Mechanical properties of seamless steel pipes

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

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

②Yield point (σs)
For metal materials with yielding phenomenon, the stress at which the specimen can continue to elongate without increasing the force (remaining constant) 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 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 initial transient effects are ignored.

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

③Elongation after fracture (σ)
In a tensile test, the percentage of the increase in the gauge length of the specimen after it is broken is called the elongation. It is represented by σ and the unit is %. The calculation formula is: σ=(Lh-Lo)/L0*100%
In the formula: Lh--the length of the gauge length after the sample is broken, mm; L0--the length of the original gauge length of the sample, mm.

④ Section shrinkage (ψ)
In the tensile test, the percentage of the maximum reduction of the cross-sectional area at the reduced diameter of the specimen after the specimen is broken and the original cross-sectional area is called the reduction of area. It is expressed in ψ and the unit is %. The calculation formula is: ψ=(S0-S1)/S0*100%

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

⑤Hardness index
The ability of a metal material to resist the indentation of a hard object is called hardness. According to different test methods and application scope, hardness can be divided into Brinell hardness, Rockwell hardness, Vickers hardness, Shore hardness, microhardness and high temperature hardness. For pipes, there are three commonly used hardnesses: Brinell, Rockwell and Vickers.

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