Introduction to Common Terms Used in Steel Pipe Standards

(2) Shape and dimension terms of steel pipes ① Nominal dimensions and actual dimensions A. Nominal dimensions: These are the nominal sizes specified in the standards; they represent the ideal sizes desired by users and manufacturers, and they are also the sizes indicated in the orders placed under contracts. B. Actual size: It is the actual size obtained during the manufacturing process, and this size is often greater than or smaller than the nominal size. This phenomenon of being greater than or less than the nominal size is called deviation. ②Deviation and tolerance A. Deviation: During the production process, it is difficult to achieve the nominal size; the actual size is often either greater than or smaller than the nominal size. Therefore, standards specify an allowable difference between the actual size and the nominal size. A positive difference is called a positive deviation, while a negative difference is called a negative deviation. B. Tolerance: The sum of the absolute values of the positive and negative deviation values specified in the standard is called the tolerance, also known as the “tolerance band”. Deviation has a direction, that is, it is expressed as “positive” or “negative” ； Tolerances have no direction, therefore it is incorrect to refer to deviation values as \"positive tolerances\" or \"negative tolerances\". ③Delivery length is also known as the customer-requested length or contract length. The standards specify the following types for delivery length: A. Normal length (also known as non-fixed length): Any length that falls within the range specified by the standards and for which no fixed length is required is considered a normal length. For example, the standards for structural pipes specify that hot-rolled (extruded, expanded) steel pipes range from 3000 mm to 12000 mm ； Cold-drawn (rolled) steel pipes: 2000 mm to 10,500 mm. B. Fixed length: The fixed length should fall within the normal range of lengths, and it is a specific length specified in the contract. However, it is practically impossible to cut pieces to an absolutely exact length in real operations; therefore, the standards specify allowable positive deviations for the length. According to the standards for structural pipes, the yield rate for pipes produced in fixed lengths is significantly lower than that for pipes of standard lengths; therefore, it is reasonable for manufacturers to request a higher price. The degree of price increase varies among different companies; generally, it is around 10% on top of the base price. C. Scale length: The scale length should fall within the normal range; the contract should specify the length of a single scale unit as well as the multiple by which it contributes to the total length (for example, 3000mm×3, meaning 3 times 3000mm, resulting in a total length of 9000mm). In practice, an allowable positive deviation of 20 mm should be added to the total length, in addition to the cutting allowance required for each individual scale length. Taking structural pipes as an example, a cut allowance is specified: 5–10 mm for an outer diameter ≤ 159 mm ； For an outer diameter > 159 mm, it is 10–15 mm. If the standard does not specify tolerances for scale length and cutting allowance, these shall be agreed upon by both the supplier and the buyer and stated in the contract. Just like with fixed-length products, using a multiple-length scale results in a significant reduction in the yield for manufacturing companies; therefore, it is reasonable for these companies to request a price increase, and the extent of this increase is roughly the same as that for fixed-length products. D. Range length: The range length is within the normal range; when the user requests a specific fixed range length, it must be specified in the contract. For example: the usual length is 3000–12000 mm, while the fixed-length ranges are 6000–8000 mm or 8000–10000 mm. It can be seen that the requirement for range length is more lenient than those for fixed-length and multiple-length specifications, but it is still much stricter than the usual length requirements; this also leads to a decrease in the yield rate for manufacturing enterprises. Therefore, it is reasonable for manufacturing companies to request a price increase; the amount of this increase is generally around 4% on top of the base price. ④Non-uniform wall thickness: The wall thickness of a steel pipe cannot be the same everywhere; there is an inherent variation in wall thickness across its cross-section and along its length, which is what is referred to as non-uniform wall thickness. To control this unevenness, some steel pipe standards specify allowable limits for wall thickness variations, generally requiring them not to exceed 80% of the wall thickness tolerance (as agreed upon through consultation between the supplier and the buyer). ⑤Ellipticity refers to the phenomenon in the cross-section of circular steel pipes where the outer diameters are not equal; in other words, there is a maximum outer diameter and a minimum outer diameter that are not necessarily perpendicular to each other. The difference between the maximum and minimum outer diameters is