JIS G3445 STKM vs G3441 STAM Mechanical Tubing Standards Comparison

Japanese industrial standards dominate the mechanical tubing supply chain for automotive chassis, hydraulic cylinders, agricultural equipment and precision machinery. Among the dozens of JIS tubing specifications, JIS G3445 (mechanical tubes) and JIS G3441 (alloy steel tubes for machine structural purposes) are the two grades most frequently requested by buyers sourcing from Chinese mills that hold JIS certification.

Despite similar scope statements, G3445 and G3441 differ significantly in chemistry, delivery condition, surface quality, dimensional tolerances and end-use suitability. Confusing the two standards on a purchase order can result in tubes that fail fit-up during assembly, crack during flaring or carry the wrong surface finish for subsequent chrome plating. This article provides a detailed comparison of STKM grades under G3445 and STAM grades under G3441, with practical procurement guidance for buyers who need to choose between them.

1. Scope and Intended Applications

JIS G3445 covers carbon steel tubes for machine structural purposes. The standard is intended for tubes used in static or moderately dynamic load-bearing applications where weldability, dimensional accuracy and moderate strength are the primary requirements. Typical end uses include automotive seat frames, conveyor rollers, bicycle frames, furniture tubing and general machinery guards.

JIS G3441 covers alloy steel tubes for machine structural purposes. The standard addresses tubes that require higher strength, better wear resistance or improved hardenability than plain carbon steel can provide. G3441 tubes are used in drive shafts, axle housings, hydraulic cylinder barrels, precision bushings and other components subjected to torsional or bending fatigue.

The critical distinction is chemistry: G3445 is a carbon-manganese system with no deliberate alloying additions beyond residual elements, while G3441 introduces chromium, molybdenum, boron or combinations thereof to achieve the property targets of each grade. Buyers should not substitute G3445 for G3441 in fatigue-critical applications without a formal design review.

2. Grade System and Chemistry Comparison

JIS G3445 uses the STKM prefix followed by a grade letter (A, B, C, D) and sometimes a heat-treatment suffix (11A, 12A, 12B, 12C, 13A, 13B, 13C). The letter indicates strength level; the numeral indicates delivery condition. JIS G3441 uses the STAM prefix followed by a grade number (28, 29, 30, 32, 33, 34, 35, 36) that correlates with the minimum tensile strength in kgf/mm².

Grade	Standard	C % max	Mn %	Cr %	Mo %	B ppm	Min Tensile (MPa)
STKM 11A	G3445	0.12	0.50 max	—	—	—	290
STKM 12B	G3445	0.20	0.25–0.60	—	—	—	340
STKM 13C	G3445	0.25	0.30–0.90	—	—	—	390
STKM 14A	G3445	0.30	0.30–1.00	—	—	—	410
STAM 29	G3441	0.28	0.30–0.90	0.20–1.00	—	—	440
STAM 30	G3441	0.28	0.30–0.90	0.20–1.00	0.15–0.35	—	490
STAM 32	G3441	0.32	0.30–0.90	0.30–1.20	—	5–20	540
STAM 34	G3441	0.34	0.30–0.90	0.30–1.50	0.15–0.45	5–20	590

Key takeaway: G3441 grades achieve their higher strength through deliberate alloying, not simply by increasing carbon. STAM 34, for example, contains chromium and molybdenum plus trace boron to maintain hardenability in thin-wall sections. This alloying strategy produces a finer microstructure after normalising and yields superior fatigue performance compared with STKM 14A at the same hardness level.

3. Delivery Condition and Heat Treatment

G3445 tubes are supplied in one of three delivery conditions indicated by the numeral suffix:

• Suffix A: As-formed (hot-finished or cold-finished, no heat treatment)
• Suffix B: Stress relieved after cold finishing
• Suffix C: Annealed after cold finishing

Suffix C (annealed) is the default for tubes that will be severely formed, bent or flared in the customer’s factory. Suffix B is preferred for welded assemblies where residual stress control is important but full softness is unnecessary. Suffix A offers the lowest cost but may exhibit ovality and springback issues in precision bending.

