Demand for stabilized austenitic seamless tubing has climbed alongside higher operating temperatures across refineries and power plants. TP 347 stainless steel and its higher carbon variant, TP347H, dominate that space. Governed by ASTM A213 and ASTM A312 specifications, both niobium-stabilized grades share adjacent UNS designations (S34700 and S34709) but differ in their carbon content. This article provides a comparison of composition, mechanical behavior, applications, and the factors that decide the selection between TP247 and TP347H.
TP347 vs TP347H Seamless Tubes: Quick Comparison
Procurement teams evaluating quotes often require a high-level summary before reviewing detailed metallurgical specifications. The table sets both grades side by side across the properties that shape purchase decisions, from carbon range to service duty.
| Property | TP347 | TP347H |
| Carbon Content | 0.08% max | 0.04% to 0.10% |
| Chromium & Nickel | 17.0 to 20.0% Cr, 9.0 to 13.0% Ni | 17.0 to 20.0% Cr, 9.0 to 13.0% Ni |
| High-Temperature Strength | Adequate up to 538°C | Higher allowable stress above 538°C |
| Creep Resistance | Moderate, grain size uncontrolled | Stronger, coarse grain extends life |
| Weldability | Wider heat input window | Tighter interpass control needed |
| Oxidation Resistance | Scaling resistant to 870°C | Scaling resistant to 870°C |
| Typical Applications | Heat exchangers, chemical process lines | Super heaters, reheater, furnace tubing |
Two differences carry real engineering weight. TP347H holds a carbon floor of 0.04% that TP347 never guarantees, and it calls for solution annealing near 1095°C against 1040°C for the standard grade. Grain size, creep rupture life, and code allowable stress follow from those two requirements. Mills supplying Stainless Steel TP347 Seamless Tubes often dual certify one heat when chemistry and grain size satisfy both designations.
Chemical Composition Compared
Carbon is the major difference between the grades. TP347 caps carbon at 0.08% with no minimum, so a mill can ship material at 0.02% and stay fully compliant. TP347H requires a carbon level between 0.04% and 0.10%. This ensures there is enough carbon to form niobium carbides, which keep the metal strong at extremely high temperatures. Chromium and nickel stay identical across both grades, which explains why aqueous corrosion behavior barely shifts between them. Niobium stabilization differs slightly. TP347 demands niobium at ten times the carbon content, TP347H at eight times, with both capped at 1.00%.
| Element | TP347 | TP347H |
| Carbon | 0.08 max | 0.04 to 0.10 |
| Chromium | 17.0 to 20.0 | 17.0 to 20.0 |
| Nickel | 9.0 to 13.0 | 9.0 to 13.0 |
| Niobium | 10 x C min, 1.00 max | 8 x C min, 1.00 max |
| Manganese | 2.00 max | 2.00 max |
| Silicon | 0.75 max | 0.75 max |
Procurement teams ordering Stainless Steel 347 Pipe alongside tubing will find ASTM A312 mirrors these limits closely, so one mill heat can cover both product forms.
Comparing Mechanical Properties and High-Temperature Performance
Room temperature minimums match across both grades. Differences surface once metal temperature climbs past 538°C, where creep governs the design calculation rather than yield strength.
Tensile Strength
Both grades meet 515 MPa minimum tensile strength in the solution annealed condition. Mill certificates commonly report 590 MPa to 620 MPa, with no consistent gap between the two.
Yield Strength
ASTM sets 205 MPa minimum yield for each grade. Coarser grain in TP347H can pull measured yield slightly lower, though both clear the specification without difficulty.
Creep Resistance
TP347H resists creep better. Its carbon floor and the ASTM grain size 7 or coarser requirement produce a structure that deforms slowly under sustained load above 550°C.
Stress Rupture Performance
Rupture life at 650°C favors TP347H by a wide margin. ASME allowable stresses reflect that advantage, permitting thinner walls at the same design pressure in creep range service.
Oxidation Resistance
Both grades resist scaling in air to roughly 870°C continuous and 925°C intermittent. Chromium drives this behavior, and chromium content stays identical in each specification.
Intergranular Corrosion Resistance
Niobium locks carbon into NbC and starves chromium carbide formation across the 425°C to 815°C sensitization band. Both grades pass ASTM A262 Practice E after welding.
