II.

Their common Cr and Ni ranges make them broadly interchangeable in corrosion resistance for non-welded, stabilized, or post-solution-annealed conditions.

How Does Carbon Content Differ Between 304H and 321H?

The carbon ranges are nominally identical-0.04% to 0.10% for both-but 321H's titanium "locks up" carbon as TiC, so the effective free carbon available for creep strengthening is lower in 321H than in 304H.

Per ASTM A240, both 304H and 321H require carbon between 0.04% and 0.10%. The lower bound ensures a minimum creep-strengthening effect; the upper bound prevents excessive carbide network formation that embrittles the matrix. How that carbon behaves, however, differs:

In 304H, most carbon remains dissolved in the austenite after solution annealing, contributing directly to solid-solution strengthening and to carbide precipitation that pins grain boundaries during service.

In 321H, titanium (5 × C minimum, typically 0.15–0.70%) reacts with carbon to form stable TiC. This reduces the carbon available for chromium carbide (Cr₂₃C₆) precipitation and also reduces the carbon available for direct matrix strengthening.

Practical consequence: 304H generally delivers slightly higher short-time elevated-temperature yield strength because more carbon stays in solid solution, whereas 321H sacrifices some of that strengthening in exchange for immunity to sensitization.

How Does Carbon Affect Creep Strength at High Temperatures?

Carbon in austenitic stainless steel raises creep strength by two mechanisms-solid-solution strengthening of the matrix and grain-boundary pinning by carbides-and both effects intensify as carbon moves from 0.04% toward 0.10%.

Creep is the slow, time-dependent deformation that occurs when a metal is held under stress at high temperature. Carbon fights creep on two fronts:

Solid-solution strengthening. Dissolved carbon distorts the austenite lattice, resisting dislocation climb-the dominant creep mechanism above 0.5 Tₘ (melting temperature in Kelvin). For 18Cr-8Ni austenite, service temperatures of 600–800°C place the alloy squarely in this regime.

Grain-boundary carbide pinning. M₂₃C₆ (mostly Cr₂₃C₆) precipitates on grain boundaries during service. A controlled dispersion pins grain boundaries, slows sliding, and extends rupture life. Too much carbide, however, forms a continuous grain-boundary film that embrittles the alloy and depletes adjacent chromium.

Stress-rupture data (ASME Section II, Table 1A) confirms the payoff. At 700°C and 100,000 hours, allowable stress values for 304H plate (ASTM A240) typically exceed those of 321H plate by roughly 5–15%, depending on product form and exact carbon/titanium balance. This is the direct, measurable result of higher free carbon in 304H.

What Role Does Titanium Play in 321H?

Titanium in 321H acts as a carbon scavenger, forming stable TiC that prevents Cr₂₃C₆ precipitation, thereby protecting grain-boundary chromium and eliminating sensitization in the heat-affected zone (HAZ) after welding.

The stabilization ratio is defined as Ti ≥ 5 × C (ASTM A240). With carbon at 0.06% (mid-range), the minimum titanium required is 0.30%, and typical specifications target 0.30–0.60% to ensure a stabilization reserve.

TiC is thermodynamically more stable than Cr₂₃C₆. It forms during solution annealing (1050–1100°C) and persists through service, so chromium carbide cannot form at the 450–850°C sensitization window the alloy encounters in service or in the HAZ.

This is why 321H does not require post-weld solution annealing to restore intergranular corrosion resistance-an advantage for field-erected vessels and large-diameter piping where furnace annealing is impractical.

The trade-off: titanium also ties up carbon that would otherwise contribute to creep strengthening, and titanium nitride/carbonitride inclusions can reduce notch toughness at room temperature.

Which Steel Has Better Creep Strength: 304H or 321H?

304H has measurably higher creep-rupture strength than 321H at 600°C and above in the base metal, because more of its carbon remains available for solid-solution strengthening and grain-boundary pinning; however, 321H retains its strength better in the weld HAZ, where 304H can lose carbon-strengthening through sensitization.

The comparison splits into two regimes-base metal and weld HAZ-and they favor different grades:

Base-metal creep-rupture. ASME Code Case and Section II allowable stress tables consistently give 304H equal or higher allowable stress than 321H from 538°C (1000°F) up to 816°C (1500°F). At 704°C (1300°F) and 100,000 hours, 304H plate allowable stress is typically 10–12% higher than 321H plate.

