Cold-drawn precision steel tubes manufactured to DIN 2391 (or its successor EN 10305-1) are the standard material for hydraulic lines, pneumatic circuits, instrumentation tubing, and mechanical guidance systems in industrial machinery, mobile equipment, and process plant. The tight dimensional tolerances — particularly outside diameter to h9 quality — allow the tube to function as a precision sliding or sealing surface without additional machining, and the consistent wall thickness ensures predictable pressure capacity and bend behaviour.
ZC Steel Pipe supplies cold-drawn precision steel tubes to DIN 2391 and EN 10305-1 in grades St 35 (E235), St 45 (E275), and St 52 (E355) across OD sizes from 6 mm to 120 mm. This guide covers the delivery conditions, dimensional tolerances, material grades, pressure ratings, and procurement requirements.
On a Southeast Asia equipment build, hydraulic lines were ordered as "DIN 2391 St 52" for a system with 90° bends at close radius. The PO did not specify a delivery condition. The mill shipped BK (as-drawn, no heat treatment) — their default and lowest-cost condition. At site assembly, the installer attempted to cold-bend the BK tube through a 90° bend on a standard pipe bender. The tube cracked at 45° on 30% of the pieces. BK St 52 has elongation of approximately 6–10% — far below the 20–25% needed for a 90° cold bend at close radius. GBK (bright annealed) has elongation above 30% and would have bent cleanly. The entire batch was scrapped and re-ordered as GBK. One word on the purchase order prevented — or in this case, failed to prevent — 100% material loss.
1. Standard Scope and Successor Standard
DIN 2391 (Seamless Precision Steel Tubes) was the German national standard that defined cold-drawn precision tube for hydraulic and mechanical applications. It has been superseded in the EU by EN 10305-1 (Seamless Cold Drawn Precision Steel Tubes), but continues to be specified in:
- Older machine and equipment drawings (German origin)
- Asian and Middle Eastern procurement specifications that reference DIN
- Replacement parts for legacy machinery originally built to DIN standards
The products are metallurgically equivalent. Both standards specify the same OD tolerances (h9), wall tolerances, delivery conditions, and base steel chemistry. When a customer specifies DIN 2391, supply of EN 10305-1 tube with the same dimensions and grade is generally acceptable and should be confirmed in writing.
2. Material Grades
| DIN 2391 Grade | EN 10305-1 Grade | Material No. | Max C% | Min Yield (MPa) | Min Tensile (MPa) |
|---|---|---|---|---|---|
| St 35 | E195 / E235 | 1.0308 | 0.17 | 195 | 320 |
| St 45 | E275 | 1.0408 | 0.21 | 275 | 410 |
| St 52-3 | E355 | 1.0580 | 0.22 | 355 | 490 |
The suffix "3" in St 52-3 indicates the sub-grade with guaranteed impact toughness. E355 is the equivalent in EN 10305-1. St 52 / E355 is the standard hydraulic tube grade — its combination of yield strength and wall tolerances gives the best pressure-per-kilogram efficiency for hydraulic system piping.
To convert between MPa/ksi, mm/inches, and bar/psi, use the Unit Converter →
3. Delivery Conditions
DIN 2391 specifies four delivery conditions (equivalent EN 10305-1 conditions in brackets):
| Condition | Description | Strength | Ductility | Typical Application |
|---|---|---|---|---|
| BK (+C) | Cold drawn, no heat treatment | Highest | Lowest | Structural, high-strength light wall |
| BKW (+LC) | Lightly cold worked after anneal | Medium-high | Medium | Intermediate |
| GBK (+A) | Cold drawn + bright annealed in HNx | Medium | High | Bending, flaring, forming |
| NBK (+N) | Normalised | As-rolled level | High | Uniform properties |
The DIN 2391 delivery condition is not a quality grade — it is a fundamentally different material state. BK (as-drawn) St 52 has yield strength approximately 35–50% higher than GBK (bright annealed) St 52 of the same OD and wall, and elongation approximately 3× lower. These are not the same tube in different finishes; they behave like different materials under bending load. A system designed with BK tube for straight runs cannot substitute GBK for the same run without recalculating pressure ratings — and a system designed with GBK for field bending cannot substitute BK without expecting field cracking. Delivery condition must be on every purchase order, not treated as a detail to confirm later.
