AISI 321H Stainless Steel: A Guide to Properties, Fabrication & Uses

AISI 321H stainless steel is a high-carbon, titanium-stabilized austenitic alloy built for extreme high-temperature and corrosive environments. It’s a step up from standard 321, with enhanced creep resistance and strength—making it a top pick for power plants, refineries, and aerospace applications. This guide breaks down everything you need to know, from its core specs to real-world use cases, to help you make informed material choices.

1. Material Overview & Key Specifications

Before using AISI 321H, it’s critical to understand its composition, standards, and basic properties. These details lay the foundation for successful application.

Chemical Composition & Standards

The AISI 321H composition is defined by two key traits: a 321H carbon range of 0.04–0.10 % (higher than standard 321’s ≤0.08% carbon) and 321H titanium stabilization (0.10–0.50% titanium). This carbon boost enhances high-temperature strength, while titanium prevents corrosion. There’s no single 321H chemical formula—instead, it adheres to global standards:

• 321H UNS S32109 (Unified Numbering System)
• 321H ASTM A240 / 321H ASME SA-240 (for plates, sheets, and strips)
• 321H EN 1.4940 equivalent (European standard for matching performance)

Physical & Mechanical Properties

AISI 321H delivers consistent strength and stability. Key metrics are organized in the table below for clarity:

A Japanese power plant, for example, specifies 321H ASTM A240 plates for boiler headers—they rely on the 515 MPa tensile strength to handle high pressure and 700 °C temperatures.

2. High-Temperature Properties & Creep Resistance

AISI 321H’s biggest advantage is its performance under sustained high heat. Its higher carbon content makes it far more resistant to creep (slow deformation) than standard 321.

Critical High-Temperature Traits

• Creep Strength: The 321H 100,000-hour creep strength (a key industry benchmark) is ~110 MPa at 650 °C—this means it can operate for decades without deforming. For shorter durations, 321H short-time tensile at 700 °C is ~300 MPa, enough for intermittent high-heat tasks.
• Oxidation Resistance: It resists rust and scaling up to 900 °C—321H oxidation resistance up to 900 °C makes it ideal for furnace liners and exhaust systems. The 321H steam oxidation rate is also low (≤0.12 mm/year at 800 °C in pure steam).
• Service Limits: The 321H continuous service limit is 870 °C (for non-stop use), and the 321H intermittent service limit (short heat bursts) is 980 °C.
• Sigma Phase Risk: 321H sigma phase embrittlement after long-term exposure (600–800 °C for 1000+ hours) can occur, but this is avoidable with proper heat treatment.

A case study: A European refinery used 321H seamless tube ASTM A213 for flare stack piping (operating at 850 °C). After 12 years, the tubes showed no creep or oxidation—saving the refinery $200,000 in replacement costs.

3. Corrosion Resistance & Stabilization Performance

While AISI 321H is known for high heat resistance, its 321H titanium stabilization also makes it highly corrosion-resistant—even in harsh environments.

Key Corrosion-Resistant Traits

• Intergranular Corrosion (IGC) Immunity: Titanium binds with carbon to form TiC (titanium carbide), preventing chromium depletion at grain boundaries. This gives 321H intergranular corrosion immunity and 321H weld decay prevention—a major upgrade over unstabillized alloys like 304.
• Other Corrosion Types: It resists 321H chloride stress corrosion cracking (common in marine settings) and 321H atmospheric corrosion (no rust in rainy or humid climates). It also has decent 321H sulfidation resistance (for refinery applications) and 321H carburization resistance (for furnace parts).
• 321H vs 321 corrosion comparison: Both offer similar corrosion resistance, but 321H’s higher carbon doesn’t reduce its ability to fight rust—making it a “best of both worlds” choice for heat and corrosion.

A U.S. chemical plant switched from 321 to 321H for acid reactor vessels—321H intergranular corrosion immunity eliminated leaks, and the higher carbon handled the 750 °C operating temperature.

4. Heat Treatment & Microstructural Control

Proper heat treatment is essential to unlock AISI 321H’s full potential. It ensures the alloy maintains strength, resists corrosion, and avoids harmful phases.

Key Heat Treatment Processes

• Solution Annealing: Heat to 1040–1120 °C, hold for 30–60 minutes, then water-quench. This dissolves unwanted carbides and restores a uniform austenitic structure—critical for 321H carbide solution treatment.
• Stabilizing Anneal: Heat to 870–900 °C to ensure titanium fully reacts with carbon. This step enhances corrosion resistance and prevents sensitization.
• Grain Size Control: The 321H grain size control ASTM 7 min (coarser grains) is required by ASME standards—coarser grains improve creep resistance at high temperatures.
• Hot Working: Use 1150–900 °C as the 321H hot working temperature for forging or rolling. This keeps the material ductile and avoids cracking.

