Carbon Steel vs Stainless Steel Pipe
Two materials whose pressure-temperature curves cross. Where each wins, what each costs, and the thermal expansion difference that makes substitution non-trivial.
- Carbon steel group
- B16.5 Group 1.1 (A105)
- Stainless group
- B16.5 Group 2.1 (F304)
- Ambient advantage
- Carbon steel
- High-temperature advantage
- Stainless, above 850 °F
Short answer. Carbon steel is stronger at ambient temperature, far cheaper, and easier to weld. Austenitic stainless starts lower but holds its rating almost flat to 1000 °F, and it resists corrosion that would consume carbon steel. Carbon steel is the default for utility, steam and clean hydrocarbon service; stainless is specified for corrosion, contamination sensitivity, cryogenic service and sustained high temperature — above about 850 °F it also carries the higher pressure rating.
The rating curves cross
This is the fact that surprises people. At 100 °F a Class 300 carbon steel flange is rated 740 psig against stainless at 720 — carbon steel wins. Carbon steel then falls away steadily as temperature rises, while austenitic stainless flattens out. By 850 °F the stainless rating is the higher of the two, and by 1000 °F it is several times higher.
Carbon steel is also limited by oxidation and graphitisation before its rating runs out. B16.5 permits it above 800 °F but advises against prolonged use there, and most specifications stop carbon steel at about that temperature. Stainless keeps going.
| Temperature | Cl 150 carbon | Cl 150 stainless | Difference | Cl 300 carbon | Cl 300 stainless | Difference |
|---|---|---|---|---|---|---|
| 100 °F | 285 psig | 275 psig | -4% | 740 psig | 720 psig | -3% |
| 200 °F | 260 psig | 230 psig | -12% | 680 psig | 600 psig | -12% |
| 300 °F | 230 psig | 205 psig | -11% | 655 psig | 540 psig | -18% |
| 400 °F | 200 psig | 190 psig | -5% | 635 psig | 495 psig | -22% |
| 500 °F | 170 psig | 170 psig | 0% | 605 psig | 465 psig | -23% |
| 600 °F | 140 psig | 140 psig | 0% | 570 psig | 440 psig | -23% |
| 650 °F | 125 psig | 125 psig | 0% | 550 psig | 430 psig | -22% |
| 700 °F | 110 psig | 110 psig | 0% | 530 psig | 420 psig | -21% |
| 750 °F | 95 psig | 95 psig | 0% | 505 psig | 415 psig | -18% |
| 800 °F | 80 psig | 80 psig | 0% | 410 psig | 405 psig | -1% |
| 850 °F | 65 psig | 65 psig | 0% | 320 psig | 395 psig | +23% |
| 900 °F | 50 psig | 50 psig | 0% | 230 psig | 390 psig | +70% |
| 950 °F | 35 psig | 35 psig | 0% | 135 psig | 380 psig | +181% |
| 1000 °F | 20 psig | 20 psig | 0% | 85 psig | 355 psig | +318% |
Group 1.1 is A105 and equivalents; Group 2.1 is F304. Type 316 is Group 2.2 and rates slightly higher when hot. Ratings are maximum allowable working gauge pressure at metal temperature.
Strength and temperature limits by grade
| Specification | Form | Tensile min | Yield min | Typical temperature range | Notes |
|---|---|---|---|---|---|
| ASTM A106 Gr B | Pipe | 60 ksi | 35 ksi | -20 to 800 °F | The workhorse seamless carbon steel pipe for high-temperature process service. |
| ASTM A53 Gr B | Pipe | 60 ksi | 35 ksi | -20 to 750 °F | Seamless or ERW. Ordinary utility service — air, water, steam, gas. Not impact-tested. |
| ASTM A333 Gr 6 | Pipe | 60 ksi | 35 ksi | -50 to 650 °F | Impact-tested carbon steel for low-temperature service. The standard cryo-adjacent carbon pipe. |
| ASTM A312 TP304 | Pipe | 75 ksi | 30 ksi | -425 to 1500 °F | General-purpose austenitic stainless. Sensitises if welded and held at 800-1500 °F. |
| ASTM A312 TP316 | Pipe | 75 ksi | 30 ksi | -425 to 1500 °F | Adds 2-3% molybdenum for chloride pitting resistance. |
| ASTM A312 TP316L | Pipe | 70 ksi | 25 ksi | -425 to 850 °F | Low-carbon 316. The default stainless for welded chemical process piping. |
| ASTM A790 S31803 | Pipe | 90 ksi | 65 ksi | -60 to 600 °F | Duplex, commonly sold as 2205 (strictly, 2205 is UNS S32205, at 95/65 ksi). Roughly twice the yield of 316L with far better chloride SCC resistance. |
Strengths are specified minimums in ksi. 1 ksi = 1000 psi = 6.895 MPa.
Carbon steel
- Cheapest material in the table by a wide margin
- Higher allowable stress at and near ambient
- Easy to weld with no special filler or purge
- Needs a corrosion allowance — typically 1/16 in
- Practical upper limit around 800 °F
- Brittle below about −20 °F unless impact tested (A333)
Austenitic stainless
- Holds its rating to 1000 °F and beyond
- No corrosion allowance needed in most clean services
- Tough to cryogenic temperatures — usable to −425 °F
- Three to five times the material cost
- Needs a back purge when welded, and sensitises if held at 800–1500 °F unless a low-carbon grade is used
- Vulnerable to chloride stress corrosion cracking
Thermal expansion is a real constraint
Austenitic stainless expands roughly 40–50% more than carbon steel per degree. A stainless line designed with a carbon steel flexibility layout will overload its anchors and nozzles. Any material substitution between the two has to go back through the stress analysis — it is not a like-for-like swap.
See also: 304 vs 316 · A106 vs A53 · full P-T ratings · material selection.
Common questions
Is stainless steel pipe stronger than carbon steel?
Not at ambient temperature — carbon steel has the higher allowable stress and the higher B16.5 rating there. Stainless becomes stronger above roughly 850 °F because it derates far more slowly.
Why does carbon steel piping stop at 800 °F?
Oxidation and graphitisation, not the pressure rating. Carbon steel scales rapidly in air above that range and long exposure can convert carbides to graphite, embrittling the steel.
How much more does stainless pipe cost?
Typically three to five times carbon steel per foot for 304/316, which is why thin-wall S-schedules are used — removing the corrosion allowance removes a large part of the cost.
Can stainless and carbon steel pipe be welded together?
Yes, with a suitable filler — usually a 309 type — and with attention to the differential thermal expansion at the joint. Dissimilar metal welds are also a galvanic couple and need consideration in wet service.
Does stainless need a corrosion allowance?
Usually not in clean service, which is the main reason thin-wall S-schedule pipe is viable in stainless and not in bare carbon steel.