347 Stainless Steel: Niobium Stabilization and High-Temperature Use

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

347 stainless steel (UNS S34700) is a niobium (columbium)-stabilized austenitic stainless steel. It is specified where a welded component must resist sensitization — the grain-boundary chromium depletion that leads to intergranular corrosion — and where the part will see elevated temperature. That combination makes 347 a workhorse in high-temperature process equipment, chemical plant, heat exchangers, and welded structures. This article explains why niobium is added, how it affects welding and sensitization, and how 347 fits into high-temperature service selection relative to 304L.

1. What Is 347 Stainless Steel?

347 is an austenitic chromium-nickel stainless steel stabilized with niobium and tantalum. Its UNS designation is S34700. The niobium/tantalum addition is not for general corrosion resistance in the way chromium or molybdenum would be — its role is to stabilize carbon so that the material retains its resistance to intergranular corrosion after welding or elevated-temperature exposure. This is why 347 appears in welded equipment, high-temperature piping, and process vessels where a stabilized grade is called for.

2. Why Is Niobium Added to 347?

At elevated temperature, carbon in stainless steel can combine with chromium to form chromium carbides at grain boundaries. This consumes chromium from the immediate surroundings, leaving a chromium-depleted zone that can no longer maintain a protective passive film as effectively, so it corrodes preferentially along the boundaries. Niobium (and tantalum) have a stronger affinity for carbon than chromium, so they form stable niobium/tantalum carbides instead, leaving the chromium available to protect the material.

This is the same stabilization concept used in 321, but with a different element: 321 uses titanium, while 347 uses niobium/columbium. Niobium carbides are generally more stable at higher temperatures than titanium carbides, which is one reason 347 is often preferred for post-weld heat treatment and higher-temperature service. The stabilization reduces sensitization risk — it does not eliminate it entirely.

Typical applications reflect this: high-temperature process piping, heat exchangers, boiler and petrochemical equipment, and welded structures that will operate at elevated temperature. In each case, the grade is chosen for the combination of weldability, stabilization, and high-temperature performance — not because it is a universal best choice for every hot application.

3. Niobium Stabilization and Sensitization

Sensitization is the process by which chromium carbides precipitate at grain boundaries during heating, depleting nearby chromium and creating zones vulnerable to intergranular corrosion. It is most relevant in the heat-affected zone of a weld and during prolonged elevated-temperature exposure. Niobium stabilization addresses the first step of this chain by capturing the carbon before it can form chromium carbides.

This is a risk-reduction mechanism, not a guarantee. Stabilization does not mean 347 can never sensitize under any condition, and it is not the same as general corrosion resistance. The benefit is specific: reduced chromium carbide precipitation in welded or heated regions.

Sensitization matters most in two places: the heat-affected zone of a weld, where the metal is briefly heated into the critical range, and long-term service at elevated temperature, where the metal spends extended time in that range. In both cases, the goal of stabilization is to keep chromium in solid solution where it can protect the surface.

4. 347 vs 304L

304L and 347 take two different routes to controlling sensitization. 304L limits carbon, so there is less carbon available to form chromium carbides. 347 keeps the carbon but stabilizes it with niobium. For ordinary welded fabrication, 304L is often sufficient and is usually easier to source. 347 becomes more relevant where the component will see elevated temperature or where a stabilized grade is specified, because its stabilization remains effective through the reheating that can cause sensitization in low-carbon grades.

The final choice depends on temperature, service environment, welding, and code requirements — not on a simple “347 is better” rule.

In practice, the decision often comes down to temperature and specification requirements. For moderate-temperature welded service, low-carbon 304L is usually the economical and readily available choice. Where the component will be reheated — by post-weld heat treatment or prolonged elevated-temperature exposure — and a stabilized grade is specified, 347 becomes the more relevant option. Availability and cost also weigh in: 304L is generally more widely stocked.

5. 347 Stainless Steel Welding

The stabilized chemistry of 347 is meaningful for welding because it reduces chromium carbide precipitation in the heat-affected zone. But stabilization does not remove the need for welding controls. Heat input, welding procedure, filler metal compatibility, and post-weld cleaning all still influence the final result, and parameters must come from a qualified welding procedure specification (WPS) and procedure qualification record (PQR).

