347H vs 347: Carbon Range Differences for ASME Boiler and Pressure Vessel Code Compliance

What Is the Difference Between 347 and 347H Stainless Steel?

347 and 347H are the same niobium-stabilized austenitic stainless steel - differentiated only by a tighter, higher carbon specification range on the H-grade, which the ASME Code treats as a materially different product for allowable-stress purposes.

Both grades share the identical base chemistry: roughly 18% chromium and 10% nickel, with niobium (plus tantalum) added specifically to tie up carbon as stable niobium carbides instead of chromium carbides. This "stabilization" is what makes the 347 family resistant to sensitization and intergranular corrosion after welding - a problem that affects unstabilized grades like 304 and 316 in the 425–870°C heat-affected zone. Where the two grades diverge is carbon content: standard 347 caps carbon at 0.08% maximum with no specified minimum, while 347H requires carbon to fall between 0.04% and 0.10%. That narrow band is a deliberate metallurgical design choice, not an arbitrary Code formality, and it is the subject of the rest of this article.

In practice, many mills produce plate, pipe, and tube to a dual-certified 347/347H chemistry that satisfies both specifications simultaneously - but a dual certification only works when the actual carbon content lands inside the narrower 347H band (0.04–0.10%). Material certified only to standard 347 chemistry can legally contain carbon anywhere from a trace amount up to 0.08%, including levels below 0.04% that would disqualify it from 347H allowable-stress tables even though it is chemically "347."

Why Does the Carbon Range Matter for Elevated-Temperature Service?

Carbon controls high-temperature creep strength in austenitic stainless steels, so 347H's 0.04% carbon floor exists to guarantee a minimum level of creep resistance that low-carbon 347 cannot promise.

At elevated temperature, austenitic stainless steels resist slow, time-dependent deformation (creep) partly through fine carbide precipitation that pins grain boundaries and slip planes. Carbon is the raw material for that precipitation. A heat of 347 produced with very low carbon - say 0.02% - will have measurably lower creep-rupture strength than a heat produced at 0.08%, even though both satisfy the same "347" chemistry certificate, because standard 347 sets no carbon floor. ASME's allowable-stress values above 1,000°F are derived from statistically validated creep-rupture data, so the Code will only publish those values for a grade whose composition is tightly enough controlled that every certified heat behaves predictably at temperature. That is exactly what the 347H carbon range guarantees, and exactly what open-ended 347 cannot.

This is the same logic ASME applies to the other H-grade families - 304H, 316H, 321H, and 309H all carry the identical 0.04–0.10% carbon requirement over their respective base grades, for the same creep-strength rationale.

What Do ASTM A240 and ASME SA-240 Specify for 347 vs 347H Composition?

ASTM A240 (mirrored by ASME SA-240) lists 347 and 347H as separate UNS-numbered grades within the same specification, distinguished by carbon range and by a different minimum niobium-to-carbon stabilization ratio.

Table 1 - 347 vs 347H chemical composition, weight % (ASTM A240 / ASME SA-240)

[Source] ASTM A240/A240M Standard Specification for Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip for Pressure Vessels; ASME Section II, Part A, SA-240.

Every element except carbon and the niobium ratio is identical between the two grades - chromium, nickel, manganese, silicon, phosphorus, and sulfur limits do not change. This confirms that 347H is not a different alloy family; it is standard 347 chemistry produced to a tighter, elevated-carbon window with a correspondingly adjusted stabilization ratio.

How Does the Niobium-to-Carbon Ratio Change Between the Two Grades?

347H requires a lower minimum niobium multiplier (8×C) than standard 347 (10×C) because 347H's higher carbon floor already supplies more carbon for niobium to stabilize, so less "safety margin" niobium is needed per unit of carbon.

Niobium stabilization works by preferentially forming niobium carbides (NbC) instead of chromium carbides (Cr23C6) during welding and elevated-temperature exposure, which prevents the chromium-depleted grain-boundary zones that cause intergranular corrosion.

The stabilization ratio (Nb+Ta as a multiple of carbon content) is set high enough to guarantee that virtually all the carbon present gets tied up as NbC rather than Cr23C6. Because standard 347 has no carbon floor, its ratio requirement (10×C minimum) is calculated conservatively against the worst case - a heat near the 0.08% carbon ceiling. 347H's narrower, and minimum-guaranteed, carbon content allows the lower 8×C multiplier while still fully stabilizing the alloy, because the metallurgical basis for the ratio scales with actual carbon present, not a theoretical maximum.

