Welded pipe is made from strip: it is bent into a cylinder and the longitudinal seam is welded by high-frequency current. Seamless pipe is made from a round billet that is pierced and rolled on a mill; it has no seam, but the wall varies more and the price per tonne is higher. For a frame, a fence, low-pressure water and lighting poles the price difference buys nothing; for steam, hydraulics, ammonia, gas under pressure and anything designed to pressure vessel and piping codes, the seam is a factor in the calculation that the designer must account for.
Standards in four systems
| System | Welded | Seamless | Grades |
|---|---|---|---|
| GOST | WeldedGOST 10704-91 dimensions, GOST 10705-80 technical conditions | SeamlessGOST 8732-78 dimensions, GOST 8731-74 technical conditions | GradesSt2sp, St3sp, 10, 20, 09G2S |
| GB (China) | WeldedGB/T 3091-2015 for low-pressure fluids | SeamlessGB/T 8163-2018 for fluids, GB/T 8162-2018 for structures | GradesQ195, Q235B, 10#, 20#, Q355B |
| ASTM | WeldedA53 Type E, electric resistance welded | SeamlessA53 Type S and A106 for high temperature | GradesGrade A, Grade B |
| EN | WeldedEN 10217-1 | SeamlessEN 10216-1 | GradesP195TR1, P235TR1, P235TR2, P265TR1 |
Note EN: the grades in EN 10217-1 and EN 10216-1 are the same, P235TR1 can be either welded or seamless, the production method is set by the standard number, not by the grade. The same with ASTM A53: Type E is welded, Type S is seamless, Grade B is common to both. In the specification the production method is written in words: “seamless, GOST 8732” or “seamless, GB/T 8163”.
A53 and A106: same mechanics, different pipes
A53 Grade B and A106 Grade B share the same numbers: yield strength not below 240 MPa, tensile strength not below 415 MPa. The difference lies elsewhere. A106 is seamless only, killed steel only, with silicon not below 0.10 %, and is intended for elevated temperature service; A53 allows a welded seam and rimming steel and is intended for general use. A mill often certifies seamless pipe with a dual designation A53 Type S / A106 Gr.B, which is acceptable if the pipe satisfies both standards; welded pipe is never certified to A106.
Weld factor and wall thickness
In a piping calculation the strength of welded pipe is multiplied by a weld joint factor: under ASME B31.3 it is 0.85 for electric resistance welded pipe and 1.00 for seamless. That is, at equal wall the welded pipe counts as 15 % weaker, and the designer either takes a thicker wall or writes seamless. The other side: the wall thickness tolerance of seamless pipe is wider than that of welded, the tables of GOST 8732 and GOST 10704 show this, and the bureau’s inspector measures the wall of seamless pipe at several points on both ends, not at one.
One mass per metre for both
Formula
m = 0.02466 · (D − t) · t
m is the mass of 1 m in kg; D the outside diameter, t the wall thickness, in mm. The coefficient is π · 7.85 / 1 000.
| D×t, mm | Mass, kg/m | Metres per tonne |
|---|---|---|
| 57×3 | Mass, kg/m4.00 | Metres per tonne250.3 |
| 76×3.5 | Mass, kg/m6.26 | Metres per tonne159.8 |
| 89×3.5 | Mass, kg/m7.38 | Metres per tonne135.5 |
| 108×4 | Mass, kg/m10.3 | Metres per tonne97.5 |
| 159×4.5 | Mass, kg/m17.1 | Metres per tonne58.3 |
| 219×6 | Mass, kg/m31.5 | Metres per tonne31.7 |
| 273×7 | Mass, kg/m45.9 | Metres per tonne21.8 |
| 325×8 | Mass, kg/m62.5 | Metres per tonne16.0 |
The formula does not know whether there is a seam: mass is calculated from outside diameter and wall. The price difference between welded and seamless pipe of one size is a difference in processing, not in metal, and it shows in the mill’s offer as a separate line. What to write into the contract: the production method in words, the dimension standard and the technical conditions standard, the grade to the mill’s standard next to the project grade, the type of test, hydraulic pressure or non-destructive testing of the seam, and the weight basis for shipment.
Grade equivalents
| GOST | GB (China) | ASTM | EN | Note |
|---|---|---|---|---|
| Steel 20 | GB (China)20# | ASTMA106 Gr.B | ENP235GH | NoteSeamless pipes. Nearest equivalents by composition and strength; test requirements differ between the standards |
| St3sp | GB (China)Q235B | ASTMA53 Gr.B | ENP235TR1 | NoteWelded pipes. A53 Gr.B is the nearest equivalent: its tensile strength is higher than that of Q235B |
Equivalents are the closest match, not identical: chemistry and tolerances differ between standards. The exact grade and standard are written into the contract.
Theoretical weight at a steel density of 7.85 g/cm³, calculated by the same function as the catalogue tables and the calculator. The actual weight of the batch is by scale and mill certificate.
