Grade 347H is a niobium-stabilized austenitic stainless steel - UNS S34709, WNR 1.4961 - manufactured to ASTM A312 (and related standards like A358, A249, and A409) in welded form. The "H" suffix designates a controlled carbon range of 0.04–0.10%, which is the single most important distinction from standard Grade 347 (S34700, carbon ≤ 0.08%).
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| Property | 347H (S34709) | 347 (S34700) | 321H (S32109) |
|---|---|---|---|
| Carbon range | 0.04–0.10% | ≤ 0.08% | 0.04–0.10% |
| Stabilizing element | Nb (≥ 10×C) | Nb (≥ 10×C) | Ti (5×C min) |
| Weld pool stability | Excellent | Excellent | Moderate (Ti burn-off risk) |
| Creep strength at 650°C | Superior | Moderate | Good |
| Sensitization resistance after welding | Excellent | Good | Good |
| Typical max service temp | ~870°C | ~870°C | ~816°C |
For welded pipe, the combination of niobium stabilization and controlled carbon is the metallurgical sweet spot. You get the weldability of a stabilized grade without the titanium burn-off risk, and you get the elevated-temperature creep resistance that justifies the premium over 304H.
Which ASTM Specification Do You Actually Need?
One of the most common specification errors we see in procurement packages is writing "ASTM A312 TP347H" when the application actually calls for a different standard. Here's how these specifications divide the work:
ASTM A312 / ASME SA-312 - The default for seamless and welded austenitic stainless steel pipe. Covers ERW (electric resistance welded) and EFW pipe in sizes from 1/8" to 30" NB. If your datasheet just says "347H welded pipe" without further qualification, this is almost certainly what's intended. Most refinery and power plant general service piping falls here.
ASTM A358 / ASME SA-358 - Electric fusion welded pipe with filler metal, intended for high-pressure and high-temperature service. This is the spec you need for larger diameters (typically 14" and above) or thicker walls where ERW isn't feasible. A358 pipe is supplied in five classes (Class 1 through Class 5) that define the radiographic examination level and whether the weld is fully x-rayed. For critical service - say, a main steam line in a combined cycle plant - you'd specify A358 Class 3 or higher, which mandates 100% radiography of the weld seam.
ASTM A249 / ASME SA-249 - Welded tubes for boiler, superheater, heat exchanger, and condenser service. The distinction between "pipe" and "tube" here is functional, not just dimensional: A249 tubes are smaller (typically 1/8" to 5" OD) and are subject to tighter tolerances, mandatory eddy current or hydrostatic testing, and a flattening test on the weld. If you're buying 347H welded superheater tubes for a boiler package, A249 is the correct call.
ASTM A409 - Large-diameter welded pipe (typically 14" to 30") for corrosive or high-temperature service in chemical, petroleum, and related applications. Less commonly specified than A358 but relevant for large-diameter process lines in petrochemical complexes.
EN 10217-7 - The European counterpart for welded stainless steel pressure tubes. If your project is in the EU and falls under the Pressure Equipment Directive (PED 2014/68/EU), the pipe must be supplied with EN 10204 3.1 certificates and may need a PED-compliant Material Certificate from a Notified Body. We'll cover this in the Europe section below.
| Standard | Form | Size range | Typical application | NDT requirement |
|---|---|---|---|---|
| ASTM A312 | ERW / EFW pipe | 1/8" – 30" NB | General process piping | Hydrostatic or NDE |
| ASTM A358 | EFW pipe (filler) | 8" – 30"+ | High-pressure, large diameter | Class-dependent radiography |
| ASTM A249 | Welded tube | 1/8" – 5" OD | Boiler, superheater, heat exchanger | Eddy current + flattening test |
| ASTM A409 | Welded pipe | 14" – 30" | Chemical process lines | Hydrostatic |
| EN 10217-7 | Welded tube | Various | EU PED pressure equipment | Per PED category |
Welded vs. Seamless: The Real Cost-Performance Decision
The seamless-vs-welded debate for 347H is not about which is "better." It's about matching the manufacturing method to the service conditions, the size range, and the budget.
