321 Stainless Steel: Titanium-Stabilized Grade for Intergranular Corrosion Resistance

What Is 321 Stainless Steel?

321 (UNS S32100) is a titanium-stabilized austenitic stainless steel developed from Type 304. Titanium is added at a level of at least five times the combined carbon and nitrogen content, so that carbon combines preferentially with titanium to form stable titanium carbides instead of chromium carbides. This prevents chromium depletion at grain boundaries when the steel is held in the sensitizing temperature range of about 450-850°C during welding or high-temperature service, giving 321 resistance to intergranular corrosion without post-weld heat treatment. The European designation is 1.4541 (X6CrNiTi18-10) per EN 10088, and the Japanese designation is SUS321 per JIS G4303/G4304.

Chemical Composition

The composition limits below follow ASTM A240.

Element (wt %)	321 (S32100)
Carbon, max	0.08
Manganese, max	2.0
Silicon, max	0.75
Phosphorus, max	0.045
Sulfur, max	0.030
Chromium	17.0-19.0
Nickel	9.0-12.0
Nitrogen, max	0.10
Titanium	5 x (C+N), min

The titanium requirement is expressed as a multiple of carbon and nitrogen because both elements must be tied up as stable compounds. The H variant 321H (UNS S32109) has a controlled carbon range of 0.04-0.10% and a titanium minimum of 4 x (C+N), which further improves creep strength for high-temperature pressure parts.

Mechanical Properties and Heat Treatment

Minimum values for annealed plate per ASTM A240 are listed below.

Property	321
Tensile strength, min (MPa)	515
0.2% yield strength, min (MPa)	205
Elongation in 50 mm, min (%)	40

321 is supplied in the solution-annealed condition, typically heated to 950-1120°C and rapidly cooled. Because titanium already stabilizes the carbon, the steel is not sensitized by subsequent heating in the 450-850°C range, and no post-weld heat treatment is required. The annealed hardness is about 201 HB maximum for plate.

High-Temperature Performance and Applications

321 offers useful creep strength and oxidation resistance for continuous service in the range of about 425-900°C, with short-term exposure possible up to about 950°C. Compared with 304, the titanium-stabilized grade shows higher creep rupture strength above roughly 550°C and lower oxidation weight gain at elevated temperature because the oxide scale remains more adherent. Typical applications are high-temperature heat exchanger tubes, furnace and cracking tubes, expansion bellows, secondary piping in nuclear plants, superheater and reheater tubes, automotive exhaust manifolds and catalytic converter shells, and components in petrochemical catalytic cracking units. For applications above about 900°C, a higher-alloyed grade such as 310S (UNS S31008) is usually specified.

Welding and Fabrication

321 has good weldability with conventional processes such as TIG, MIG, SMAW and SAW. The titanium addition is prone to oxidation and burnout at very high weld pool temperatures, so welding should be carried out with controlled heat input, short arc residence time and adequate argon shielding, keeping the titanium content above the minimum specified after welding. Filler metal is typically matched to the base material. The steel forms well by bending, deep drawing and spinning, and it can be machined in the annealed condition; work hardening is comparable to other austenitic grades. Passivation with a nitric acid solution after fabrication restores the chromium-rich oxide film and improves resistance to surface contamination.

Frequently Asked Questions

Q1. What does titanium do in 321 stainless steel?
Titanium has a higher affinity for carbon than chromium, so it forms stable titanium carbides and prevents chromium carbide precipitation at grain boundaries, eliminating sensitization.

Q2. Is post-weld heat treatment required for 321?
No. The titanium stabilization means 321 can be welded and used in the sensitizing temperature range without losing intergranular corrosion resistance.

Q3. What is the difference between 321 and 304?
321 adds titanium and is resistant to intergranular corrosion after welding and high-temperature service, while 304 can sensitize in the 450-850°C range.

Q4. Can 321 replace 316L in chloride service?
No. 321 contains no molybdenum, so its resistance to chloride pitting is similar to 304 and lower than 316L; it is selected for high-temperature strength, not for marine environments.

Q5. What is the service temperature range of 321?
Continuous service is generally 425-900°C with short-term exposure up to about 950°C; above that, 310S is recommended.

Q6. Why is titanium burnout a concern when welding 321?
Titanium oxidizes readily at high weld pool temperatures; low heat input, short arc time and argon shielding keep the residual titanium above the specified minimum.