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[Summary]Seamless steel pipe is divided into two types of hot rolled (extruded) seamless steel pipe and cold drawn (rolled) seamless steel pipe due to their different manufacturing processes.

Seamless tube knowledge

[Summary]Seamless steel pipe is divided into two types of hot rolled (extruded) seamless steel pipe and cold drawn (rolled) seamless steel pipe due to their different manufacturing processes.

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Classification of seamless pipes

Seamless steel pipe is divided into two types of hot rolled (extruded) seamless steel pipe and cold drawn (rolled) seamless steel pipe due to their different manufacturing processes. Cold-drawn (rolled) tubes are also divided into two types of round tubes and shaped tubes. Seamless steel pipe, and because of its different uses and divided into a number of varieties as follows.

1.GB/T8162-2008 (seamless steel pipe for structural use). Mainly used for general structure and mechanical structure. Its representative materials (grades): carbon steel 20, 45 steel; alloy steel Q345, 20Cr, 40Cr, 20CrMo, 30-35CrMo, 42CrMo, etc.

2.GB/T8163-2008 (seamless steel pipe for conveying fluids). Mainly used in engineering and large equipment on the transport of fluid pipeline. The representative materials (grades) are 20, Q345, etc.

3.GB3087-2008 (seamless steel pipe for low and medium pressure boilers). Mainly used for industrial boilers and living boilers to transport low and medium pressure fluid piping. The representative material is 10, 20 steel.

4.GB5310-2008 (seamless steel pipe for high pressure boilers). Mainly used for power station and nuclear power station boilers on the high temperature, high pressure transmission fluid collection box and pipeline. The representative materials are 20G, 12Cr1MoVG, 15CrMoG, etc.

5.GB5312-2009 (carbon steel and carbon manganese steel seamless steel pipe for ships). Mainly used for ship boilers and superheaters with I, II pressure-resistant tubes, etc.. The representative material is 360, 410, 460 steel grade, etc.

6.GB6479-2013 (seamless steel pipe for high pressure fertilizer equipment). Mainly used for fertilizer equipment on the transport of high temperature and high pressure fluid pipeline. The representative materials are 20, 16Mn, 12CrMo, 12Cr2Mo, etc.

7.GB9948-2013 (seamless steel pipe for petroleum cracking). Mainly used in petroleum refinery boilers, heat exchangers and their transport fluid pipelines. Its representative materials are 20, 12CrMo, 1Cr5Mo, 1Cr19Ni11Nb, etc.

8.GB18248-2008 (seamless steel pipe for gas cylinder). Mainly used for the production of various gas, hydraulic cylinders. Its representative material is 37Mn, 34Mn2V, 35CrMo, etc.

In addition, there are GB/T17396-1998 (hot-rolled seamless steel tubes for hydraulic pillars), GB3093-1986 (high-pressure seamless steel tubes for diesel engines), GB/T3639-1983 (cold-drawn or cold-rolled precision seamless steel tubes), GB/T3094-1986 (cold-drawn seamless steel tubes shaped steel tubes), GB/T8713-1988 (hydraulic and pneumatic cylinders GB/T8713-1988(Precision seamless inner diameter steel tubes for hydraulic and pneumatic cylinders), GB13296-1991(Stainless steel seamless steel tubes for boilers and heat exchangers), GB/T14975-1994(Stainless steel seamless tubes for structures), GB/T14976-1994(Stainless steel seamless tubes for fluid transportation), GB/T5035-1993(Seamless steel tubes for automobile semi-axial casing), API SPEC5CT-2012 (casing and oil pipe specification), etc.

Common terms used in steel pipe standards - General

①Delivery condition

It refers to the final plastic deformation or final heat treatment state of the delivered product. Generally not after heat treatment delivery is called hot-rolled or cold-drawn (rolled) state manufacturing state; after heat treatment delivery is called heat treatment state, or according to the category of heat treatment is called normalized (normalized), tempered, solid solution, annealed 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 for its calculation is as follows (delivery by theoretical weight is required and must be specified in the contract).

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

W=0.02466*(D-S)*S

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, known as 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 by the standard provisions of the test, 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 guarantee conditions: the standard in addition to the basic guarantee conditions, there are "according to the requirements of the demand side, by mutual agreement between supply and demand, and specified in the contract" or "when the demand side requirements ......, should be specified in the contract "; there are also customers, the basic guarantee conditions in the standard to tighten the requirements (such as composition, mechanical properties, size deviation, etc.) or increase the test items (such as steel oval, 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 in the contract specified. 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.

④Batch

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 and GB/T3077 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.

Common terms used in steel pipe standards - shape, size

① Nominal size and actual size

A, nominal size: is the nominal size specified in 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 specify 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-fold 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).

⑥Bendiness

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, its unit mm / m, expressed as 2.5mm / m. This method is also applicable 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 should be: 0.03 ÷ 8m × 100% = 0.375%

(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 in the "positive, negative" deviation are out of the phenomenon.

Common terms used in steel pipe standards - chemical analysis

The chemical composition of steel is an important factor in the quality and end-use performance of steel, 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 product composition, also known as verification analysis, is the chemical composition of the finished steel according to the prescribed method (GB/T222), drilled or shaved samples 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 laboratory analysis of the same sample results are significantly different and beyond the two laboratories allowed analytical error, or 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 are the basis for the final decision.

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