Why One Stainless Standard Is Not Enough
Stainless steel pipe and tubing are covered by several ASTM specifications that overlap in appearance but differ in product form, testing regime, permitted grades and intended service. A pipe ordered to A312 cannot simply be replaced by tubing ordered to A269, even when both are TP316L and both are welded, because the dimensional tolerances, the pressure test and the acceptance criteria are written for different applications.
Standard Overview
| ASTM standard | Product form | Principal grades | Intended service |
|---|---|---|---|
| A312 | Seamless and welded austenitic pipe | TP304, TP304L, TP316, TP316L, TP321, TP347, TP310S | High-temperature and corrosive process piping |
| A213 | Seamless ferritic and austenitic alloy steel tubes | T5, T11, T22, T91, TP304H, TP347H | Boiler, superheater and heat-exchanger tubes |
| A269 | Seamless and welded austenitic tubing | TP304, TP304L, TP316, TP316L | General-purpose, instrumentation and mechanical tubing |
| A790 | Seamless and welded ferritic and austenitic pipe | S31803, S32205, S32750 | Duplex and super duplex piping in chloride service |
| A789 | Seamless and welded duplex tubing | S31803, S32205, S32750 | Heat exchangers in chloride-bearing water |
| A358 | Electric fusion welded austenitic pipe | TP304, TP316, TP321 | Large-diameter high-temperature piping |
Mechanical Properties of the Common Grades
| Grade | Minimum yield strength | Minimum tensile strength | Typical field |
|---|---|---|---|
| TP304 | 205 MPa / 30 ksi | 515 MPa / 75 ksi | General corrosion resistance, food and dairy |
| TP304L | 170 MPa / 25 ksi | 485 MPa / 70 ksi | Welded fabrication without post-weld anneal |
| TP316 | 205 MPa / 30 ksi | 515 MPa / 75 ksi | Molybdenum-bearing, chemical and marine service |
| TP316L | 170 MPa / 25 ksi | 485 MPa / 70 ksi | Pulp, paper, pharmaceutical and desalination |
| TP321 | 205 MPa / 30 ksi | 515 MPa / 75 ksi | Stabilised against sensitisation, elevated temperature |
| TP347 | 205 MPa / 30 ksi | 515 MPa / 75 ksi | Niobium-stabilised, steam and furnace service |
| S32205 duplex | 450 MPa / 65 ksi | 620 MPa / 90 ksi | Chloride pitting and stress corrosion resistance |
Chemistry control separates the L grades from the standard grades. TP304 and TP316 limit carbon to 0.08 % while TP304L and TP316L limit carbon to 0.030 %, which reduces the risk of chromium carbide precipitation during welding. Chromium is 18.0 to 20.0 % in the 304 family, and the 316 family adds 2.00 to 3.00 % molybdenum with nickel in the range 10.0 to 14.0 %. Duplex grades combine ferrite and austenite in roughly equal proportions and contain about 22 % chromium with molybdenum and nitrogen additions.
Testing and Inspection Expectations
A312 and A790 require a hydrostatic test or an approved non-destructive substitute, plus flattening or reverse bend tests on welded pipe and an eddy current or ultrasonic examination of the weld.
A213 requires a hydrostatic test and specifies minimum wall and average wall options, with additional eddy current testing for many orders.
A269 places the emphasis on dimensional control and cleanliness for tubing, with pressure test requirements that differ from A312.
Duplex grades need a check on the ferrite to austenite ratio and are normally supplied with corrosion test data for chloride environments.
Selection and Fabrication Notes
Choose the product form before the grade. Heat-exchanger tubes are ordered to A213 or A789, process piping to A312 or A790, and instrument tubing to A269. Stabilised grades such as TP321 and TP347 are selected when the component operates in the sensitising temperature range for long periods. Welding of the L grades and of duplex grades requires controlled heat input and interpass temperature, and duplex pipe is normally welded with a nickel-bearing filler to balance the phase ratio in the weld metal. Pipe that will be bent or expanded must be ordered with the correct wall tolerance, since A213 offers both minimum wall and average wall options.
FAQ
Q: What is the difference between ASTM A312 and ASTM A269?
A312 applies to pipe for pressure and high-temperature service with a rigorous pressure test and weld examination regime, while A269 applies to general-purpose tubing where dimensional control and surface finish dominate.
Q: When should ASTM A790 be used instead of A312?
A790 should be used when the service contains chlorides and stress corrosion cracking or pitting is a risk, because duplex grades offer higher strength and better chloride resistance than the austenitic grades.
Q: Which grades are covered by ASTM A213?
A213 covers ferritic alloy grades such as T5, T11, T22 and T91 for boiler and superheater tubing together with austenitic grades such as TP304H and TP347H.
Q: Is TP316L stronger than TP316?
No. The L grades have a lower carbon level and correspondingly lower minimum yield and tensile strength, which is the trade-off for improved weldability without post-weld heat treatment.
Q: What does the H suffix mean in grades such as TP304H?
The H suffix marks a high-carbon austenitic grade intended for elevated-temperature service where creep and rupture strength matter more than weldability.
Q: What tests should be requested for duplex pipe?
A hydrostatic or non-destructive test, an impact test where specified, a phase balance determination and, in chloride service, corrosion test data. A hardness survey is often added when sour service applies.
