Two Stabilized Grades, One Critical Difference
AISI 348 (UNS S34800, ASTM A182 F348) and AISI 347 (UNS S34700, ASTM A182 F347) are both niobium-stabilized austenitic stainless steels with nearly identical chemistry and mechanical properties. The critical difference is that AISI 348 has a maximum cobalt content of 0.20 wt%, while AISI 347 has no cobalt limit. AISI 348 also counts tantalum (Ta ≤ 0.10%) alongside niobium in its stabilization ratio.
When to choose AISI 348: Required for nuclear reactor service (ASME Section III) and any application where cobalt content must be controlled. In all other applications — petrochemical, oil & gas, cryogenic, welded fabrications — AISI 347 is the economical default with identical corrosion resistance and mechanical performance.
AISI 347 and AISI 348 are both niobium (columbium)-stabilized austenitic stainless steels derived from the 18Cr-8Ni base composition of Type 304. They share the same fundamental purpose: preventing sensitization — the precipitation of chromium carbides (Cr₂₃C₆) at grain boundaries — that makes unstabilized grades like 304 and 316 vulnerable to intergranular corrosion (IGC) and polythionic acid stress corrosion cracking (PTA-SCC).
In the vast majority of industrial environments, AISI 347 and AISI 348 are functionally interchangeable. Both offer excellent corrosion resistance, strong weldability without post-weld heat treatment, and reliable mechanical properties at elevated temperatures up to 816 °C (1500 °F). Most procurement engineers default to 347 because it is more widely available and modestly less expensive.
But there is one environment where AISI 347 is explicitly prohibited and AISI 348 is mandated: nuclear reactor systems under ASME Section III. The reason comes down to a single specification requirement — the maximum cobalt content limit.
AISI 348 (UNS S34800) differs from AISI 347 (UNS S34700) in two ways: it has a maximum cobalt content of 0.20 wt% (vs. no limit in 347), and it counts tantalum alongside niobium in its stabilization ratio. The cobalt restriction makes 348 mandatory for nuclear reactor service; in all other applications, 347 is the economical default with identical performance.
Chemical Composition: Every Element Compared
The table below shows the full composition requirements per ASTM A182 for Grade F347 (UNS S34700) and Grade F348 (UNS S34800). All values are weight percent (wt%). Differences between the two grades are highlighted in color.
| Element | F347 / UNS S34700 | F348 / UNS S34800 | Engineering Significance |
|---|---|---|---|
| Carbon (C) | 0.08 max | 0.08 max | Identical — low C reduces sensitization risk |
| Manganese (Mn) | 2.00 max | 2.00 max | Identical |
| Silicon (Si) | 1.00 max | 1.00 max | Identical |
| Chromium (Cr) | 17.0 – 19.0 | 17.0 – 19.0 | Identical — passive corrosion resistance |
| Nickel (Ni) | 9.0 – 13.0 | 9.0 – 13.0 | Identical — austenite stabilizer |
| Niobium (Nb) | 10×C min, 1.10 max (Nb only) | 10×C min, 1.10 max (Nb + Ta combined) | 348 counts Ta with Nb toward stabilization ratio |
| Tantalum (Ta) | Not specified | 0.10 max (separate limit) | Unique to 348 — Ta counted in Nb+Ta stabilizer balance |
| Cobalt (Co) | No limit specified | 0.20 max ← CRITICAL | The critical difference. Co restriction enables nuclear service |
| Phosphorus (P) | 0.045 max | 0.045 max | Identical |
| Sulfur (S) | 0.030 max | 0.030 max | Identical |
The two grades differ in only two respects: tantalum (Ta) inclusion in Grade 348’s stabilizer balance, and the maximum cobalt content of 0.20 wt% in F348 absent from F347. All other elements — carbon ceiling, chromium range, nickel range, minimum niobium relative to carbon — are identical. This is why the two grades are functionally equivalent outside nuclear procurement.
Why the 0.20% Cobalt Limit in AISI 348 Matters
This is the heart of the AISI 347 vs. AISI 348 question. Natural cobalt consists almost entirely of the stable isotope &sup5;&sup9;Co. When exposed to neutron flux in a reactor, it undergoes neutron capture and becomes radioactive cobalt-60:
Cobalt-60 in nuclear plant structural materials causes: elevated radiation dose to workers; higher ALARA program costs (more shielding, shorter exposure windows); long-term contamination of coolant systems; and increased decommissioning complexity at end of plant life.
