Why this guide exists

A hydraulic cylinder that leaks at 220 bar, an automotive shaft that warps after welding, a bearing race that cracks under cyclic load — in a large share of our incoming RFQs, the root cause is not the steel grade. It is the delivery condition of the tube. The buyer ordered E355+N, the mill shipped E355+C and called it "close enough." The drawing called for honed ID Ra 0.4 µm, the mill delivered as-drawn Ra 1.6 µm. The drawing called for a 3.2 MTC, the mill issued a 2.2 test report and a separate chemistry sheet.

For buyers who work with carbon steel pipe for non-pressure, mechanical and hydraulic applications, the standards landscape is wider than it looks. EN 10305-1, EN 10305-4, DIN 2391, JIS G3445, ASTM A519, GB/T 8713 and GB/T 3639 all describe tubes that look similar in a catalogue page but behave very differently in a cylinder bore or a shaft seat. This guide walks through the standards, the grades, the delivery conditions, the inspection points and the mill-direct sourcing path that EZ Steel Industrial has refined since 1994.

1. The standards landscape, mapped to your drawing

If your drawing is stamped to an ISO or DIN norm, you are in the EN 10305 family. If it is stamped to ASTM/ASME, you are in A519. If your OEM customer is a Japanese hydraulic cylinder maker, you will see JIS G3445. If the project ships domestically inside China, you will see GB/T 8713 or GB/T 3639. The table below maps the most common cross-references we see in RFQs.

DIN 2391 has been formally superseded by the EN 10305 series in the EU, but a large share of European buyers — and almost all German and Italian hydraulic cylinder plants — still write "DIN 2391 St 37.4 NBK" on their drawings. Treat DIN 2391 as a legacy designation and map it to the appropriate EN 10305 part and delivery condition before you issue the RFQ.

2. Delivery condition is the single most important line on the RFQ

The same nominal grade (say, E355 or St 52) can be supplied in six different metallurgical conditions. Each one has a different yield strength, a different elongation, a different residual stress level and a different ID surface. A wrong line item on the PO is a wrong tube at the cylinder hone machine.

EN 10305-1 delivery condition code table

3. The four ID surface and tolerance realities

A honed hydraulic cylinder running at 200–350 bar typically needs ID roughness Ra 0.2–0.4 µm and ID roundness under 0.02 mm on the bore. As-delivered cold-drawn tube sits at Ra 0.8–1.6 µm and ID roundness 0.05–0.10 mm. The hone machine is doing the final work — but it cannot fix a tube that is out of round at the mill end. Here is the practical sequence we see working at the cylinder plant:

• Step 1 — mill end: EN 10305-1 E355+N, OD tolerance ±0.10 mm on small sizes, ID tolerance ±0.15 mm, wall tolerance ±10% (standard) or ±7.5% (special).
• Step 2 — mill end (after honing): Customer hone shop reduces ID Ra to 0.2–0.4 µm and corrects ID roundness to < 0.02 mm.
• Step 3 — end-of-line: Chromed ID rod (where used) is a separate part; the cylinder barrel itself is a honed E355+N tube with a honed ID.
• Step 4 — final assembly: Welded-end clevis eye, port drilling, and tie-rod threads are done at the cylinder builder; the tube was supplied as a straight length, not as a finished cylinder.

If you skip the delivery condition conversation and just order "E355 honed," a less rigorous mill may ship +A (GBK) annealed — softer, easier to hone, but the welded end fitting will sink the heat-affected zone into the bore.

