Pipe Bending Technology And Pipework Fabrications (1)
- May 30, 2018 -

Pipe Bending Technology & Pipework Fabrications (1)


1. Pipe bending should be made with tubes with positive tolerances. The relationship between the bending radius and the wall thickness of the straight pipe should comply with the requirements in Table 1. 


Table 1 Relationship Between Bending Radius And Wall Thickness Of Straight Pipe

Bend Radius RThickness Of Pipe 
R≥6Do1.06td
6Do>R≥5Do1.08td
5Do>R≥4Do1.14td
4Do>R≥3Do1.25td


2. Bend radius of pipe bending shall comply with the design documents and the relevant national standards.When there is no regulation, the bending radius of the high-pressure steel pipe should be 5 times greater than the outer diameter of the pipe, and the bending radius of other pipes should be 3.5 times greater than the outer diameter of the pipe.


3. When a seam pipe is bent, the welding seam should avoid the hand pull area or pressing area.


4. Metal tubes should be cold-bent or hot-bent within the allowable range of their material properties.


5. When high-alloy steel pipes or non-ferrous metal pipes are used to make pipe bending, mechanical methods are preferred;When sand fills the pipe during pipe bending process, it must not be struck with a hammer. When the lead pipe is heated and bent, it shall not be filled with sand.


6. After the metal tube is bent hot or cold, it should be heat treated according to the design documents.When the design documents are not specified, the following requirements shall be complied:


●Except the situation of maintaining temperature above 900 ℃ in initial pipe bending processing, the carbon steel pipe with a nominal thickness of more than 19mm shall be bent according to Table 2:


Table 2 Basic Requirements For Heat Treatment Of Pipes

Parent material category

Nominal thickness t(mm)

Minimum prescribed tensile strength of parent material (Mpa)

Heat treatment temperature (℃)

Constant temperature time (mim/mm)

The shortest constant temperature time (h)

Carbon steel(C)≤19AllNo Requirement--
Carbon manganese steel(C-Mn)>19All600-6502.41
Chrome molybdenum alloy steel (C-Mo、Mn-Mo、Cr-Mo) Cr≤0.5%≤19≤490No Requirement--
>19All600-7202.41
All>490600-7202.41
Chrome molybdenum alloy steel (C-Mo) 0.5%<Cr≤2%≤13≤490No Requirement--
>13All700-7502.42
All>490700-7502.42
Chrome molybdenum alloy steel (Cr-Mo) 2.25%≤Cr≤3%≤13AllNo Requirement--
>13All700-7602.42
Chrome molybdenum alloy steel(Cr-Mo)3%<Cr≤10%

All

All
700-7602.42
Martensitic stainless steelAllAllNo Requirement--
Ferritic stainless steelAllAllNo Requirement--
Austenitic stainless steel

All

AllNo Requirement--
Low temperature nickel steel (Ni≤4%)≤19AllNo Requirement--
>19All600-640
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Note: The heating rate and cooling rate of the heat treatment  for pipe bending should meet the following requirements:

1) When the heating temperature is increased to 400℃, the heating rate should not exceed (205 x 25/t)℃/h and must not exceed 205℃/h.

2) The constant cooling rate should not exceed (260×25/t)℃/h, and should not exceed 260℃/h, and it can be cooled naturally below 400°C.


7. When carbon steel, carbon manganese steel, chromium molybdenum alloy steel and low temperature nickel steel pipes is bending, whose nominal diameter is greater than or equal to 100mm, or whose nominal thickness is greater than or equal to 13mm carbon steel, shall be heat treated according to the following provisions:

1) When hot bending, complete annealing, normalizing and tempering or tempering should be done according to the design documents.

2) In cold pipe bending, heat treatment shall be carried out in accordance with Table 2 above.


8. After the pipe is bent, clean the inner and outer surfaces. The quality of bends should meet the following requirements:

1) There must be no defects such as cracks, burns, and delamination.

2) The height of the inner fold of the elbow should not exceed 3% of the outer diameter of the pipe, and the distance between waves (Figure 3) should not be less than 12 times as the height of the inner fold.The inner fold height should be calculated as follows:


pipe bending










In formula:

A: fold height (mm);

B: outside diameter of folding bulge (mm);

D: outside diameter of folding indentation (mm);

C: outside diameter of adjacent folding bulge (mm);


pipe bending

9. The roundness of the elbow should meet the following requirements:


1) The roundness of the elbow should be calculated as follows:


pipe bending

In formula:

A: the roundness of the elbow (%)

B: maximum measured outside diameter of the same section (mm)

C: minimum measured outer diameter of the same section (mm)


2) For bends subjected to internal pressure, the roundness should not exceed 8%; For bends subjected to external pressure, the roundness should not exceed 3%.