What DIN 2391 Covers
DIN 2391 is the German standard for seamless circular steel tubes for precision applications, issued in two parts: Part 1 covering dimensions and Part 2 covering technical delivery conditions. Its scope is cold-drawn seamless tube in carbon and low-alloy steel, supplied in the bright, close-tolerance condition for mechanical, hydraulic and automotive use. In European practice the standard has been superseded by EN 10305-1 for seamless precision steel tubes and by the related parts of that series covering welded precision tube and hydraulic lines, but DIN 2391 remains a common reference in purchase orders and drawings, and the steel grade names St 35, St 45 and St 52 still circulate alongside the EN designations E235, E260 and E355 respectively.
Steel Grades and Chemistry
| Grade (DIN 2391) | EN 10305-1 Equivalent | Typical Chemistry Limits |
|---|---|---|
| St 35 | E235 | C 0.17 max, Si 0.35 max, Mn 1.20 max, P 0.025 max, S 0.015 max |
| St 52 | E355 | C 0.22 max, Si 0.55 max, Mn 1.60 max, P 0.025 max, S 0.015 max |
The grades are non-alloy structural steels selected for a defined minimum yield strength rather than for corrosion resistance or elevated temperature strength. Because the tubes are cold drawn to close tolerance, the steel is bought as killed, close-grained material with controlled sulphur to avoid stringers that open up during drawing and to keep the surface free of laps and seams. Residual elements are limited by the steelmaker to protect weldability and to keep the response to heat treatment consistent from lot to lot.
Mechanical Properties and Delivery Condition
| Grade | Condition | Minimum Yield Strength | Tensile Strength Range | Elongation |
|---|---|---|---|---|
| E235 | Normalised (+N) | 235 MPa | 360 to 520 MPa | 25 % minimum |
| E355 | Normalised (+N) | 355 MPa | 490 to 630 MPa | 22 % minimum |
Delivery condition is part of the designation and must be stated on the order. Cold drawn and hard (+C) tube has the highest strength and the lowest ductility; cold drawn and soft (+LC) is partly recrystallised; stress relieved (+SR) removes residual stress after forming while retaining most of the cold-worked strength; annealed (+A) and normalised (+N) give the lowest strength with the best ductility and dimensional stability. Tubes for hydraulic cylinders are frequently ordered stress relieved to limit distortion after machining, while tubes that will be bent or flared are ordered annealed.
Dimensions, Tolerances and Testing
The dimensional system of the standard is built around outside diameter with a closely controlled wall, using tolerance classes for both diameter and wall so that a tube can be matched to the machining allowance of the component. Straightness, end squareness and internal cleanliness are specified because the tubes are used as hydraulic cylinders or as machined components where an eccentric bore would affect the finished part. The usual acceptance tests are a tension test, a flattening or bending test to check ductility, and a drift expanding or flaring test for tubes intended for forming operations. A hydrostatic test or an eddy current test is applied to tubes for hydraulic service, and a material inspection certificate to EN 10204 is issued, most commonly as a 3.1 certificate for pressure-containing applications.
Typical Applications
The main use is in machinery and vehicle engineering: hydraulic cylinder barrels and piston rods, pneumatic cylinder tubes, shock absorber and damper tubes, drive shafts, steering columns and fuel or oil transfer lines. Because the bore is smooth and closely toleranced, a hydraulic cylinder barrel can often be honed with the minimum of stock removal, and the close wall tolerance keeps the wall thickness variation small enough for high pressure service. Tubes for low pressure fluid transfer are sometimes supplied with a coating or with an internal surface treatment for corrosion protection, since the base grade is a plain carbon steel and has no intrinsic corrosion resistance. Automotive tube in particular is ordered in very long lengths with tight straightness limits so it can be fed directly into automated cutting, bending and forming equipment.
Frequently Asked Questions
Q: Is DIN 2391 still a current standard?
A: It has been superseded in Europe by the EN 10305 series, principally EN 10305-1 for seamless precision steel tubes, but it remains widely referenced in purchase orders and existing drawings. In practice a DIN 2391 order is fulfilled to EN 10305-1 with the corresponding grade designation.
Q: What is the difference between St 35 and St 52 tube?
A: St 35 corresponds to E235 with a minimum yield strength of 235 MPa, and St 52 corresponds to E355 with a minimum yield strength of 355 MPa. The higher strength of St 52 allows a thinner wall for the same working pressure in hydraulic cylinders.
Q: What does the plus sign in the delivery condition mean?
A: It identifies the final manufacturing condition, for example +N for normalised, +C for cold drawn and hard, +A for annealed and +SR for stress relieved. Each condition gives a different combination of strength, ductility and residual stress.
Q: Which delivery condition is used for hydraulic cylinder barrels?
A: Stress relieved or normalised tube is common, because it combines adequate strength with dimensional stability during machining. Cold drawn and hard tube is used only where the extra strength is needed and the component will not be subsequently machined or welded.
Q: Can precision tubes be welded?
A: Yes. E235 and E355 are weldable non-alloy steels, and welding procedures are normally qualified to the applicable pressure equipment or machinery welding code. The heat of welding locally alters the properties of cold drawn tube, so a post-weld stress relief may be required.
Q: What documentation is supplied with the tubes?
A: A material inspection certificate to EN 10204 is normally issued, with the 3.1 type used where the tube will contain pressure or form part of a safety-related assembly.