1. Brand and Chemical Constituents
The grades and chemical compositions of galvanized steel pipes shall conform to the grades and chemical compositions of black Pipe Steels specified in GB 3091.
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 3091的规则。
4. Mechanical properties of steel pipes before galvanizing should conform to the rules of GB 3091.
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 diameter 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.
Mechanical property of steel is an important index to ensure the ultimate performance (mechanical property) of steel, 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)
During the tensile process, the maximum force (Fb) that the specimen receives at break is the stress (_) obtained by the original cross-sectional area (So) of the specimen, which is called tensile strength (_b) in units of N/mm2 (MPa). It represents the greatest ability of metal materials to withstand damage under tension. The calculation formula is as follows:
In the formula: Fb - the maximum force accepted when the specimen is broken, N (Newton); So - the original cross-sectional area of the specimen, mm2.
(2) yield point (_s)
For metal materials with yielding phenomena, 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 maximum stress before the initial drop of the force due to the initiation of the specimen, and downward yield point (_sl): the minimum stress in the yield stage when the initial instantaneous stress is not taken into account.
The formula for calculating the yield point is as follows:
In the formula: 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 breaking (_)
In the tensile test, the percentage of the length of the standard distance increased by the sample after breaking to the length of the original standard distance is called elongation. In_, the unit is. The calculation formula is as follows:
In the formula: L1 - the length of standard distance after specimen breaking, mm; L0 - the original length of standard distance of specimen, mm.
(4) Section shrinkage (_)
In tensile test, the percentage of the maximum reduction of cross-sectional area at the shrinkage of specimen after breaking to the original cross-sectional area is called the cross-sectional shrinkage rate. In_, the unit is. The calculation formula is as follows:
Formula: S0 - the original cross-sectional area of the specimen, mm2; S1 - the minimum cross-sectional area at the shrinkage point of the specimen after breaking, mm2.
The ability of metal materials to resist the collapse of hard objects is called hardness. The hardness can be divided into Brinell hardness, Rockwell hardness, Vickers hardness, Shore hardness, microhardness and high temperature hardness. Brinell hardness, Rockwell hardness and Vickers hardness are commonly used for pipes.
A. Brinell Hardness (HB)
A steel ball or cemented carbide ball with a certain diameter is pressed into the surface of the sample with regular experimental force (F). The experimental force is removed after the regular adherence time, and the indentation diameter (L) of the sample surface is measured. Brinell hardness is a quotient obtained by dividing the experimental force by the surface product of indented spheres. In terms of HBS (steel ball), the unit is N/mm2 (MPa).