Technical requirements for hot-dip galvanized steel tubes

Hot-dip galvanizing is to pickle the steel pipe first. In order to remove the iron oxide on the surface of the steel pipe, after the pickling, it is cleaned in a tank of ammonium chloride or zinc chloride aqueous solution or a mixed aqueous solution of ammonium chloride and zinc chloride, and then sent to In the hot dip plating tank.Hot dip galvanizing has the advantages of uniform coating, strong adhesion, long service life and strong corrosion resistance.

1. Brand and chemical composition

The grade and chemical composition of the steel for galvanized steel pipes should comply with the grade and chemical composition of the steel for black pipes as specified in GB 3092.

2. Manufacturing method

The manufacturing method of the black pipe (furnace welding or electric welding) is selected by the manufacturer. Hot-dip galvanizing is used for galvanizing.

3. Thread and pipe joints

3.1 For galvanized steel pipes delivered with threads, the threads should be machined after galvanizing. The thread should comply with YB 822 regulations.

3.2 Steel pipe joints should comply with YB 238; malleable cast iron pipe joints should comply with YB 230.

4. Mechanical properties The mechanical properties of steel pipes before galvanizing should meet the requirements of GB 3092.
5. Uniformity of the galvanized layerGalvanized steel pipe should be tested for the uniformity of the galvanized layer. The steel pipe sample shall not become red (copper-plated) after being immersed in copper sulfate solution for 5 consecutive times.
6. Cold bend test The galvanized steel pipe with a nominal diameter of not more than 50mm should be subjected to a cold bend test. The bending angle is 90°, and the bending radius is 8 times the outer diameter. There is no filler during the test, and the weld of the sample should be placed on the outside or upper part of the bending direction. After the test, there should be no cracks and peeling of the zinc layer on the sample.
7. Water pressure test The water pressure test should be carried out in the clarinet. Eddy current flaw detection can also be used instead of the water pressure test. The test pressure or the size of the comparison sample for eddy current testing shall meet the requirements of GB 3092.

The mechanical properties of steel is an important index to ensure the final use performance (mechanical properties) of the steel, and it depends on the chemical composition of the steel and the heat treatment system. In the steel pipe standard, according to different application requirements, the tensile properties (tensile strength, yield strength or yield point, elongation), hardness, toughness indicators, and high and low temperature properties required by users are specified.

①Tensile strength (σb)

In the tensile process, the maximum force (Fb) that the sample bears when it breaks is the stress (σ) obtained from the original cross-sectional area (So) of the sample, which is called the tensile strength (σb), and the unit is N/mm2 (MPa). It represents the maximum ability of a metal material to resist damage under tensile force. The calculation formula is:

In the formula: Fb-the maximum force that the sample bears when it is broken, N (Newton); So-the original cross-sectional area of ​​the sample, mm2.

②Yield point (σs)

For a metal material with a yield phenomenon, the stress at which the sample can continue to elong without increasing the force during the stretching process (maintaining constant) 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 force drops for the first time; Lower yield point (σsl): the minimum stress in the yield stage when the initial transient effect is not taken into account.

The calculation formula of the yield point is:

Where: Fs–yield force (constant) during the tensile process of the specimen, N (Newton) So–the original cross-sectional area of ​​the specimen, mm2.

③Elongation after breaking (σ)

In the tensile test, the percentage of the length of the gauge length increased after the sample is broken to the original gauge length is called the elongation. Expressed by σ, the unit is %. The calculation formula is:

In the formula: L1-the gauge length of the specimen after breaking, in mm; L0-the original gauge length of the specimen, in mm.

④Reduction of area (ψ)

In the tensile test, the percentage of the maximum reduction of the cross-sectional area at the reduced diameter of the sample after the sample is broken to the original cross-sectional area is called the reduction of area. Expressed in ψ, the unit is %. The calculation formula is 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 of the sample after it is broken, mm2.

⑤ Hardness index

The ability of metal materials to resist the indentation of hard objects on the surface is called hardness. According to the different test methods and scope of application, the hardness can be divided into Brinell hardness, Rockwell hardness, Vickers hardness, Shore hardness, micro hardness and high temperature hardness. There are three commonly used pipes: Brinell, Rockwell, and Vickers hardness.

1. Brinell hardness (HB)

Use a steel ball or cemented carbide ball of a certain diameter to press into the surface of the sample with the specified test force (F), remove the test force after the specified holding time, and measure the diameter of the indentation on the surface of the sample. (L) The Brinell hardness value is the quotient obtained by dividing the test force by the spherical surface area of ​​the indentation. Expressed in HBS (steel ball), the unit is N/mm2 (MPa).