When welded 347H pipe is the right choice:
- Large diameters (14" and above): Seamless pipe above 16" is rare and prohibitively expensive. EFW welded pipe is the standard for large-diameter process lines in refineries and petrochemical plants.
- Tight wall thickness tolerances: ERW pipe, because it's formed from precision-rolled strip, typically holds ±10% wall tolerance compared to ±12.5% for seamless (per ASTM A312). For heat exchanger tubes where thermal efficiency depends on consistent wall thickness, this matters.
- Cost-sensitive applications at moderate pressures: Welded 347H pipe typically runs 15–25% below the price of equivalent seamless material. For a project buying 200 tonnes of pipe, that's a six-figure delta.
- Faster lead times for standard sizes: ERW production from coil is a continuous process. A mill with coil in stock can produce and ship standard sizes in 2–3 weeks, versus 6–10 weeks for seamless extrusion.
When you should not accept welded 347H:
- ASME B31.3 "Category M" fluid service (highly toxic)
- Cyclic service with severe thermal shock (rapid temperature transients > 200°C)
- Any application where the client's specification explicitly prohibits welded pipe - and some do, particularly in sour service (NACE MR0175) environments where HIC testing of the weld seam is required
The weld seam itself is the key variable. A properly fabricated 347H welded pipe - with full penetration, internal bead control, solution annealing after welding, and 100% radiography - will perform identically to seamless in most high-temperature service. The problem is that "properly fabricated" is doing a lot of work in that sentence. We'll cover the quality control checkpoints below.
Quality Control Checkpoints That Actually Matter for 347H Welded Pipe
Most generic QC checklists for stainless steel pipe are copy-pasted from 304/316 specifications and miss the checkpoints that matter specifically for 347H welded pipe. Here's what a competent inspection plan should cover:
1. Chemical Composition Verification
The carbon content must fall within 0.04–0.10%. This sounds obvious, but we've seen mills produce heats that sit at 0.038% - technically "347" territory, not "347H." At the low end of the carbon range, the creep advantage over standard 347 disappears. At the high end (> 0.10%), you risk sensitization during welding because there aren't enough Nb atoms to form NbC with all the available carbon. The Nb/C ratio should be ≥ 10, but some specifications allow up to 1.0% Nb maximum regardless of carbon. Check both the absolute carbon value and the Nb/C ratio on the MTC.
2. Weld Seam Integrity
For ERW pipe: the weld seam should be normalized or solution-annealed after welding. A cold-welded ERW seam in 347H is a liability - the HAZ will have sensitized grain boundaries that will corrode under thermal cycling. Verify that the heat treatment temperature and cooling method (water quench) are recorded on the MTC.
For EFW pipe (A358): the weld seam must be 100% radiographed for Class 3 and above. Review the radiographic reports and ensure that acceptance criteria match the project specification (typically ASTM E273 for ERW, ASME Section VIII Article 4 for EFW).
3. Intergranular Corrosion Testing
ASTM A262 Practice E (Strauss test) is the standard method for evaluating intergranular corrosion resistance in stabilized stainless steels. For 347H welded pipe, the test should be performed on a specimen that includes the weld seam and HAZ - not just base metal. A base-metal-only test will pass even if the weld HAZ is sensitized.
Some clients (particularly in the nuclear and refinery sectors) require ASTM A262 Practice C (Huey test), which is more aggressive. Confirm which practice is specified before ordering.
4. Mechanical Testing at Elevated Temperature
Standard ASTM A312 requires tensile and yield testing at room temperature. For 347H pipe going into high-temperature service, specify elevated-temperature tensile testing (at the design temperature, typically 550–650°C) and, if the service is critical, stress-rupture testing per ASTM E139. These tests are not standard and must be explicitly added as supplementary requirements (S-requirements) in the purchase order.
5. PMI on Every Pipe
For Middle East projects, 100% PMI is non-negotiable. But PMI only tells you the alloy chemistry - it won't distinguish 347 from 347H because the difference is in carbon content, which handheld XRF or OE spectrometers can't reliably measure. PMI confirms you have a niobium-bearing 347 family grade; the carbon content verification must come from the MTC. This is why some Aramco projects require both PMI and a review of the mill's heat analysis certificate for every pipe in the shipment.