Weld-HAZ behavior. In 304H, the HAZ experiences carbide dissolution during welding and re-precipitation during cooling. If cooling is slow (thick sections), a sensitized zone forms, locally depleted in chromium and weakened in creep. In 321H, titanium re-forms TiC preferentially, preserving grain-boundary integrity and HAZ creep strength.

Practical takeaway: For seamless, thin-wall components (tubing, thin pipe), 304H's higher base-metal creep strength usually wins. For thick-wall, welded, or field-fabricated pressure vessels, 321H's weld-HAZ stability often wins the life-cycle argument.

How Do They Compare in Sensitization and Intergranular Corrosion Resistance?

321H is far more resistant to sensitization and intergranular corrosion (IGC) than 304H, because titanium stabilization prevents the chromium-depletion mechanism that drives 304H sensitization in the 425–900°C range.

Sensitization is the loss of corrosion resistance caused by chromium carbide precipitation at grain boundaries. The two grades handle it very differently:

Sensitization mechanism. When 304H is slow-cooled through 425–900°C, Cr₂₃C₆ precipitates on grain boundaries. Adjacent chromium drops below the 10.5% passivation threshold, leaving those zones vulnerable to IGC in acidic or chloride-bearing media.

321H immunity. TiC forms at higher temperature and is more stable than Cr₂₃C₆, so chromium carbides cannot form and grain boundaries retain full chromium content. The alloy passes ASTM A262 Practice A/E tests even after sensitizing heat treatment.

Service relevance. In refinery sour service, polythionic acid (PTA) cracking during shutdowns is a recognized failure mode for sensitized 304H. NACE SP0170 explicitly recommends stabilized grades (321H, 347H) over 304H for equipment subject to PTA cracking.

Huey test (ASTM A262 Practice C). 304H generally requires solution annealing after welding to pass; 321H passes in the as-welded condition.

What Are the ASME and ASTM Standards for 304H and 321H?

Both alloys are governed by essentially the same set of ASTM material specifications and ASME BPVC Section II allowable-stress tables, but the specific UNS designation, stabilization requirement, and product-form scope differ.

The table below summarizes the governing standards and key requirements for each grade:

The ASME allowable-stress basis (Section II, Appendix 1) for both grades is the lower of: (1) two-thirds of 0.2% yield at temperature, (2) 100% of average stress to produce 1% creep in 100,000 hours, or (3) 67% of average stress to rupture in 100,000 hours. At temperatures above roughly 538°C, the creep-rupture criterion dominates-so the higher creep strength of 304H directly translates into higher allowable stress in base metal.

How Do Welding Considerations Differ?

321H is easier to weld for thick sections and field fabrication because it resists sensitization without post-weld heat treatment (PWHT), whereas 304H may require solution annealing after welding to restore corrosion resistance; 321H also requires a specialized filler.

Welding high-carbon austenitic stainless steel is where the two grades diverge most sharply:

Filler metal. 304H is typically welded with ER308H filler, matching the parent metal's carbon range. 321H is welded with ER347 (niobium-stabilized) filler, not ER321, because titanium does not transfer well across the arc; niobium provides equivalent stabilization in the weld metal.

Sensitization after welding. 304H HAZ sensitizes if cooling through 425–900°C is slow (thick wall, high heat input). Mitigation: solution anneal at 1050–1100°C followed by rapid quench-a furnace operation impractical for large pressure vessels. 321H HAZ does not sensitize because TiC re-forms preferentially.

PWHT requirement. ASME Section VIII does not mandate PWHT for either grade, but service environments (sour, PTA, caustic) often require it. For 304H, the required PWHT is full solution anneal; for 321H, a stress-relief at 850–900°C is often sufficient and avoids the distortion risk of full anneal.

Hot cracking risk. Both grades are austenitic with 5–10% ferrite in weld metal, so hot cracking is modest. 321H's titanium nitride inclusions can act as crack initiators in highly constrained joints-requiring controlled heat input and interpass temperature.

Which Is Better for Pressure Vessel Applications?

