Physical properties of steel

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

① Tensile strength（ σ b）

The maximum force (Fb) that the specimen bears when it is pulled apart during the tensile process is the stress obtained from the original cross-sectional area (So) of the specimen（ σ）， Is called tensile strength（ σ b) , in N/mm2 (MPa). It indicates the maximum ability of metal materials to resist damage under tensile force. The calculation formula is:

Where: Fb - maximum force borne by the sample when it is pulled off, N (Newton); So -- original cross-sectional area of the sample, mm2.

② Yield point（ σ s）

For metal materials with yield phenomenon, the stress when the specimen can continue to stretch without increasing (keeping constant) the force in the tensile process is called yield point. If the force decreases, the upper and lower yield points shall be distinguished. The yield point is expressed in N/mm2 (MPa).

Upper yield point（ σ Su): the maximum stress before the first drop of force due to yield of the sample; Lower yield point（ σ Sl): the minimum stress in the yield stage when the initial instantaneous effect is not considered.

The calculation formula of yield point is:

Where: Fs - yield force (constant) in the tensile process of the sample, N (Newton) So - original cross-sectional area of the sample, mm2.

③ Elongation after fracture（ σ）

In the tensile test, the percentage between the increased length of the gauge length and the original gauge length after the specimen is broken is called the elongation. with σ Is expressed in%. The calculation formula is:

Where: L1 -- gauge length of sample after breaking, mm; L0 -- Original gauge length of the sample, mm.

④ Reduction of area（ ψ）

In the tensile test, the percentage between the maximum reduction of the cross-sectional area at the reduced diameter and the original cross-sectional area after the specimen is broken is called the reduction of area. with ψ Is expressed in%. The calculation formula is as follows:

Where: S0 -- original cross-sectional area of the sample, mm2; S1 -- The minimum cross-sectional area of the reduced diameter after the specimen is pulled off, 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 application scope, hardness can be divided into Brinell hardness, Rockwell hardness, Vickers hardness, Shore hardness, microhardness and high-temperature hardness. There are three kinds of hardness commonly used for pipes: Brinell hardness, Rockwell hardness and Vickers hardness.