The calculation of the width of the tube blank of the used roll forming machine is based on the allowance of the welded pipe, the allowance of sizing and the allowance of forming.

Welding allowance

The welded tube blank is formed into a cylinder and then enters the induction coil. Due to the skin effect and proximity effect of the current, the edge of the tube blank is heated to a semi-melted or molten state. Under the pressure of the squeeze roller, the molten metal at the two edges crystallizes together. to achieve the purpose of welding.

In order to weld firmly, during the extrusion process, a small amount of molten metal is squeezed out of the weld to form burrs attached to the upper and lower sides of the weld.

If the negligible factor in engineering, the oxidation of the edge of the tube blank during the welding process is excluded, the amount of extruded metal can be regarded as the welding allowance.

Sizing allowance

After welding into a tube, the roundness and diameter of the steel pipe are different from the standard requirements, and the diameter reduction and rounding process must be carried out to make the outer diameter of the steel pipe meet the standard requirements. Therefore, it is necessary to leave a certain reduction and finishing allowance on the width of the strip, that is, the sizing allowance.

Forming allowance

During the forming process of the steel pipe, the outer wall of the pipe blank has been strictly limited in a certain shape of the roll groove. The inner wall of the tube blank is less and less restricted as the edge of the tube blank rises, and it is not restricted after it is rolled into a cylindrical shape. In the process of completing the inward bending deformation of the tube blank under this condition, there is a slight thickening phenomenon, thus making the tube blank narrow. In order to offset the narrowing of the tube blank due to the thickening, it is necessary to add a certain allowance to the width of the strip, that is, the forming allowance.

The tube blank is constantly stretched throughout the forming process of the used roll forming machine, which also narrows the strip. Under normal conditions, the stretch will not exceed the elastic limit of the steel. This can be proved from the obvious yield point in the elongation test of the low carbon steel finished pipe. Therefore, the amount of narrowing caused by stretching will basically disappear as the tube blank is rolled into a cylinder. Therefore, this factor can be ignored when considering the forming allowance.

By measuring the thickness of the welded pipe blank and the wall thickness of the finished pipe, it is easy to obtain the wall thickness increase during the production of the welded pipe.

The increase in the thickness of the finished tube compared to the blank tube is not solely caused by the forming process. The reducing and finishing process of the sizing process will cause a slight thickening of the pipe wall while drawing the steel pipe to elongate it. Therefore, the increase in the wall thickness of the finished pipe is the sum of the increase in the wall thickness during the forming process and the increase in the wall thickness during the sizing process.

The empirical formulas commonly used by manufacturers in calculating the width of blanks are as follows. However, the selection range of the relevant coefficients in these formulas is large, and the calculation is more complicated.

At present, the commonly used empirical formulas for calculating the width of blanks mainly include the following:

Formula 1: Blank width = π (finished tube outer diameter + sizing allowance - blank thickness) + welding and forming allowance coefficient × blank thickness

In the formula, the sizing allowance is taken as a coefficient of 0.7~1.5mm.

The welding and forming allowance coefficient is 0.5~2.5mm. The thinner the tube wall, the larger the diameter and the lower the carbon content, the larger the value. Or determined according to production experience.

This formula is applicable to welded pipes whose outer diameter is less than or equal to 114mm.

Formula 2: Blank width = π (finished tube outer diameter - blank thickness) + forming allowance + welding allowance + sizing allowance

This formula applies to the circular deformation method.

The values of forming allowance, welding allowance and sizing allowance are shown in the table below.

Formula 3: Blank width = (Finished tube outer diameter - Blank thickness) × 1.046~1.05

The selection of the range of 1.046~1.05 in the formula needs to be determined according to production experience. When the outer diameter of the finished pipe is greater than 40mm, the coefficient is 1.04~1.045; when the outer diameter of the finished pipe is smaller or equal to 40mm, the coefficient is 1.046~1.05. This formula applies to thin-walled tubes.

Two empirical formulas with simple calculation methods and reliable results are recommended here.

Formula 1: Blank width = π (finished tube outer diameter - blank thickness) + 3mm

The scope of application of this formula: straight seam welded pipes with a thickness between 0.7 and 2 and an outer diameter below φ50.

In the formula, π (the outer diameter of the finished pipe - the thickness of the billet) refers to the width of the theoretical non-deformation layer during the forming process of the steel pipe. This is a necessary basic dimension in the calculation of the blank width of the straight seam welded pipe. The constant 3mm refers to the sum of the required allowance for the steel pipe in the process of forming, welding and sizing.

Formula 2: Blank width = π (finished tube outer diameter - tube blank thickness) + (0.438 tube blank thickness) + (0.035 finished tube outer diameter) + 2.07

This formula has no restrictions on the use of used roll forming machine.

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