ASME BPVC Section III and IAEA nuclear safety guidelines require materials in reactor coolant systems to have restricted cobalt content. ASTM A182 Grade F348 with Co ≤ 0.20% is one of the approved forging specifications. Grade F347 has no cobalt limit and cannot be substituted for F348 in nuclear applications.
Do not accept Grade F347 as a substitute for F348 on nuclear purchase orders — even if the supplier claims the heat analysis shows cobalt below 0.20%. Only a forging certified to the F348 specification (UNS S34800) with the cobalt limit documented on an EN 10204 Type 3.1 or 3.2 MTC satisfies the requirement. The specification number controls, not the actual analysis result.
Nb+Ta vs. Nb-Only: Does the Dual Stabilizer Make a Difference?
In Grade 347, only niobium (Nb) counts toward the stabilization ratio — minimum 10× the carbon content. In Grade 348, both niobium and tantalum are counted together toward the same 10×C minimum, with tantalum separately limited to 0.10% maximum.
How Sensitization Occurs
Sensitization occurs when carbon migrates to austenite grain boundaries and combines with chromium to form Cr₂₃C₆ in the critical range of 427–816 °C (800–1500 °F), depleting chromium from the adjacent zone and creating a corrosion-vulnerable “chromium-depleted zone.” Niobium and tantalum are stronger carbide formers than Cr₂₃C₆; they preferentially form stable NbC and TaC, keeping chromium in solution.
In standard industrial service, the tantalum addition in Grade 348 produces no measurable improvement over Grade 347 in IGC resistance. Both grades pass ASTM A262 Practice E corrosion testing equivalently. The tantalum in 348 arises from scrap selection constraints when cobalt is restricted — the specification accounts for it rather than engineering it as a performance benefit.
Mechanical Properties: AISI 347 vs. AISI 348 Side by Side
Because the base alloy chemistry is identical between the two grades, minimum mechanical properties per ASTM A182 are the same in all respects.
| Property | F347 Minimum | F348 Minimum | Notes |
|---|---|---|---|
| Tensile Strength | 515 MPa (75 ksi) | 515 MPa (75 ksi) | Identical |
| 0.2% Yield Strength | 205 MPa (30 ksi) | 205 MPa (30 ksi) | Identical |
| Elongation (2 in.) | 30% min | 30% min | Identical |
| Reduction in Area | 50% min | 50% min | Identical |
| Hardness | ≤ 223 HBW | ≤ 223 HBW | Identical |
In practice, forgings produced to ASTM A182 F348 typically test well above these minimums. Actual tensile strength, yield strength, elongation, and hardness values from production Mill Test Certificates are listed on our AISI 348 forged parts product page, along with available sizes, forms, and how to request a certified test report with your order.
Heat Treatment Requirements for F347 and F348 Forgings
Both grades follow the same heat treatment sequence per ASTM A182. The stabilization anneal (Step 04) is mandatory when specified by the purchaser or required by the application code (e.g., ASME Section III).
Where Each Grade Excels in Industrial Service
- Oil refinery piping exposed to polythionic acid (PTA-SCC during shutdowns)
- Petrochemical heat exchangers, vessels & flanges at 427–816 °C
- Exhaust systems, furnace components & recuperators
- Aircraft exhaust manifolds & engine hot section components
- Cryogenic service down to −196 °C
- Chemical process reactors, acid digesters & pressure vessels
- Welded fabrications where PWHT is impractical or prohibited
- Food processing & pharmaceutical vessel fabrication
- Nuclear reactor primary coolant system piping, flanges & nozzles
- Reactor pressure vessel internals (ASME Section III Class 1, 2, 3)
- Spent fuel storage & handling equipment
- Any 347 application where cobalt activation must also be controlled
- Defense and research reactor components
- Radiation environments where Co-60 buildup must be minimized
- Specifications explicitly calling out UNS S34800 or A182 F348
Nuclear Applications: Why AISI 348 Is the Required Grade
Nuclear power plants operate under strict regulatory frameworks — the USNRC in the United States, the IAEA internationally, and national nuclear authorities in each country — that mandate material traceability, chemistry control, and conformance to ASME Section III.
