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What is the National Standard for Straightness of Seamless Steel Tubes?

Date:2021-07-21    keywords:straightness of seamless steel pipes, pipe standard

In the production process of seamless steel tubes, the steel tube body will have different degrees of curve in the longitudinal direction (ie, the length direction). However, the degree of curve of some seamless steel tubes can be intuitively seen with the naked eye. This is necessary to be seamless. The steel pipe is already a substandard product. As long as it is a regular manufacturer, such a product is definitely not allowed to leave the factory, and some require special equipment to measure its straightness. The degree of this curve expressed by numbers is called steel pipe bending or steel pipe straightness. For the straightness of steel pipes, the requirements are not high in general pipelines for fluid transportation. This kind of pipes mainly require the concentricity of the nozzles to facilitate the welding of the pipes. However, if steel pipes are used in processing machinery and equipment, especially rubber rollers, central shafts, etc., they have higher requirements for straightness or bending, and this is also a very important index for them. Because it involves a processing volume problem, the processing volume directly affects the production cost. If it cannot be processed, the steel pipe will be a waste product. Therefore, manufacturers like the production of roller body equipment pay special attention to the straightness of the steel pipe when buying seamless steel tubes.


Seamless tube straightening machine


Generally speaking, there are two types of seamless steel tube straightness or bending calculation:
1. Partial curvature of seamless steel tube or straight seam steel tube: that is, use a one-meter straight ruler to measure the maximum bend of the steel pipe, and measure the chord height (mm), which is the local curvature value, and the unit is mm /m, means such as 2.5mm/m. This method is also suitable for tube end curvature.

2. The total bending degree of the seamless steel tube full length: Use a thin rope to pull tight from both ends of the pipe, measure the maximum chord height (mm) of the steel pipe bend, and then convert it into a percentage of the length (in meters), which is The overall curvature of the steel pipe in the longitudinal direction.

For example: the length of the steel pipe is 10m, and the measured maximum chord height is 30mm, the total length of the pipe should be:

             0.03÷10m×100%=0.3%

Let's take a look at the straightness requirements on the national standard?
Seamless steel tubes mainly have two delivery states: annealed and cold. In the annealed state, the standard for judging the straightness of seamless steel pipes is GB/T 8162 and GB/T 8163, as shown below:

Bending degree of steel pipe


Nominal wall thickness of steel pipe S/mm
Bend per meter/(mm/m)
≤15
≤1.5
>15~30 
≤2.0
>30 or D≥351
 ≤3.0

(1) The full-length curvature of the steel pipe should not be greater than 0.15% of the total length of the steel pipe.

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