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Introduction to common terms used in steel pipe standards

[Summary]Is the final plastic deformation or final heat treatment of the delivery product state.

Introduction to common terms used in steel pipe standards

[Summary]Is the final plastic deformation or final heat treatment of the delivery product state.


(1)General terms

①Delivery state

Is the final plastic deformation or final heat treatment of the delivery product state. Generally not after heat treatment delivery is called hot rolling or cold drawing (rolling) state or manufacturing state; after heat treatment delivery is called heat treatment state, or according to the category of heat treatment is called normalized (normalized), tempering, solution, annealing state. When ordering, the delivery state should be specified in the contract.

②Delivery by actual weight or delivery by theoretical weight

Actual weight - when delivered, the product weight is according to the weighing (overweight) weight delivery;

Theoretical weight - when delivered, the weight of the product is the weight calculated according to the nominal size of the steel. The formula is as follows (for those requiring delivery by theoretical weight, this must be stated in the contract).

The theoretical weight of steel pipe per metre (steel density 7.85kg/dm3) is calculated as follows


Where: W - theoretical weight per metre of steel pipe, kg/m;

D - the nominal outer diameter of the steel pipe, mm;

S - the nominal wall thickness of the steel pipe, mm.

③ Guarantee conditions

According to the current standard of the project to test and ensure compliance with the provisions of the standard, called the guarantee conditions. Guarantee conditions are further divided into.

A, the basic guarantee conditions (also known as must guarantee conditions). Whether or not the customer in the contract specified. Are required to carry out the test according to the provisions of the standard, and to ensure that the test results comply with the provisions of the standard.

Such as chemical composition, mechanical properties, size deviation, surface quality and flaw detection, hydraulic test or flattening or flaring and other process performance test, are mandatory conditions.

B, the agreement to ensure that the conditions: the standard in addition to the basic guarantee conditions, there are still "according to the requirements of the demand side, by mutual agreement between supply and demand, and in the contract note?quot; or "when the demand side requirements ......, should be indicated in the contract"; there are customers, the standard in the basic guarantee conditions put forward to tighten the requirements (such as composition, mechanical properties, size (such as composition, mechanical properties, dimensional deviations, etc.) or additional inspection items (such as steel pipe ellipticity, uneven wall thickness, etc.). The above terms and requirements, when ordering, by the supply and demand sides to negotiate, sign the technical agreement of supply and specify in the contract. Therefore, these conditions are also known as the conditions of the agreement guarantee. Products with agreed guarantee conditions are generally subject to a price increase.


The "lot" in the standard refers to a unit of inspection, i.e. the inspection lot. If the delivery unit group batch, called the delivery batch. When the delivery batch is large, a delivery batch can include several inspection batches; when the delivery batch is small, an inspection batch can be divided into several delivery batches.

The composition of the "batch" usually has the following provisions (see the relevant standards):.

A, each batch should be the same grade (steel grade), the same furnace (tank) number or the same parent furnace number, the same specification and the same heat treatment system (furnace) of the steel pipe composition.

B, for good carbon steel structural pipe, fluid pipe, can be different furnace (tank) of the same grade, the same specification and the same heat treatment system (furnace) of steel pipe composition.

C, welded steel pipe each batch should be the same grade (steel grade), the same specification of steel pipe composition.

⑤ excellent steel and advanced excellent steel

In GB/T699-1999 and GB/T3077-1999 standard, its grade with "A" behind the word, for the senior excellent steel, and vice versa for the general excellent steel.

High quality steel is better than good steel in some or all of the following ways.

A. Reduces the range of constituent content;

B, reduce the harmful elements (such as sulphur, phosphorus, copper) content;

C, to ensure a high degree of purity (requiring less non-metallic inclusions);

D. Ensure high mechanical and technological properties.

(vi) Longitudinal and transverse

The standard said longitudinal is parallel to the processing direction (i.e., along the processing direction); transverse is perpendicular to the processing direction (processing direction that is the axial direction of the steel pipe).

Do the impact test, longitudinal specimen fracture because of the direction perpendicular to the processing. Therefore, called transverse fracture; transverse specimen fracture because of the processing direction parallel, so called longitudinal fracture.

(2) steel pipe shape, size terminology

① nominal size and actual size

A, nominal size: is the nominal size of the standard, is the ideal size of the user and production enterprises hope to get, is also the contract specified in the order size.

B. Actual size: it is the actual size obtained in the production process, which is often larger or smaller than the nominal size. This phenomenon of larger or smaller than the nominal size is called deviation.

②Deviation and tolerance

A, deviation: in the production process, because the actual size is difficult to achieve the nominal size requirements, that is, often larger or smaller than the nominal size, so the standard provides for the actual size and the nominal size of the difference between the allowed value. The difference is called positive deviation, the difference is called negative deviation.

B. Tolerance: The sum of the absolute values of the positive and negative deviations specified in the standard is called the tolerance, also called the "tolerance zone".