What Are the Mechanical Property Differences at Room and Elevated Temperature?

At room temperature, 347 and 347H share identical minimum tensile and yield strength requirements; the practical difference only appears at elevated temperature, where 347H's higher, controlled carbon content delivers measurably higher creep-rupture strength.

Table 2 - Room-temperature minimum mechanical properties (ASTM A240 / ASME SA-240)

[Source] ASTM A240/A240M minimum property requirements, UNS S34700 / S34709.

The room-temperature numbers are identical by design - the Code does not need a strength differentiator at ambient conditions, because sensitization and creep are not active failure modes there. The differentiation only matters once service temperature climbs above roughly 1,000°F (538°C), which is precisely the threshold where ASME Section II, Part D stops publishing allowable stress values for standard 347 altogether and continues them for 347H, reflecting the creep-rupture data supporting the H-grade's guaranteed carbon floor.

Does Grain Size Affect ASME Code Qualification for 347H?

Yes - ASME Section II, Part D requires 347H material to meet either a coarse grain size (ASTM No. 7 or coarser) or a minimum elevated-temperature creep-rupture strength before it can be assigned the higher allowable stress values published for use above 1,000°F.

Grain size interacts with creep resistance in the opposite direction from most mechanical properties: coarser grains generally improve high-temperature creep-rupture life because there is less grain-boundary area available for creep cavitation and boundary sliding to initiate cracking. ASME's footnoted requirement gives fabricators two qualification paths for the highest published 347H allowable stresses - either demonstrate the coarse grain structure directly, or submit supplemental creep-rupture test data proving the material meets the required strength regardless of grain size.

Buyers specifying 347H for the most demanding superheater or reheater service should confirm on the purchase order which qualification path the mill intends to certify against, since it affects which stress table column applies to the final component design.

What Are the ASME Section I and Section II, Part D Requirements for 347H?

ASME Section I (Power Boilers) and Section II, Part D permit 347H for pressure-retaining components at temperatures where standard 347 has no listed allowable stress, making 347H the only Code-compliant choice for boiler components operating above roughly 1,000°F (538°C).

Section II, Part D contains the allowable stress tables that every other ASME construction code (Section I, Section VIII, and the B31 piping codes) references when an engineer sizes wall thickness for a given pressure, temperature, and material. For 347H, Part D publishes allowable stress values in temperature increments extending well above 1,000°F, consistent with the alloy's Code-recognized creep strength. For standard 347, Part D's stress table generally stops at or near 1,000°F.

This is not a drafting oversight - it reflects that the Code committee has validated creep-rupture data for 347H's controlled carbon range but has not validated an equivalent basis for open-specification 347 at those temperatures. A design engineer who specifies plain 347 for a superheater tube operating at 1,050°F has no Code-sanctioned allowable stress value to design against; the material is simply not qualified for that service.

When Must You Use 347H Instead of 347 Under ASME B31.1/B31.3?

347H is required whenever the piping design temperature exceeds the highest temperature listed for standard 347 in the applicable B31 allowable-stress table - in practice, any power boiler, process heater, or high-temperature process piping system operating above about 1,000°F (538°C).

ASME B31.1 (Power Piping): main steam, hot reheat, and superheater outlet piping in fossil and combined-cycle power plants routinely operate at 1,000–1,050°F, placing them squarely in 347H-only territory.

ASME B31.3 (Process Piping): high-temperature reactor feed and effluent lines in refining and petrochemical service can exceed 1,000°F, particularly in catalytic reforming and hydroprocessing units.

Below the ~1,000°F threshold, both grades are Code-qualified and the choice becomes an economic one - see Section 11 below.

Design engineers should always verify the current-edition Part D stress table for the exact temperature and grade combination on their project, since Code editions are revised periodically and the precise cutoff temperature can shift slightly between editions.

What Applications Use 347H vs Standard 347?

347H is specified almost exclusively for elevated-temperature, Code-stamped pressure equipment, while standard 347 covers the broader range of general corrosion-resistant, weld-fabricated equipment where creep strength is not a governing design factor.

Table 3 - Typical application split between 347 and 347H

How Does Sensitization Risk Differ Between 347 and 347H?

