With the acceleration of social development, the use of hot-dip galvanized steel pipes has gradually increased. So how much do you know about hot-dip galvanized steel pipes? What are the classification and production principles of hot-dip galvanized steel pipes? Today, I will take you into the world of hot-dip galvanized steel pipes and learn about its secrets.
1. Overview of galvanized pipe
In order to improve the corrosion resistance of steel pipes, general steel pipes are galvanized. Galvanized pipe (galvanized steel pipe) is divided into two types: hot-dip galvanized and cold-galvanized. The cost of cold galvanized pipe is low, the surface is not very smooth, and its corrosion resistance is much different than that of hot-dip galvanized pipe, and it has been gradually eliminated.
Galvanized pipes are mostly used in old housing construction in our country, and iron pipes commonly used for gas and heating are also galvanized pipes. Galvanized pipe is used as a water pipe. With the accumulation of years of use, a large amount of rust is generated in the pipe, which not only pollutes sanitary ware, but also contains bacteria that breed on the uneven inner wall. More seriously, the rust causes the heavy metal content in the water to be too high, which seriously harms the human body. of health. In the 1960s and 1970s, some developed countries began to develop new types of pipes and gradually banned galvanized pipes. Four ministries and commissions including the Ministry of Construction of China have also expressly stipulated that galvanized pipes will be banned from 2000. At present, galvanized pipes are rarely used for cold water pipes in newly built communities, and galvanized pipes are used for hot water pipes in some communities.
2. Classification and characteristics of galvanized pipes
Galvanized steel pipe is divided into hot-dip galvanized steel pipe and cold-galvanized steel pipe, among which cold-galvanized steel pipe has been gradually eliminated.
1. Hot-dip galvanized pipe / hot-dip galvanized steel pipe
Hot-dip galvanizing is to make the molten metal react with the iron matrix to produce an alloy layer, so that the matrix and the coating are combined. The manufacture of hot-dip galvanized pipe is to first pickle the steel pipe to remove iron oxide on the surface of the steel pipe. After 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 the hot dip coating tank.
Hot-dip galvanized steel pipe is a complex physical and chemical reaction between the steel pipe matrix and the molten plating solution to form a corrosion-resistant zinc-iron alloy layer with a tight structure. The alloy layer is integrated with the pure zinc layer and the steel pipe matrix, so the hot-dip galvanized steel pipe has the advantages of uniform coating, strong adhesion and long service life.
2. Cold galvanized pipe/cold galvanized steel pipe
Cold galvanizing is electro-galvanizing. The amount of galvanizing is very small, only 10-50g/m2. Its corrosion resistance is much different than that of hot-dip galvanized pipes. In order to ensure the quality, most regular galvanized pipe manufacturers do not use electro-galvanizing (cold galvanizing). Only those small enterprises with outdated equipment use electrogalvanizing, and of course their prices are relatively cheap. The zinc layer of cold galvanized steel pipe is layered independently from the steel pipe base. Due to the thin zinc layer, the zinc layer simply adheres to the steel pipe substrate and is easy to fall off, so the corrosion resistance of cold galvanized steel pipe is poor. Therefore, in newly built houses, the use of cold galvanized steel pipes as water supply and gas supply pipes has been prohibited.
3. The production process of galvanized pipe
The production process of galvanized steel pipe belongs to the secrets of various manufacturers, so only the general process flow can be provided: a. round steel preparation; b. heating; c. hot rolling perforation; d. cutting head; e. pickling; f. repairing Grinding; g. lubrication; h. cold rolling; i. degreasing; j. solution heat treatment; k. straightening; l. pipe cutting; m. pickling; n. finished product inspection.
Fourth, the technical requirements and standards of galvanized pipes
1. Grade and chemical composition of galvanized pipe
The grade and chemical composition of the steel for galvanized steel pipe shall conform to the grade and chemical composition of the steel for black pipe specified in GB-3092.
2. Manufacturing method of galvanized pipe
The manufacturing method of galvanized pipe (furnace welding or electric welding) is selected by the manufacturer. Galvanizing is mostly hot-dip galvanizing.
3. Threads and pipe joints of galvanized pipes
(1) For galvanized steel pipes delivered with threads, the threads shall be machined after galvanizing, and the threads shall comply with the provisions of YB-822;
(2) Steel pipe joints shall comply with the regulations of YB-238, and malleable iron pipe joints shall comply with the regulations of YB-230.
4. Specifications and dimensions of galvanized pipes
Nominal wall thickness (mm): 2.02.52.83.23.53.84.04.5 Weight coefficient c: 1.0641.0511.0451.0401.0361.0341.0321.028
5. Mechanical properties of galvanized steel pipe
The mechanical properties of steel pipes before galvanizing should meet the requirements of GB-3092. The mechanical properties of steel are important indicators to ensure the final use performance (mechanical properties) of steel, which depends on the chemical composition and heat treatment system of the steel. In the steel pipe standard, according to different use requirements, the tensile properties (tensile strength, yield strength or yield point, elongation), hardness and toughness indicators, as well as high and low temperature properties required by users are specified.
6. The uniformity of the galvanized layer of the galvanized pipe
Galvanized steel pipes should be tested for the uniformity of the galvanized layer. The galvanized steel pipe sample shall not turn red (copper-plated color) after being continuously dipped in copper sulfate solution for 5 times.
7. Cold bending test of galvanized pipe
Galvanized steel pipes 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. During the test without filler, the weld of the sample should be placed on the outer side or upper side of the bending direction. After the test, there should be no cracks and peeling of the zinc layer on the sample.
8. Hydrostatic test of galvanized pipe
The hydrostatic test of the galvanized steel pipe should be carried out on the black pipe, and the hydrostatic test can also be replaced by eddy current flaw detection. The test pressure or eddy current inspection comparison sample size shall comply with the provisions of GB-3092.