4. EN 10305-4 vs GB/T 8713 for hydraulic cylinder service

EN 10305-4 is the European standard for "Seamless cold-drawn tubes for hydraulic and pneumatic power systems — Part 4: Tubes for hydraulic and pneumatic systems." The Chinese equivalent, GB/T 8713, covers "Hydraulic cylinder — Tubes with precise inside diameters." Both specify tight ID tolerances and a normalized or stress-relieved delivery condition, but they differ in three practical areas:

• ID tolerance class: EN 10305-4 offers tighter ID tolerances on the honed product than the GB/T 8713 as-delivered tube (the GB standard expects the hone shop to do most of the work).
• Steel grade coverage: EN 10305-4 explicitly lists E215, E235, E255, E355, E410; GB/T 8713 lists 20#, 35#, 45#, Q345B, 27SiMn. For high-pressure mobile hydraulics, E355+N is the closest cross to Q345B normalized.
• Surface roughness specification: EN 10305-4 calls for a defined as-delivered ID roughness; GB/T 8713 leaves it as a mill-customer agreement. In practice, both are honed by the cylinder builder before final assembly.

If your hydraulic cylinder is going into a European OEM machine, write EN 10305-4 E355+N on the RFQ. If it is going into a Chinese-built machine for domestic use, GB/T 8713 20# or Q345B is acceptable. If the same machine is exported to both regions, run the order as EN 10305-4 to keep one specification and one certificate template.

5. The ASTM A519 reality for North American buyers

ASTM A519 / ASME SA519 covers "Seamless carbon and alloy steel mechanical tubing." SAE 1020 and SAE 1026 are the two most-requested grades we see in A519 RFQs from the US and Canada. SAE 1020 is the default for low-stress bushings, spacers and sleeves. SAE 1026 is the standard for hydraulic cylinder barrels up to about 200 bar, because its higher carbon gives a slightly stronger weld zone.

The common cross-table for A519 mechanical grades:

For mobile hydraulics on North American-built machines, SAE J525 ( annealed, soft for tube flaring) and SAE J524 (hard drawn, for bending) are the two supplementary tube standards usually called out on the drawing alongside A519. They specify the flaring/bending test, surface roughness and hardness band that A519 alone does not.

6. JIS G3445 for the Japan-Korea supply chain

JIS G3445 covers "Carbon steel tubes for machine structural purposes." The grade series is STKM 11A → 12A → 13A → 13C → 14A → 15A → 16A → 17A → 18A, with STKM 11A being the lowest carbon (general machine parts) and STKM 18A the highest. The "A" suffix is aluminum-killed fine-grain steel; "C" (as in STKM 13C) is the quenched-and-tempered variant. STKM 13A and STKM 13C are the two workhorses for agricultural machinery, hydraulic cylinder barrels and drive shafts in Japan, Korea, Thailand and Indonesia.

For honed cylinder service, JIS G3445 STKM 13C + SR is the common choice. For general machine frames and brackets, STKM 11A or STKM 12A as-rolled or +A annealed is the default. EZ Steel Industrial supplies the full STKM grade range with JIS-compliant test certificates and full chemical / mechanical traceability.

7. The certificate question: EN 10204 3.1 vs 3.2

For mechanical and hydraulic service, the inspection certificate you receive with the shipment is as important as the tube itself. The two certificate types you will see are:

• EN 10204 3.1 (formerly 3.1.B): Mill-issued test report, validated by the mill's authorized inspection representative independent of production. Accepted by most OEM and Tier-1 hydraulic cylinder builders.
• EN 10204 3.2 (formerly 3.1.C / 3.2): Mill-issued report validated by an independent third party (the buyer's inspector, a notified body like TÜV, BV, DNV, SGS, or Lloyd's). Required for pressure-service components under PED, for nuclear, and for many oil & gas projects.

For honed hydraulic cylinder service sold into agricultural, construction and material handling machines, 3.1 is the default and is what we ship. For pressure-bearing service, sour service, nuclear, or any project with a third-party inspector on site, we issue 3.2 with the inspector's stamp. The certificate must show: heat number, chemical composition, mechanical properties (tensile, yield, elongation), impact test (if specified), hardness (if specified), NDT results (if specified), and dimensional report.

