321 Stainless Steel (UNS S32100): The Titanium-Stabilized Solution for Welded High-Temp Service

Overview and Designations

321 is an 18-8 austenitic stainless steel stabilized with titanium. It is designed for welded components that operate at high temperature, where standard or low-carbon grades can lose corrosion resistance through sensitization. The grade is specified as ASTM A240 Grade 321 for plate and ASTM A213 Grade TP321 for tube, with UNS S32100 and EN 1.4541 (X6CrNiTi18-10) as equivalents.

Chemical Composition

Chemical composition, wt% (ASTM A240 Grade 321)

Element	Content
C	≤0.08
Mn	≤2.00
P	≤0.045
S	≤0.030
Si	≤0.75
Cr	17.0–19.0
Ni	9.0–12.0
Ti	≥5 × C

Mechanical and Physical Properties

Mechanical properties, minimum values, plate (ASTM A240)

Property	Value
Tensile strength	515 MPa
Yield strength	205 MPa
Elongation in 2 in	40%

Typical physical properties

Property	Typical value
Density	7.9 g/cm³
Melting range	≈1400–1425 °C
Elastic modulus	≈193 GPa
Thermal conductivity at 100 °C	≈16 W/(m·K)

Stabilization Mechanism and Service Range

In 321, titanium combines preferentially with carbon to form stable titanium carbides, so carbon does not deplete chromium at grain boundaries. This prevents the sensitization that leads to intergranular corrosion. While the low carbon of 304L prevents sensitization during a single welding event, components that repeatedly cycle through the critical 450–850°C range remain at risk over time. The titanium carbides in 321 are thermally stable, providing continuous long-term protection even under thousands of thermal cycles, which is why 321 is preferred for exhaust manifolds, ducting and heat exchanger parts. In service, the grade is generally rated for continuous oxidation up to about 870°C, with the exact limit depending on the application and environment.

Welding, Fabrication and Procurement

The main fabrication challenge is preserving the stabilizing effect of titanium during welding. Use excellent shielding gas coverage, with argon backing for the root pass to prevent titanium oxidation. Matching filler metal, normally ER321, should be used, and heat input should be controlled to keep the heat-affected zone narrow. Post-weld heat treatment is not typically required for thin sections, but may be recommended for heavy weldments in highly corrosive service. For continuous service above about 800°C under significant stress, Grade 347, which is stabilized with niobium, is often preferred because niobium carbides are more stable at extreme temperatures and give better long-term creep strength. When ordering, specify the full designation such as ASTM A240 Grade 321 or UNS S32100, and require the mill test report to confirm the titanium content and a Ti/C ratio of at least 5. Define the maximum operating temperature and thermal cycling profile in the specification so the correct grade is delivered and documented.

Frequently Asked Questions