The mechanical properties of steel are important indicators to ensure the ultimate performance (mechanical properties) of steel, which depends on the chemical composition of the steel and the heat treatment system. In the steel pipe standard, tensile properties (tensile strength, yield strength or yield point, elongation), hardness and toughness indexes, and high and low temperature properties required by users are specified according to different use requirements.
1. Tensile strength (σb)
The maximum force (Fb) that the specimen is subjected to during the drawing process, divided by the original cross-sectional area (So) of the specimen, is called the tensile strength (σb). The unit is N/mm2 (MPa). It represents the maximum ability of a metal material to resist damage under tensile forces. The calculation formula is:
Where: Fb--the maximum force to be taken when the sample is broken, N (Newton); So--the original cross-sectional area of the sample, mm2.
2. Yield point (σs)
The metal material with yielding phenomenon, the force of the sample does not increase (maintains constant) during the stretching process, and the stress at the time of elongation can be continued, which is called the yield point. If the force drops, 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 first drop of force; the lower yield point (σsl): the minimum stress in the yielding phase when the initial transient effect is not counted.
The calculation formula for the yield point is:
Where: Fs--the yield force (constant) during the tensile process of the specimen, N (Newton) So--the original cross-sectional area of the specimen, mm2.
3. Elongation after break (σ)
In the tensile test, the percentage of the length increased by the gauge length after the sample is broken and the length of the original gauge length is called the elongation. Expressed in σ, the unit is %. The calculation formula is:
Where: L1--the length of the gauge length after the sample is broken, mm; L0--the original gauge length of the sample, mm.
4. Area shrinkage rate (ψ)
In the tensile test, the percentage of the maximum reduction in cross-sectional area at the reduced diameter after the sample is broken and the original cross-sectional area is called the reduction in area. Expressed in ψ, the unit is %. Calculated as follows:
Where: 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.
5. Hardness index
The ability of a metal material to resist the indentation of a hard object is called hardness. According to the test method and the scope of application, the hardness can be divided into Brinell hardness, Rockwell hardness, Vickers hardness, Shore hardness, microhardness and high temperature hardness. Commonly used for pipes are Brinell, Rockwell and Vickers hardness.