JIS G3445 steel tubes are widely specified for machine structural applications, but a grade name on a certificate isn’t enough to guarantee performance. Over years of verifying cold drawn tube shipments, I’ve learned that dimensional precision and heat treatment documentation matter as much as the chemical analysis. This guide moves beyond standard property tables to give engineers and buyers a practical path for comparing, selecting, and sourcing JIS G3445 tubing with confidence.
JIS G3445 Grade Overview and Key Specifications
JIS G3445 covers carbon steel tubes intended for machine structural parts such as shafts, gears, and transmission components. The standard is built around a series of STKM grades that balance strength, ductility, and weldability. The table below lists the commonly supplied grades and their mechanical requirements.
| Grade | Tensile Strength (MPa) | Yield Point (MPa) | Elongation (%) | Typical Hardness (HB) |
|---|---|---|---|---|
| STKM 11A | ≥ 290 | ≥ 175 | ≥ 35 | 100–145 |
| STKM 12A | ≥ 370 | ≥ 215 | ≥ 30 | 115–160 |
| STKM 13A | ≥ 440 | ≥ 255 | ≥ 25 | 130–175 |
| STKM 14A | ≥ 510 | ≥ 295 | ≥ 20 | 150–195 |
Note: Chemical composition limits (C, Mn, P, S) tighten progressively across the grades, and most tubes are supplied in the as‑drawn or normalized condition unless otherwise specified.
In practice, the three most commonly ordered grades are STKM 12A and STKM 13A for general mechanical parts, and STKM 14A when higher strength is needed. Standard size ranges from our mill run from 10 mm to 108 mm OD, with wall thicknesses between 1 mm and 20 mm. Dimensional tolerance on the outer diameter is held to ±0.1 mm through cold drawing, which meets or exceeds the JIS nominal limits for mechanical tubes.
Cold Drawing and Heat Treatment: Performance Drivers
The properties of a JIS G3445 tube are not solely decided by the melt analysis; they are shaped by the cold drawing reduction and the heat treatment cycle that follows. During cold drawing, the wall is compressed and the grain structure elongates, raising tensile strength and hardness while lowering elongation. A well‑controlled drawing pass can add 10–15% to the tensile figure, but if the reduction ratio is pushed too hard without intermediate annealing, residual stress and uneven hardness become problems.
We have seen STKM 13A tubes that landed at opposite ends of the allowable hardness range simply because one supplier used a single heavy drawing pass and another applied a two‑pass schedule with stress relieving. That variation can affect machinability and fatigue life in the final component. The safest specification is to ask for tubes in the normalised or stress‑relieved condition, which restores uniform grain structure and eases subsequent bending or machining.
If your design operates near the upper limits of the material, or you need to guarantee a tight hardness window, it is worth discussing the drawing and heat treatment route with the supplier before the order is placed. Reach out at [email protected] and we can review your parameters against the achievable mechanical range.
JIS G3445 vs International Standards: A Comparison
Buyers who source tubes globally often need to map JIS G3445 against ASTM A519 (USA), EN 10305‑1 (Europe), and DIN 2391 (Germany). While the grades don’t translate one‑to‑one, the table below gives a practical cross‑reference based on typical tensile strength bands.
| Tensile Range (MPa) | JIS G3445 | ASTM A519 | EN 10305‑1 | Typical Use |
|---|---|---|---|---|
| 290–370 | STKM 11A | MT1010 | E235–E275 | Low‑stress housings, spacers |
| 370–440 | STKM 12A/13A | 1026 | E275–E355 | Shafts, bushings, couplings |
| 440–510 | STKM 13A/14A | 1026/4130 | E355–E460 | Transmission parts, cylinder barrels |
| 510–650 | STKM 14A (Q&T) | 4130 | E460+ | Steering knuckles, high‑load pins |
The biggest divergence is in dimensional tolerances. EN 10305‑1 can demand ±0.05 mm on OD in its premium classes, whereas JIS G3445 typically works with ±0.1 mm. That may sound negligible, but in pilot‑bore assemblies, it can mean the difference between a slip fit and a press fit. When a drawing calls out JIS G3445 but demands EN‑level precision, the supplier must be capable of extending the cold drawing sequence, which adds production passes and cost. We frequently combine a JIS chemical specification with an EN tolerance class for European customers who require both.