G3441 tubes are supplied in the normalized condition as standard. Some grades (STAM 33, 34, 35, 36) can also be supplied in the quenched-and-tempered condition if specified in the purchase order. Normalised G3441 tubes display a fine ferrite-pearlite microstructure with ASTM grain size 8–10, which is finer than typical G3445 tubes and explains their superior toughness and fatigue resistance.

4. Dimensional Tolerances and Surface Quality

Both standards reference JIS G3442 (dimensional tolerances for mechanical tubes), but G3441 imposes tighter limits because alloy tubes are often used in precision-fit applications such as hydraulic cylinders and bearing housings.

Tolerance	G3445 (STKM)	G3441 (STAM)
OD ≤ 30 mm	±0.20 mm	±0.15 mm
OD 30–50 mm	±0.25 mm	±0.20 mm
OD > 50 mm	±0.30 mm	±0.25 mm
Wall thickness	±10 %	±8 %
Out-of-roundness	≤ 60 % of OD tolerance	≤ 50 % of OD tolerance
Surface roughness Ra	≤ 6.3 μm	≤ 3.2 μm

G3441 also requires 100 % eddy-current testing (ECT) or ultrasonic testing (UT) for wall thickness below 3.0 mm, whereas G3445 permits spot-checking at 10 % frequency for tubes above 3.0 mm wall. If your application involves thin-wall tubing (< 2.5 mm) for weight-sensitive structures, specify G3441 tolerances even if the chemistry is carbon-only — the tighter process control reduces fit-up scrap by 30–50 %.

5. Mechanical Property Comparison at Room Temperature

The mechanical-property gap between G3445 and G3441 widens as strength increases. At the low end, STKM 11A and STAM 28 are comparable in tensile strength but STAM 28 offers better elongation due to its controlled normalising practice. At the high end, STAM 36 delivers 690 MPa tensile strength with 15 % elongation — a combination impossible to achieve with plain carbon steel.

Grade	Tensile (MPa)	Yield (MPa)	Elongation (%)	Hardness HV max
STKM 11A	290 min	—	35 min	120
STKM 12B	340 min	175 min	35 min	140
STKM 13C	390 min	215 min	30 min	160
STKM 14A	410 min	245 min	25 min	170
STAM 29	440 min	295 min	25 min	180
STAM 30	490 min	345 min	20 min	200
STAM 34	590 min	440 min	18 min	240
STAM 36	690 min	540 min	15 min	280

Buyers should note that elongation values in JIS standards are measured on No. 11 test pieces (50 mm gauge length, proportional to 14 mm×14 mm cross-section). If your design code uses ASTM E8 test pieces with 50 mm gauge length on 12.5 mm wide strip, the elongation will read 2–4 % higher. Always specify the test-piece geometry in the purchase order to avoid disputes.

6. Weldability and Formability Guidance

G3445 STKM grades are readily welded by gas metal arc welding (GMAW), tungsten inert gas welding (TIG) and resistance spot welding. Preheat is generally unnecessary for wall thickness below 6 mm, but tack welds on STKM 14A should be preheated to 100°C–150°C to avoid HAZ cracking in high-carbon heat-affected zones.

G3441 STAM grades require more care during welding because the alloying elements increase hardenability. STAM 30 and above should be welded with low-hydrogen electrodes (E7018-H4R or equivalent) with preheat of 150°C–200°C and immediate post-weld heat treatment (PWHT) at 620°C–650°C for 1 hour per 25 mm wall. Without PWHT, the HAZ hardness can exceed 350 HV and become susceptible to delayed hydrogen cracking.