8. Common sourcing mistakes and how to avoid them

Mistake 1 — Ordering a grade, forgetting the delivery condition

"We need E355 cold drawn" is not a complete specification. Add the EN 10305-1 suffix: +C, +LC, +SR, +A, +N or +Q. The same grade in +C versus +N can swing elongation from 6% to 22%.

Mistake 2 — Specifying the wrong ID tolerance band

EN 10305-1 offers three tolerance classes per table. For honed cylinder service, the tightest class is required. For a bushing or sleeve where the part will be reamed anyway, the standard class is fine. The cost difference is real, and so is the rework cost of getting it wrong.

Mistake 3 — Treating cold-drawn and honed as the same product

"Cold drawn" describes the manufacturing process (cold reduction over a mandrel or plug). "Honed" is a downstream finishing operation. Most honed tubes you will receive are cold drawn first, then honed. The mill's job is to deliver the as-drawn tube to a defined ID tolerance and surface condition. The cylinder builder's hone shop finishes the bore.

Mistake 4 — Ignoring the end-fitting weld procedure

If the cylinder builder will weld a SAE 1026 clevis eye to a SAE 1026 tube barrel, the tube must be in +N or +SR condition. Welding a +C hard-drawn tube to a fitting pulls the heat-affected zone down into the bore; the cylinder will warp. Specify +N on the RFQ, and the clevis welding step becomes a non-event.

Mistake 5 — Bundling mechanical tube with pressure pipe in one RFQ

The standards, the test requirements, the certificates and the mill heat treatment paths are different. Bundling them in a single RFQ for a "discount" usually costs you more in rework and audit time than it saves. Keep them as separate line items with their own specification, certificate and inspection plan.

9. Mill-direct sourcing from EZ Steel Industrial

EZ Steel Industrial has been producing carbon and carbon-alloy carbon steel pipe and tube since 1994 out of a 500+ person facility in Changsha, China, with annual output above 480,000 tonnes and full-cycle control from billet to finished tube. For cold-drawn mechanical and hydraulic tube, our production chain runs:

• Steel sourcing: Qualified round billet from EAF + LF + VD producers; full ladle analysis on every heat.
• Hot piercing & hot rolling: Continuous mill tube to hot-rolled hollows at the required OD and wall.
• Cold drawing: Cold reduction over plug or mandrel on draw benches to the final cold-drawn OD / wall and surface.
• Heat treatment: Stress-relief, normalizing, annealing or quenching-and-tempering furnaces, selected by the delivery condition on the PO.
• Straightening & cutting: Rotary straightener + cold saw cut to length, 0.5 mm/m typical straightness on cold-drawn tube.
• Inspection & testing: Hydrostatic test (where specified), eddy current or ultrasonic NDT, hardness survey, ID surface roughness, OD/ID roundness, dimensional report.
• Marking, packing, export: Each tube ink-marked with grade, size, heat number and standard; bundle-packed or crated per ISPM 15 export rules; MTC issued as EN 10204 3.1 by default, 3.2 on request.

Because we control the cycle, we can hold tighter heat-to-batch chemistry and mechanical consistency than a stockist pulling from two or three unnamed mills. For a hydraulic cylinder builder running thousands of honed barrels per month, that chemistry consistency is the difference between a stable honing cycle and a daily adjustment.

10. How to write the RFQ so the mill can answer in 24 hours

A precise RFQ eliminates a round of clarification email and gets you a faster, firmer quote. Use this eight-line template:

1. Standard and grade (e.g. EN 10305-1 E355+N, or JIS G3445 STKM13C+SR, or ASTM A519 SAE 1026+SR)
2. OD × wall × length (with tolerance class, e.g. OD ±0.10 mm, wall ±10%, length ±5 mm)
3. Delivery condition code (+C, +LC, +SR, +A, +N, +Q)
4. Quantity (pieces or tonnes; specify heat lot size if relevant)
5. Inspection plan (EN 10204 3.1 or 3.2, NDT method, hydro test, mechanical tests, impact if required)
6. End finish (plain end, beveled, swaged, threaded)
7. Marking & packing (ink marking template, bundle or crate, ISPM 15, seaworthy export packing)
8. Delivery terms (port of destination, Incoterms 2020, target month, drawing reference)

Send the drawing, the project specification and the inspection plan with the RFQ. Most of our cold-drawn mechanical tube RFQs are turned around with a firm quote within one working day, with samples shipped from stock within five working days and production lots within 25–35 days depending on grade and delivery condition.

