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

① tensile strength (σb)

The maximum force (Fb) of the specimen during the tensile process, divided by the stress (σ) obtained by the original cross-sectional area (so) of the specimen, is called the tensile strength (σb), and the Unit is n/mm² (MPa). It represents the maximum ability of metal materials to resist damage under tensile forces. The calculation formula is:

In the formula: the maximum force of the fb--specimen when it is pulled off, N (Newton); The original cross-sectional area of so--specimen was mm².

② yield Point (σs)

In the case of metallic materials with yielding phenomena, the stress of the specimen during the tensile process is not increased (constant), and the yield point is also maintained. If the force is reduced, the upper and lower yield points should be differentiated. The unit of the yield point is n/mm² (MPa).

The upper yield point (ΣSU): The maximum stress before the first descent of the specimen yielding force; The lower yield point (ΣSL): When the initial instantaneous effect is disregarded, the minimum stress in the yield phase is minimized.

The formula for the yield point is:

In the formula: The yield force (constant) in the tensile process of fs--specimen, the original cross-sectional area of N (Newton) so--specimen, mm².

③Elongation after breaking (σ)

In tensile test, the percentage of the length of the specimen and the length of the original standard distance is called elongation rate. In σ, the unit is%. The calculation formula is:

In the formula: the l1--specimen pulls the mark distance length, mm; l0--specimen original standard distance length, MM.

④ section Shrinkage Rate (PSI)

In tensile test, the maximum reduction of cross-sectional area and the percentage of the original cross-sectional area are referred to as the sectional shrinkage rate. In the case of PSI, the unit is%. The calculation formula is as follows:

In the formula: the original cross-sectional area of s0--specimen, mm²; The minimum cross-sectional area of the s1--specimen at the shrinkage diameter is mm².

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