ASME BPVC Section III incorporates ASTM A182 as an approved material specification. For applications requiring stabilized austenitic stainless steel forgings with restricted cobalt, Grade F348 (UNS S34800) is specified. Grade F347 is not a permissible substitute due to the absence of a cobalt limit in its specification.
Typical Nuclear Components Using ASTM A182 F348 Forgings
- Reactor coolant pump (RCP) casings and impellers
- Pressurizer surge line and spray line nozzle forgings
- Reactor pressure vessel (RPV) outlet nozzle safe ends
- Control rod drive mechanism (CRDM) penetration flanges
- Steam generator inlet and outlet nozzle forgings (Class 1 boundary)
- Residual heat removal (RHR) and ECCS valve bodies
Typical Documentation Required for Nuclear-Service F348 Forgings
The following represents typical purchaser requirements seen in nuclear procurement. Specific documentation requirements vary by project, customer QA program, and applicable code. Confirm your exact requirements with your quality/engineering team and state them on the purchase order.
- EN 10204 Type 3.1 MTC (standard) or Type 3.2 (third-party witness inspection, available on request)
- Chemical analysis with cobalt content reported to sufficient decimal places to verify Co ≤ 0.20%
- Full heat treatment records with time-temperature documentation
- Grain size certification per ASTM E112 (when specified)
- Non-destructive examination per customer-specified procedures (UT, PT, PMI)
- ASTM A262 Practice E intergranular corrosion test (when specified by purchaser)
Decision Matrix: AISI 347 or AISI 348?
| Selection Criterion | AISI 347 / F347 | AISI 348 / F348 |
|---|---|---|
| Nuclear reactor service (ASME Section III) | ❌ Not permitted — no Co limit | ✅ Required |
| Cobalt must be ≤ 0.20 wt% | ❌ Not guaranteed by specification | ✅ Specification requirement |
| Oil & gas / petrochemical service | ✅ Preferred — lower cost | ✅ Acceptable (over-specified) |
| Refinery PTA-SCC prevention | ✅ Preferred — equivalent performance | ✅ Acceptable |
| Welded fabrication without PWHT | ✅ Preferred | ✅ Acceptable |
| Elevated temp service 427–816 °C | ✅ Preferred — lower cost | ✅ Equivalent performance |
| Cryogenic service to −196 °C | ✅ Preferred | ✅ Acceptable |
| Radiation environment, Co-60 concern | ❌ Uncontrolled Co content | ✅ Required |
| Budget-sensitive procurement | ✅ Lower cost, better stock | ⚠ 15–30% premium over 347 |
| Spec states UNS S34800 / A182 F348 | ❌ Cannot substitute | ✅ Required by specification |
| ASTM A262 IGC testing required | Both — equivalent performance | Both — equivalent performance |
If your specification states UNS S34800, ASTM A182 F348, or AISI 348 — you must order Grade 348. Supplying Grade 347 as a substitute is a non-conformance, even if the heat analysis shows low cobalt, because the F347 specification does not require cobalt to be controlled or documented. To request certified AISI 348 (UNS S34800) forgings with documented cobalt content, contact Jiangsu Liangyi.
Cost and Availability: What Engineers and Buyers Should Expect
Why AISI 348 Costs More Than AISI 347
1. Controlled scrap selection: To meet Co ≤ 0.20%, steelmakers must avoid high-cobalt scrap sources (tool steel, superalloy scrap), restricting the charge mix and increasing raw material cost.
2. Lower production volume: 348 is primarily specified for nuclear service — a comparatively small market — so global melt volumes are substantially lower than for 347, reducing economies of scale.
AISI 348 forgings typically price 15–30% above equivalent AISI 347 forgings. For non-nuclear applications, specifying 348 “to be safe” is a cost premium with no technical benefit.
Delivery and Lead Times
Grade 348 is not a standard stock item. At Jiangsu Liangyi, F348 open-die forgings and seamless rolled rings typically run 8–16 weeks lead time. For nuclear orders requiring 3.2 MTC witness inspection, allow 2–4 additional weeks.
If your project requires Grade 348 with a firm delivery schedule, engage your forging manufacturer during the engineering phase — before the bill of materials is frozen. Late-stage F348 orders can extend to 20+ weeks during high nuclear industry activity periods.