A Procurement Engineer’s Checklist for JIS G3445 Tubes
Over the years, I have distilled the verification process into a handful of checks that catch most problems before they reach the production floor:
- Mill Test Certificate (MTC) – Ask for a certificate that lists the actual heat number, chemical analysis, tensile test results, and heat treatment state. A generic “conforms to JIS G3445” statement is not enough.
- Dimensional Verification – Spot‑check OD, wall thickness, and straightness on the first shipment. Hold all dimensions against the drawing; ±0.1 mm is achievable, but if the tube is to be honed later, concentricity matters as much as OD tolerance.
- Heat Treatment Condition – Confirm whether the tubes are as‑drawn, normalised, stress‑relieved, or quenched and tempered. This directly affects machinability and dimensional stability during subsequent manufacturing.
- Surface and Visual Inspection – Look for scale, pitting, score marks, and end‑face squareness. Deep scratches can act as stress risers on a rotating shaft.
- Traceability – Ensure each tube bundle or piece carries a heat number that ties back to the MTC. This is a non‑negotiable for ISO‑certified supply chains.
- Supplier Certification – The mill should hold ISO 9001 at a minimum and be capable of performing Positive Material Identification (PMI) and Non‑Destructive Testing (NDT) when required. A supplier like Tenjan Steel Tube includes PMI and NDT reports as standard with every shipment of JIS G3445 material.
Running through this checklist before accepting a delivery can save days of rework and supplier back‑and‑forth.
From Spec to Supply: Securing Your JIS G3445 Order
The most common pain point buyers share is not finding a source; it is receiving tubes that drift outside tolerance mid‑batch or arrive without the heat treat condition that was promised. Unplanned delays then ripple through machining, assembly, and delivery schedules.
Because JIS G3445 tubes are used in parts like gearbox shafts, suspension components, and hydraulic cylinder glands, the cost of a failed quality lot often dwarfs the tube price itself. Tenjan Steel Tube addresses this by controlling the entire process from raw steel selection through cold drawing, heat treatment, and final inspection, all under a single ISO‑certified quality system. That eliminates the blame‑shifting that can happen when drawing, heat treatment, and inspection are performed at separate subcontractors.
If you are about to issue a purchase order, send your part number, required grade, dimensions, and quantity to [email protected], or call +86 13401309791 for a same‑day quotation. We will also provide a free preliminary review of your material certification requirements before you commit.
Questions Engineers Ask About JIS G3445 Tubes
What is the difference between JIS G3445 and JIS G3444?
JIS G3444 covers general structural carbon steel tubes, while G3445 is written specifically for machine structural parts. The G3445 grades impose tighter controls on tensile strength, yield point, and elongation, and are more often used where dynamic loading, fatigue, or subsequent heat treatment is involved.
Can JIS G3445 tubes be welded?
Yes. The low‑carbon STKM grades (up to STKM 13A) weld readily with standard gas metal arc or resistance welding processes. For STKM 14A, especially in the as‑drawn or higher‑strength condition, a low‑hydrogen procedure and post‑weld stress relief are recommended to avoid cracking in the heat‑affected zone.
What surface finish is normal for JIS G3445 tubes?
The typical delivery condition is a cold‑drawn surface with a light protective oil. The inside surface will show slight drawing marks but should be free of gouges and heavy scale. For parts that require a smoother bore (for example, a bushing press‑fit), an additional honing or polished finish must be specified at the order stage.
How can I verify that a received batch really meets JIS G3445?
Start with the MTC; check that the heat number on the tubes matches the certificate and that the test results fall within the standard’s limits. Then, perform a random dimensional check on 3‑5% of the pieces and a Brinell or Rockwell hardness test on one sample per heat. For high‑criticality parts, a PMI analysis provides the final chemical confirmation. At Tenjan, all JIS G3445 shipments include the PMI report, so there is no need to negotiate for it later. Share your specific requirements and we will confirm what compliance documentation is available before the order ships.
