SUS321 vs 304: How Titanium Alloying Improves High-Temperature Resistance

SUS321 and SUS304 in the Austenitic Family

SUS321 and SUS304 are both austenitic stainless steels defined by JIS G4303 and G4304, and they appear similar at first glance. The decisive difference is titanium: SUS321 carries a titanium addition of at least five times the carbon content, typically 0.4 to 0.8 percent, while SUS304 contains no titanium. Because titanium has a stronger affinity for carbon than chromium, it forms stable titanium carbides and leaves chromium in solid solution, protecting the grain boundaries. Internationally, SUS321 corresponds to UNS S32100, EN 1.4541 (X6CrNiTi18-10) and GB 06Cr18Ni11Ti, while SUS304 corresponds to UNS S30400, EN 1.4301 (X5CrNi18-10) and GB 06Cr19Ni10.

Composition: The Role of Titanium

The table compares composition limits per JIS G4303. Both grades share the same carbon ceiling, but only SUS321 specifies titanium, which locks up the carbon and prevents chromium depletion at high temperature.

Element	SUS321	SUS304
Carbon, max	0.08	0.08
Silicon, max	1.00	1.00
Manganese, max	2.00	2.00
Phosphorus, max	0.045	0.045
Sulfur, max	0.030	0.030
Nickel	9.00-13.00	8.00-10.50
Chromium	17.00-19.00	18.00-20.00
Titanium	5 x C, min	-

High-Temperature Performance

EN 10095, the European standard for heat-resisting steels, lists a maximum service temperature in air of 850 degrees C for 1.4541 compared with 800 degrees C for 1.4301, reflecting the stabilised grade better resistance to oxidation and scale spalling. The titanium-stabilised structure also delays carbide coarsening and sigma-phase embrittlement, so SUS321 holds strength longer under sustained load. Published creep-rupture data commonly cited for these grades show SUS321 retaining roughly 60 MPa at 700 degrees C for 1000 hours versus about 40 MPa for SUS304 under the same conditions; these are typical datasheet values for comparison and must be confirmed against current producer data or code allowables, such as ASME Section II, before design.

Intergranular Corrosion and Weldability

When either grade is held in the sensitisation range of about 450 to 850 degrees C, carbon can precipitate as chromium carbide at grain boundaries, depleting adjacent chromium and making the material susceptible to intergranular corrosion. In SUS321 the titanium carbides form first, so chromium remains in solution and the material is effectively immune to this attack, even in the weld heat-affected zone and in long-term high-temperature service. As a result, SUS321 requires no post-weld solution annealing, which simplifies fabrication of large welded structures such as furnace tubes and heat-exchanger tube sheets. SUS304, by contrast, may require solution annealing at about 1050 degrees C with water quenching after welding, or the substitution of low-carbon SUS304L with carbon 0.030 percent maximum.

Application Scenarios and Selection Guide

SUS321 is the natural choice where the operating temperature exceeds about 500 degrees C, where sustained high-temperature stress is present, or where complex welded structures cannot be easily solution-annealed. Typical applications include catalytic cracking unit components in the 600 to 800 degrees C range, annealing-furnace muffle tubes in oxidising atmospheres, high-temperature heat-exchanger tube sheets, and steam piping in power plants. SUS304 remains a cost-effective material for general acid and alkali piping below 450 degrees C, room-temperature condensate lines, LNG storage at cryogenic temperature, and non-critical equipment supports. For chloride-rich environments such as seawater, neither grade is suitable; molybdenum-bearing SUS316 or higher-alloyed grades should be selected instead.

Frequently Asked Questions

Q1. What does titanium do in SUS321?
Titanium preferentially forms stable carbides, preventing chromium-carbide precipitation at grain boundaries and eliminating intergranular corrosion and chromium depletion at high temperature.

Q2. Can SUS321 be used above 850 degrees C?
EN 10095 lists 850 degrees C as the maximum service temperature in air for 1.4541; above this, oxidation and loss of mechanical strength accelerate rapidly.

Q3. Does SUS321 need post-weld heat treatment?
No. The titanium stabilisation keeps the weld heat-affected zone immune to sensitisation, so solution annealing after welding is not required.

Q4. Is SUS321 more expensive than SUS304?
Yes, because of the titanium addition and tighter processing control. The premium is justified where high-temperature or welded service demands stabilisation.

Q5. Which is better for seawater, SUS321 or SUS304?
Neither. Both lack molybdenum; use SUS316 or a duplex grade for chloride or marine environments.

Q6. What are the equivalents of SUS321?
UNS S32100, EN 1.4541 (X6CrNiTi18-10), GB 06Cr18Ni11Ti, and in older British designations 321S31 or similar per relevant standards.