How to Order ASTM A182 F347 and F348 Forgings Correctly
A complete purchase order for ASTM A182 forgings must state the specification (ASTM A182 / ASME SA182), grade (F347 or F348 — never accept substitution), UNS number (S34700 or S34800), forging form, nominal dimensions, heat treatment requirements, MTC type (EN 10204 3.1 standard / 3.2 available on request), and any applicable project codes or NDT procedures.
For Grade F348, also state the maximum cobalt content on the purchase order — typically Co ≤ 0.20%, though some nuclear projects require ≤ 0.10%. This limit must appear on the Mill Test Certificate itself. A verbal assurance or an unmarked analysis does not satisfy the requirement.
For available forging forms, size ranges, and a quotation within 24 hours, contact Jiangsu Liangyi via the Contact Us page.
Frequently Asked Questions About AISI 348 vs. AISI 347
AISI 348 (UNS S34800, ASTM A182 F348) and AISI 347 (UNS S34700, ASTM A182 F347) are both niobium-stabilized austenitic stainless steels with nearly identical chemistry and mechanical properties. The critical difference is that AISI 348 has a maximum cobalt content of 0.20 wt%, while AISI 347 has no cobalt limit. AISI 348 also counts tantalum (Ta, max 0.10%) alongside niobium in its stabilization ratio. The cobalt restriction makes AISI 348 mandatory for nuclear reactor service, where cobalt-60 activation under neutron flux creates long-term radiation hazards to personnel and equipment.
AISI 348 is required in nuclear reactor service because its specification (ASTM A182 F348 / UNS S34800) imposes a maximum cobalt content of 0.20 wt%. When cobalt-59 is exposed to neutron flux, it activates into cobalt-60, a high-energy gamma emitter with a 5.27-year half-life. Cobalt-60 increases radiation dose to workers, contaminates coolant systems, and complicates decommissioning. AISI 347 has no cobalt limit and cannot be substituted for F348 in nuclear applications under ASME Section III.
No. Even if a specific heat of AISI 347 has cobalt below 0.20%, the ASTM A182 F347 specification does not require cobalt to be controlled, reported, or certified. A forging certified to F347 cannot formally satisfy an F348 purchase order. For nuclear applications, the specification number (F348 / UNS S34800) and a documented cobalt limit must both appear on the EN 10204 Type 3.1 or 3.2 Mill Test Certificate.
Per ASTM A182, AISI 348 (F348) forgings in solution-annealed condition must meet: Tensile Strength ≥ 515 MPa (75 ksi), 0.2% Yield Strength ≥ 205 MPa (30 ksi), Elongation ≥ 30%, Reduction in Area ≥ 50%, Hardness ≤ 223 HBW. These minimum values are identical to those of AISI 347 (F347), as the two grades share the same base alloy chemistry.
The maximum cobalt content in AISI 348 (UNS S34800, ASTM A182 F348) is 0.20 wt%. This restriction prevents neutron activation of cobalt-59 into radioactive cobalt-60 in nuclear reactor environments. AISI 347 (UNS S34700) has no specified cobalt limit.
The UNS designation for AISI 348 is UNS S34800 (ASTM A182 Grade F348). The UNS designation for AISI 347 is UNS S34700 (ASTM A182 Grade F347). Always verify that the UNS number on the Mill Test Certificate matches your purchase order to confirm the correct grade was delivered.
ASTM A182 F348 forgings are first cooled below 538 °C (1000 °F) after hot working, then solution annealed at 1040–1120 °C (1900–2050 °F) followed by rapid quenching. When specified, a stabilization anneal at 815–870 °C (1500–1600 °F) for minimum 2 h/in (4.7 min/mm) of thickness is performed. The stabilization anneal drives carbon into stable NbC and TaC, maximizing resistance to intergranular corrosion in service and after welding.
Yes. Jiangsu Liangyi Co., Limited supplies AISI 348 (UNS S34800, ASTM A182 F348) forgings with EN 10204 Type 3.1 Mill Test Certificates as standard. EN 10204 Type 3.2 (third-party witness inspection) is available upon customer request — please specify your preferred inspection body and requirements at time of order. The company is ISO 9001:2015 certified and has been producing AISI 348 stainless steel forgings since 1997.