1, brand and chemical composition
The grade and chemical composition of the steel for galvanized pipe 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 pipe 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 50 mm 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 on the sample and the same phenomenon as the zinc layer peeling off.
7. Hydraulic test The hydraulic 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 of steel (mechanical properties), 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.
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 force first drops; Lower yield point (σsl): The minimum stress in the yielding phase when the initial transient effect is not counted.
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