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For precision-driven applications where superior surface quality and reliable formability are paramount, the DIN 2391-1 St35 Honed Seamless Steel Tube establishes the benchmark. Engineered to the exacting German DIN standard, this low-carbon steel tube undergoes a precision honing process that delivers an exceptionally smooth, ready-to-use internal surface, eliminating the need for further finishing. While not chosen for ultimate tensile strength, St35's exceptional ductility and outstanding cold-forming capabilities make it the ideal choice for medium-duty hydraulic cylinders, precision pneumatic actuators, and low-stress structural components where ease of welding, bending, and flanging is critical. If you require a seamless honed tube that guarantees consistent performance, enhances seal longevity, and streamlines your assembly process without compromising on metallurgical integrity, the St35 grade provides an optimal balance of performance, manufacturability, and cost-efficiency for a wide range of industrial systems.
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DIN 2391 St35 Pipe Specification
| DIN Standard | DIN 2391 |
| Available Grade | ST35 |
| Outer Dimensions | 4.0 To 60.0 MM |
| Size in Mm | Outer diameter (4-190) x Wall thicknes (0.5-15) |
| Wall Thickness | 0.5 To 8 mm |
| Maximum Length | Maximum 6000mm |
ST 35 NBK Seamless Pipe Chemical Resistance Guide
| Standard | Grade | C | Mn | Si | S maximum | P maximum |
| DIN2391 | St35 | ≤0.17 | ≥0.4 | ≤0.35 | 0.025 | 0.025 |
ST35BK Smls Pipe Mechanical Strength
| Cold drawn(Mandatory) | Cold drawn(Soft) | Cold Drawn and Stress Release | Normalizing | Anneal | ||||||||
| BK1) | BKW1) | (BKS) | (NBK)1) | (GBK)1) | ||||||||
| Grade | Tensile | Elongation | Tensile | Elongation | Tensile | Yield point | Elongation | Tensile | Yield point | Elongation | Elongation | Tensile |
| N/mm2 | % | N/mm2 | % | N/mm2 | N/mm2 | % | N/mm2 | N/mm2 | % | % | N/mm2 | |
| St35 | 480 | 6 | 420 | 10 | 420 | 315 | 35 | 340~470 | 235 | 25 | 14 | 315 |
Equivalent of ST35.4 Cold Finished Tube
| Outer Diameter Tolerance (MM) | 4-30 | 31-40 | 41-50 | 51-60 | 61-70 | 71-80 | 81-90 | 91-100 |
| ±0.08 | ±0.15 | ±0.20 | ±0.25 | ±0.30 | ±0.30 | ±0.40 | ±0.45 | |
| Wall Thickness Tolerance | ±10% | |||||||
DIN 2391-2 Grade St 35 Carbon Steel Seamless Tubes Weight Chart
| Wall Thickness (mm) | Weigh (kg/m) | Outer Diameter(mm) |
|---|---|---|
| 4 | 3.410 | 75 |
| 1 | 0.300 | 12 |
| 1.2 | 0.358 | 14 |
| 1.5 | 0.505 | 18 |
| 5 | 4.290 | 90 |
| 0.5 | 0.116 | 8 |
| 4 | 3.170 | 70 |
| 1.5 | 0.561 | 20 |
| 4.5 | 3.761 | 80 |
| 1 | 0.251 | 10 |
| 2.5 | 1.661 | 45 |
| 3 | 2.070 | 50 |
| 1.5 | 0.616 | 22 |
| 5 | 4.784 | 100 |
| 0.5 | 0.087 | 6 |
| 2 | 1.018 | 30 |
| 2.5 | 1.454 | 40 |
| 3 | 2.290 | 55 |
| 3.5 | 2.602 | 60 |
| 2 | 1.165 | 35 |
| 3.5 | 2.832 | 65 |
| 1.5 | 0.725 | 25 |
| 0.5 | 0.058 | 4 |
DIN 2391 St35 Gbk Welded Tube Size
| Wall Thickness | Outer Diameter Sizes in Inch |
|---|---|
| .012 | 1/8 |
| .010 | 1/8, 1/16, 3/16 |
