St35.8 Seamless Boiler Tubes: Composition, Properties and Tolerances
Nov 13, 2023
What the St35.8 Designation Covers
St35.8 is the classic German designation for a seamless carbon steel boiler tube, published in DIN 17175 and used for low and medium pressure steam, feedwater and heating circuits. The number in the grade name refers to the minimum yield strength level in the old DIN system, while the suffix identifies the steel as a boiler tube quality. When a project is written to current European practice, the same service is usually ordered against EN 10216-2, where the P235GH grade occupies the equivalent position of a low-carbon, non-alloy pressure tube for elevated temperature service.
Because the two systems are commonly mixed on the same enquiry, a purchase order should always name one standard, the grade under that standard, and the delivery condition. A seamless tube made to GB 3087 in China is frequently offered for the same low pressure boiler duty, and its Grade 10 chemistry and mechanical properties are a useful reference point for buyers comparing offers.
Reference Chemical Composition
The table below gives the reference composition of GB 3087 Grade 10, the grade most often supplied when an St35.8 seamless boiler tube is requested without a European designation.
| Element | C | Si | Mn | P | S | Cr | Cu | Ni |
|---|---|---|---|---|---|---|---|---|
| Content (%) | 0.07–0.13 | 0.17–0.37 | 0.38–0.65 | ≤0.030 | ≤0.030 | 0.3–0.65 | ≤0.25 | ≤0.30 |
The low carbon range keeps the tube readily weldable and gives good ductility after bending and expanding, which matters when tubes are rolled into boiler drums or bent into superheater coils. Phosphorus and sulphur are held at 0.030% maximum each, so hot working and welding remain predictable.
Mechanical Properties
| Standard | Grade | Tensile strength (MPa) | Yield strength (MPa) | Elongation (%) |
|---|---|---|---|---|
| GB 3087 | 10 | 335–475 | ≥195 | ≥24 |
These values are quoted with the property level applying to the tube as delivered. Where a project specifies a hot finished or normalised delivery condition, the tensile range is checked on the finished tube rather than on the mother material, and test frequency is taken from the standard.
Dimensional Tolerances for Hot Rolled and Cold Drawn Tube
| Delivery condition | Dimension | Size range (mm) | Tolerance, ordinary | Tolerance, advanced |
|---|---|---|---|---|
| Hot rolled tube | Outside diameter | ≤159 | ±1.0% (min ±0.50 mm) | ±0.75% (min ±0.40 mm) |
| Outside diameter | >159 | ±1.0% | ±0.90% | |
| Wall thickness | ≤20 | +15.0% (min +0.45 mm) / −12.5% (min −0.35 mm) | ±10% (min ±0.30 mm) | |
| Wall thickness | >20 | ±12.5% | ±10% | |
| Cold drawn tube | Outside diameter | 10–30 | ±0.40 mm | ±0.20 mm |
| Outside diameter | >30–50 | ±0.45 mm | ±0.25 mm | |
| Outside diameter | >50 | ±1.0% | ±0.75% | |
| Wall thickness | 1.5–3.0 | +15% / −10% | ±10% | |
| Wall thickness | >3.0 | +12.5% / −10% | ±10% |
The advanced tolerance columns are used for tube that will be expanded, bent on tight radii or fitted into tube sheets, where a small diameter spread keeps tooling loads even across the bundle.
Surface Condition, Testing and Marking
Inner and outer surfaces are free of cracks, folds, laps and heavy scale; minor dressing marks are permitted within the depth allowed by the standard.
Hydrostatic testing is applied when the order calls for it, and wall thickness verification is carried out on each lot.
Flattening, flaring and bending tests are used to confirm ductility of the finished tube.
Marking on each bundle or tube includes the standard, the grade, the heat number and the size, so the material can be traced to its mill certificate.
Packaging and Transport
Before packing, the tube surface is cleaned and inspected for defects, grease and debris. Tube is cut to the ordered length and tied into bales with steel strapping or plastic banding, and each bundle is secured so that the tubes cannot shift against one another in transit. For long distance shipment or storage in a damp environment, the surface is protected with anti-rust oil or a protective film, and the bundle is covered so that condensation does not form on the steel.
Frequently Asked Questions
Q: What is St35.8 seamless boiler tube used for?
It is used for low and medium pressure steam and hot water service, feedwater lines, heating coils and general boiler tubing where a weldable low-carbon seamless tube is required.
Q: Which current standard replaces the St35.8 designation?
DIN 17175 has been superseded by the European pressure tube standards; in EN 10216-2 the P235GH grade covers the same low-carbon, non-alloy pressure service, so the two are quoted side by side on many orders.
Q: What tensile and yield values should the tube meet?
For the comparable GB 3087 Grade 10 supply, tensile strength is 335–475 MPa, minimum yield strength is 195 MPa and elongation is at least 24%.
Q: How tight are the dimensional tolerances?
Hot rolled tube holds outside diameter to about ±1.0% and wall to +15% / −12.5% in the ordinary class, while cold drawn tube of 10–30 mm outside diameter is held to ±0.40 mm.
Q: Is the tube supplied in the hot rolled or cold drawn condition?
Either condition can be supplied. Hot rolled tube suits larger diameters and thicker walls, and cold drawn tube is chosen for smaller diameters, thinner walls and a smoother surface.
Q: How is the tube protected during shipment?
Tube is cleaned, cut to length, strapped into stable bundles and, for damp routes or long storage, coated with anti-rust oil or covered with a protective film before it leaves the mill.
