The mechanical properties of steel are important indicators to ensure the final performance of seamless pipe, which depends on the chemical composition and heat treatment system of the steel. In the seamless pipe standard, according to different use requirements, the tensile properties, hardness and toughness indicators, as well as the high and low temperature properties required by users, are specified.
① Tensile strength
During the tensile process, the maximum force that the sample bears when it is broken is the stress obtained from the original cross-sectional area of the sample, which is called tensile strength, and the unit is N/mm2. It represents the maximum ability of a metal material to resist damage under tension. The calculation formula is:
In the formula: Fb–the maximum force that the sample bears when it is pulled off, N; So–the original cross-sectional area of the sample, mm2.
②Yield point
For metal materials with yielding phenomenon, the stress at which the specimen can continue to elongate without increasing the force during the stretching 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.
Upper yield point: The maximum stress before the specimen yields and the force drops for the first time; Lower yield point: The minimum stress in the yield stage when initial transient effects are ignored.
The formula for calculating yield point is:
In the formula: Fs–yield force of the sample during tensile process, NSo–the original cross-sectional area of the sample, mm2.
③Elongation after fracture
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:
In the formula: L1–the gauge length of the specimen after breaking, mm; L0–the original gauge length of the specimen, mm.
④Section shrinkage
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 is called the reduction of the area. It is expressed in ψ and the unit is %. Calculated as follows:
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 scope of application, 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.