GBK (bright annealed) is the most commonly ordered condition for hydraulic tube that will be bent or end-formed on site. Annealing in a hydrogen-nitrogen atmosphere produces a clean, oxide-free internal and external surface without scale — important for hydraulic circuit cleanliness.
BK (hard, as-drawn) is ordered where the tube is used straight (no bending) and maximum strength from cold work is desired.
4. Dimensional Tolerances
DIN 2391 and EN 10305-1 define three OD tolerance classes:
| Class | OD Tolerance (typical, 10–30 mm) | Application |
|---|---|---|
| D8 (coarser) | ±0.10 mm | Light-duty structural |
| D9 | ±0.052 mm | Standard hydraulic tube |
| H9 (preferred) | +0 / −0.052 mm (one-sided minus) | Sliding fits, cylinder OD guides |
Standard precision tube is supplied to OD tolerance class D9 or h9. For most hydraulic fitting assemblies, D9 is sufficient. For applications where the tube OD functions as a sliding fit or bearing surface, h9 (unilateral minus tolerance) must be specified.
Wall thickness tolerance: ±10% of specified wall (minimum ±0.1 mm) for standard tube. Wall tolerance class T9 applies; T8 (tighter, ±7.5%) can be ordered by agreement.
Straightness: 1.0 mm per metre maximum (standard). For automatic CNC loading or precision assembly: 0.5 mm/m by agreement.
When NOT to Use BK (As-Drawn) Condition Precision Tube
| Scenario | Risk | Correct Approach |
|---|---|---|
| Any tube that will be cold-bent or end-formed on site | BK elongation ~6–10%; cracks at close-radius bends | Specify GBK (bright annealed) for all tube requiring field bending or flaring |
| Hydraulic systems requiring internal cleanliness | BK tube has mill scale that generates particles in service | Specify GBK (scale-free bright annealed) for all hydraulic tube; confirm oil preservation and end caps |
| OD tolerance class not specified | Mill ships D8 (coarser) or D9 — not h9 if sliding fit is required | For sliding fits or bearing surfaces, explicitly specify "OD tolerance class h9" |
| "DIN 2391" without successor standard clarification | Some mills supply EN 10305-1; older drawings may specify DIN 2391 | Confirm in writing that EN 10305-1 is an acceptable equivalent; both are technically the same product |
| Small OD tube (≤12 mm) without end caps and preservation | Fine bore contamination from transit damages servo valves and instrumentation | Specify "cleaned, oil-preserved internally, end-capped, and sealed" for all instrumentation tube |
| St 52 BK in high-cycle fatigue hydraulic circuits | As-drawn surface condition retains cold-work surface stresses that accelerate fatigue crack initiation | For fatigue-critical circuits, specify NBK or GBK which have more uniform surface stress state |
5. Common Size Range
The following sizes represent the most active range of DIN 2391 precision tube in hydraulic service applications (OD × wall in mm):
| OD (mm) | Common WT (mm) | OD × WT |
|---|---|---|
| 6 | 1.0 | 6 × 1 |
| 8 | 1.0 | 8 × 1 |
| 10 | 1.0 | 10 × 1 |
| 12 | 1.5 | 12 × 1.5 |
| 16 | 2.0 | 16 × 2 |
| 20 | 2.0 | 20 × 2 |
| 25 | 2.5 | 25 × 2.5 |
| 30 | 3.0 | 30 × 3 |
| 38 | 3.0 | 38 × 3 |
| 50 | 4.0 | 50 × 4 |
Other OD/WT combinations are available per DIN 2393 preferred dimension tables.