There is no universal winner; 304H is preferred for thin-wall, high-temperature components where base-metal creep strength governs design (superheater tubes, furnace coils), while 321H is preferred for thick-wall, welded, or sour-service pressure vessels where weld-HAZ integrity and corrosion resistance govern (refinery drums, reactor internals, catalytic cracker regenerator shells).

The right choice depends on which failure mode actually governs the design. Use the following decision guide:

Choose 304H when:

Design is governed by creep-rupture stress and the component is seamless or has few welds (tube, pipe, coil).

Post-weld solution annealing is feasible (shop fabrication, small vessels).

Service is oxidizing, low-sulfur, and free of polythionic acid exposure.

Minimum wall thickness and weight are premium design goals (higher allowable stress yields thinner wall).

Choose 321H when:

The vessel is thick-wall, field-welded, or too large to furnace-anneal.

Service includes shutdown exposure to polythionic acid (refinery FCC, coker, hydrotreater).

The design must survive cyclic temperature service without sensitization-driven cracking.

Code or end-user specification (e.g., NACE SP0170, API RP 939-A) mandates a stabilized grade.

Rule of thumb: Below roughly 538°C, both grades are over-specified-standard 304/321 or 316L suffices. Between 538°C and roughly 750°C, 304H is usually the creep-efficient choice. Above 750°C, consider 800H/HT, 310S, or nickel alloys (600/601).

What Are the Cost Differences?

321H typically costs 15–30% more than 304H per unit weight due to titanium addition, lower mill production volume, and tighter quality-control requirements for the stabilization ratio; however, life-cycle cost analysis often favors 321H when PWHT, inspection, and outage risk are included.

Material price is only one component of total cost of ownership. The breakdown:

Raw-material premium. Titanium-bearing ferroalloys and the narrower heat chemistry window (Ti must be ≥ 5×C but not so high as to cause TiN inclusions) raise melting cost.

Mill availability. 304H is produced in far higher tonnage globally, so lead time and pricing are more competitive. 321H is a specialty grade with fewer qualified producers.

Fabrication cost. 304H may require full solution anneal after welding-an additional furnace cycle, transport, and potential distortion rework. 321H is usually as-welded or stress-relieved, cutting fabrication cost.

Life-cycle cost. An unplanned refinery outage caused by 304H PTA cracking can cost millions per day. The modest material premium for 321H is often recovered many times over in outage avoidance.

How to Choose Between 304H and 321H: Decision Checklist

Make the selection in four ordered steps-(1) identify the governing failure mode, (2) check the temperature window, (3) evaluate weldability and PWHT feasibility, (4) confirm code and end-user mandates-then specify UNS, product form, and stabilization or carbon testing requirement.

A structured approach prevents the most common specification errors:

Step 1 - Identify the governing failure mode. If creep-rupture governs (thin-wall, high-temperature), lean 304H. If IGC/PTA cracking governs (thick-wall, shutdown exposure), lean 321H.

Step 2 - Check the temperature window. Below 538°C, neither H-grade is needed. From 538°C to 750°C, both fit; above 750°C, upgrade the alloy family.

Step 3 - Evaluate weldability/PWHT. If post-weld solution anneal is impractical (large field-fabricated vessel), choose 321H. If shop-fabricated thin-wall, choose 304H.

Step 4 - Confirm code and end-user mandates. NACE SP0170 (PTA cracking), API RP 939-A (refinery heater tubes), and many operator specs force the choice.

Then specify on the drawing and purchase order: UNS number, ASTM product specification, H-grade carbon range, stabilization ratio (for 321H), and required corrosion test (ASTM A262).

Conclusion

304H and 321H are close metallurgical cousins: both are 18Cr-8Ni austenitic stainless steels with controlled elevated carbon for high-temperature strength. The decision between them turns on a single trade-off-higher base-metal creep strength (304H) versus superior weld-HAZ stability and corrosion resistance (321H). For thin-wall, seamless, high-temperature tubing where creep governs design, 304H is usually the right call.

For thick-wall, field-welded, or shutdown-exposed pressure vessels, 321H usually pays back its premium in reliability and life-cycle cost. Engineers who make this selection deliberately-using the ASME allowable-stress tables, NACE and API guidance, and the service-environment profile-will specify the right alloy the first time and avoid the costly surprise of an out-of-service vessel.