(1) General Terms

① Delivery Status

Refers to the final plastic deformation or final heat treatment state of the delivered product. Generally, products delivered without heat treatment are referred to as hot-rolled or cold-drawn (rolled) state or manufacturing state; those delivered after heat treatment are referred to as heat-treated state, or according to the type of heat treatment, referred to as normalizing, quenching and tempering, solution treatment, or annealing state. The delivery status must be specified in the contract when ordering.

② Delivery by Actual Weight or Theoretical Weight

Actual Weight -- At the time of delivery, the product weight is based on the weighed (weighbridge) weight;

Theoretical Weight -- At the time of delivery, the product weight is calculated based on the nominal dimensions of the steel. The calculation formula is as follows (those requiring delivery by theoretical weight must specify it in the contract):

The theoretical weight of steel pipes per meter (density of steel is 7.85kg/dm3) calculation formula:

W=0.02466*(D-S)*S

Where: W -- Theoretical weight of steel pipe per meter, kg/m;

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

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

③ Guarantee Conditions

Inspection is conducted according to the current standard's specified items and guarantees compliance with the standard's regulations, referred to as guarantee conditions. Guarantee conditions are further divided into:

A. Basic Guarantee Conditions (also known as Mandatory Conditions). Regardless of whether the customer specifies it in the contract, inspection must be conducted according to standard regulations, and the inspection results must comply with the standard.

Such as chemical composition, mechanical properties, dimensional deviations, surface quality, as well as non-destructive testing, hydraulic testing, or flattening or expansion tests, all belong to mandatory conditions.

B. Agreed Guarantee Conditions: In addition to basic guarantee conditions in the standard, there are also "according to the buyer's requirements, negotiated by both parties, and specified in the contract" or "when the buyer requests... it should be specified in the contract"; some customers may impose stricter requirements on the basic guarantee conditions in the standard (such as composition, mechanical properties, dimensional deviations, etc.) or add inspection items (such as ovality of steel pipes, uneven wall thickness, etc.). The above clauses and requirements are negotiated by both parties at the time of ordering, signing a technical supply agreement, and specifying in the contract. Therefore, these conditions are also known as agreed guarantee conditions. Products with agreed guarantee conditions generally require an additional charge.

④ Batch

The term "batch" in the standard refers to a unit of inspection, i.e., an inspection batch. If grouped by delivery unit, it is called a delivery batch. When the delivery batch is large, one delivery batch may include several inspection batches; when the delivery batch is small, one inspection batch may be divided into several delivery batches.

The composition of a "batch" usually has the following regulations (see relevant standards for details):

A. Each batch should consist of steel pipes of the same grade, same furnace (tank) number, or the same mother furnace number, the same specification, and the same heat treatment system (furnace batch).

B. For excellent carbon structural pipes and fluid pipes, they can consist of steel pipes of the same grade, same specification, and the same heat treatment system (furnace batch) from different furnaces (tanks).

C. Each batch of welded steel pipes should consist of steel pipes of the same grade and same specification.

⑤ Excellent Steel and High-Quality Excellent Steel

In the GB/T699-1999 and GB/T3077-1999 standards, those with an "A" after the grade are high-quality excellent steel, while those without are general excellent steel.

High-quality excellent steel is superior to excellent steel in part or all of the following aspects:

A. Narrowing the range of component content;

B. Reducing the content of harmful elements (such as sulfur, phosphorus, copper);

C. Ensuring higher purity (requiring low content of non-metallic inclusions);

D. Ensuring higher mechanical properties and process performance.

⑥ Longitudinal and Transverse

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

When conducting impact tests, the fracture of the longitudinal sample is perpendicular to the processing direction, hence referred to as transverse fracture; the fracture of the transverse sample is parallel to the processing direction, hence referred to as longitudinal fracture.

(2) Steel Pipe Shape and Dimension Terms

① Nominal Size and Actual Size

A. Nominal Size: The nominal size specified in the standard, which is the ideal size that users and manufacturers hope to obtain, and is also the ordered size specified in the contract.

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

② Deviation and Tolerance

A. Deviation: During the production process, due to the difficulty in achieving the nominal size requirements, the actual size is often larger or smaller than the nominal size, so the standard specifies that there can be a difference between the actual size and the nominal size. A positive difference is called positive deviation, and a negative difference is called negative deviation.

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

Deviation has directionality, represented as "positive" or "negative"; tolerance does not have directionality, therefore, referring to deviation values as "positive tolerance" or "negative tolerance" is incorrect.

③ Delivery Length

Delivery length is also known as user-required length or contract length. The standard has the following provisions regarding delivery length:

A. Normal Length (also known as Non-Standard Length): Any length within the length range specified by the standard and without fixed length requirements is referred to as normal length. For example, the structural pipe standard specifies: hot-rolled (extruded, expanded) steel pipes 3000mm to 12000mm; cold-drawn (rolled) steel pipes 2000mm to 10500mm.

