ASTM A312 Stainless Steel Pipe Grades: TP304, TP316, TP316L, TP321, TP347 Compared
ASTM A312 covers seamless and welded austenitic stainless steel pipe in sizes from NPS 1/8 to NPS 30. With several grades available — TP304, TP316, TP316L, TP321, TP347 — choosing the right one depends on your corrosion environment, temperature, welding requirements and budget. This guide compares each grade side by side and shows which one fits your application.
Common ASTM A312 Grades at a Glance
| Grade | UNS No. | Key Alloying Elements | Min Tensile (MPa) | Min Yield (MPa) | Key Advantage |
|---|---|---|---|---|---|
| TP304 | S30400 | 18Cr-8Ni | 515 | 205 | General purpose, lowest cost |
| TP316 | S31600 | 16Cr-10Ni-2Mo | 515 | 205 | Chloride / acid resistance |
| TP316L | S31603 | 16Cr-10Ni-2Mo (C ≤ 0.03) | 485 | 170 | Weldability, low-carbon variant |
| TP321 | S32100 | 17Cr-9Ni + Ti | 515 | 205 | Intergranular corrosion resistance at high temp |
| TP347 | S34700 | 17Cr-11Ni + Nb | 515 | 205 | Creep resistance at elevated temperature |
Values are per ASTM A312 / A240. Always confirm the exact heat analysis against the applicable specification.
When to Choose Each Grade
TP304 — General Purpose
TP304 is the most common stainless pipe grade worldwide. It handles food processing, potable water, pharmaceutical piping, and general chemical service well. Choose TP304 when chloride levels are low, no aggressive acids are present, and cost is a priority.
TP316 — Marine and Chemical
The 2% molybdenum in TP316 gives it significantly better pitting and crevice corrosion resistance in chloride environments — seawater, coastal atmospheres, pulp and paper, and most chemical processing. If your pipe will see salt spray or chloride-bearing fluids, TP316 is usually the safe choice.
TP316L — Welded and Low-Carbon
TP316L is the low-carbon variant (C ≤ 0.03%) of TP316. The reduced carbon minimizes carbide precipitation in the heat-affected zone during welding, so TP316L is preferred when heavy-section welding is required or when the pipe will be used in the as-welded condition. The trade-off is slightly lower tensile strength (485 vs 515 MPa).
TP321 — High Temperature
TP321 is stabilised with titanium, which ties up carbon and prevents intergranular corrosion after prolonged exposure to 425–815 °C. It is the standard choice for exhaust systems, thermal cracking furnaces and refinery equipment where TP316 may sensitise under sustained heat.
TP347 — Elevated Temperature and Creep
TP347 is stabilised with niobium (columbium), offering similar high-temperature stability to TP321 but with better creep strength at temperatures above 540 °C. It is commonly specified for petrochemical reactor internals, high-temperature heat exchangers, and power plant piping.
Composition and Mechanical Properties in Detail
| Property | TP304 | TP316 | TP316L | TP321 | TP347 |
|---|---|---|---|---|---|
| Cr (%) | 18.0–20.0 | 16.0–18.0 | 16.0–18.0 | 17.0–19.0 | 17.0–19.0 |
| Ni (%) | 8.0–10.5 | 10.0–14.0 | 10.0–14.0 | 9.0–12.0 | 9.0–13.0 |
| Mo (%) | — | 2.0–3.0 | 2.0–3.0 | — | — |
| C max (%) | 0.08 | 0.08 | 0.03 | 0.08 | 0.08 |
| Ti or Nb | — | — | — | Ti ≥ 5×C | Nb ≥ 10×C |
| Min elongation (%) | 35 | 35 | 35 | 35 | 35 |
Mechanical values are minima; actual mill test results are typically higher. The full chemical ranges and supplementary requirements are listed in ASTM A240 and cross-referenced in A312.
How ASTM A312 Relates to Other Specifications
ASTM A312 is a pipe specification — nominal pipe sizes, schedules, and designed for connection by welding, threading or flanging. It is often confused with two related tubing specifications:
- ASTM A213 — seamless tubing for boilers and heat exchangers (smaller OD, tighter OD tolerance)
- ASTM A269 — welded and seamless tubing for general service (instrumentation, food, pharma)
A312 pipe sizes run from NPS 1/8 to NPS 30; A213 and A269 tubing sizes are 1/4“ to 5“ OD. If you need a pipe connected with standard pipe fittings (butt-weld, socket-weld, threaded), specify A312. If you need a tube for a tube-and-shell heat exchanger or instrumentation line, specify A213 or A269 accordingly.
FAQ
Can TP316L replace TP316 in most applications?
Yes. TP316L is fully interchangeable with TP316 for most services and is often preferred because of its superior weldability. The slightly lower min tensile strength is rarely a design limitation. Check your project specification before substituting, as some standards distinguish the two grades.
Does ASTM A312 cover seamless pipe only?
No. ASTM A312 covers both seamless and welded austenitic stainless steel pipe. When a buyer requires seamless only, the grade is typically listed as “TP304 SMLS” or “TP316 SMLS” on the purchase order. Seamless pipe generally costs more than welded pipe of the same grade and size.
What surface finish does A312 require?
A312 itself does not specify a surface finish. The buyer specifies the required finish — pickled and passivated, mill finish (2B), or polished (180 grit, 240 grit, mirror) — on the purchase order. Most industrial piping is supplied with a pickled and passivated finish unless otherwise stated.
