AISI 348 vs. 347 Stainless Steel:
What's the Difference and When Should You Choose 348?

Background: What Are Stabilized Austenitic Stainless Steels?

Standard 304 and 316 austenitic grades suffer from sensitization — the precipitation of chromium carbides (Cr₂₃C₆) at grain boundaries when heated between 425 °C and 870 °C. This depletes chromium near the grain boundary and causes intergranular corrosion (IGC) in the heat-affected zone (HAZ) of welds, also known as weld decay.

The stabilized grades solve this by adding an element that binds carbon more strongly than chromium does. Carbon bonds preferentially to the stabilizer — keeping chromium in solid solution and preserving corrosion resistance through and after welding. Three main stabilized grades exist:

• AISI 321 — titanium (Ti) stabilized; effective, but Ti volatility during arc welding can cause porosity in thick sections
• AISI 347 — niobium (Nb, also called columbium) stabilized; superior weld performance; widely specified for pressure vessels, heat exchangers, and power-generation piping
• AISI 348 — also niobium stabilized; chemically nearly identical to 347, but with two additional elemental ceilings on tantalum and cobalt, introduced exclusively for nuclear service

All three are fully austenitic, non-magnetic in the annealed condition, and weldable without post-weld heat treatment in most applications.

Side-by-Side Chemistry: Where 347 and 348 Diverge

The table below compares AISI 347 and AISI 348 per ASTM A182 / ASTM A240. All values are maximum wt% unless a range is stated.

★ = the only two elements where 347 and 348 differ. Grade 348 is not a higher-performance material in any general engineering sense; it carries stricter purity controls on two trace elements for one specific reason: reducing radioactive activation in neutron environments.

Why Tantalum and Cobalt Are Restricted in AISI 348

The Cobalt Problem: Cobalt-60 (Co-60)

Cobalt-59, the only stable isotope of cobalt, has a large thermal neutron capture cross-section of approximately 37 barns. When any component containing Co-59 is exposed to neutron flux inside or adjacent to a reactor core, the reaction Co-59(n,γ)Co-60 occurs. Co-60 is a gamma emitter with a 5.27-year half-life producing two high-energy gamma rays (1.17 MeV and 1.33 MeV).

Co-60 is the dominant source of radioactive contamination in Light Water Reactor (LWR) primary coolant systems and accounts for the majority of occupational radiation dose during outage maintenance. Even trace levels of Co-59 in large forgings — valve bodies, pump casings, flanges — accumulate significant Co-60 inventory over a 40–60 year plant lifetime. Grade 348's limit of Co ≤ 0.20 wt% balances radiological control with metallurgical practicality.

The Tantalum Problem: Tantalum-182 (Ta-182)

Tantalum-181, the only naturally occurring stable isotope of tantalum, activates to Ta-182 under neutron irradiation. Ta-182 emits gamma radiation across multiple energies and has a 114.4-day half-life — long enough to create significant shielding requirements during reactor outages. The restriction of Ta ≤ 0.10 wt% limits this activation pathway without affecting the Nb stabilization function.

Mechanical Properties: Functionally Identical

Because Ta and Co serve no structural role, the elemental restrictions in Grade 348 have zero measurable effect on mechanical performance. Values below are ASTM A182 / ASTM A240 minima in the solution-annealed condition:

Corrosion Resistance: Equal Across Common Environments

Both grades exhibit equal corrosion resistance in the majority of industrial environments. The shared niobium stabilization prevents sensitization and makes both suitable for:

• Exposure to the 425–870 °C sensitization temperature range — heat-affected zones of welds, boiler and heat exchanger service
• Mildly acidic aqueous media — acetic acid, phosphoric acid at low-to-moderate concentrations
• Oxidizing high-temperature atmospheres up to approximately 870 °C continuous / 925 °C intermittent
• Steam systems, high-temperature water, and condensate return lines

Important limitation: neither 347 nor 348 contains molybdenum. In chloride-rich or strongly reducing environments, specify 316L, 317L, or a duplex grade. The PREN for 347/348 is approximately 18–20, versus 24–26 for 316L.

Welding and Fabrication

Both grades weld with the same processes and filler metals:

• Non-nuclear service: ER347 is the standard matching filler for both grades
• Nuclear service: ER347 filler selected and certified to meet Co ≤ 0.20 % and Ta ≤ 0.10 % — the weld deposit must satisfy the same nuclear restrictions as the base metal

In nuclear applications, using standard unrestricted ER347 filler with Grade 348 base metal re-introduces cobalt through the weld deposit, defeating the purpose of specifying 348. Your Welding Procedure Specification (WPS) and Procedure Qualification Record (PQR) must address filler metal compliance explicitly.