Long Term Thermal Exposure
TP347 loses ductility slowly through thousands of hours near 600°C. TP347H holds its structure better, which is why boiler codes list it for sustained high-temperature duty.
TP347 vs TP347H Seamless Tubes for Different Industrial Applications
Grade selection follows service temperature more than industry label. Each application below names the better-suited grade and gives the reasoning behind that match.
Heat Exchangers
TP347 suits most shell and tube service. Metal temperature in condensers and process coolers stays below 538°C, so the guaranteed carbon floor of TP347H returns nothing measurable.
Super Heaters
TP347H governs here. Metal temperature reaches 600°C to 650°C, creep rupture strength sets the wall thickness, and the coarse grain of the H grade extends element life.
Petrochemical Plants
Both grades appear on one site. Reformer and cracking furnace tubes take TP347H for creep duty, while cooler process and transfer lines run on TP347 without penalty.
Power Generation
TP347H carries the load in boiler circuits. ASME assigns it higher allowable stresses above 538°C, letting designers trim wall thickness on reheater and header tubing at equal pressure.
Chemical Processing
TP347 covers reactor and transfer lines. Corrosion performance outweighs creep strength at these temperatures, and chromium and nickel stay identical.
High-Temperature Process Piping
TP347H handles sustained exposure above 550°C. Its coarse grain resists slow deformation across thousands of operating hours, which standard TP347 cannot promise from one heat to the next.
Fabrication route matters as much as temperature. Tight U-bends and heavy welding can still favour TP347 near the crossover point.
Specification and Fabrication Considerations Compared
ASTM A213 covers seamless tubes for boiler, super heater, and heat exchanger duty, while ASTM A312 covers pipe. Both list 347 and 347H, and mills routinely dual certify. Stainless steel seamless tube sizes run from 3.2 mm to 127 mm outside diameter with walls between 0.4 mm and 12.7 mm, covering most exchanger and boiler layouts.
Welding needs attention either way. Niobium raises the risk of weld metal fissuring, so welders keep heat input low and hold interpass temperature under 150°C. TP347H allows less latitude here because of its carbon floor. Solution annealing separates the two once more, 1040°C minimum for TP347 against 1095°C for TP347H, and that gap affects mill lead time.
Key Factors to Consider Before Choosing Between TP347 and TP347H
These considerations resolve most specification requirements. Evaluating them against operational service data ensures an optimal selection rather than simply relying on previously ordered grades.
- Check the maximum metal temperature in service, because TP347H earns its price premium only above 538°C.
- Estimate how many hours the tube holds at temperature, since short excursions rarely justify the H grade.
- Review welding volume and joint design, as higher carbon narrows the acceptable heat input window during fabrication.
- Compare the required allowable stress against wall thickness, because stronger creep values can cut material weight noticeably.
- Assess the corrosive media separately, since chromium and nickel levels stay identical and offer no real differentiator.
- Confirm which code governs the project, as ASME Section I and B31.1 treat the two grades differently.
- Plan around design life, because thousands of hours near 600°C reward the coarser grain of TP347H.
Which Grade Should You Choose?
TP347
TP347 is recommended for applications where service temperatures remain below 538°C and extensive fabrication is required. Heat exchanger bundles, condenser tubing, and chemical transfer lines all remain within this group. The open carbon range makes bending and welding more forgiving, and stock sits with more suppliers, which shortens delivery on replacement work.
TP347H
TP347 should be specified when operating temperatures do not exceed 538°C and the project entails substantial fabrication work. Super heater and reheater elements, catalytic reformer tubes, and high-temperature headers qualify. The guaranteed carbon floor and coarse grain deliver creep rupture life that TP347 cannot match, and ASME allowable stresses let designers trim wall thickness at the same pressure.
Neither grade wins outright. Operating temperature, governing code, and fabrication load decide the answer, and a documented review of service conditions beats habit every time.
Conclusion
Carbon separates TP347 from TP347H, and that single variable drives high-temperature strength and creep rupture life. Both grades share chromium and nickel content, both resist oxidation to 870°C, and both use niobium to block chromium carbide precipitation. Base the final call on operating temperature, fabrication demands, applicable standards, and expected service life. MBM Tubes manufactures stainless steel seamless tubes, welded tubes, and U tubes to ASTM A213 and ASTM A312 requirements.