B. Fixed Length: The fixed length should be within the usual length range and is a specific length dimension required in the contract. However, it is unlikely to cut out an absolute fixed length in actual operations, so the standard specifies allowable positive deviation values for fixed lengths.

According to the structural pipe standard:

The yield rate of producing fixed length pipes is significantly lower than that of producing usual length pipes, so it is reasonable for manufacturers to request a price increase. The price increase varies among companies, generally around 10% on top of the base price.

C. Multiple Length: The multiple length should be within the usual length range, and the contract should specify the single multiple length and the multiple that constitutes the total length (for example, 3000mm×3, which means three times 3000mm, totaling 9000mm). In actual operations, an allowable positive deviation of 20mm should be added to the total length, plus a cutting allowance for each single multiple length. For structural pipes, the cutting allowance is specified as: outer diameter ≤ 159mm is 5-10mm; outer diameter > 159mm is 10-15mm.

If there are no specifications for multiple length deviations and cutting allowances in the standard, it should be negotiated between the supply and demand parties and noted in the contract. The multiple length, like the fixed length, will significantly reduce the yield rate for manufacturers, so it is reasonable for manufacturers to request a price increase, which is generally similar to the price increase for fixed lengths.

D. Range Length: The range length is within the usual length range. When the user requests a specific range length, it must be specified in the contract.

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

It can be seen that the range length requirements are more lenient than fixed and multiple lengths, but much stricter than the usual length, which will also lead to a decrease in yield rate for manufacturers. Therefore, it is reasonable for manufacturers to request a price increase, generally around 4% on top of the base price.

④ Uneven Wall Thickness

The wall thickness of steel pipes cannot be the same everywhere; there is an objective phenomenon of varying wall thickness in its cross-section and longitudinal body, known as uneven wall thickness. To control this unevenness, some steel pipe standards specify allowable indicators for uneven wall thickness, generally not exceeding 80% of the wall thickness tolerance (to be implemented after negotiation between supply and demand parties).

⑤ Ovality

In the cross-section of round steel pipes, there is a phenomenon of varying outer diameters, meaning there are maximum and minimum outer diameters that are not necessarily perpendicular to each other. The difference between the maximum and minimum outer diameters is called ovality (or non-roundness). To control ovality, some steel pipe standards specify allowable indicators for ovality, generally not exceeding 80% of the outer diameter tolerance (to be implemented after negotiation between supply and demand parties).

⑥ Curvature

Steel pipes exhibit a curved shape along their length, and the degree of this curvature is referred to as curvature. The standards generally define two types of curvature:

A. Local Curvature: A one-meter long straight ruler is used to measure the maximum curvature of the steel pipe, measuring its chord height (mm), which is the value of local curvature, expressed in mm/m, such as 2.5mm/m. This method is also applicable to the curvature at the ends of the pipe.

B. Total Curvature: A thin rope is pulled tight from both ends of the pipe, measuring the maximum chord height (mm) at the curvature of the steel pipe, then converting it into a percentage of the length (in meters), which represents the total curvature in the length direction of the steel pipe.

For example: if the length of the steel pipe is 8m and the maximum chord height measured is 30mm, then the total curvature of the pipe should be:

0.03÷8m×100%=0.375%.

⑦ Dimension Tolerance

Dimension tolerance, or the allowable deviation beyond standard dimensions, primarily refers to the outer diameter and wall thickness of the steel pipe. It is common for people to refer to dimension tolerance as "tolerance exceeding limits," but this terminology is not precise; it should be called "deviation exceeding limits." The deviation here can be "positive" or "negative," and it is rare for both positive and negative deviations to exceed limits in the same batch of steel pipes.

(3) Chemical Analysis Terminology

The chemical composition of steel is one of the important factors related to the quality and final performance of steel materials, and it is also the main basis for formulating heat treatment systems for steel materials and even final products. Therefore, in the technical requirements section of steel material standards, the first item often specifies the applicable grades (steel grades) and their chemical compositions, listed in tabular form, which serves as an important basis for manufacturers and customers to accept steel and its chemical composition. ① Melting Composition of Steel

The chemical composition specified in general standards refers to the melting composition, which is the chemical composition during the mid-stage of pouring after the steel has been melted. To ensure it has a certain representativeness, representing the average composition of that furnace or ladle, the sampling standard method specifies that the molten steel is cast into small ingots in a sample mold, and samples are taken by planing or drilling, analyzed according to the specified standard method (GB/T223), and the results must meet the standard chemical composition range, which is also the basis for customer acceptance.

② Finished Product Composition

Finished product composition, also known as verification analysis composition, is obtained by drilling or planing samples from finished steel materials according to specified methods (GB/T222) and analyzing them according to the specified standard method (GB/T223). Due to the uneven distribution of alloy elements in steel during crystallization and subsequent plastic deformation (segregation), deviations between the finished product composition and the standard composition range (melting composition) are allowed, and these deviation values must comply with the provisions of GB/T222.

