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Mechanical Properties Of Seamless Steel Tubes

Jul 24, 2017

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

① Tensile strength (σb)

The maximum force (Fb) of the specimen in the drawing process, divided by the original cross-sectional area (So) of the specimen, is called the tensile strength (σb) in units of N / mm2 (MPa). It represents the maximum capacity of metal materials to resist damage under tension.

② yield point (σs)

With the yield phenomenon of the metal material, the sample in the tensile process does not increase (to maintain a constant) can continue to stretch the stress, called the yield point. If the force falls, it should distinguish between the upper and lower yield points. The unit of yield point is N / mm2 (MPa).

(Σs): the maximum stress before the first drop of the specimen; the lower yield point (σsl): the minimum stress in the yield phase when the initial transient effect is not counted.

The yield point is calculated as:

Where: Fs - the tensile strength of the sample during the test (constant), N (Newton) So - the original sample cross-sectional area, mm2.

③ elongation after breaking (σ)

In the tensile test, the length of the gauge length after the specimen is pulled off is the percentage of the length of the original gauge length, called the elongation. Expressed in σ, in%. The formula is: σ = (Lh-Lo) / L0 * 100%

Where: Lh - the length of the specimen after the withdrawal from the gauge length, mm; L0 - the original sample length length, mm.

④ section shrinkage (ψ)

In the tensile test, the maximum reduction in the cross-sectional area at the reduced diameter and the percentage of the original cross-sectional area after the specimen is broken is called the section shrinkage. Expressed in ψ, in%. Calculated as follows:

Where: S0 - the original cross-sectional area of the sample, mm2; S1 - the minimum cross-sectional area of the specimen after the break, mm2.

⑤ hardness indicators

The ability of a metal material to resist the surface 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, micro hardness and high temperature hardness. For the pipe commonly used are Brinell, Rockwell, Vickers hardness of three.

A, Brinell hardness (HB)

(F) is pressed against the pattern surface with a predetermined test force (F) with a certain diameter ball or carbide ball. After the specified holding time, the test force is removed and the indentation diameter (L) of the sample surface is measured. Brinell hardness is calculated by dividing the test force by the indentation spherical surface area. It is expressed in HBS (steel ball) in units of N / mm2 (MPa).

The formula is:

Where: F - into the metal sample surface test force, N; D - test ball diameter, mm; d - indentation average diameter, mm.

Determination of Brinell hardness is more accurate and reliable, but the general HBS only applies to 450N / mm2 (MPa) the following metal materials, for hard steel or thin plate is not applicable. In the steel pipe standard, Brinell hardness is the most widely used, often with indentation diameter d to represent the hardness of the material, both intuitive and convenient.

Example: 120HBS10 / 1000/30: indicates a Brinell hardness value of 120 N / mm2 (MPa) measured for 30 seconds (seconds) under a test force of 1000 Kgf (9.807 KN) with a diameter of 10 mm.


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