(1) Nominal size and actual size:

①Nominal size of seamless steel pipe is the nominal size specified in the standard, is the ideal size desired by users and production enterprises, and is also the order size indicated in the contract.
②Actual size: the actual size obtained in the production process, which is often greater than or less than the nominal size. This phenomenon of being larger or smaller than the nominal size is called deviation.

(2) Deviation and tolerance

①Deviation: in the production process of smls pipe , because the actual size is difficult to meet the nominal size requirements, that is, often greater than or less than the nominal size, so the standard stipulates that there is a difference between the actual size and the nominal size. A positive difference is called a positive deviation, and a negative difference is called a negative deviation.

②Tolerance: the sum of the absolute values of positive and negative deviations specified in the standard is used for tolerance, also called “tolerance zone”.

Deviation is square, namely “positive” or “negative”; The tolerance is not directional, so it is wrong to call the deviation value “positive tolerance” or “negative tolerance”.

(3) Delivery length:

Delivery length is also called user required length or contract length. The standard has the following provisions on delivery length:

① usually length (also known as non-fixed length) : where the length in the standard length range and no fixed length requirements, are called usually long. For example, structural pipe standards: hot rolled (extruded, expanded) steel pipe 3000mm~12000mm; Cold drawn (rolled) steel pipe 2000mm~10500mm.

② Measuring length: measuring length should be in the usual length range, which is a fixed length size required in the contract. However, it is impossible to cut out the absolute length in practice, so the standard provides the allowable positive deviation value for the length of the length.

Due to the production of fixed length tube than the usual length tube of the yield of a larger decline, the production enterprises put forward a markup request is reasonable, generally about 10% on the basis of the base price.

③The length of the double ruler: the length of the double ruler should be within the range of the usual length. The contract should indicate the length of the single ruler and the multiple of the total length (for example, 3000mmx3, that is, the triple multiple of 3000mm, the total length is 9000mm). In practice, the allowable positive deviation of 20mm should be added on the basis of the total length, and the margin of incision should be left for each haploid length. Taking the structural pipe as an example, the allowance for incision is specified: the outer diameter ≤159mm is 5~10mm; Outer diameter & gt; 159 mm for 10 to 15 mm.

If there is no stipulation IN the STANDARD on the deviation of The Times length and the cutting allowance, it shall be negotiated by the supply and demand parties and indicated in the contract. The double length is the same as the fixed length, which will also bring a significant reduction in the finished product rate to the production enterprise, and its markup range is basically the same as that of the fixed length.

④, the range length: the range length is usually within the range of length, when the user requires a fixed range length, it needs to be noted in the contract.

For example, the length of steel pipe is usually 3000 to 12000mm, and the length of the range is 6000 to 8000mm or 8000-1 0000mm.

Although the range length is looser than the fixed and double length, it is much stricter than the usual length, which will also bring the reduction of the finished product rate to the production enterprises. Therefore, it is reasonable for the production enterprises to raise the price, and the range of the price increase is generally about 4% on the base price.

(4) Uneven wall thickness:

The wall thickness of smls pipe cannot be the same everywhere, and there is an objective phenomenon of wall thickness inequality in the cross section and the longitudinal pipe body, that is, wall thickness inequality. In order to control this inhomogeneity, the allowable index of wall thickness inhomogeneity is stipulated in some steel pipe standards, which generally does not exceed 80% of the wall thickness tolerance (to be negotiated by the supply and demand parties).

(5) Ellipticity:

In the cross section of circular steel pipe, there is the phenomenon of unequal outside diameters, that is, there are the maximum and minimum outside diameters that are not necessarily perpendicular to each other, then the difference between the maximum and minimum outside diameters is the ellipticity (or not roundness). In order to control ellipticity, some steel pipe standards stipulate the allowable index of fineness roundness, which is generally not more than 80% of the outside diameter tolerance (implemented after negotiation between the supply and demand parties).

(6) Bending degree

The steel pipe is curvilinear in the direction of length, and its curvilinear degree is denoted by numbers. The bending degree specified in the standard is generally divided into the following two types:

①Local bending degree: one meter long ruler is used to measure the maximum bending of the steel pipe, and the chord height (mm) is measured, which is the value of the local bending degree. The unit is mm/m, and the method of expression is as follows: 2.5mm/m. This method is also suitable for pipe end bending.

② the total length of bending:

The maximum chord height (mm) at the bending point of the steel pipe is measured by pulling a string from both ends of the pipe, and then the percentage of length (in meters) is converted, which is the bending degree of the full length of the steel pipe.

For example, if the length of the precision steel pipe is 8m and the maximum chord height measured is 30mm, the bending degree of the full length of the pipe should be 0.03÷8mx0100%=0.375%.

(7) Size out of tolerance:

Size deviation or allowable deviation of size beyond standard. The “size” here refers mainly to the outer diameter and wall thickness of the pipe. It is usually called “tolerance out of order”, the deviation and tolerance of the same name is not strict, should be called “deviation out of order”. Here the deviation may be “positive” or “negative”, rarely in the same batch of steel pipe “positive” and “negative” deviation is out of line.

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