Post-Weld Heat Treatment (PWHT)

Neither grade requires PWHT for corrosion resistance when the delta ferrite content of the weld deposit falls within 4–12 FN per the WRC-1992 diagram. Stress-relief annealing may be required by the governing code (ASME VIII Div. 1, EN 13445) based on wall thickness and service temperature — always consult the applicable code.

Forging Considerations for A182 F347 and F348

From a hot-working standpoint, both grades are processed identically. At Jiangsu Liangyi's Jiangyin facility, we produce open-die forgings and seamless rolled rings in UNS S34800 under the same parameters as 347:

• Forging temperature range: 1,150–1,260 °C initial; finish above 950 °C to preserve full austenite
• Solution annealing: 1,010–1,120 °C, water quench or rapid air cool — identical for both grades
• Post-forging cooling: Cool below 538 °C before annealing, per ASTM A182
• Product forms: Open-die forgings, seamless rolled rings, forged bars, round billets
• Weight range: 30 kg to 30,000 kg per piece

The critical difference in Grade 348 production is charge chemistry and raw material traceability. To certify Co ≤ 0.20 % and Ta ≤ 0.10 %, the melt must use controlled virgin nickel and pre-screened stainless scrap. These limits cannot be corrected post-melting. PMI verification is performed at the billet stage before forging begins.

Applicable Standards and International Cross-References

The European EN system does not maintain a separate designation for Grade 348. When specifying 348 under EN standards, reference UNS S34800 alongside the applicable EN standard (EN 10088-3 for bar/plate, EN 10222-5 for forgings), and state the Co and Ta limits as supplementary purchase order requirements.

Cost and Availability

AISI 347 is a high-volume commodity grade, widely stocked. AISI 348 is a specialty product with a narrower supply base due to controlled charge chemistry and individual element certification for Co and Ta.

• Price premium: A182 F348 forgings typically carry a 10–25 % premium over equivalent F347 parts — reflecting scrap selection costs and additional analytical testing, not a different base alloy
• Lead times: Grade 347 is often available from stock as bar or plate; 348 forgings are almost always made-to-order, with minimum lead times of 6–14 weeks depending on weight and complexity
• Minimum quantities: Heat-lot certification for 348 may impose an effective minimum charge of several hundred kilograms

When Should You Specify Each Grade?

The decision rule is simple: Is radioactive activation of cobalt or tantalum a design constraint? If yes — mandate Grade 348. If no — specify Grade 347. It is lower cost, more readily available, and performs identically in every non-nuclear application.

A Note on H Grades: F347H and F348H

Both grades are available in "H" (high-carbon) variants with carbon controlled to 0.04–0.10 % C (versus ≤ 0.08 % for standard) and minimum grain size of ASTM No. 7 or coarser. This controlled microstructure improves creep resistance at elevated temperatures. F347H and F348H are specified in high-temperature power piping (ASME B31.1 / B31.3) where the allowable stress tables show a creep advantage for H grades above approximately 550 °C. The nuclear Co and Ta restrictions in F348H are identical to standard F348.

Writing the Purchase Order

An incomplete PO for A182 F348 forgings is a common source of quality disputes. A well-specified purchase order should include:

1. Material standard and grade: ASTM A182 / A182M Grade F348 (UNS S34800)
2. Heat treatment condition: Solution annealed per ASTM A182
3. Elemental restriction: Co ≤ 0.20 wt%, Ta ≤ 0.10 wt% — actual measured values to be individually reported on MTC
4. MTC type: EN 10204 Type 3.1; or Type 3.2 if third-party witness certification is required
5. Testing: Full heat and product chemical analysis; tensile test; hardness; IGC per ASTM A262 Practice E if code-required; UT per ASTM A388
6. Dimensional standard: ASME B16.5 / B16.47 for flanges, or your referenced drawing and revision number
7. Supplementary requirements: Any applicable code requirements (ASME Sec. III, PED, API, etc.) or customer-specific inspection hold points

At Jiangsu Liangyi Co., Limited, every shipment of A182 F348 material includes full heat analysis with Co and Ta values individually reported on the EN 10204 3.1 MTC. We hold ISO 9001:2015 certification. Third-party inspection can be coordinated upon customer request — customers typically nominate their own preferred inspection body, and we facilitate access and documentation accordingly. View full specifications, available sizes, and request a quote for F348 forgings.

Summary: AISI 347 vs. 348 at a Glance