Other Considerations

• Sensitization Avoidance: Avoid heating to 450–850 °C for long periods—321H sensitization temperature avoidance prevents chromium depletion.
• Residual Stress Relief: Heat to 450–600 °C to reduce stresses from welding or forming. Use a 321H controlled cooling rate (slow cooling) to avoid warping.

5. Welding, Fabrication & Machining Guidelines

Welding and machining AISI 321H require simple adjustments to preserve its properties. Follow these best practices for success.

Welding Tips

• Consumables: Use 321H welding consumables ER321 (for TIG/MIG welding) to match the base metal’s carbon and titanium content. This ensures the weld has the same high-temperature strength as the parent material.
• Preheat & PWHT: 321H preheat temperature is typically not required for thicknesses up to 25 mm. For thicker parts, 321H post-weld heat treatment (PWHT) (600–650 °C) helps restore 321H HAZ creep strength (heat-affected zone) and relieve stress.
• 321L vs 321H welding differences: 321L uses low-carbon filler (ER321L), while 321H needs ER321—using the wrong filler reduces creep resistance. Also, 321H requires slower cooling to avoid cracking.

Machining & Forming

• Speeds & Feeds: 321H machining speeds and feeds should be 10–15% lower than carbon steel. For example, use 80–100 m/min speed with 321H tool life coated carbide (TiAlN coatings work best—they last 2x longer than uncoated tools).
• Formability & Distortion: 321H formability limits are similar to standard 321—deep drawing is possible with lubrication. For 321H distortion control, use clamping during welding and slow cooling. For high-temperature vessels, follow 321H joint design for high-temperature vessels (thicker welds for strength).

6. Product Forms, Sizes & Supply Chain

AISI 321H is available in a wide range of forms to fit nearly any high-temperature project.

Common Product Forms

• Plates: 321H stainless steel plate thicknesses range from 3 mm to 200 mm, including 321H pressure vessel plate (certified to ASME SA-240 for high-pressure use). Many suppliers offer 321H custom cut plate to reduce waste.
• Pipes & Tubes: 321H pipe schedule chart includes Schedule 40 to Schedule 160; 321H seamless tube ASTM A213 is used for boiler and superheater tubes.
• Bars & Fittings: 321H round bar stock (10 mm to 300 mm diameter), 321H flat bar sizes (5 mm x 20 mm to 20 mm x 100 mm), and 321H forged fittings (elbows, tees for piping).
• Coils: 321H coil suppliers provide coils (1000–3000 mm width) for large-scale fabrication.

Supply Chain Tips

Work with 321H global stockists to ensure quick delivery—many keep common sizes (e.g., 6 mm plate, 2-inch pipe) in stock. For custom parts, choose suppliers with experience in 321H pressure vessel plate to meet safety standards.

Yigu Technology’s Perspective

At Yigu Technology, we recommend AISI 321H for clients needing high heat and corrosion resistance. We source 321H ASTM A240 plates and 321H seamless tube ASTM A213 from certified mills, ensuring compliance with ASME/EN standards. For power plant clients, we prioritize 321H 100,000-hour creep strength testing to guarantee long-term reliability. Our team also advises on welding—using 321H welding consumables ER321 is key to avoiding post-weld failures. For projects where durability at 600+ °C matters, AISI 321H is a cost-effective, low-maintenance choice.

FAQ

1. What’s the difference between AISI 321 and 321H?
   The main difference is carbon content: 321 has ≤0.08% carbon, while 321H has 0.04–0.10% carbon. This makes 321H high-temperature strength and creep resistance better than 321. Both have titanium stabilization (so similar corrosion resistance), but 321H is better for long-term high-heat service.
2. Do I need to preheat AISI 321H before welding?
   No—321H preheat temperature is not required for thicknesses up to 25 mm. For thicker parts (over 25 mm), a low preheat (100–150 °C) can reduce cracking risk. Always use 321H welding consumables ER321 and optional PWHT to restore HAZ strength.
3. Can AISI 321H be used in marine environments?
   Yes—its 321H chloride stress corrosion cracking resistance makes it suitable for marine parts like exhaust elbows. However, 316Ti or 317L are better for fully submerged parts (more molybdenum resists pitting). Use 321H for marine parts that also need high heat resistance (e.g., engine exhausts).