Mechanical properties of steel is to ensure the ultimate use of steel performance (mechanical properties) an important indicator, it depends on the chemical composition of steel and heat treatment system. In the steel pipe standard, according to different requirements, the provisions of the tensile properties (tensile strength, yield strength or yield point, elongation) and hardness, toughness indicators, as well as user requirements of high and low temperature performance.
① Tensile strength (σb)
The maximum stress (Fb) of the specimen during the tensile process, and the stress (σ) obtained from the original cross-sectional area (So) of the specimen are called the tensile strength (σb) in units of N / mm2 (MPa). It represents the maximum resistance of a metal material against tensile failure. The formula is:
Where: Fb - the maximum force when the specimen pulled off, N (Newton); So - the original sample cross-sectional area, mm2.
② yield point (σs)
With the phenomenon of yield of metal materials, the sample in the tensile force does not increase (to maintain a constant) can continue to stretch when the stress, called the yield point. If the force occurs when the decline, you should distinguish between the upper and lower yield point. Yield point in units of N / mm2 (MPa).
(Σsl): the minimum stress in the yield phase when the initial transient effect is not taken into account. The upper yield point (σsu) is the maximum stress at which the specimen is yielded for the first time.
Yield point is calculated as:
Where: Fs - the sample during the tensile yield strength (constant), N (Newton) So - sample the original cross-sectional area, mm2.
③ elongation after fracture (σ)
In the tensile test, the sample after the breakage of the gauge length of the increase in the length of the original gauge length percentage, called the elongation. Expressed in [sigma], in%. The formula is:
Where: L1 - the length of the gauge after the specimen is pulled apart, in mm; L0 - the original gauge length of the specimen, in mm.
④ section shrinkage (ψ)
In the tensile test, the sample after breaking the diameter of the cross-sectional area of ??the largest reduction in the original cross-sectional area with the percentage, called the cross-section shrinkage. In ψ, in units of%. Calculated as follows:
Where: S0 - sample of the original cross-sectional area, mm2; S1 - sample cut off after the reduction of the smallest cross-sectional area, mm2.
⑤ hardness index
The resistance of a metal material to the ability of a hard object to crush the surface, known as hardness. According to the test method and the scope of application of different hardness can be divided into Brinell hardness, Rockwell hardness, Vickers hardness, Shore hardness, microhardness and high temperature hardness. Commonly used for the pipe is Brinell, Rockwell, Vickers hardness of three.
A, Brinell hardness (HB)
With a certain diameter of the ball or carbide ball, with a specified test force (F) pressed into the surface of the sample, after a specified time to remove the test force to measure the surface indentation indentation diameter (L). The Brinell hardness value is the quotient of the test force divided by the indentation spherical surface area. In HBS (steel ball), said unit is N / mm2 (MPa).
Standard Specification for Zinc - Coated Welded Steel Pipes for Low Pressure Fluid Conveying
1, the uniformity of galvanized layer: steel pipe samples in copper sulfate solution, continuous impregnation 5 times shall not be red (copper)
2, the surface quality: galvanized steel pipe surface should be a complete galvanized layer, no uncoated spots and bubbles exist, allowing a rough surface and the presence of local zinc.
3, zinc coating weight: According to the demand side, galvanized steel pipe can be used for zinc layer weight determination, the average should be not less than 500g / square meters, any sample shall not be less than 480g / square meter.
