Tube Bending Technology And Tube Fabrication (3)
- Jun 01, 2018 -

Tube Bending Technology And Tube Fabrication (3)


17. Terms And Definitions

The following terms and definitions apply to this standard.


● Bend: circular arc tube parts to change the flow direction of the medium in the pipeline, and the bending radius is not less than 5 times of the outer diameter of its mother pipe with straight sections at both ends.

●  Arc: bending section of bends (elbow)

●  Bend angle: the plane angle formed by the direction change of the whole bend.

●  Bend radius: the distance from the bending center to its central axis.

●  Outer arc side: the lateral bending part of a bend.

●  Inner arc side: the inner bending part of the elbow

●  Neutral face: the section where the wall thickness does not change and the shape changes on the curved surface of the bent pipe during tube bending.

● Mother pipe: straight pipe for making an elbow

● Straight section: a straight pipe section between both ends of the bent pipe

● Induction heating bending: induction heating method is used to form a narrow annular heating zone on the circumference of the steel pipe, and the process of continuous bending is performed while the steel pipe is moving.

● Manufacturing procedure specification: technical documents stipulating the material properties of the mother pipe, bending process, heat treatment equipment and processes, non-destructive testing, bevel type and dimensions of pipe end, including the evaluating results of inspection machine.

● Transition zone: an area near the starting bend point or ending bend point that is from the end part of tube that has not been heated to the beginning part of tube that has beet heated at bending temperature.

● Reduction rate of wall thickness:

(The minimum wall thickness of the mother tube - The minimum wall thickness of elbow outer arc) ÷ The minimum wall thickness of the mother tube  %

● Testing pipe: pipe material for first testing

● Bending pipe production: bending pipe production in accordance with MPS approved by the buyer's written approval.


18. Symbols During Tube Bending

A----Elongation rate, %;

D----Outside diameter, mm;

DN---Nominal diameter, mm;

ƒ-----Wave distance of bend pipe, mm;

h------Wave degree of bend pipe,mm;

L------Straight length section of bend pipe, mm;

Q------Perpendicularity of the end of bend pipe, mm;

R------Bending radius of the elbow, mm;

Rm-----Tensile strength, MPa;

Rt0.5------The yield limit of 0.5% of the total elongation, MPa;

r----Outer radius of mother tube, mm;

So------The initial cross section area within gauge length of the tensile testing part, mm²;

Tdmin------Minimum design temperature specified by the purchaser, ℃;

t-------Nominal wall thickness of mother tube, mm;

t1-------The minimum wall thickness of the mother tube, mm;

tH------The minimum wall thickness of the outer arc side of the elbow, mm;

ti-------Minimum wall thickness of the inner arc of elbow, mm;

tmin-------Calculation of wall thickness based on the straight section of bent pipe calculated by GB50251 or GB50253;

U---------Plane degree of bend pipe, mm;

ɑ---------Bending angle;

tube bending-----Flexural flatness, mm;


19. Pipe Wall Thickness

Minimum wall thickness of outer arc

Tube Bending

Minimum wall thickness of inner arc

Tube Bending


20. Basic Information Provided By The Purchaser

A tube bending name;

B The number of tube bending parts;

C the mother tube provided by the purchaser or manufacturer;

D tube bending material;

E The size of the tube bending from purchaser, including:

1) nominal diameter DN.

2) the outer diameter is D.

3) minimum wall thickness ti.

4) the radius of the tube bending.

5) tube bending angle.

6) straight length L


21. Bending Parameter Deviation


Table 5 Permissible Deviation Of Bending Parameters

Parameter namePermissible deviation of bending parameter
Heating modeNot allowed to change
Welding procedure specification for welding seam of mother pipe and welding materialsNot allowed to change
The nominal diameter of the mother tubeNot allowed to change
Bending speed ±2.5mm/min
Bending temperature25 ℃
Cooling medium flow or pressure10%
The type of cooling mediumnot allowed to change
Cooling medium temperature15 ℃
Induction coil designnot allowed to change
Induction heating frequency20%


22. Tube Bending And Sizing After Bending

After tube bending, the elbow should not be reheated or fixed, except for the overall heat treatment over the tube phase transition temperature.

Cold sizing of the elbow is allowed, and no heat treatment can be carried out.Heat treatment is required and cold shaping should be carried out before heat treatment.Permanent deformation should not be greater than 1.5% of the outer diameter before cold forming. The calculation is as follows:

tube bending

In formula:

A-----External diameter of cold sizing, mm;

B------Outer diameter of cold sizing, mm;


Tube end:

When there are provisions on the groove of the pipe end in design diagram, it should conform to the design requirements.

The bevel of the bend pipe should be machined.

When the thickness of the elbow is greater than that of 2.0mm thick straight pipe, the inner edge should be adopted, and the inner slope should not be greater than 15 degrees.

Inside and outside welds that less than 100mm from surface of tube ends should be cleaned The height of welding seam should be 0mm~0.5mm after clearance, and it is smooth over the surface of the adjacent tube.The remaining wall thickness of the adjacent pipe after grinding of surface should not be less than 95% of the specified wall thickness.


23. Marks

Mark position:

After the elbow is inspected, make marks outside and inside of pipe starting from 100mm from the end of tube.

Identification content:

If the order contract is stipulated, the contract shall be executed according to the contract; if there is no special provision in contract, the execution shall be carried out according to Fig. 4.

tube bending


A-----bending grade (when the mother pipe adopts GB/T9711);

B----- standard number for execution / standard name;

C----- bending steel grade;

D------ external diameter (D);

E------- straight segment wall thickness (T);

F-------- tube bending radius;

G-------- tube bending angle;

H-------- bent pipe number;

I---------- project name / Purchaser code;

J---------- manufacturer name / trademark;

K---------- manufacturing date;


24. Die Marks Printing

A clear and durable brush mark should be used for template spray. The font color should be easy to identify; cold and hot stamping hammer marks shall not be allowed.


25. The Transportation, Storage And Protection Of The Bend Pipe:

● Carrying

Nylon sling or hooks with rubber pads shall be used for handling in workshops and receiving and dispatching areas.The use of a hook with a low melting point metal is not allowed. The manufacturer shall submit the written handling method to the purchaser for approval.

● Transport

The manufacturer shall submit the complete shipping instructions before shipment to the purchaser for approval.Shipment should be at least in line with the requirements of railway transportation, highway transportation or sea transportation. The proposed method should include the necessary calculation method and stacking layout, bearing belt location, cushion block and fastening belt.

Trucks or trailers should be cleaned before loading bends.

The weld of the elbow shall not be in contact with any part of the isolating block. The weld shall not contact any part of the railway carriage and the trailer, and the pipe shall not be directly contacted with the sharp object in the carriage.

● Storage

The storage of finished tube bending products should be prevented from deformation, damage and corrosion.The manufacturer shall submit a written storage method to the purchaser for approval.

The manufacturer shall submit the method and drawings for the stacked and fixed bends to be approved for purchaser's approval.

There should be no contact between metal and metal between adjacent tube bending parts.For all bends, there should be no direct contact between the elbow and the tight chain or between the elbow and the clapboard.

● Protection

The elbow (bends) should be handled carefully during storage, handling and transportation to avoid damage.The ends of the elbow (bends) shall be protected by protective cover of pipe end and must not be removed during transportation and storage.