Sensitization resistance is essentially equivalent between the two grades because both rely on the same niobium-stabilization mechanism; 347H's slightly higher carbon content is fully offset by its own correspondingly higher niobium addition.

A common misconception is that raising the carbon floor for 347H must increase sensitization risk, since carbon is the element responsible for chromium-carbide-driven intergranular attack in unstabilized grades like 304 or 316. That logic does not carry over to the 347 family because niobium - not carbon control alone - is doing the stabilizing work.

As long as the niobium content satisfies its specified multiple of the actual carbon present (8×C minimum for 347H, 10×C minimum for standard 347), essentially all available carbon is tied up as stable NbC rather than migrating to grain boundaries as Cr23C6. Both grades, properly certified to their respective ASTM A240 ratios, offer equivalent resistance to weld-decay sensitization in the 425–870°C range.

What Is the Cost and Availability Difference?

347H typically carries a modest cost premium over standard 347 - driven by tighter melt-chemistry control and mandatory creep/grain-size documentation - and mill lead times can run longer because fewer heats are produced specifically to the narrower carbon band.

Because many mills melt to a chemistry that satisfies both specifications at once, the price gap for stock plate and pipe is often smaller than the nominal alloy surcharge difference suggests. The larger cost and schedule driver is documentation: 347H orders typically require supplemental certified test reports covering the grain-size or creep-rupture qualification path discussed in Section 6, along with tighter chemical-composition reporting on the mill test certificate. Buyers should request dual 347/347H certification whenever the application allows it - this preserves flexibility to use the same stocked material for both standard and elevated-temperature scopes without carrying two separate inventories.

How to Select and Order the Correct Grade for Your Project?

Selection comes down to a single question - does any credible operating scenario push the component above roughly 1,000°F (538°C)? - and the purchase order should state the grade, specification, carbon range, and certification requirements explicitly rather than relying on a generic "347" callout.

Confirm maximum design temperature (including upset/excursion conditions, not just normal operating temperature) against the current ASME Section II, Part D stress table.

If any credible condition exceeds standard 347's listed temperature range, specify 347H explicitly by UNS S34709 and ASTM A240/A312/A213/A182 as applicable to the product form.

Request dual 347/347H mill certification where available, to preserve inventory flexibility.

For 347H, specify which qualification path (coarse grain size or creep-rupture test data) the mill will certify against.

Request an EN 10204 3.1 mill test certificate showing actual heat chemistry, and 3.2 third-party witnessed certification for critical Code-stamped components.

Verify Nb+Ta content on the certificate satisfies the correct stabilization ratio for the grade ordered.

Frequently Asked Questions About 347 vs 347H

Q1: Can 347 be substituted for 347H in an ASME Code application?

A: Only if the maximum design temperature stays within the range where ASME Section II, Part D publishes an allowable stress value for standard 347. Above that temperature, the Code provides no basis for designing with 347, regardless of the actual carbon content of the material on hand.

Q2: Is 347H stronger than 347 at room temperature?

A: No. ASTM A240 sets identical minimum tensile strength, yield strength, and elongation requirements for both grades at room temperature. The strength difference only becomes relevant at elevated temperature, where 347H's controlled carbon floor supports higher creep-rupture allowable stresses.

Q3: Does 347H cost significantly more than 347?

A: Typically only a modest premium, and often less than expected, because many mills melt to a chemistry satisfying both specifications simultaneously. The larger cost factor is the supplemental grain-size or creep-rupture documentation required for full 347H Code qualification.

Q4: What welding filler metal is used for 347 and 347H?

A: Both grades are welded with ER347 or E347 filler metal (AWS A5.9 / A5.4), which carries matching niobium stabilization. The same filler is appropriate for both grades since their base chemistries are otherwise identical.

Q5: How can I verify a mill certificate qualifies material as 347H and not just 347?

A: Check that the reported carbon content falls within 0.04–0.10% (not just below 0.08%), confirm the niobium-plus-tantalum content satisfies the 8×C minimum ratio, and confirm the certificate references either a coarse grain size result (ASTM No. 7 or coarser) or supplemental creep-rupture test data if the material is intended for the highest Part D stress table entries.

Q6: Does JN Alloy / EETA supply dual-certified 347/347H material?

A: Yes. EETA stocks and supplies plate, pipe, tube, fittings, and forgings in both 347 and dual-certified 347/347H, each traceable to heat number with full mill test certification. Contact our technical sales team to confirm current stock and certification options for your project.