The finished product composition of steel materials is mainly used for acceptance of steel quality by the user department or quality inspection department. Generally, manufacturers do not conduct finished product analysis (except when requested by users), but they should ensure that finished product analysis complies with standard regulations.

③ Arbitration Analysis

When there are significant differences in the analysis results of the same sample by two laboratories that exceed the allowable analytical error of both laboratories, or when there are differing opinions between the manufacturer and the user department, or between the buyer and the seller regarding the finished product analysis of the same sample or batch of steel, a third party with rich analytical experience (such as the China Iron and Steel Research Institute or a qualified inspection department) can conduct a re-analysis, known as arbitration analysis. The result of the arbitration analysis serves as the final basis for judgment. (4) Mechanical Performance Terminology

The mechanical properties of steel are important indicators that ensure the final performance (mechanical properties) of steel, which depend on the chemical composition and heat treatment system of the steel. In the steel pipe standards, tensile properties (tensile strength, yield strength or yield point, elongation) as well as hardness, toughness indicators, and high and low temperature performance as required by users are specified according to different usage requirements.

① Tensile Strength (σb)

The maximum force (Fb) that the sample bears at the moment of fracture during the tensile process, divided by the original cross-sectional area of the sample (So), is called the tensile strength (σb), with units of N/mm2 (MPa). It indicates the maximum ability of metal materials to resist failure under tensile forces. The calculation formula is:

In the formula: Fb -- the maximum force borne by the sample at fracture, N (Newton);

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

② Yield Point (σs)

For metallic materials that exhibit yield phenomena, the stress at which the sample can continue to elongate while the force remains constant (not increasing) during the tensile process is called the yield point. If the force decreases, the upper and lower yield points should be distinguished. The unit of the yield point is N/mm2 (MPa).

Upper Yield Point (σsu): the maximum stress before the force first decreases when the sample yields;

Lower Yield Point (σsl): the minimum stress during the yield stage when the initial instantaneous effect is not considered.

The calculation formula for the yield point is:

In the formula: Fs -- the yield force during the tensile process of the sample (constant), N (Newton);

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

③ Elongation after Fracture (σ)

In the tensile test, the percentage of the increase in length of the gauge length after the sample fractures compared to the original gauge length is called the elongation. It is represented by σ, with units of %. The calculation formula is:

In the formula: L1 -- the gauge length after the sample fractures, mm;

L0 -- the original gauge length of the sample, mm.

④ Reduction of Area (ψ)

In the tensile test, the percentage of the maximum reduction in cross-sectional area at the necking point after the sample fractures compared to the original cross-sectional area is called the reduction of area. It is represented by ψ, with units of %. The calculation formula is as follows:

In the formula: S0 -- the original cross-sectional area of the sample, mm2;

S1 -- the minimum cross-sectional area at the necking point after the sample fractures, mm2.

⑤ Hardness Index

The ability of metal materials to resist indentation on their surface by hard objects is called hardness. Depending on the testing method and applicable range, hardness can be divided into Brinell hardness, Rockwell hardness, Vickers hardness, Shore hardness, microhardness, and high-temperature hardness, among others. For pipes, the commonly used types are Brinell, Rockwell, and Vickers hardness.

A. Brinell Hardness (HB)

Using a steel ball or hard alloy ball of a certain diameter, a specified test force (F) is applied to press into the surface of the sample. After a specified holding time, the test force is removed, and the diameter of the indentation (L) on the surface of the sample is measured. The Brinell hardness value is the quotient of the test force divided by the surface area of the indentation ball. It is represented as HBS (steel ball), with units of N/mm2 (MPa).

The calculation formula is:

In the formula: F -- the test force applied to the surface of the metal sample, N;

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

d -- the average diameter of the indentation, mm.

The determination of Brinell hardness is relatively accurate and reliable, but generally, HBS is only applicable to metal materials below 450 N/mm2 (MPa). It is not suitable for harder steels or thinner plates. In the steel pipe standards, Brinell hardness is the most widely used, often represented by the indentation diameter d, which is both intuitive and convenient.

For example: 120HBS10/1000130: indicates that the Brinell hardness value measured with a 10mm diameter steel ball under a test force of 1000Kgf (9.807KN) held for 30 seconds is 120N/mm2 (MPa).

B. Rockwell Hardness (HK)

The Rockwell hardness test is similar to the Brinell hardness test, both being indentation test methods. The difference is that it measures the depth of the indentation. That is, under the action of the initial test force (Fo) and the total test force (F) in succession, the indenter (diamond cone or steel ball) is pressed into the surface of the sample. After a specified holding time, the main test force is removed, and the hardness value is calculated using the measured increment of the residual indentation depth (e). Its value is a dimensionless number, represented by the symbol HR, with 9 scales used: A, B, C, D, E, F, G, H, K. Among them, the scales commonly used for hardness testing of steel are generally A, B, C, namely HRA, HRB, HRC.