6. Hydraulic Pressure Rating
Working pressure (Pw) can be estimated from:
Pw = (2 × S × t) / (D − t) × (1 / SF)
Where:
- S = allowable stress (MPa) — for St 52/E355 BK: approximately 240 MPa; for GBK: approximately 220 MPa
- t = specified wall thickness (mm)
- D = outside diameter (mm)
- SF = safety factor (typically 2.5–4 for static service per ISO 4413 / EN 982)
Hydraulic Working Pressure — Step-by-Step Calculation
For St 52 (E355) GBK tube: 25 × 2.5 mm (OD × WT), safety factor SF = 4:
Step 1 — Identify allowable stress for GBK condition: St 52 GBK: S ≈ 220 MPa (yield stress / safety factor not yet applied — S here is allowable fibre stress for the formula)
Step 2 — Apply working pressure formula: Pw = (2 × S × t) / (D − t) × (1 / SF) Pw = (2 × 220 × 2.5) / (25 − 2.5) × (1 / 4) Pw = 1,100 / 22.5 × 0.25 Pw = 48.89 × 0.25 = 12.2 MPa = 122 bar
Step 3 — Compare to system requirement: Standard mobile hydraulic system: 200–350 bar → this tube is adequate for systems up to 122 bar at SF=4. For 200 bar with SF=4: use 25 × 4.0 mm wall → Pw = (2 × 220 × 4.0) / (25 − 4.0) / 4 = 1,760 / 21 / 4 = 20.95 MPa = 210 bar ✓
Step 4 — Check BK condition for comparison: St 52 BK: S ≈ 240 MPa (higher due to cold work) Pw (BK) = (2 × 240 × 2.5) / 22.5 / 4 = 13.3 MPa = 133 bar BK gives 9% higher pressure rating — but at the cost of formability. Use BK only for straight runs requiring maximum pressure efficiency.
A simplified design table for St 52 GBK tube (SF = 4) for common hydraulic sizes:
| OD × WT (mm) | Max Working Pressure (bar) |
|---|---|
| 10 × 1 | 78 |
| 16 × 2 | 98 |
| 20 × 2.5 | 98 |
| 25 × 3 | 96 |
| 30 × 3.5 | 93 |
| 38 × 4 | 84 |
7. Internal Cleanliness for Hydraulic Service
Hydraulic circuit cleanliness is critical for valve and pump reliability. DIN 2391 / EN 10305-1 tubes in GBK (bright annealed) condition are:
- Scale-free on internal and external surfaces
- Oil-preserved internally (typically with rust-preventive oil)
- End-capped to prevent contamination during transit
For critical hydraulic assemblies (servo valves, high-pressure manifolds), specify cleanliness class per ISO 4406:1999 (e.g. 16/14/11 at delivery). Tubes meeting this requirement are pickled, flushed, dried, and preserved in a controlled environment.
8. Applications
| Application | Typical Grade and Condition | Key Requirement |
|---|---|---|
| Machine tool hydraulic lines | St 52 GBK | Clean bore, bendable |
| Mobile equipment hydraulic circuits | St 52 GBK | Pressure rating, formability |
| Instrumentation lines | St 35 GBK | Thin wall, small OD |
| Pneumatic cylinders | St 45 or St 52 GBK | Bore finish |
| Structural tubular members | St 52 BK | Strength, tight OD tolerance |
| Automotive suspension arms | St 52 GBK or NBK | Formability, consistent wall |
| Shock absorber bodies | St 52 BK or GBK | Wall uniformity, straightness |
9. Procurement Checklist
Procurement trap — delivery condition omitted:
Wrong PO: "DIN 2391 St 52 precision tube, 25 mm × 2.5 mm, 4 m cut lengths, EN 10204 2.2."
What ships: St 52 BK (as-drawn, no heat treatment) — the mill's default lowest-cost condition. Hardness 34–38 HRC. Elongation 6–10%. This tube will crack if bent more than approximately 15° at close radius. If the system requires 90° bends, 100% of the bent pieces will be scrapped.
Correct PO: "DIN 2391 (or EN 10305-1) St 52-3 / E355, delivery condition GBK (bright annealed), OD tolerance class D9, 25 mm OD × 2.5 mm WT, 4 m cut lengths. Tube to be internally oil-preserved and end-capped. EN 10204 3.1 MTC with chemistry, mechanical properties, and delivery condition confirmation."