Deviation is directional, i.e. "positive" or "negative"; tolerance is not directional, therefore, the deviation value is called "positive tolerance" or "negative tolerance" is wrong.

③Delivery length

The delivery length is also called the user requirement length or contract length. There are several provisions in the standard for the delivery length as follows.

A, the usual length (also known as non-fixed length): where the length of the standard length range and no fixed length requirements, are known as the usual length. For example, structural pipe standards: hot-rolled (extruded, expanded) steel pipe 3000mm ~ 12000mm; cold-drawn (rolled) steel pipe 2000mmm ~ 10500mm.

B, fixed length: fixed length should be within the usual length range, is a fixed length of a certain size required in the contract. But the actual operation are cut out of the absolute fixed length is unlikely, so the standard for the fixed length of the allowed positive deviation value.

Take the standard for structural tubes as follows.

Production of fixed-length tubes than the usual length of the tube into a larger rate of decline, the production enterprises to raise the price request is reasonable. The rate of price increase is not consistent among enterprises, and is generally about 10% on the basis of the base price.

C, times the length: times the length of the length should be within the usual length, the contract should indicate the single times the length and constitute a multiple of the total length (for example, 3000mm × 3, that is, 3 times 3000mm, the total length of 9000mm). In practice, an allowable positive deviation of 20mm should be added to the total length, plus a margin of cut should be left for each single multiple length. In the case of structural tubes, for example, it is stipulated that the margin of notch should be left: 5-10mm for OD≤159mm; 10-15mm for OD>159mm.

If the standard does not have the deviation of the length and cutting allowance, the supply and demand sides should negotiate and specify in the contract. Double length scale with the same fixed length, will bring to the production enterprises into a substantial reduction in the rate of material, so the production enterprises to raise the price is reasonable, the rate of increase with the fixed length rate of increase is basically the same.

D, range length: range length in the usual length range, when the user requires a fixed range length, need to be specified in the contract.

For example: the usual length is 3000 to 12000mm, while the range of fixed length is 6000 to 8000mm or 8000 to 10000mm.

It can be seen that the range length is more lenient than the fixed and multi-length requirements, but much stricter than the usual length, but also to the production enterprises will bring the reduction of the material rate. Therefore it makes sense for the producer to propose a price increase, which is generally about 4% on the base price.

④Uneven wall thickness

Steel pipe wall thickness cannot be the same everywhere, in its cross-section and longitudinal tube body objective wall thickness inequality phenomenon, that is, uneven wall thickness. In order to control this unevenness, there are steel pipe standards in the wall thickness unevenness of the allowable indicators, the general provisions of not more than 80% of the wall thickness tolerance (after consultation between the supply and demand sides to implement).

⑤ Ellipticity

In the cross-section of the round steel pipe there is the phenomenon of unequal outside diameter, that is, there is not necessarily perpendicular to each other's maximum outside diameter and minimum outside diameter, the maximum outside diameter and minimum outside diameter difference that is the ellipticity (or not roundness). In order to control the ellipticity, some steel pipe standards specify the allowable index of ellipticity, generally specified as not more than 80% of the outer diameter tolerance (after consultation between supply and demand).


Steel pipe is curved in the direction of the length, with the number of its curvature is called bend. The standard bending degree is generally divided into the following two kinds.

A, local curvature: a metre-long straightedge against the maximum bend in the steel pipe, measuring its chord height (mm), that is, the value of the local curvature, the unit of mm / m, expressed as 2.5mm / m. This method also applies to the end of the pipe curvature.

B, the total length of the bend: a thin rope, from the two ends of the tube tension, measuring the maximum chord height at the bend of the steel pipe (mm), and then converted into the length (in meters) of the percentage, that is, the length of the steel pipe direction of the full-length bend.

For example: the length of the steel pipe is 8m, measured the maximum chord height of 30mm, then the full length of the tube bending degree should be.


(vii) Dimensional deviation

Dimensional deviation or dimensional exceedance of the standard allowable deviation. Here the "size" mainly refers to the outer diameter and wall thickness of the pipe. Usually some people are used to call the dimensional deviation "tolerance out", this deviation and tolerance equivalent to the call is not strict, should be called "deviation out". Here the deviation may be "positive", may also be "negative", rarely in the same batch of steel pipe "positive, negative" deviation are out of the phenomenon.

(3) Chemical analysis terminology

The chemical composition of steel is related to the quality of steel and end-use performance of one of the important factors, but also the preparation of steel, and even the final product heat treatment system is the main basis. Therefore, in the technical requirements of the steel standard part, often the first item on the applicable grade of steel (steel grade) and its chemical composition, and in the form of a table included in the standard, is an important basis for manufacturers and customers to accept the chemical composition of steel and steel. ① Steel melting composition

The chemical composition specified in the general standard refers to the melting composition. It refers to the chemical composition of steel smelting is completed, pouring in the middle of the chemical composition. In order to make it representative, that is, on behalf of the average composition of the furnace or tank, in the standard method of sampling, the steel will be cast into a small ingot in the sample mould, on which the sample chip is shaved or drilled, according to the standard method specified (GB/T223) for analysis, the results must be in line with the standard chemical composition range, but also the basis for customer acceptance.