11. Beyond the tube: connecting to pipe fittings and steel flanges

A honed cylinder barrel is rarely the only part on the order. Most hydraulic power units and skid-mounted compressor packages also need a forged or welded pipe fittings set (DIN 2353 compression fittings, SAE J514 O-ring boss fittings, JIS B2351 37° flare fittings), plus steel flanges at the pump inlet, valve block and reservoir connection. Bundling the tube, fittings and flanges under one purchase order has three practical benefits:

• One MTC pack per shipment, one customs entry, one incoming-inspection audit.
• Coordinated lead time — the tube, the fittings and the flanges arrive on the same truck.
• Material traceability link — same project, same heat-number cross-reference if the OEM needs it for a quality claim.

For skid builders, EPC contractors and OEM hydraulic power-unit shops, this is where mill-direct sourcing from a single bundled supplier pays for itself.

12. Common questions we receive in technical RFQ reviews

Can E355+N be hot-dip galvanized after honing?

Yes, but the galvanizing bath temperature (≈ 450 °C) will temper the normalized microstructure and may ｄｒｏｐ the yield strength by 5–10%. If the cylinder builder is going to galvanize the finished assembly, the spec should be written as "galvanizing-compatible" +N, or the galvanizing should be done before final honing.

What is the typical honing allowance on EN 10305-1 honed tube?

Allow 0.15–0.30 mm of stock on the ID for honing, depending on the starting ID tolerance. For a tight 50 mm ID honed to Ra 0.2 µm, request ID tolerance ±0.10 mm from the mill and plan 0.20 mm of honing stock.

Is +SR (stress-relieved) the same as +N (normalized)?

No. +SR is a sub-critical stress relief that lowers residual stress without changing the grain structure. +N is a full austenitize-and-normalize that refines the grain and gives a tougher, more uniform microstructure. For welded-end cylinders, +N is the safer choice. For drive shafts that will not be welded, +SR is usually sufficient.

Can ASTM A519 SAE 1026 and EN 10305-1 E255 be used interchangeably?

In low-pressure mechanical service, yes. In pressure-bearing or welded hydraulic cylinder service, no — the carbon, manganese and residual element limits differ, and the weld procedure must be qualified to one of them, not both. Pick one standard and stay on it for the life of the part.

What is the smallest OD you can cold-draw?

Down to 6 mm OD with 1 mm wall is routine; 4 mm OD with 0.5 mm wall is achievable but is at the limit of draw bench control. For sub-6 mm OD, welded-and-drawn tube (EN 10305-2) is often a more practical choice than seamless.

13. The bottom line

A cold-drawn seamless carbon steel pipe for a hydraulic cylinder barrel, a drive shaft, a bearing sleeve or an injection-molding barrel is a defined engineering product, not a commodity. The grade on the drawing is the start of the specification, not the end. The delivery condition, the ID tolerance, the surface roughness, the certificate type, the NDT scope and the end-fitting weld plan are all part of the same conversation. Get those right on the RFQ and the mill quote comes back fast, the lot arrives on time, and the honed bore does not surprise you at the cylinder test stand.

EZ Steel Industrial supplies the EN 10305, DIN 2391 legacy, JIS G3445, ASTM A519, GB/T 8713 and GB/T 3639 cold-drawn carbon and alloy mechanical tube range from a single mill in Changsha, with EN 10204 3.1 / 3.2 certification, full NDT, hydrostatic testing and project-level bundle packing for OEM, EPC and skid-builder customers worldwide.