- Standard: DIN 2391 or EN 10305-1 (state which; note they are equivalent in product)
- Grade: St 35 (E235) / St 45 (E275) / St 52 (E355)
- Delivery condition: BK / GBK / NBK (GBK is standard for most hydraulic applications)
- OD × wall thickness (mm)
- OD tolerance class: D9 (standard) or h9 (for sliding fits)
- Wall tolerance class: T9 (standard) or T8 (tighter, by agreement)
- Length: mill random (typically 4–8 m) or cut-to-length
- End condition: open ends, end-capped, or degreased and capped
- Internal cleanliness: standard (oil-preserved) or specify ISO 4406 class for servo/critical service
- Mill test certificate: EN 10204 2.2 (standard) or 3.1 (required for pressure system compliance)
Failure Modes
Failure Mode 1 — BK tube cracked during field bending
Mechanism: St 52 BK (as-drawn) precision tube is shipped for a hydraulic system with multiple 90° bends. The as-drawn condition leaves the steel in a work-hardened state with elongation of approximately 8%. Cold bending to 90° at a bend radius of 2× OD (close radius) requires the outer surface to stretch approximately 15% — well above the 8% elongation limit. The tube cracks at the bend at approximately 45° deflection.
Diagnostic: Visual crack on the outer surface of the bend at the maximum tension zone. Metallurgical examination shows the fracture initiates at the cold-drawn surface. Hardness test confirms BK condition (34+ HRC). Delivery condition on MTC confirms BK — the PO did not specify a condition.
Fix: All tube specified for bending, flaring, or end-forming must be ordered as GBK (bright annealed, elongation >30%) or NBK. Replace the entire BK batch with GBK of the same OD/wall/grade. Revise the purchase order template to include delivery condition as a mandatory field.
Failure Mode 2 — Mill scale contamination from non-GBK tube in hydraulic system
Mechanism: Standard-condition (BK) DIN 2391 tube is used in a servo-hydraulic circuit. BK tube retains mill scale on the internal surface. During commissioning flushing, scale particles are dislodged and circulate through the system. Particles of 50–200 µm pass through the 25 µm filter installed for initial flushing and accumulate in the servo valve orifices. Two servo valves are damaged beyond repair within 40 hours of operation.
Diagnostic: Servo valve malfunction. Flushing circuit analysis shows ISO 4406 cleanliness class of 22/20/16 — far above the 16/14/11 required for servo valve service. Internal inspection of removed tube sections confirms scale on the bore.
Fix: All hydraulic tube for servo and proportional valve circuits must be GBK (bright annealed in HNx atmosphere, scale-free), oil-preserved internally, and supplied with end caps intact. Specify ISO 4406 cleanliness class at delivery for critical circuits and verify on a representative tube sample before installation.
Failure Mode 3 — OD tolerance class not specified; tube does not fit coupling
Mechanism: Precision tube is ordered for a system using compression-type fittings (e.g. Swagelok or Parker type). The coupling is designed for h9 OD tolerance on the tube. The PO specifies "DIN 2391 St 52 GBK" but does not state an OD tolerance class. The mill ships D9 tolerance (±0.052 mm, bilateral). The coupling's ferrule requires h9 (0/−0.052 mm, unilateral minus). Some tubes are at the high end of the D9 range — 0.052 mm over nominal — and cannot be fully inserted into the coupling before the ferrule engages. Approximately 15% of fittings leak during pressure testing.
Diagnostic: Pressure test reveals leaks at compression fittings. Measurement of tube OD confirms some pieces are at the top of D9 tolerance (+0.050 mm). The coupling manufacturer's specification requires h9 (tube OD at or below nominal). The standard D9 tolerance allows tubes above nominal.
Fix: For all systems using compression-type fittings, specify OD tolerance class h9 (one-sided minus tolerance) — this ensures tube OD is always at or below nominal and never protrudes above nominal to prevent ferrule seating. Confirm the required OD tolerance class with the fitting manufacturer before ordering tube.
Frequently Asked Questions
What does DIN 2391 cover?