For flaring and expansion tests, G3445 Suffix C tubes can withstand a 1.2×OD flare without cracking. G3441 tubes in the normalised condition tolerate 1.1×OD flare. If your application requires tube-end flaring to form hydraulic-seal interfaces, specify G3445 Suffix C or request a special annealing cycle on G3441 tubes.

7. Procurement Checklist for JIS Mechanical Tubing

When drafting a purchase order for JIS G3445 or G3441 tubing, include the following clauses to eliminate ambiguity and reduce inspection disputes:

• Standard and year: JIS G3445:2019 or JIS G3441:2019 (earlier editions have different sulfur limits)
• Grade and suffix: e.g., STKM 13C or STAM 30 (normalized)
• Dimensions: OD × wall × length with tolerances (standard or special)
• Surface finish: Black (as-rolled), pickled, oiled, phosphated or chrome-plated pre-treatment
• Ends: Plain cut, deburred, chamfered 45°, or threaded
• Marking: Grade, heat number, mill logo stamped every 1 metre
• Test reports: Chemical, tensile, flattening, flaring and ECT/UT reports per heat
• Certification: EN 10204 3.1 (mill) or 3.2 (inspection-body witness)
• Third-party inspection: SGS, BV or TÜV witness at mill before shipment
• Packaging: Hexagonal bundle, steel strapping, VCI film, wooden cradle for sea freight

8. FAQ

Q1: Can G3445 STKM 13C replace G3441 STAM 29 in a drive-shaft application?

No. STAM 29 has 27 % higher yield strength (295 MPa vs 215 MPa) and better fatigue resistance due to its fine normalised microstructure. Substituting STKM 13C would reduce drive-shaft life by 40–60 % under torsional fatigue loading. Specify the correct grade from the beginning.

Q2: Is JIS certification mandatory for Chinese mills supplying G3441?

JIS certification is not legally mandatory for export from China, but Japanese OEMs and Tier-1 suppliers typically require it. Mills without JIS certification can still produce to G3441 chemistry and property targets, but they cannot stamp the JIS mark on the tubes. For non-Japanese end markets, specify “to JIS G3441 chemical and mechanical requirements” without requiring the JIS mill mark.

Q3: What is the price premium of G3441 over G3445?

G3441 tubes cost 15–30 % more than equivalent-strength G3445 tubes because of alloying-element additions, tighter process control and 100 % NDE coverage. For STAM 34 vs STKM 14A, the premium is typically $180–$280 per metric ton. The premium is justified in fatigue-critical applications where field failures would cost far more than the material savings.

Q4: Can G3441 tubes be chrome plated directly?

Yes, but surface preparation is critical. Normalised G3441 tubes have a surface roughness of 3–6 μm Ra, which is too coarse for direct hard-chrome plating. Specify a fine-turning or centreless grinding operation to achieve 0.4–0.8 μm Ra before plating. Alternatively, request a phosphated pre-treatment from the mill to improve chrome adhesion.

Q5: How do I verify the JIS certificate is authentic?

Request the mill’s JIS registration number and verify it on the Japan Iron and Steel Federation (JISF) website or through the certification body (JQA, JICQA or equivalent). For EN 10204 3.2 certificates, the third-party inspection body’s accreditation (ISO 17020) should also be checked on the body’s website.

Conclusion

JIS G3445 STKM and JIS G3441 STAM mechanical tubing serve different segments of the machinery and automotive markets. G3445 is the cost-effective choice for static structures, furniture and general fabrications where moderate strength and excellent weldability are sufficient. G3441 is the fatigue-resistant choice for drive trains, hydraulic systems and precision mechanisms where alloying additions deliver the strength and toughness that plain carbon steel cannot achieve.

At Huaxia-Steel, we stock and source both JIS G3445 and G3441 mechanical tubing from JIS-certified mills with full dimensional inspection, chemical verification and mechanical testing. Contact our sales engineers for grade-selection advice, tolerance optimisation and competitive quotations for your next mechanical tubing project.