347 vs 347H Stainless Steel: Welding, Creep and Code Selection

Written by: Emma, Technical Sales Engineer  |  Reviewed by: Ethan, Materials Engineer  |  Updated: August 2026

347 and 347H are both niobium (columbium)-stabilized austenitic stainless steels, sharing the same stabilization mechanism that resists weld decay and intergranular corrosion. The difference between them is carbon. 347H carries a controlled higher carbon range, and that difference exists for one main reason: improved high-temperature strength and creep performance for code-governed elevated-temperature service. This article explains what actually changes between 347 and 347H, how it affects welding and creep, and how buyers should decide which grade their design requires.

1. What Is the Difference Between 347 and 347H?

347 (UNS S34700) and 347H (UNS S34709) are both niobium/columbium-stabilized austenitic stainless steels. The defining difference is the carbon range: 347H has a controlled higher carbon content, which is intended to improve elevated-temperature strength. It is not a claim that 347H outperforms 347 in every property — the higher carbon is aimed specifically at high-temperature mechanical behavior, not at corrosion resistance or weldability.

2. Niobium Stabilization and Sensitization

Niobium is added to 347 for the same reason titanium is added to 321: to stabilize carbon. At elevated temperature, carbon can combine with chromium to form chromium carbides at grain boundaries, depleting the surrounding metal of chromium and leaving it vulnerable to intergranular corrosion — the mechanism behind weld decay in the heat-affected zone. Niobium has a stronger affinity for carbon than chromium, so it forms stable niobium carbides instead, leaving the chromium available to maintain the passive film.

This is a different control mechanism from 304L, which simply limits carbon. It is also different from 321, which uses titanium. Niobium carbides are generally more stable at higher temperatures than titanium carbides, which is why 347 is often preferred for post-weld heat treatment and higher-temperature service. As with all stabilized grades, this is a risk-reduction mechanism — not a guarantee that 347 can never sensitize under any condition.

3. Carbon Content and High-Temperature Strength

Carbon is a powerful strengthening element in austenitic stainless steel at elevated temperature, which is why 347H — with its controlled higher carbon range — is referenced in high-temperature structural applications. The higher carbon supports the material’s resistance to the time-dependent deformation that becomes the limiting factor in long-term high-temperature service.

However, carbon is not the only variable. Actual performance also depends on temperature, product form, heat treatment, processing, and service conditions. A higher-carbon grade does not automatically mean higher performance in every mechanical property, and selection should not be reduced to a single carbon number.

In metallurgical terms, carbon strengthens austenitic stainless steel at elevated temperature by helping the material resist time-dependent deformation. This is the specific reason the higher-carbon 347H is favored for long-term high-temperature structural service. However, grain structure, processing history, and product form also influence the final behavior, so carbon content alone does not determine the outcome.

4. 347 vs 347H: Creep and Stress-Rupture

Creep is the time-dependent, permanent deformation of a material under sustained stress at elevated temperature. In long-term high-temperature service, a component can deform slowly or eventually rupture under stress well below its room-temperature yield strength. Stress-rupture performance — the stress a material sustains for a given time before failure — is therefore a central concern for pressure components that operate hot for long periods.

It is important to keep the related terms separate:

• Tensile strength / yield strength: short-time mechanical properties.
• Creep strength: the stress a material sustains while limiting time-dependent deformation at temperature.
• Stress-rupture performance: the stress a material sustains for a given time before rupture at temperature.
• Allowable stress: the code design value at the design temperature, which reflects long-term high-temperature behavior.

347H is generally the more relevant choice when long-term high-temperature loading, creep, or code allowable stress controls the design. For applications where the primary concern is stabilization and welding rather than sustained high-temperature load, ordinary 347 is often sufficient.

In code-governed design, the allowable stress at the design temperature already accounts for long-term high-temperature behavior such as creep and rupture. This is why allowable stress typically decreases as temperature rises, and why code design values are not interchangeable with room-temperature mechanical properties or a datasheet “maximum temperature.”

5. Welding Performance

As a niobium-stabilized austenitic grade, 347 offers useful resistance to chromium carbide precipitation in the weld heat-affected zone, which is why it appears in welded equipment that will see elevated temperature or post-weld heat treatment. But stabilization does not remove the need for welding controls. Heat input, welding procedure, filler metal compatibility, interpass control, and post-weld cleaning all still matter, and parameters must come from a qualified welding procedure specification (WPS) and procedure qualification record (PQR).

The higher carbon in 347H should not be read as “347H cannot be welded,” nor should 347H’s high-temperature strength advantage be read as a blanket improvement in welding performance over 347. Filler metal selection depends on the welding process, base material, design requirements, and the applicable AWS or ASME code — not on the grade name alone.

