JIS G 3461 Carbon Steel Tube Buyer's Guide: STB340, STB410 and STB510 for Boilers, Heat Exchangers and Mill-Direct Sourcing

1. What JIS G 3461 actually covers

JIS G 3461 is the Japanese Industrial Standard for carbon steel tubes used in boilers and heat exchangers. The current edition is JIS G 3461:2023, which replaced the 2019 revision; the 2023 ｕｐｄａｔｅ tightened the chemical ranges, clarified the heat-treatment table and aligned the test methods with the rest of the JIS G 3460 family. The standard covers both seamless and electric-resistance-welded (ERW) carbon steel tubes intended for service at elevated temperature and pressure — typically economizer inlet lines, superheater tubing, water-wall panels, air-preheater connections, and the tube bundle of shell-and-tube exchangers.

Three boundary conditions matter when buyers first meet the standard:

In other words, the moment your datasheet says "boiler," "superheater," "economizer" or "shell-and-tube heat exchanger" and the medium is water / steam on the tube side, JIS G 3461 is the standard your tube should be quoted to.

2. The three grades: STB340, STB410, STB510

JIS G 3461 defines exactly three strength grades, and the number in the grade name is the minimum tensile strength in MPa. There is no "STB360," no "STB420" — these are the only three options the standard offers.

2.1 STB340 — the workhorse for low-to-medium pressure boilers

STB340 is the most common grade in the family. With a minimum tensile strength of 340 MPa and a minimum yield of 175 MPa, it covers the bulk of fire-tube boilers, auxiliary heat exchangers, and lower-temperature superheater sections. It is the grade most stockists carry on the shelf and the grade that most "carbon-steel boiler tube" RFQs in the Asian market default to.

2.2 STB410 — for higher-pressure and higher-temperature service

STB410 lifts the minimum tensile to 410 MPa and the minimum yield to 250 MPa. It is the grade typically specified for water-wall tubes, economizer inlet headers, and the higher-pressure side of industrial boilers. When a Japanese utility or EPC writes "JIS G 3461" without a grade suffix on a power-plant RFQ, it is almost always STB410 they actually mean.

2.3 STB510 — for high-temperature headers and severe service

STB510 takes the minimum tensile to 510 MPa and is reserved for high-temperature headers, superheater outlet sections and other locations where the combination of internal pressure and elevated metal temperature would over-stress an STB340 or STB410 tube. STB510 is the least-stocked of the three and is almost always a mill-direct inquiry rather than a stockist call.

3. Chemical composition across the three grades

The JIS G 3461 chemistry is intentionally narrow — these are carbon-manganese steels, with low residual P and S, and no deliberate alloy addition. The 2023 edition tightened the upper limit on phosphorus in some thicknesses and clarified the silicon range.

Two things worth noting in the table. First, the carbon cap for STB340 and STB410 is identical at 0.18% — the strength difference between the two comes from the wider Mn range in STB410 and from the heat-treatment condition, not from carbon. Second, STB510 allows a higher carbon ceiling (0.25%) to reach the 510 MPa tensile floor; that is the metallurgical reason STB510 is also the grade most often supplied in the normalized condition.

4. Mechanical properties and how they map to ASTM

The mechanical table is the part of JIS G 3461 that gets quoted into purchasing documents most often, and the part that frequently causes confusion when buyers cross-walk to ASTM.

The ASTM cross-walk most B2B buyers actually need looks like this:

If you have a project that mixes JIS and ASTM datasheets — common in Southeast-Asian combined-cycle plants built under Japanese EPC licenses — the cross-walk above will let you keep both specs honest without ordering two separate batches.

5. Manufacturing routes: seamless vs ERW, hot vs cold

JIS G 3461 lets you buy the same grade in four different process routes, and the way the tube is made has real consequences for surface finish, dimensional tolerance and price. The 2023 edition keeps the same symbol convention as the 2019 version, which is genuinely useful when you read old datasheets.

A practical rule of thumb: if the tube will see internal pressure above about 10 MPa or if the design code requires 100% radiographic / ultrasonic inspection of the weld line, specify the seamless route. For lower-pressure economizer and air-preheater service, ERW is acceptable and noticeably cheaper.