6. Dimensional and Visual Inspection
Check the weld reinforcement height (typically ≤ 1.5 mm for ERW, ≤ 3 mm for EFW per ASME), ovality (≤ ±1% of OD for pipe ≤ 4", per ASTM A312), and wall thickness at a minimum of three points per pipe - including directly on the weld seam. Undercut on the weld seam's external surface is a rejectable defect if it exceeds 0.8 mm or 10% of wall thickness, whichever is less.
Frequently Asked Questions
Q: Can 347H welded pipe replace 321H in high-temperature service?
In most cases, yes. 347H offers equivalent or superior creep resistance, and its niobium stabilization is more reliable in the as-welded condition than 321H's titanium stabilization. The main reason to prefer 321H is cost - 321H is typically 8–12% cheaper because ferro-titanium is less expensive than ferro-niobium. If the service temperature is below 540°C and the specification allows either grade, 321H may be the economical choice. Above 540°C, particularly in cyclic service, 347H is the better engineering decision.
Q: What's the maximum diameter for ERW 347H pipe?
Most ERW lines can produce up to 24" (610 mm) OD, though availability above 16" is limited because fewer mills maintain the tooling. Above 24", or for wall thicknesses beyond SCH 80S, EFW (A358) is the standard manufacturing method. For diameters above 30", fabricated pipe made from rolled plate with longitudinal seam welds may be the only option.
Q: Does 347H welded pipe require post-weld heat treatment?
No. The niobium stabilization prevents sensitization during welding, so post-weld solution annealing is not required for corrosion resistance. However, if the pipe is cold-bent after welding (for example, into a U-tube for a heat exchanger), a stress-relief or solution anneal may be specified to restore dimensional stability. Follow the project's welding procedure specification (WPS) and the applicable construction code (ASME B31.3, EN 13480, etc.).
Q: How do I verify that a pipe marked "347H" actually meets the H-grade carbon range?
Check the heat analysis on the MTC. The carbon content must be 0.04–0.10%. If the MTC shows carbon at 0.03% or below, the material is standard 347, not 347H, regardless of the marking. PMI (XRF or OE) cannot measure carbon, so the MTC is the only documentation that proves the H-grade designation. For critical applications, send a sample to a laboratory for combustion analysis (LECO) to independently verify the carbon content.
Q: What surface finish is standard for 347H welded pipe?
The default is pickled and passivated (annealed and pickled, or "AP" finish). This removes the oxide scale formed during solution annealing and restores the passive chromium oxide layer. For process piping in refinery and petrochemical service, AP finish is standard. For heat exchanger tubes, a bright annealed (BA) finish may be specified for tighter surface cleanliness. For architectural or food-grade applications, polished finishes (240 grit, 320 grit, or mirror) are available but rarely needed for 347H's typical service.
Q: What is the lead time for 347H welded pipe from Chinese vs. Indian mills?
Chinese mills with 347H coil stock can typically produce and ship standard sizes (2"–12" SCH 40S/80S) in 3–4 weeks. Non-standard sizes or EFW pipe requiring plate procurement add 2–3 weeks. Indian mills typically quote 6–10 weeks for standard sizes and 10–14 weeks for non-standard, but they often offer more flexible payment terms and may have lower minimum order quantities. For Middle East destinations, total door-to-door lead time including ocean freight and customs clearance is typically 6–8 weeks from China and 8–12 weeks from India.
Q: Can 347H welded pipe be used in sour service (H₂S-containing environments)?
NACE MR0175 / ISO 15156 does not specifically list 347H, but it does cover austenitic stainless steels with the applicable chemical composition and hardness requirements. 347H can qualify for sour service if the hardness is ≤ 22 HRC (per NACE MR0175 Table A.2) and the material is in the solution-annealed condition. However, many operators prohibit welded pipe in sour service unless the weld seam is 100% NDT-examined and the HAZ hardness is verified. Always check the specific operator's sour service specification before specifying 347H welded pipe for H₂S service.