Regardless of grade, post-weld surface condition matters for corrosion performance. Heat tint, oxide scale, and contamination should be addressed as the service requires, with pickling or passivation where specified. Niobium stabilization does not eliminate the need for proper post-weld cleaning.

6. When Should Buyers Choose 347 or 347H?

For ordinary stabilized service where welding and general high-temperature use are the main concerns, 347 is often sufficient. When the design is driven by long-term high-temperature loading, creep, or code allowable stress, 347H becomes the grade to evaluate. The decision should follow the design temperature, sustained stress, creep requirement, service environment, welding, product form, and applicable code — not a simple “347H is the premium option” assumption.

In practice, the cost and availability of 347 versus 347H are often similar, and the deciding factor is whether the design documentation or code actually requires the higher-carbon grade. Buyers should start from the design requirements — temperature, stress, and code — and work backward to the grade, rather than defaulting to 347H because it sounds more capable.

7. ASTM, UNS and Code Selection

When purchasing 347 or 347H, confirm the grade, the UNS designation (S34700 for 347, S34709 for 347H), the applicable ASTM product specification, product form, dimensions, heat treatment / condition, finish, material test certificate (MTC), heat-number traceability, intended service temperature, and any applicable ASME or code requirements.

Different product forms reference different ASTM standards — for example, ASTM A240 for plate, sheet, and strip; ASTM A312 for pipe; and ASTM A213 for seamless boiler and heat-exchanger tube. It is important not to confuse the levels: the ASTM product specification, the UNS designation, and the ASME code allowable-stress requirements are different things. Code allowable stresses are design values within a code context, not general commercial datasheet figures.

Example purchase specification (illustrative only, not ASTM or ASME standard text): “347H stainless steel plate, UNS S34709, ASTM A240/A240M, [dimensions], annealed, specified finish, EN 10204 Type 3.1 MTC with heat-number traceability.” This is a purchasing template, not a standard requirement; the actual grade, dimensions, finish, testing, and acceptance requirements must be confirmed against the project specification and applicable code.

Where ASME code design applies, the allowable-stress values come from the applicable code section and edition for the specific product form and design temperature. These are engineering design inputs, not general material-specification values, and they should be taken from the governing code rather than from a general datasheet.

8. Common Purchasing Mistakes

• Reading 347H as simply “a more advanced 347.”
• Looking only at carbon and ignoring the complete grade requirements.
• Using room-temperature strength to judge creep performance.
• Confusing stabilization resistance with creep resistance.
• Assuming 347H is suitable for every high-temperature scenario.
• Ignoring the product form and the applicable ASTM specification.
• Ignoring ASME or code requirements where code design applies.
• Ordering only “347H” without UNS, standard, condition, and MTC requirements.

9. FAQ

Q1: What is the difference between 347 and 347H?
Both are niobium-stabilized austenitic grades; 347H (UNS S34709) has a controlled higher carbon content than 347 (UNS S34700), aimed at improved high-temperature strength.

Q2: Why is niobium added to 347?
Niobium forms stable carbides in preference to chromium carbides, reducing chromium depletion at grain boundaries and the associated sensitization and weld decay.

Q3: How does 347 differ from 304L and 321?
304L controls sensitization by limiting carbon, 321 uses titanium stabilization, and 347 uses niobium/columbium stabilization — three different routes to controlling the same risk.

Q4: Why does 347H have higher carbon?
The higher carbon improves high-temperature strength and creep performance, which is why 347H is referenced in elevated-temperature code applications.

Q5: Is 347H always stronger than 347?
Not in every respect. 347H’s advantage is mainly long-term high-temperature strength; it is not a universal upgrade of 347.

Q6: What is creep strength?
Creep strength is the stress a material can sustain while limiting time-dependent deformation at elevated temperature, under sustained load.

Q7: Can I use room-temperature tensile strength to judge high-temperature performance?
No. At elevated temperature, long-term creep behavior — not short-time tensile strength — usually controls the design.

Q8: When should I choose 347H instead of 347?
When the design is driven by long-term high-temperature loading, creep, or code allowable stress. For ordinary stabilized service, 347 is often sufficient.

Q9: What are the UNS numbers for 347 and 347H?
347 is UNS S34700 and 347H is UNS S34709. Confirm the UNS along with the applicable ASTM product specification on the order.

Q10: What should an RFQ for 347 or 347H include?
Grade, UNS, applicable ASTM specification and edition, product form, dimensions, condition, finish, intended service temperature, MTC, heat-number traceability, and any applicable code requirements.