Hot-dip galvanized pipes are galvanized for general steel pipes in order to improve the corrosion resistance of steel pipes. The galvanized steel pipe is divided into two types: hot-dip galvanizing and electro-galvanizing. The hot-dip galvanizing layer is thick, the cost of electro-galvanizing is low, and the surface is not very smooth galvanized welded square steel pipes manufacturer. Here are some technical requirements for hot-dip galvanized steel pipes:
1, brand and chemical composition
The grade and chemical composition of the steel for galvanized steel pipe shall comply with the grade and chemical composition of the steel for black pipe specified in GB 3092.
2, manufacturing methods
The manufacturing method of the black tube (furnace welding or electric welding) is selected by the manufacturer. Galvanizing is performed by hot dip galvanizing.
3, thread and pipe joints
3.1 Galvanized steel pipes with thread delivery, thread should be made after galvanizing. Threads shall comply with the provisions of YB 822.
3.2 Steel pipe joints shall comply with the provisions of YB 238; malleable cast iron pipe joints shall comply with the provisions of YB 230.
4. Mechanical properties The mechanical properties of steel pipes before galvanizing shall comply with the provisions of GB 3092.
5. Uniformity of galvanized layer The galvanized steel pipe shall be tested for the uniformity of the galvanized layer. The steel pipe sample shall not be reddened (copper plating) by continuously immersing it in the copper sulfate solution for 5 times.
6. Cold bending test The galvanized steel pipe with a nominal diameter of not more than 50mm shall be tested for cold bending. The bending angle is 90° and the bending radius is 8 times the outer diameter. The test shall be carried out without filler and the weld of the specimen shall be placed on the outside or the upper part of the bending direction. After the test, there should be no cracks in the sample and the same phenomenon as the zinc layer peeling off.
7. Hydrostatic test The hydrostatic test shall be carried out in a black tube, or an eddy current test may be used instead of a hydrostatic test. The test pressure or eddy current test comparison sample size should meet the requirements of GB 3092.
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) of the specimen during the drawing process, the stress (σ) obtained from the original cross-sectional area (So) of the specimen, called the tensile strength (σb), in 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. galvanized tubing for sale
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 section 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.
A, Brinell hardness (HB)
Press the steel ball or carbide ball of a certain diameter into the surface of the sample with the specified test force (F). After the specified holding time, remove the test force and measure the indentation diameter (L) on the surface of the sample. The Brinell hardness value is the quotient obtained by dividing the test force by the spherical surface area of the indentation. It is expressed by HBS (steel ball) and the unit is N/mm2 (MPa).