Views: 1 Author: Site Editor Publish Time: 2023-07-24 Origin: Site
Galvanized pipe, also known as galvanized steel pipe, is divided into two types: hot-dip galvanized and electro-galvanized. Hot-dip galvanized galvanized layer is thick, and has the advantages of uniform coating, strong adhesion, and long service life. The cost of electro-galvanizing is low, the surface is not very smooth, and its corrosion resistance is much worse than that of hot-dip galvanized steel pipe.
1. Grade and chemical composition: The grade and chemical composition of steel for galvanized steel pipes shall comply with the grade and chemical composition of steel for black pipes specified in GB 3091.
2. Manufacturing method: The manufacturing method of the clarinet (furnace welding or electric welding) is selected by the manufacturer. Galvanizing adopts the hot-dip galvanizing method.
3. Threads and pipe joints: 3.1 For galvanized steel pipes delivered with threads, the threads should be machined after galvanizing. The thread should comply with the provisions of YB 822. 3.2 Steel pipe joints shall comply with the provisions of YB 238; malleable iron pipe joints shall comply with the provisions of YB 230.
4. Mechanical properties: The mechanical properties of steel pipes before galvanizing should meet the requirements of GB 3091.
5. Uniformity of galvanized layer Galvanized steel pipe should be tested for uniformity of galvanized layer. The steel pipe sample shall not turn red (copper-plated color) after being continuously dipped in the copper sulfate solution for 5 times.
6. Cold bending test The galvanized steel pipe with a nominal diameter not greater than 50mm should be subjected to a cold bending test. The bending angle is 90°, and the bending radius is 8 times the outer diameter. No filler is used 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 off of the zinc layer on the sample.
7. Hydrostatic test The hydrostatic test should be carried out on the clarinet, and eddy current flaw detection can also be used instead of the hydrostatic test. Test pressure or eddy current testing comparison sample size should comply with the provisions of GB 3092.
The mechanical properties of steel are important indicators to ensure the end-user performance (mechanical properties) of steel, which depends on the chemical composition and heat treatment system of steel. In steel pipe standards, tensile properties (tensile strength, yield strength or yield point, elongation), hardness, toughness, and high and low-temperature properties required by users are specified according to different use requirements.
①Tensile strength (σb): During the stretching process, the force (Fb) that the sample bears when it breaks, and the stress (σ) obtained from the original cross-sectional area (So) of the sample is called the tensile strength (σb), and the unit is N/mm2 (MPa). It represents the ability of metal materials to resist damage under tension. The calculation formula is: In the formula: Fb--the force borne by the sample when it is broken, N (Newton); So--the original cross-sectional area of the sample, mm2.
②Yield point (σs): For metal materials with yield phenomenon, the stress at which the sample can continue to elongate without increasing the force (keep constant) during the stretching process 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 stress before the sample 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 ignored.
The formula for calculating the yield point is:
In the formula: Fs--the yield force (constant) during the tensile process of the sample, N (Newton) So--the original cross-sectional area of the sample, mm2.
③Elongation after a break (σ): In the tensile test, the percentage of the increased length of the gauge length after the sample is broken and the original gauge length is called elongation. Expressed in σ, the unit is %. The calculation formula is: In the formula: L1--the gauge length of the sample after breaking, mm; L0--the original gauge length of the sample, mm.
④Reduction of area (ψ): In the tensile test, the percentage of the reduction of the cross-sectional area at the reduced diameter of the sample after the sample is broken and the original cross-sectional area is called the reduction of area. Expressed by 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 of the reduced diameter of the sample after breaking, mm2.
⑤Hardness Index: The ability of metal materials to resist the indentation of hard objects on the surface is called hardness. According to different test methods and scope of application, hardness can be divided into Brinell hardness, Rockwell hardness, Vickers hardness, Shore hardness, microhardness, and high-temperature hardness. For pipes, there are three types of hardness commonly used: Brinell, Rockwell, and Vickers.
A. Brinell hardness (HB): Use a steel ball or 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 indentation diameter (L) on the sample surface. 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).
Galvanized welded steel pipe for low-pressure fluid transmission (hot-dip galvanized part)
1. The uniformity of the galvanized layer: the steel pipe sample shall not turn red (copper-plated color) after being dipped continuously in the copper sulfate solution for 5 times
2. Surface quality: The surface of galvanized steel pipe should have a complete galvanized layer, and there must be no uncoated black spots and bubbles, and small rough surfaces and local zinc tumors are allowed.
3. Weight of galvanized layer: according to the requirements of the buyer, galvanized steel pipe can be used for the determination of the weight of the galvanized layer, and the average value should not be less than 500g/square meter, and any sample should not be less than 480g/square meter.