SUS321 Stainless Steel Pipe: Specifications, Sizes and Standards

What Is SUS321 Stainless Steel Pipe?

SUS321 is the Japanese Industrial Standard designation of the titanium-stabilized austenitic stainless steel equivalent to AISI 321 and EN 1.4541 (X6CrNiTi18-10). Titanium is added at 5 times the carbon plus nitrogen content, up to a maximum of 0.70%, which fixes carbon as stable titanium carbides. The steel therefore keeps its corrosion resistance after welding and during continuous service in the 450-870 degrees C range, where unstabilized grades such as 304 would suffer chromium carbide precipitation and intergranular attack. SUS321 pipe is a standard choice for heat exchangers, furnace parts, superheater tubing and exhaust systems.

Equivalent Grade Designations

Designation System	Grade
ASTM / AISI	321 (UNS S32100)
EN	X6CrNiTi18-10 (1.4541)
JIS	SUS321
BS (Old British)	321S31 (58B, 58C)
Swedish SS	2337

Applicable Standards

Product Form	Standard
Seamless boiler and heat exchanger tubes	ASTM A213 / ASME SA213, JIS G3459
Welded and seamless pipe	ASTM A312 / ASME SA312, JIS G3463
Welded heat exchanger tubes	ASTM A249, EN 10217
Chemical composition of bar and plate	JIS G4303, ASTM A240
Inspection certificates	EN 10204 3.1 / 3.2

Chemical Composition (ASTM A312 TP321)

Composition limits per ASTM A312 for grade TP321, in weight percent:

Element	Limit (%)
Carbon (C)	0.08 max
Manganese (Mn)	2.0 max
Silicon (Si)	1.0 max
Phosphorus (P)	0.045 max
Sulfur (S)	0.030 max
Chromium (Cr)	17.00-19.00
Nickel (Ni)	9.00-12.00
Nitrogen (N)	0.10 max
Titanium (Ti)	5 x (C+N) min, 0.70 max

Mechanical and Physical Properties

Property	Value
Density	8.0 g/cm3
Melting Point	1457 degrees C
Tensile Strength	515 MPa min
Yield Strength (0.2% offset)	205 MPa min
Elongation	35% min

The HS code for SS 321 pipe is 73044900. Creep resistance data published for TP321 show, for example, a 100000-hour rupture strength of about 65 MPa at 600 degrees C and about 22 MPa at 700 degrees C, which is why the grade is widely used for furnace and heat exchanger service in the creep range.

Size Chart and Dimensional Tolerances

Representative ASTM A312 TP321 seamless pipe sizes (outside diameter in mm, wall thickness in mm):

NPS	OD (mm)	SCH 5S	SCH 10S	SCH 40S	SCH 80S
1/4	13.72	-	1.65	2.24	3.02
1/2	21.34	1.65	2.11	2.77	3.73
1	33.40	1.65	2.77	3.38	4.55
2	60.33	1.65	2.77	3.91	5.54
4	114.30	2.11	3.05	6.02	8.56

Outside-diameter tolerances per ASTM A312 for seamless TP321 pipe range from plus 0.4 mm / minus 0.8 mm for NPS 1/8 to 1-1/2 up to plus 4.8 mm / minus 0.8 mm for NPS over 34 to 48. Wall thickness tolerance is plus 20% / minus 12.5% for seamless pipe; welded pipe follows the tolerances of ASTM A999.

Fabrication and Service Notes

SUS321 pipe is available in seamless and welded forms, with manufacturing by hot or cold forming and delivery surfaces of cold finish, bright annealed, polished, annealed and pickled. The stabilized chemistry means no post-weld heat treatment is required for normal service, and the recommended solution annealing temperature for the standard carbon grade is 1040 degrees C minimum (the high-carbon H variant, TP321H, is annealed at 1090 degrees C minimum). For the highest creep strength above about 500 degrees C, specify TP321H per ASTM A213.

Frequently Asked Questions

What is SUS321 stainless steel equivalent to? SUS321 is equivalent to AISI 321, UNS S32100, EN 1.4541 (X6CrNiTi18-10), BS 321S31 and Swedish SS 2337.

Why is titanium added to SUS321 pipe? Titanium combines with carbon and nitrogen to form stable compounds, preventing chromium depletion at grain boundaries and protecting against intergranular corrosion after welding and service between 450 and 870 degrees C.

Which standards cover SUS321 pipe? Seamless tubes: ASTM A213/ASME SA213 and JIS G3459. Welded pipe: ASTM A312/ASME SA312 and JIS G3463. Heat exchanger tubes: ASTM A249 and EN 10217. Inspection certificates per EN 10204 3.1/3.2.

What is the maximum service temperature of SUS321 pipe? Continuous service is typically limited to about 870 degrees C by oxidation; above roughly 650 degrees C, design is governed by creep rupture data, and TP321H is preferred for the highest temperatures.

Does SUS321 pipe require post-weld heat treatment? No. The titanium stabilization protects the weld heat-affected zone from sensitization, so SUS321 pipe is normally used in the as-welded condition without PWHT.

What is the difference between TP321 and TP321H? TP321 has 0.08% carbon maximum and is annealed at 1040 degrees C minimum; TP321H has 0.04-0.10% carbon and is annealed at 1090 degrees C minimum, giving higher creep strength for elevated-temperature design.