SUS321 Stainless Steel: Titanium-Stabilized Grade for High-Temperature and Welded Service

What Is SUS321?

SUS321 is a titanium-stabilized austenitic stainless steel specified in JIS G4303/G4304 (SUS321), ASTM A240 (plate, sheet and strip, grade 321), ASTM A312 (pipe) and EN 10088 (1.4541, X6CrNiTi18-10). Its defining feature is a titanium addition, typically 5 x (C+N) minimum up to 0.70% maximum, which has a higher affinity for carbon than chromium. During welding or heating in the sensitizing range of roughly 425-815°C, titanium reacts with carbon to form stable titanium carbides, leaving chromium in solid solution and preserving corrosion resistance at grain boundaries.

Chemical Composition (wt%)

Element	Specified Range (JIS G4304 / ASTM A240)
Carbon, max	0.08
Silicon, max	1.00
Manganese, max	2.00
Phosphorus, max	0.045
Sulfur, max	0.030
Chromium	17.00 - 19.00
Nickel	9.00 - 12.00
Titanium	5 x (C+N) min, 0.70 max (ASTM A240)
Iron	Balance

Mechanical Properties (Annealed)

Property	Minimum Value	Reference Standard
Tensile strength	515 MPa	ASTM A240
Yield strength (0.2% offset)	205 MPa	ASTM A240
Elongation	40%	ASTM A240
Hardness	217 HBW max	ASTM A240

Performance Advantages

The stabilization mechanism gives SUS321 three practical advantages over unstabilized 304. First, it resists intergranular corrosion after welding without post-weld heat treatment, which simplifies fabrication of vessels and piping. Second, it retains useful strength and oxidation resistance up to about 870°C in air, with intermittent exposure to about 925°C, making it suitable for exhaust and heat-transfer components. Third, in moderate chloride environments it is less susceptible to stress corrosion cracking than 304, although it is not a substitute for molybdenum-bearing grades in seawater or strong chloride service.

Equivalent Grades

ASTM A240/A312 321, UNS S32100, EN 1.4541 (X6CrNiTi18-10), JIS SUS321, GB 06Cr18Ni11Ti (GB/T 20878, S32168). The high-carbon variant 321H (UNS S32109, EN 1.4941) with carbon 0.04-0.10% is used where creep strength at 538-870°C is required.

Welding and Fabrication

SUS321 welds readily by TIG, MIG and MMA. Because of titanium stabilization, post-weld heat treatment is generally unnecessary, which is a major cost advantage for field-erected equipment. When joining SUS321 to carbon or low-alloy steel, a nickel-rich austenitic filler such as ER309L is recommended to avoid martensite formation and to provide adequate toughness in the joint. For welded structures that cannot be heat-treated after fabrication, SUS321 is a reliable choice.

Typical Applications

Aircraft and automotive exhaust systems, heat exchangers and condensers, chemical and petrochemical processing equipment, pressure vessels, steam piping, expansion joints, and food processing machinery. The grade is also used for furnace components operating below the oxidation limit of the material.

FAQ

Q1: How does titanium stabilization improve corrosion resistance? Titanium combines with carbon in preference to chromium, forming stable titanium carbides. This prevents chromium depletion at grain boundaries during welding or heating, eliminating sensitization and intergranular corrosion in the heat-affected zone.

Q2: What is the difference between SUS321 and SUS304? SUS321 adds titanium for stabilization, giving better resistance to intergranular corrosion after welding and better high-temperature strength up to about 870°C. SUS304 is cheaper and adequate for mild, non-welded or low-temperature service.

Q3: Can SUS321 be used at high temperature? Yes, continuously to about 870°C in oxidizing atmospheres, with short excursions to about 925°C. Above this range, grain growth and oxidation cause rapid loss of properties.

Q4: Can SUS321 be welded to other steels? Yes. It welds to other austenitic grades and, with high-nickel filler such as ER309L, to carbon and low-alloy steels. Stabilization avoids sensitization in the heat-affected zone.

Q5: How does SUS321 perform in chloride environments? Better than 304 against chloride stress corrosion cracking, but inferior to molybdenum-bearing grades such as 316/316L for pitting and crevice corrosion. For seawater or concentrated chloride service, 316, 904L or duplex grades are more appropriate.

Q6: When should 321H be specified instead of 321? When the component is designed for creep at 538-870°C, such as pressurized piping and superheater tubes; the higher carbon range of 321H (0.04-0.10%) provides the necessary creep-rupture strength.