304 vs 321 Stainless Steel: Standard vs Stabilized Austenitic Alloys

Grade Overview and Designations

Grade 304 (UNS S30400) is the classic 18/8 austenitic stainless steel used in food processing, kitchenware, architecture and general industry. Grade 321 (UNS S32100) has a similar base of 18% chromium and 9% nickel but adds titanium, at least five times the carbon content, as a stabilizing element. The titanium addition does not significantly change room-temperature strength or formability. 321 is specified for welded assemblies in aerospace, exhaust and chemical equipment that will operate at high temperature.

Designation cross-reference

Grade	UNS	EN 10088	JIS
304	S30400	1.4301	SUS304
321	S32100	1.4541	SUS321

Chemical Composition and Mechanical Properties

The tables below give the specified composition and minimum mechanical properties for plate according to ASTM A240.

Chemical composition, wt% (ASTM A240)

Element	304 (S30400)	321 (S32100)
C	≤0.08	≤0.08
Cr	18.0–20.0	17.0–19.0
Ni	8.0–10.5	9.0–12.0
Mn	≤2.00	≤2.00
Si	≤0.75	≤0.75
P	≤0.045	≤0.045
S	≤0.030	≤0.030
Ti	-	≥5 × C

Mechanical properties, minimum values, plate (ASTM A240)

Property	304	321
Tensile strength, MPa	515	515
Yield strength, MPa	205	205
Elongation in 2 in, %	40	40

Sensitization and the Stabilization Mechanism

When standard 304 is heated between 425°C and 850°C, carbon migrates to the grain boundaries and combines with chromium to form chromium carbides. The surrounding metal is depleted of chromium, and the chromium-depleted path becomes susceptible to intergranular corrosion, a condition known as sensitization or weld decay in the heat-affected zone. In 321, titanium has a stronger affinity for carbon than chromium does. It forms stable titanium carbides, keeping chromium in solution so the protective oxide film stays uniform.

When 321 Is Required

Specify 321 for any component that will be welded, cannot receive post-weld heat treatment, and will subsequently operate in the carbide precipitation range or above 400°C. Typical parts include aircraft exhaust manifolds, expansion bellows, heat exchanger tubing and furnace internals. For parts in continuous high-temperature service between 400°C and 900°C, long-term exposure can sensitize 304, so 321 or another stabilized grade is required.

Cost, Drawbacks and Selection Guideline

The main drawback of 321 is higher material cost due to the titanium addition. During welding, titanium can oxidize if shielding is inadequate, losing its stabilizing effect at the weld seam, so careful welding procedures are needed. For applications that are neither welded nor exposed to high temperature, 321 provides no functional benefit. The rule is simple: if the component is welded and will be used above 400°C or held in the 425–850°C range, use 321. For room-temperature or occasional moderate heat without welding, 304 is the correct and more economical choice.

Frequently Asked Questions