Post-weld surface condition also matters. Heat tint, oxide scale, and surface contamination can reduce corrosion resistance in service, and they should be addressed as the application requires — with pickling or passivation where specified. A stabilized grade still needs proper post-weld cleaning.

Filler metal selection depends on the welding process, base material, design requirements, and the applicable AWS or ASME code. It cannot be read off the grade name alone. For applications where the weld will see elevated temperature or post-weld heat treatment, the filler and procedure must be qualified for those conditions.

6. 347 for High-Temperature Service

347 is frequently used in elevated-temperature service because its stabilization helps maintain resistance to intergranular corrosion after exposure to temperature. But high-temperature suitability is not a single number, and several properties must be considered separately: oxidation resistance (how the surface withstands scaling), mechanical strength, creep performance (long-term deformation under sustained load), and sensitization resistance. Niobium stabilization supports the last of these; it does not by itself establish the others.

There is no single, context-free maximum operating temperature for 347. The usable temperature depends on product form, stress, design life, oxidation, and the applicable code allowable stress. Any specific temperature, allowable-stress, or creep figure must be read against its source standard, product form, and test conditions.

To read high-temperature suitability correctly, keep the properties separate. Oxidation resistance describes how the surface withstands scaling in a given atmosphere; mechanical strength describes short-time load-bearing; creep and stress-rupture describe long-term deformation and rupture under sustained stress at temperature; and sensitization resistance is the stabilization benefit. A grade can be strong in one of these and limited in another, so they must be evaluated individually.

347 also has a higher-carbon variant, 347H, which is aimed at elevated-temperature strength and creep performance for code-governed applications. Buyers focused on long-term high-temperature loading should evaluate 347H; those focused on stabilization and general high-temperature use may find 347 sufficient.

7. ASTM Standards and Procurement

When purchasing 347, confirm the grade, UNS (S34700), the applicable ASTM product specification, product form, dimensions, heat treatment / condition, surface finish, material test certificate (MTC), heat-number traceability, intended service temperature, and any supplementary testing required.

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 any ASME/code requirements are different things and operate at different levels of a specification.

Example purchase specification (illustrative only, not ASTM or ASME standard text): “347 stainless steel plate, UNS S34700, 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.

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

• Describing 347 as simply “321 with titanium” and confusing niobium stabilization with titanium stabilization.
• Assuming 347 can never sensitize after welding.
• Confusing stabilization resistance with general corrosion resistance.
• Treating high-temperature suitability as unlimited high-temperature use.
• Writing only “347” without the UNS number and product specification.
• Ignoring MTC and heat-number traceability requirements.
• Ignoring ASME or code requirements where code design applies.

9. FAQ

Q1: What is 347 stainless steel?
347 (UNS S34700) is a niobium/columbium-stabilized austenitic stainless steel designed to resist sensitization and intergranular corrosion after welding or elevated-temperature exposure.

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

Q3: What is sensitization?
Sensitization is the formation of chromium carbides at grain boundaries during heating, which depletes nearby chromium and makes those zones vulnerable to intergranular corrosion.

Q4: How does 347 differ from 304L?
304L controls sensitization by limiting carbon, while 347 stabilizes carbon with niobium. Both target the same risk through different mechanisms.

Q5: Is 347 the same as 321?
No. Both are stabilized grades, but 321 uses titanium while 347 uses niobium/tantalum. Niobium carbides are generally more stable at high temperature.

Q6: Can 347 be welded?
Yes, using conventional austenitic stainless steel processes, but heat input, procedure, filler compatibility, and post-weld cleaning must follow a qualified WPS/PQR.

Q7: Does stabilization mean 347 never sensitizes?
No. Stabilization reduces the risk, but it is not an unconditional immunity — welding practice and service conditions still matter.

Q8: Is 347 suitable for all high-temperature applications?
No. High-temperature suitability depends on oxidation, strength, creep, and code allowable stress — there is no single maximum operating temperature.

Q9: Which ASTM standard applies to 347?
The applicable standard depends on product form — for example, A240 for plate/sheet/strip, A312 for pipe, or A213 for tube. Confirm the correct standard and edition.

Q10: What should an RFQ for 347 include?
Grade 347, UNS S34700, applicable ASTM specification and edition, product form, dimensions, condition, finish, MTC, heat-number traceability, and any supplementary testing or code requirements.