DIN 2391 (Seamless Precision Steel Tubes — Technical Delivery Conditions) specifies seamless cold-drawn precision steel tubes for mechanical engineering and instrumentation applications, particularly hydraulic and pneumatic systems. The standard covers OD from 4 mm to 50 mm (Part 1: soft and annealed grades) and from 4 mm to 200 mm (Part 2: harder grades), with tighter dimensional tolerances than standard structural tube. DIN 2391 has largely been superseded by the harmonised European standard EN 10305-1 (seamless cold-drawn) for new designs, but remains widely referenced in older specifications and machine drawings, particularly in Asia and the Middle East.
What are the DIN 2391 delivery conditions and how do they differ?
DIN 2391 specifies four delivery conditions: BK (bright hard — cold drawn without subsequent heat treatment, highest strength but lowest ductility), BKW (bright hard, lightly cold worked — intermediate), GBK (bright annealed — cold drawn then annealed in controlled atmosphere, soft with good formability), and NBK (normalised — heat treated to restore mechanical properties similar to hot-rolled). BK is used where the highest strength from cold work is required. GBK is used where tube is to be subsequently bent, flared, or formed. NBK provides uniform properties similar to the annealed condition.
What are the dimensional tolerances for DIN 2391 precision tubes?
DIN 2391 precision tubes are manufactured to significantly tighter tolerances than standard structural tube. For a typical OD range of 10–30 mm, outside diameter tolerance is ±0.05 mm (h9 quality), wall thickness tolerance is ±10% of the specified wall with a minimum of ±0.1 mm, and straightness is typically 1 mm per metre or better. These tolerances allow the tube to be used directly as a sliding or clearance fit without further OD grinding, which is the key advantage over standard seamless pipe feed stock.
What materials are specified in DIN 2391?
DIN 2391 Part 1 covers soft and half-hard grades based on carbon steel material numbers: St 35 (1.0308, C≤0.17%, equivalent to P235T), St 45 (1.0408, C≤0.21%, equivalent to P265T), and St 52-3 (1.0580, C≤0.22% with microalloying, higher yield). These designations correspond to EN 10305-1 grades E235, E275, and E355 respectively in the harmonised version of the standard. The number in the grade designation (35, 45, 52) refers to the minimum tensile strength in kgf/mm² (or approximately the minimum tensile strength in MPa when multiplied by 10), not the carbon content.
What is the difference between DIN 2391 and EN 10305-1?
EN 10305-1 (Seamless Cold Drawn Steel Tubes for Precision Applications) is the harmonised European standard that replaced DIN 2391 for new designs in EU countries. EN 10305-1 covers the same product with updated material designations: E195 (St 35), E235 (St 37), E275 (St 45), E355 (St 52). Tolerances in EN 10305-1 are essentially the same as DIN 2391. For projects using European standards, specify EN 10305-1; for projects referencing older German machine drawings or Asian markets where DIN 2391 is still specified, supply to DIN 2391 — the product is essentially identical.
What OD and wall combinations are commonly available for DIN 2391 tubes?
DIN 2391 precision tubes are commonly available from 4×0.5 mm (OD × WT) to 200×15 mm, with the most active range being 6–80 mm OD and 1–10 mm wall thickness. Standard combinations follow DIN 2393 preferred dimensions tables. The most common hydraulic tube sizes are 6×1, 8×1, 10×1, 12×1.5, 16×2, 20×2, 25×2.5, and 30×3 (OD×WT in mm). Availability of very small or very large sizes should be confirmed with the manufacturer.
What hydraulic pressure can DIN 2391 precision tubes withstand?
Hydraulic pressure rating depends on OD, wall thickness, and material grade. For St 52 (E355) grade tube in condition BK, the allowable working pressure can be calculated from: P = (2 × S × t) / (D − t), where S is the allowable stress (approximately 240 MPa for E355 BK), t is wall thickness, and D is OD. For example, a 25×3 mm tube in E355 BK: P = (2 × 240 × 3) / (25 − 3) = 65.5 MPa (655 bar). This is well within the normal hydraulic system range. Always apply a safety factor per the applicable machine design standard (typically 2.5–4× for static service).