② Finished product composition

Finished components, also known as verification analysis components, are the chemical composition obtained by drilling or shaving samples from finished steel according to the prescribed method (GB/T222) and analysed according to the prescribed standard method (GB/T223). Steel in the crystallization and later plastic deformation, because of the uneven distribution of alloying elements in the steel (segregation), so allow the finished composition and the standard composition range (melting composition) between the existence of deviations, the deviation value should be consistent with the provisions of GB/T222.

The finished composition of steel is mainly for the use of the department or quality inspection department to accept the quality of steel, the production enterprises generally do not do the finished product analysis (except for user requirements), but should ensure that the finished product analysis in line with the standard provisions.

③ Arbitration analysis

As a result of two laboratories to analyse the same sample results are significantly different and beyond the two laboratories allowed to analyze the error, or the production enterprises and the use of departments, demand and supply side of the same sample or the same batch of steel analysis of finished products have different views, can be analyzed by a third party with extensive experience in the unit (such as the China Iron and Steel Research Institute or the inspection department with commercial inspection qualifications) for re-analysis, known as arbitration analysis Arbitration analysis. The results of the arbitration analysis shall be the basis for the final decision. (4) Mechanical properties terminology

Steel mechanical properties is to ensure that the steel end-use properties (mechanical properties) of important indicators, it depends on the chemical composition of steel and heat treatment system. In the steel pipe standard, according to the different use requirements, specify the tensile properties (tensile strength, yield strength or yield point, elongation) and hardness, toughness indicators, and user requirements of high and low temperature performance, etc.

① Tensile strength (σb)

Specimen in tension, the maximum force (Fb), out of the original cross-sectional area of the specimen (So) stress (σ), known as the tensile strength (σb), the unit is N/mm2 (MPa). It represents the maximum capacity of a metal material to resist damage in tension. The calculation formula is

Where: Fb - the maximum force to which the specimen is subjected when it is pulled, N (Newton);

So - the original cross-sectional area of the specimen, mm2.

②Yield point (σs)

With the yield phenomenon of metal materials, the specimen in the stretching process of the force does not increase (remain constant) can continue to elongate the stress, called the yield point. If a fall in force occurs, a distinction should be made between the upper and lower yield points. The unit of yield point is N/mm2 (MPa).

Upper yield point (σsu): the maximum stress before the specimen yields and the force falls for the first time;

Lower yield point (σsl): the minimum stress in the yielding phase when the initial transient effect is not taken into account.

The formula for calculating the yield point is

Where: Fs - specimen yield force (constant) during tension, N (Newton);

So - the original cross-sectional area of the specimen, mm2.

③ Elongation after break (σ)

In the tensile test, the percentage increase in the length of the specimen after it has been pulled from its mark to the length of the original mark is called the elongation. It is expressed as σ in %. The formula is calculated as

Where: L1 - the length of the specimen after the specimen is pulled, mm;

L0 - specimen original pitch length, mm.

④ Fractional shrinkage rate (ψ)

In the tensile test, the maximum shrinkage of the cross-sectional area of the specimen at its shrinkage after pulling off as a percentage of the original cross-sectional area is called the fractional shrinkage rate. Expressed as ψ in %. The calculation formula is as follows

Where: S0 - the original cross-sectional area of the specimen, mm2;

S1 - the minimum cross-sectional area at the shrinkage of the specimen after pulling off, mm2.

⑤ Hardness index

The ability of a metal material to resist the indentation of a surface by a hard object is called hardness. Depending on the test method and the scope of application, hardness can be divided into Brinell hardness, Rockwell hardness, Vickers hardness, Shore hardness, microhardness and high temperature hardness. For tubes are generally used Brinell, Rockwell, Vickers hardness three.

A. Brinell hardness (HB)

Using a steel or carbide ball of a certain diameter, press the specified test force (F) into the surface of the specimen, remove the test force after the specified holding time and measure the indentation diameter (L) on the surface of the specimen. The Brinell hardness value is the quotient obtained by dividing the test force by the spherical surface area of the indentation. Expressed as HBS (steel balls) in N/mm2 (MPa).

Its calculation formula is

Where: F - the test force pressed into the surface of a metal specimen, N;

D - diameter of the steel ball for the test, mm;

d - the average diameter of the indentation, mm.

The determination of Brinell hardness is more accurate and reliable, but generally HBS is only applicable to metal materials up to 450 N/mm2 (MPa) and is not applicable to harder steel or thinner plates. In steel pipe standards, Brinell hardness is the most widely used and is often expressed in terms of the indentation diameter, d, of the material


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