what constitutes ellipticity (or lack of roundness). To control ovality, some steel pipe standards specify allowable limits for ovality, generally requiring it not to exceed 80% of the outer diameter tolerance (as agreed upon through consultation between the supplier and the buyer). ⑥Curvature: The steel pipe is curved along its length, and the degree of this curvature is expressed numerically; this is what is referred to as curvature. The bending degrees specified in the standards are generally divided into the following two types: A. Local bending degree: A 1-meter-long straight ruler is placed against the most bent part of the steel pipe, and the height of the chord at that point (in mm) is measured; this value represents the local bending degree, with the unit being mm/m, expressed for example as 2.5 mm/m. This method is also applicable to the bend degree at the pipe end. B. Total bending degree over the full length: A thin rope is used to pull taut from both ends of the pipe; the maximum chord height at the bent portion of the steel pipe is measured in millimeters, and this value is then converted into a percentage of the length in meters – this represents the total bending degree along the length of the steel pipe. For example: if the length of the steel pipe is 8 meters and the maximum chord height measured is 30 mm, then the bending degree of the entire pipe should be: 0.03 ÷ 8 meters × 100% = 0.375%. ⑦ Dimensional tolerances: Dimensional tolerances refer to deviations from the allowable limits specified for dimensions. The “dimensions” mentioned here refer mainly to the outer diameter and wall thickness of the steel pipe. Often, when the dimensions are out of spec*, it is casually referred to as \"tolerance exceeding limits.\" Equating deviation with tolerance is an imprecise term; it should be called \"deviation exceeding limits.\" The deviation here can be either \"positive\" or \"negative\"; it is rare for both positive and negative deviations to be out of range in the same batch of steel pipes. This post was last edited by htsky77 on 2009-3-22 08:42]

(3) Terms in chemical analysis: The chemical composition of steel is one of the important factors affecting the quality of steel and its ultimate performance; it also serves as the main basis for determining the heat treatment procedures for steel, as well as for the final products. Therefore, in the technical requirements section of steel standards, the first item usually specifies the grade of steel applicable to it as well as its chemical composition, which is listed in a table within the standard; this serves as an important basis for manufacturing companies and customers to verify the chemical composition of steel. ①The melting composition of steel: The chemical composition specified in general standards refers to the melting composition. It refers to the chemical composition at the middle stage of casting, after steel smelting is complete. To ensure a certain degree of representativeness, that is, to reflect the average composition of the furnace or tank, the standard sampling method specifies that the molten steel be cast into small ingots in sample molds, from which sample chips are planed or drilled off. These samples are then analyzed using the specified standard method (GB/T223), and the results must fall within the specified chemical composition range; this range serves as the basis for customer acceptance. ②Composition of the finished product: The composition of the finished product, also known as the composition obtained through verification analysis, is the chemical composition determined by drilling or planing samples from the finished steel product using specified methods (GB/T222), and then analyzing those samples according to prescribed standard methods (GB/T223). During crystallization and subsequent plastic deformation of steel, due to the uneven distribution of alloying elements in the steel (segregation), it is permissible for there to be deviations between the composition of the final product and the standard composition range (melting composition); such deviations must comply with the provisions of GB/T222. The finished product composition of steel is primarily used by the end-users or quality inspection departments to assess the quality of the steel. Manufacturing companies generally do not conduct analyses on the finished products (except when requested by the customers), but they must ensure that such analyses comply with relevant standard specifications. ③Arbitration analysis: When there are significant differences in the results obtained by two laboratories analyzing the same sample, and these differences exceed the allowable analytical errors for each laboratory, or when there are disagreements between the manufacturing company and the users, or between the buyers and suppliers regarding the analysis of the same sample or batch of steel products, a third-party authority with extensive analytical experience (such as the China Iron and Steel Research Institute or inspection agencies with customs inspection qualifications) can conduct a re-analysis; this is what is known as arbitration analysis. The results of the arbitration analysis serve as the final basis for judgment.

