Comparison of 321 and 321H Stainless Steel: Standard vs High-Carbon Titanium-Stabilized Grade

Introduction: Two Titanium-Stabilized Grades, One Carbon Difference

Grades 321 (UNS S32100) and 321H (UNS S32109) are titanium-stabilized austenitic stainless steels designed for elevated-temperature service. Both use titanium to combine with carbon and prevent chromium carbide precipitation, so both resist intergranular corrosion after welding. The core difference is carbon content: 321H carries a controlled range of 0.04 to 0.10 percent, which raises creep strength at high temperature, while 321, with 0.08 percent maximum carbon, gives better toughness and marginally better intergranular corrosion resistance.

Chemical Composition Compared

Element (wt %)	321 (ASTM A240)	321H (ASTM A240/A213)
Carbon (C)	0.08 max	0.04 - 0.10
Manganese (Mn)	2.00 max	2.00 max
Phosphorus (P)	0.045 max	0.045 max
Sulfur (S)	0.030 max	0.030 max
Silicon (Si)	0.75 max	0.75 max
Chromium (Cr)	17.00 - 19.00	17.00 - 19.00
Nickel (Ni)	9.00 - 12.00	9.00 - 12.00
Titanium (Ti)	5 x (C+N) min, 0.70 max	4 x C min, 0.60 max
Iron (Fe)	Balance	Balance

Mechanical Properties Compared (Annealed)

Property	321	321H
Tensile strength	515 MPa min	515 MPa min
Yield strength (0.2% offset)	205 MPa min	205 MPa min
Elongation	40% min	40% min
Hardness	201 HB max	201 HB max

Carbon, Creep Strength and Design Allowables

The H suffix exists for one reason: long-term creep strength. At temperatures above about 540 C, the higher carbon of 321H strengthens the matrix and stabilizes the carbide structure, giving higher allowable design stresses than 321 in boiler and pressure-vessel codes. 321H is therefore specified for superheater and reheater tubes, hot steam piping and furnace components that carry sustained load at temperature, typically in the 500 to 900 C range.

Toughness and Corrosion Resistance

321 keeps carbon lower, so it retains better impact toughness, including in low-temperature service down to about -196 C, and slightly better intergranular corrosion resistance than 321H. For components that alternate between low and high temperature, or where toughness dominates the design, 321 is the preferred grade.

Cross-Standard Equivalents

Standard	321	321H
UNS	S32100	S32109
JIS	SUS321	SUS321H
EN 10088	1.4541 (X6CrNiTi18-10)	-
EN 10028-7	-	1.4940 (X6CrNiTi18-10 H)

Applications

321: automotive exhaust manifolds, chemical heat exchanger tubes, components alternating between low and high temperature

321H: boiler superheater and reheater tubes, hot steam piping, petrochemical furnace internals, nuclear secondary-circuit piping

FAQ

1. What is the role of titanium in 321 and 321H?

Titanium combines preferentially with carbon to form stable titanium carbides, preventing chromium carbide precipitation at grain boundaries. This is the mechanism that gives both grades resistance to intergranular corrosion after welding.

2. Can 321H be used in low-temperature service?

It can be used down to about -196 C, but its low-temperature toughness is lower than that of 321. For alternating low- and high-temperature duty, 321 is recommended.

3. What filler metal is used for 321 and 321H?

ER321 wire maintains titanium in the weld metal and is the matching filler for both grades. ER347 is an alternative when titanium transfer across the arc is a concern.

4. What is the cost difference between 321 and 321H?

321H is somewhat more expensive than 321 because of the tighter carbon control and the additional testing for high-temperature service. The premium is typically in the single-digit to low-double-digit percent range depending on product form.

5. What is the correct EN equivalent of 321H?

For plate and strip, EN 10028-7 lists 1.4940 (X6CrNiTi18-10 H). Note that 1.4542 is the EN number for 17-4 PH precipitation-hardening steel, not for 321H.

6. How are 321 and 321H high-temperature components maintained?

Remove oxide scale periodically by mechanical or chemical means, avoid rapid cooling after high-temperature service to prevent thermal shock, and carry out scheduled non-destructive examination of welds.