What SA268 covers
ASME SA268 covers seamless and welded ferritic and martensitic stainless steel tube for general service. The grades most often specified for heat transfer are:
- TP439, 18% chromium, titanium stabilized. The most common ferritic condenser tube.
- TP444, 18Cr-2Mo, for higher chloride tolerance approaching 316L.
- TP409, 11% chromium, for low-cost, low-corrosivity service.
- TP410, 12% chromium martensitic, where hardness and wear matter.
Because ferritic steels have a body-centered cubic structure, they are essentially immune to chloride stress corrosion cracking, the failure mode that takes out austenitic tube in warm, evaporating, or crevice-prone water.
Typical mechanical properties, TP439
| Property | Minimum |
|---|---|
| Tensile strength | 60,000 psi (415 MPa) |
| Yield strength | 30,000 psi (205 MPa) |
| Elongation in 2" | 20% |
Where it is used
- Power station surface condensers on fresh and low-chloride cooling water.
- Feedwater heaters and moisture separator reheaters.
- Refinery and chemical plant water-cooled exchangers where 304 has cracked.
- Automotive and appliance heat exchangers.
Limits
Ferritic grades lose toughness in heavy sections and above about 700 degrees F. For thin-wall condenser tube those limits rarely matter. For seawater, choose copper-nickel, super-ferritic, duplex, or titanium instead.
Grade (TP439, TP409, TP444, TP410, or other), welded or seamless, OD, wall, length, quantity, and the cooling water chemistry. Ferritic grades are chloride-tolerant but not immune; tell us the chloride level.