ASME SA213 UNS S34709 / TP347H Stainless Steel Seamless Tube Pickled Annealed Heat Exchanger Tube

ASME SA213 TP347H / UNS S34709 is a Nb-stabilized austenitic stainless steel seamless tube with high-temperature creep strength and sensitization resistance. This material is very common used in heat exchangers. EN 1.4961 is a related European grade, but it has different Cr/Ni chemistry. They can be dual-certified only with tight chemistry control. For ASME work, order and certify to ASME SA213 TP347H UNS S34709 — do not accept EN 1.4961 alone as a substitute.

SCC limits: TP347H resists polythionic acid SCC if properly solution-annealed, but it is not immune to chloride SCC.

When engineers choose TP347H?

1. High-temperature creep service
   Above about 550–600°C, 304H and 316H allowable stresses drop rapidly. TP347H has higher allowable stress in the 500–700°C range, so it can allow thinner walls, higher pressure, or longer life.

2. Polythionic acid SCC risk
   In refinery hydroprocessing, catalytic cracking, and similar units, polythionic acid can form during shutdown. Sensitized 304H/316H can crack. TP347H is stabilized with Nb, so it resists sensitization and is a standard choice.

3. Welded construction without PWHT
   TP347H is often used where post-weld heat treatment is impractical. The Nb stabilization reduces chromium carbide precipitation in the HAZ, lowering sensitization risk.

4. Long-term exposure to 425–815°C
   In this sensitization range, unstabilized grades can form chromium carbides at grain boundaries. TP347H ties up carbon as NbC, maintaining corrosion resistance.

5. Power plant superheaters/reheaters
   For steam temperatures around 540–620°C, TP347H is a common upgrade from 304H/316H when creep strength is limiting.

6. Cost-effective step below 310H or nickel alloys
   If 310H or Alloy 800H is over-specified, TP347H can be a lower-cost alternative — provided chlorides and sulfidation are not severe.

Application

• Refinery heat exchangers and furnace tubes

• Power plant superheaters and reheaters

• Petrochemical steam/gas heat exchangers

• High-temperature steam service

• Applications needing resistance to polythionic acid SCC