| .065 | 3/4, 2 1/2, 1 1/4, 1 5/8, 1/2, 5/16, 7/8, 1, 1 3/4, 3/8, 16, 5/8, 1 1/2, 2, 3, 1/4 |
| .020 | 1/16, 3/8, 5/16, 1/4, 1/8, 3/16 |
| .125 | 3, 3/4, 1 1/4, 1, 1 1/2, 3 1/4, 2 |
| .120 | 1 1/2, 1/2, 2, 3/4, 3, 2 1/4, 1, 5/8, 2 1/2, 7/8, 1 1/4 |
| .016 | 1/8, 3/16 |
| .028 | 3/16, 3/8, 5/16, 3/4, 1 1/2, 1/2, 1/4, 1, 2, 1/8 |
| .035 | 1/4, 5/8, 3/4, 3/8, 2 1/4, 16, 3/16, 1 5/8, 1/2, 2, 7/16, 5/16, 1, 1/2, 7/8, 1 1/4, 1/8, 1 |
| .134 | 1 |
| .049 | 1 5/8, 2, 1 1/8, 1 1/4, 5/16, 3/4, 2 1/4, 1, 5/8, 1/2, 16, 3/16, 7/8, 3/8, 1/4, 1 1/2 |
| .083 | 3/4, 1 1/4, 1/2, 1/4, 5/8, 7/8, 2 1/2,3, 1 7/8, 1, 1 5/8, 3/8, 1 1/2, 2 |
| .095 | 1, 1-1/4, 1-1/2, 5/8, 2, 1/2, |
| .109 | 1/2, 1, 2, 1-1/4, 3/4, 1-1/2, |
| .250 | 3 |
| .375 | 3 1/2 |
Carbon Steel DIN 2391 ST35 Round Pipe Sizes
| 1/8 Inch IPS(.405 Inch Outer Diameter) | 3 1/2 Inch IPS( 4.000 Inch Outer Diameter) |
| 3/8 Inch IPS(.675 Inch Outer Diameter) | 5 Inch IPS( 5.563 Inch Outer Diameter) |
| Schedule 40, 80 | Schedule 10, 40, 80, 160, XXH |
| 1/4 Inch IPS(.540 Inch Outer Diameter) | 4 Inch IPS(4.500 Inch Outer Diameter) |
| Schedule 10, 40, 80 | Schedule 10, 40, 80, 160, XXH |
| Schedule 10, 40, 80, 160, XXH | Schedule-40 (.375) |
| Schedule 10, 40, 80 | Schedule 10, 40, 80, 160, XXH |
| 1/2 Inch IPS(.840 Inch Outer Diameter) | 6 Inch IPS(6.625 Inch Outer Diameter) |
| Schedule 5, 10, 40, 80, 160, XXH | Schedule 5, 10, 40, 80, 120, 160, XXH |
| 3/4 Inch IPS(1.050 Inch Outer Diameter) | 8 " IPS(8.625 Inch Outer Diameter) |
| Schedule 10, 40, 80, 160, XXH | Schedule 5, 10, 40, 80, 120, 160, XXH |
| 1 Inch IPS:(1.315′ Outer Diameter) | 10 Inch IPS(10.750 Inch Outer Diameter) |
| Schedule 5, 10, 40, 80, 160, XXH | Schedule 10, 20, 40, 80 (.500), TRUE 80(.500) |
| 2 Inch IPS(2.375 Inch Outer Diameter) | 16 Inch IPS(16.000 Inch Outer Diameter) |
| 1-1/4 Inch IPS(1.660 Inch Outer Diameter) | 12 Inch IPS(12.750 Inch Outer Diameter) |
| 1-1/2 Inch IPS(1.900 Inch Outer Diameter) | 14 Inch IPS(14.000 Inch Outer Diameter) |
| Schedule 10, 40, 80, 160, XXH | Schedule10 (.188), Schedule40 (.375) |
| Schedule 10, 40, 80, 160, XXH | Schedule 10, 20, 40(.375), TRUE40(.406), Schedule80(.500) |
| Schedule 10, 40, 80, 160, XXH | Schedule10(.188), Schedule40(.375) |
| 2 1/2 Inch IPS(2.875 Inch Outer Diameter) | 18 Inch IPS (18.000 Inch Outer Diameter) |
| 3 Inch IPS( 3.500 Inch Outer Diameter) | Schedule 5, 10, 40, 80, 160, XXH |
Frequently Asked Questions (FAQ)
1. Q: St35 is a lower-strength steel. Why would I specify it over higher-strength grades like St52 for a hydraulic cylinder barrel?
A: The primary advantage of St35 is not strength, but its exceptional formability and weldability. Its low carbon content (typically ≤0.17%) makes it supremely suited for applications requiring secondary operations like flanging, dishing, or extensive welding. This eliminates the risks of cracking or heat-affected zone (HAZ) brittleness, which are more prevalent in higher-carbon steels. For medium-pressure systems where fatigue life depends more on surface finish and geometric accuracy than on ultimate material strength, St35 provides a reliable and cost-effective solution.