6. Heat treatment: why it matters for STB510 in particular

The heat-treatment matrix is the part of JIS G 3461 that has changed most across recent editions, and the part that decides whether a delivery is acceptable for high-temperature service. STB340 and STB410 are typically delivered in the as-manufactured condition with a low-temperature anneal or normalize as needed, while STB510 must be normalized across all process routes. The 2023 edition tightened the normalize temperature window for STB510 to keep the grain size consistent.

In practice, if you are buying STB510 you should always request the heat-treatment record on the MTC and confirm the normalize temperature and the hold time. The cost difference between "as-rolled" and "properly normalized" STB510 is meaningful, and a shipment that arrives as-rolled when the datasheet says normalized is one of the most common acceptance failures we see.

7. Cross-walks you will actually need on a B2B RFQ

Most JIS G 3461 RFQs do not arrive alone. They arrive next to an EN spec, a GB spec, or an ASTM spec on the same project, and the buyer needs to know whether the same heat can satisfy two or three of them. Here is the cross-walk table we hand to procurement teams most often:

If you would like the full size-and-tolerance table for any of these cross-walks, our EN 10216-2 Steel Tube and GB 5310 Steel Tube reference pages give them side by side.

8. Inspection, testing and MTC: what a mill-direct shipment should carry

JIS G 3461 calls out a clear inspection regime that any mill-direct delivery should be able to satisfy without negotiation. The 2023 edition was tightened on hydrostatic test pressure calculation and on the eddy-current / ultrasonic test scope, so older MTC templates may need an ｕｐｄａｔｅ.

On documentation, the standard expects an MTC to EN 10204 3.1 for routine shipments. Where the project specification calls for independent witnessing — power-plant, nuclear auxiliary, refinery heater — you should ask for EN 10204 3.2 with a third-party inspection body (BV, SGS, TUV, etc.) countersigning the certificate.

9. Sizes, tolerances and the unit-mass rule

JIS G 3461 covers outside diameters from 15.9 mm up to 139.8 mm as the table in the standard lists, with wall thicknesses from 1.2 mm to 12.5 mm. For diameters or walls outside that range, most mills will accept a special-order inquiry but you should expect a longer lead time and possibly a different certification basis.

The unit-mass rule in the standard is one of the most-asked-about details: the trading mass for hot-finished seamless tubes is the table value + 15%, cold-finished seamless is table + 10%, and ERW is table + 9%. The 2023 edition clarified that this is a contractual trading rule, not a tolerance on actual mass, so it is the figure you use for freight and customs declarations.

10. Sourcing from a mill-direct manufacturer

The single biggest difference between a mill-direct shipment of JIS G 3461 tube and a stockist resale is documentation. A mill-direct shipment will carry a single MTC tied to a single heat number with full traceability, while a stockist resale often bundles mixed-heat lots under one certificate and may not be able to produce the original heat-number file on request. For a routine fire-tube boiler that is acceptable; for a power-station water-wall panel or a refinery exchanger it is not.

As a mill-direct carbon-steel tube manufacturer, we hold JIS G 3461 in our standard carbon & carbon alloy steel pressure-tube program. Every order is produced to a single EN 10204 3.1 / 3.2 MTC, hydrostatically tested to the JIS formula, and 100% eddy-current tested. We can also supply the matching pipe fittings, pipe flanges and stud-bolt / gasket sets so the boiler outlet piping arrives on one MTC family.

11. Common pitfalls when buying JIS G 3461 from a new supplier

12. Where JIS G 3461 sits in our product program

JIS G 3461 is one of the standards we run as a stock rolling program within our carbon & carbon alloy steel pressure-tube family. It sits next to our high-pressure carbon and alloy steel pipe program, our GB 5310 alloy steel tube creep-resistant line, and our JIS G 3461 / G 3463 boiler and heat-exchanger tube family. If you are specifying a JIS-standard boiler or heat-exchanger bundle, we can quote the tube, the matching BW fittings and the pipe flanges against a single mill package, with one documentation set per heat.