Stainless Steel 321H

321H (UNS S32109) is the high-carbon variant of Type 321, a titanium-stabilized austenitic stainless steel. Carbon is controlled at 0.04-0.10%, and titanium is specified at 5 times (C+N) with a maximum of 0.70%, so the higher carbon does not lead to chromium-carbide precipitation. The result is a grade with markedly better creep strength than 321 above about 800 C, while keeping the intergranular-corrosion resistance that titanium stabilization provides.

Chemical Composition (ASTM A240, UNS S32109)

Element (wt%)	321H (UNS S32109)
C	0.04-0.10
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+N) min to 0.70
N	≤0.10

Mechanical Properties

Annealed plate per ASTM A240 UNS S32109: tensile strength ≥515 MPa, yield strength ≥205 MPa, elongation ≥40%. Tube and pipe per ASTM A213/A312 (TP321H): tensile strength ≥515 MPa, yield strength ≥205 MPa, elongation ≥35%.

Why the H Variant Exists

Creep strength in the 800-900 C range

For boiler superheaters, turbine internals and furnace components, the design criterion above about 800 C is creep rupture rather than room-temperature strength. The extra carbon in 321H forms a dense dispersion of titanium carbides that pins dislocations, slowing creep dramatically compared with 321.

Stabilization still works

Titanium is specified in proportion to carbon plus nitrogen, with a 5(C+N) minimum. At the top of the carbon range, titanium must reach about 0.50% or more, which is enough to bind essentially all carbon as titanium carbide. Because carbon is locked up as TiC, chromium carbides do not form at grain boundaries, and the grade keeps its resistance to intergranular corrosion after welding.

Designations and Cross-Reference

321H is specified as UNS S32109 to ASTM A240 (plate), A213 (tube) and A312 (pipe). JIS G4304 covers the base grade SUS321, and EN 10088-2 covers Type 321 as 1.4541 (X6CrNiTi18-10); the H variant itself has no direct JIS or EN 10088 designation, so it should be ordered to the ASTM and UNS reference. GB/T 3280 06Cr18Ni11Ti is the Chinese designation for the 321 base composition.

Fabrication and Welding

321H is welded with stabilized austenitic filler of the AWS ER321 or ER347 type, which restores titanium or niobium stabilization in the weld metal. Solution annealing before heavy forming is commonly carried out around 950-1100 C followed by water quenching. For long-term 800-900 C service, keep heat input moderate during welding and remove surface oxide before service.

FAQ

1. What does 321H offer over 321?

Higher minimum carbon, 0.04-0.10% versus not more than 0.08%, and consequently better creep and rupture strength for long-term service at 800-900 C.

2. Does the higher carbon cause intergranular corrosion?

No. Titanium is specified at 5(C+N) minimum, which binds the extra carbon as stable titanium carbides and prevents chromium depletion at grain boundaries.

3. What is the practical service limit?

For stressed components, 321H is commonly rated for continuous service around 850-900 C in published engineering data. Above that range, higher-alloy heat-resisting grades should be considered.

4. Which filler metal is used for welding 321H?

Stabilized austenitic filler of the AWS ER321 or ER347 type, selected so the weld metal retains stabilization and matches the elevated-temperature properties of the base metal.

5. 321H or 347H?

347H (UNS S34709) uses niobium instead of titanium and is often preferred for heavy welded sections. 321H is frequently the lower-cost choice for boiler tubing where welding is limited.