The tensile strength of seamless steel pipe

The mechanical properties of seamless steel pipes are important indicators to ensure the end-use properties (mechanical properties) of steel, which depend on the chemical composition and heat treatment system of steel. In the steel pipe standard, according to different use requirements, the tensile properties required by users (tensile strength, yield strength or yield point, elongation), hardness, toughness index, high and low temperature performance, etc. are specified.

①Tensile strength (σb)
During the stretching process, the maximum force (Fb) that the sample bears when it breaks is divided by the original cross-sectional area (So) of the sample is the stress (σ), which is called the tensile strength (σb), and the unit is N/ mm2 (MPa). It represents the maximum ability of a metal material to resist damage under tension.

② Yield point (σs)
For metal materials with yield phenomenon, the stress at which the sample can continue to elongate without increasing the force (remaining constant) during the stretching process is called the yield point. If the force drops, 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 drops for the first time; lower yield point (σsl): the minimum stress in the yield stage when the initial transient effect is not considered.

The calculation formula of yield point is: σs=Fs/So
In the formula: Fs——the yield force (constant) during the stretching process of the sample, N (Newton) So——the original cross-sectional area of the sample, mm2.

③Elongation after break (σ)
In a tensile test, the percentage of the increased length of the gauge length after the specimen breaks to the original gauge length is called the elongation. Expressed by σ, the unit is %. The calculation formula is: σ=(Lh-L0)/L0*100%

In the formula: Lh - the gauge length of the sample after fracture, mm; L0 - the original gauge length of the sample, mm.

④ area reduction (ψ)
In the tensile test, the percentage of the maximum reduction of the cross-sectional area of the sample at the shrinkage of the sample after fracture to the original cross-sectional area is called the reduction of area. Expressed by ψ, the unit is %. The calculation formula is as follows: ψ=(S0-S1)/S0*100%

In the formula: S0—the original cross-sectional area of the sample, mm2; S1—the minimum cross-sectional area of the sample at the reduced diameter after breaking, mm2.

⑤ Hardness index
The ability of metal materials to resist the indentation of hard objects on the surface is called hardness. According to different test methods and scope of application, hardness can be divided into Brinell hardness, Rockwell hardness, Vickers hardness, Shore hardness, microhardness and high temperature hardness. Commonly used pipes are Brinell hardness, Rockwell hardness and Vickers hardness.

A. Brinell hardness (HB)
Use a steel ball or hard alloy ball of a certain diameter to press into the surface of the sample with the specified test force (F), remove the test force after the specified holding time, and measure the surface indentation diameter (L) of the sample. The Brinell hardness value is the quotient of the test force divided by the surface area of the indentation ball. Expressed in HBS (steel ball), the unit is N/mm2 (MPa).

The measurement of Brinell hardness is more accurate and reliable, but generally HBS is only suitable for metal materials below 450N/mm2 (MPa), not for harder steel or thinner plates. In the steel pipe standard, Brinell hardness is the most widely used, and the hardness of the material is usually expressed by the indentation diameter d, which is intuitive and convenient.

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

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