JIS G3461 STB340S is a seamless carbon steel tube specified under the Japanese Industrial Standard for boiler and heat exchanger service, with a minimum tensile strength of 340 MPa and a minimum yield strength of 175 MPa. The "S" designation in STB340S denotes the seamless product route. Within JIS G3461, both hot-finished and cold-drawn seamless variants are permitted, but the cold-drawn route is the practical default when dimensional consistency at the tube-to-tubesheet interface matters.
The chemistry is deliberately lean: carbon capped at 0.18%, manganese at 0.30–0.60%, and phosphorus and sulfur each at 0.035% maximum — a composition that prioritizes weldability and forming behavior over elevated-temperature strength.
The parameter that most directly governs fitness-for-service is not the tensile number but the continuous metal temperature ceiling. JIS G3461 STB340S is a low-to-medium pressure carbon steel; once continuous service approaches approximately 450 °C, creep strength becomes the controlling limit and a chromium-molybdenum grade should be evaluated instead. This is a standard-boundary statement, not a marketing qualification.
| Parameter | JIS G3461 STB340S | STB410 (same standard, for contrast) |
|---|---|---|
| Min. tensile strength | 340 MPa | 410 MPa |
| Min. yield strength | 175 MPa | 255 MPa |
| Min. elongation (OD ≥ 20 mm) | ≥ 30% | ≥ 30% |
| Typical service role | Economizer, low-pressure waterwall, shell-and-tube bundles | Subcritical superheater, condensate return |
Table data drawn from JIS G3461 grade envelopes.
In a fixed-tubesheet or U-tube bundle, the recurring failure mode is not bulk rupture — it is progressive wall loss at the tube-to-tubesheet joint and mid-span vibration wear. Cold-drawn STB340S tubes address this through dimensional discipline rather than alloy upgrading.
When the outside diameter tolerance is held to the SA-450/SA-450M framework — for example, ±0.10 mm on OD below 1 in, and +20%/0% on wall thickness up to 1½ in — the radial clearance between tube and tubesheet hole is tightened. This reduces the crevice geometry that accelerates differential aeration corrosion and improves the repeatability of roller-expansion joints. In economizer and low-pressure waterwall service, the same tight wall control supports predictable heat-transfer surface area without resorting to thicker, heavier tubes.
Tubes are manufactured and inspected to JIS G3461:2019, the current edition, with dimensional and testing requirements aligned to ISO 9329-1. Non-destructive examination options include eddy current, ultrasonic, and hydrostatic testing. Yuhong Group supplies STB340S under a quality system covering ISO 9001, with third-party certification available to ABS, DNV, GL, NK, PED, and AD2000 upon project request.
The base grade is STB340; the "S" suffix in mill documentation typically identifies the seamless delivery condition. Always confirm the product route — hot-finished versus cold-drawn — on the mill test certificate, because the dimensional tolerance and surface condition differ between the two.
For continuous metal temperatures approaching 450 °C, carbon steel creep strength becomes marginal; JIS G3461 does not position STB340S as a high-temperature creep-grade tube. Above that envelope, evaluate a Cr-Mo specification rather than extending the STB340S rating.
Require the JIS G3461 mill test certificate with heat number, chemistry, tensile and yield results, elongation, and the NDT method actually performed. For export projects, confirm whether third-party inspection and hydrostatic test reports are required at the time of inquiry.
Tubes are supplied in plywood cases, iron cases, or bundles with plastic end caps, sized for container or breakbulk shipment.
For a firm quotation, specify OD × WT, total quantity, product route (hot-finished or cold-drawn), NDT requirements, and destination port. MOQ, FOB basis, and lead time are confirmed against the final dimensional schedule — MOQ is quantity-dependent, FOB terms are quoted from the nominated Chinese load port, and lead time is allocated after drawing and inspection requirements are locked.