2. Q: How does the honing process specifically benefit the performance of St35 in pneumatic applications?
A: In pneumatics, where lubrication is often minimal ("lube-free" systems), the honed internal surface is critical. The cross-hatched pattern (typically Rz 4-10 µm) acts as a micro-reservoir for any available lubricant, drastically reducing friction and stick-slip effect. For St35, this honed finish ensures smooth piston travel and extends the life of seals, compensating for the system's low viscosity air and making it ideal for high-cycle pneumatic cylinders.
3. Q: The St35 grade has excellent weldability. What is the recommended welding procedure for assembling these honed tubes into a manifold or reservoir?
A: Due to its low carbon equivalent (Ceq), St35 is highly weldable with all standard processes (MIG, TIG, MMA). For optimal results, we recommend using a matching or slightly under-matched filler metal (e.g., SG2, ER70S-6). While pre-heating is generally not required, it is crucial to protect the honed ID from spatter and oxidation. Using an argon back-purge is the best practice to prevent the formation of scale on the critical internal surface, which would compromise seal integrity.
4. Q: For a custom application, can the St35 honed tube be cold-bent after the honing process, and what are the limitations?
A: Yes, the high ductility and elongation (>22%) of St35 make it one of the most suitable seamless grades for post-honing bending. The key limitation is preserving the honed ID surface. We recommend a bending radius no tighter than 3x the tube's outer diameter and the use of mandrel bending to support the inner wall and prevent wrinkling or ovality, which would create an uneven surface for the piston seal.
5. Q: How does the normalization heat treatment, mandated by DIN 2391-1, contribute to the stability of St35 honed tubes during machining?
A: Normalization refines the grain structure of the steel, resulting in a uniform and fine-grained ferrite-pearlite microstructure. This process eliminates internal stresses from the cold-drawing operation, ensuring dimensional stability during subsequent machining (e.g., turning, grooving). This prevents part distortion and guarantees that the precise honed ID remains concentric with the machined OD features, which is vital for proper cylinder assembly and function.
6. Q: In corrosive environments (e.g., high humidity, certain coolants), what are the best practices for using St35 tubes, given they are not stainless?
A: While St35 has limited inherent corrosion resistance, its performance can be significantly enhanced. For internal corrosion, specify the tubes to be filled with an inhibited, water-glycol-based hydraulic fluid. For external protection, phosphate coating (e.g., phosphatizing) is an excellent and cost-effective pre-treatment that provides a superior key for subsequent paint or powder coating, greatly improving durability in harsh conditions.
7. Q: What non-destructive testing (NDT) methods are most appropriate for quality control on St35 honed tubes, particularly for validating the internal surface?
A: Beyond standard dimensional checks, two NDT methods are key:
Eddy Current Testing: Ideal for high-speed detection of surface and near-surface flaws on the ID and OD, such as micro-cracks or inclusions.
Borescope Inspection: For a direct visual assessment of the honed surface finish, verifying the absence of pits, tears, or tooling marks. This is often specified for critical applications to ensure the surface is free of defects that could damage seals.
8. Q: From a fatigue life perspective, how does the seamless nature of the St35 tube benefit a cylinder undergoing high cyclic loading?
A: A seamless tube has a homogeneous, continuous grain structure around its entire circumference. This eliminates the potential weak point of a longitudinal weld seam, which can be a nucleation site for fatigue cracks under repeated pressure cycles. For St35, this means the fatigue performance is predictable and uniform, leading to a longer and more reliable service life in dynamic applications.
9. Q: Can St35 honed tubes be successfully used in applications involving high water content fluids (HFA, HFB)?
A: Yes, they are commonly used in such systems. However, the low corrosion resistance of uncoated St35 requires careful fluid management. The inhibitor package in the water-based fluid must be maintained at the correct concentration. As an upgrade, specifying a phosphate-coated ID can provide an additional layer of protection, dramatically increasing the tube's service life with high-water-content fluids.
10. Q: When would I specify a "Honed and Hardened" tube over a standard St35 honed tube?
A: Specify a hardened tube (often made from higher grades like C45 or 42CrMo4) when your application involves:
Abrasive Contamination: Grit or contaminants in the hydraulic fluid that could score a softer surface.
Very High Side Loads: Where the piston rod imposes significant bending forces on the tube, risking galling or deformation of the ID.
Extreme Cycle Life Requirements: Hardened tubes generally offer superior wear resistance.
For all other applications where conditions are clean, pressures are medium, and formability is key, St35 remains the optimal and economical choice.
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