1. Brand and Chemical Constituents
The grades and chemical compositions of galvanized steel pipes shall conform to the grades and chemical compositions of the black Pipe Steels specified in GB 3092.
2. Manufacturing methods
The manufacturing method of black pipe (furnace welding or electric welding) is chosen by the manufacturer. Hot dip galvanizing is used for galvanizing.
3. Thread and pipe fittings
3.1 带螺纹交货的镀锌钢管，螺纹应在镀锌后车制。螺纹应契合YB 822的规则。
3.1 For galvanized steel pipes delivered with threads, threads shall be made after galvanizing. Threads shall conform to the rules of YB 822.
3.2 钢制管接头应契合YB 238的规则；可锻铸铁管接头应契合YB 230的规则。
3.2 Steel pipe joints shall conform to the rules of YB 238; malleable cast iron pipe joints shall conform to the rules of YB 230.
4、力学性能 钢管镀锌前的力学性能应契合GB 3092的规则。
4. Mechanical properties of steel pipes before galvanizing should conform to the rules of GB 3092.
5. The average galvanized steel pipe should be tested for the average galvanized layer. The steel tube specimen shall not turn red (copper-plated) after five consecutive dips in copper sulfate solution.
6. Galvanized steel pipes with nominal caliber less than 50 mm should be cold-bended. The bending angle is 90 degrees and the bending radius is 8 times of the outer diameter. There is no filler in the experiment. The weld of the sample should be placed on the outside or upper part of the bending direction. After the experiment, there should be no cracks and zinc layer spalling in the sample.
7. Hydraulic pressure experiment should be stopped in the black tube, and eddy current testing can be used instead of hydraulic pressure experiment. Test pressure or eddy current testing shall conform to the rule of GB 3092 as compared with sample size.
The mechanical property of steel is an important index to ensure its performance (mechanical property), which depends on the chemical composition and heat treatment system of steel. In the steel pipe specification, according to different application requirements, the tensile properties (tensile strength, yield strength or yield point, elongation) and hardness, toughness index, as well as the high and low temperature properties requested by users are regulated.
(1) Tensile strength (_b)
The stress (_) obtained from the original cross-sectional area (So) of the specimen is called tensile strength (_b) in units of N/mm2 (MPa). It represents the ability of metal materials to withstand destruction under tension. The calculation formula is as follows:
In the formula: Fb - the strength accepted by the specimen at break, N (Newton); So - the original cross-sectional area of the specimen, mm2.
(2) yield point (_s)
For metal materials with yielding phenomenon, the stress at which the specimen can continue to elongate without increasing the force during the tensile process (insisting on constant) is called the yield point. If the force attacks and falls, the upper and lower yielding points should be distinguished. The unit of yield point is N/mm2 (MPa).
Upper yield point (su): The large stress before the initial drop of the force due to the initiation of the specimen; Downward yield point (sl): The small stress in the yield stage when the initial instantaneous stress is not taken into account.