(4) Terms related to mechanical properties: The mechanical properties of steel are important indicators that ensure its performance in actual use (mechanical characteristics), and they depend on the chemical composition of the steel as well as the heat treatment process applied to it. In steel pipe standards, depending on various application requirements, specifications are set for tensile properties (tensile strength, yield strength or yield point, elongation), as well as hardness and toughness indicators, in addition to high and low temperature performance as required by the users. ①Tensile strength (σb) is the stress (σ) resulting from the maximum force (Fb) exerted on the specimen during stretching until it breaks, calculated using the original cross-sectional area of the specimen (So). It is denoted as tensile strength (σb), with units of N/mm2 (MPa). It represents the maximum capacity of a metal material to resist failure under tensile stress. The calculation formula is: Where: Fb -- the maximum force exerted on the specimen at the time of fracture, in N (newtons) ； So--the original cross-sectional area of the specimen, in mm2. ②Yield point (σs): For metal materials that exhibit yield behavior, the stress at which the specimen can continue to elongate even though the applied force remains constant during stretching is known as the yield point. If the force decreases, the upper and lower yield points should be distinguished. The unit of yield strength is N/mm2 (MPa). Upper yield point (σsu): the maximum stress before the force begins to decrease as the specimen yields ； Lower yield point (σsl): The minimum stress during the yield stage, when initial transient effects are not taken into account. The formula for calculating the yield point is: Where: Fs -- the yield force during the tensile testing of the specimen (constant), N (newtons) ； So--the original cross-sectional area of the specimen, in mm2. ③Elongation at break (σ) In tensile testing, the percentage increase in the gauge length of a specimen after it breaks, relative to its original gauge length, is referred to as elongation. Denoted by σ, with the unit being %. The calculation formula is: Where: L1 -- the gauge length of the specimen after it breaks, in mm ； L0--Original gauge length of the specimen, mm. ④Reduction of area (ψ) In tensile testing, it refers to the percentage by which the cross-sectional area at the narrowed portion of the specimen decreases compared to its original cross-sectional area, after the specimen is broken. Denoted by ψ, in percentage. The calculation formula is as follows: Where: S0 -- the original cross-sectional area of the specimen, in mm2 ； S1--The minimum cross-sectional area at the necking site after the specimen breaks, in mm2. ⑤Hardness index: The ability of a metal material to resist the indentation of its surface by hard objects is called hardness. Depending on the testing method and scope of application, hardness can be further divided into Brinell hardness, Rockwell hardness, Vickers hardness, Shore hardness, microhardness, and high-temperature hardness, among others. For pipes, the three commonly used hardness tests are Brinell, Rockwell, and Vickers. A. Brinell hardness (HB): A steel ball or cemented carbide ball of a certain diameter is pressed into the surface of the specimen using a specified testing force (F); after a specified holding time, the testing force is removed, and the diameter of the indentation on the specimen’s surface (L) is measured. The Brinell hardness value is the quotient obtained by dividing the testing force by the surface area of the spherical indenter. Denoted as HBS (steel ball), with the unit being N/mm2 (MPa). Its calculation formula is: Where: F -- the testing force applied to the surface of the metal specimen, N ； D--Diameter of the test steel ball, mm ； d--average indentation diameter, mm. Measuring Brinell hardness is relatively accurate and reliable, but HBS is generally only suitable for metal materials with a strength of 450 N/mm2 (MPa) or less; it is not applicable to harder steels or thinner sheets. In steel pipe standards, Brinell hardness is the most widely used; it is often used to indicate the hardness of a material based on the diameter of the indentation d, which is both intuitive and convenient. Example: 120HBS10/1000130: This indicates that the Brinell hardness value, measured by using a steel ball with a diameter of 10 mm under a testing force of 1000 Kgf (9.807 KN) for 30 seconds, is 120 N/mm2 (MPa). B. Rockwell hardness (HK) The Rockwell hardness test, like the Brinell hardness test, is also a indentation testing method. The difference is that it measures the depth of the indentation. That is, under the sequential action of the initial testing force (Fo) and the total testing force (F), the indenter (a steel cone or steel ball from a steel plant) is pressed into the surface of the specimen; after a specified holding time, the main testing force is removed, and the hardness value is calculated using the measured increase in residual indentation depth (e). Its value is an unknown number, denoted by the symbol HR, and there are 9 scales used, namely A, B, C, D, E, F, G, H, and K. The scales commonly used for steel hardness testing are generally A, B, and C, namely HRA, HRB, and HRC. The hardness value is calculated using the following formula: When testing with scales A and C, HR=100-e; when testing with scale B, HR=130-e. Here, e represents the increase in residual indentation depth, expressed in the specified unit of 0.002 mm. In other words, when the indenter moves axially by one unit (0.002 mm), it corresponds to a change in Rockwell hardness by that amount. The larger the e-value, the lower the hardness of the metal; conversely, the higher the e-value, the higher the hardness. The applicable ranges of the above three scales are as follows: HRA (diamond cone indenter): 20–88; HRC (diamond cone indenter): 20–70; HRB (steel ball indenter with a diameter of 1.588 mm): 20–100. The Rockwell hardness test is a widely used method, and among its variants, HRC is the most commonly used in steel pipe standards, second only to Brinell hardness HB. Rockwell hardness can be used to measure metal materials ranging from very soft to very hard. It overcomes the shortcomings of the Brinell method; it is simpler than Brinell, as the hardness value can be read directly from the dial of the hardness tester. However, due to its small indentation, its hardness value is less accurate than that of the Brinell method. C. Vickers hardness (HV): The Vickers hardness test is also a type of indentation test. In this test, a diamond indenter in the shape of a regular tetrahedron with an angle of 136° between its opposite faces is pressed into the test surface using a specified testing force (F). After a predetermined holding time, the testing force is removed, and the lengths of the two diagonals of the resulting indentation are measured. The Vickers hardness value is the quotient obtained by dividing the testing force by the surface area of the indentation. Its calculation formula is: Where: HV -- the symbol for Vickers hardness, N/mm2 (MPa) ； F--test force, N ； d--Arithmetic mean of the two diagonals of the indentation, in mm. The test force F used for Vickers hardness testing is available in six levels: 5 (49.03), 10 (98.07), 20 (196.1), 30 (294.2), 50 (490.3), and 100 (980.7) Kgf(N), with a hardness measurement range of 5 to 1000 HV. Example of notation: 640HV30/20 indicates that the Vickers hardness value, determined by applying a testing force of 30 Hgf (294.2 N) for 20 seconds, is 640 N/mm2 (MPa). The Vickers hardness method can be used to determine the hardness of very thin metal materials and surface layers. It possesses the main advantages of the Brinell and Rockwell methods while overcoming their fundamental disadvantages, but it is not as simple as the Rockwell method. The Vickers method is rarely used in steel pipe standards. ⑥Impact toughness index: Impact toughness reflects a metal’s ability to resist external impact loads. It is generally expressed by the impact toughness value (ak) and the impact energy (Ak), with units of J/cm2 and J respectively. Impact toughness or impact energy tests (referred to simply as “impact tests”) are divided into three types based on the testing temperature: room temperature, low temperature, and high temperature impact tests ； Based on the shape of the specimen notch, it can be further divided into two types of impact tests: \"V\"-notch and \"U\"-notch impact tests. Impact test: An experiment in which a specimen of specific dimensions and shape (10×10×55 mm), with a \"U\"-shaped or \"V\"-shaped notch at the middle of its lengthwise direction and a notch depth of 2 mm, is subjected to an impact load on a specified testing machine, resulting in fracture at the notch. A. Impact absorption energy Akv(u) – the energy absorbed by a metal specimen of certain size and shape when it breaks under an impact load. The unit is joule (J) or Kgf·m. B. Impact toughness value akv(u) – the quotient obtained by dividing the impact absorption energy by the cross-sectional area at the bottom of the notch in the specimen. The unit is joules per square centimeter (J/cm2) or kilogram-force meters per square centimeter (Kgf·m/cm2). The calculation formula is: Where: Akv(u) -- the work absorbed when the specimen breaks, in Kgf·m (J) ； S -- Cross-sectional area at the bottom of the specimen notch, in cm2. The temperature for the room-temperature impact test is 20±5°C ； The temperature range for the low-temperature shock test is