cold galvanized steel pipe and hot-dip galvanized steel pipe.

Hot dip galvanized pipe

Hot-dip galvanized pipe 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. Hot-dip galvanizing is to pickle the steel pipe first. In order to remove the iron oxide on the surface of the steel pipe, after pickling, it is cleaned in the tank of ammonium chloride or zinc chloride aqueous solution or mixed aqueous solution of ammonium chloride and zinc chloride, and then sent to the in the hot dip bath. Hot-dip galvanizing has the advantages of uniform coating, strong adhesion and long service life. The hot-dip galvanized steel pipe substrate undergoes complex physical and chemical reactions with the molten plating solution to form a corrosion-resistant zinc-iron alloy layer with a compact structure. The alloy layer is integrated with the pure zinc layer and the steel pipe substrate, so it has strong corrosion resistance.

Cold galvanized pipe
Cold galvanized pipe is electro-galvanized, the amount of galvanized is very small, only 10-50g/m2, its corrosion resistance is much different than that of hot-dip galvanized pipe. In order to ensure the quality, most regular galvanized pipe manufacturers do not use electro-galvanized (cold plating). Only those small enterprises with outdated equipment use electrogalvanizing, and of course their prices are relatively cheap. The Ministry of Construction has officially issued an order to eliminate cold galvanized pipes with outdated technology, and not to use cold galvanized pipes for water and gas pipes. The galvanized layer of cold galvanized steel pipe is an electroplating layer, and the zinc layer is independently layered with the steel pipe substrate. The zinc layer is thin, and the zinc layer simply adheres to the steel pipe substrate and is easy to fall off. Therefore, its corrosion resistance is poor. In newly built houses, it is forbidden to use cold galvanized steel pipes as water supply pipes.

The use of galvanized steel pipe

The production process of galvanized pipe has the following production steps:
a. Round steel preparation; b. heating; c. hot-rolled piercing; d. cut the head; e. Pickling; f. Grinding; g. Lubrication; h. cold rolling
; i. Degreasing; j. Solution heat treatment; k. Straightening; l. Cut the tube; m. Pickling; n. Product testing.
Technical requirements that galvanized pipes should meet
1. Grades and chemical compositions
The grades and chemical compositions of steel for galvanized pipes should conform to the grades and chemical compositions of steel for black pipes specified in GB 3092.
2. Manufacturing method
The manufacturing method of the black pipe (furnace welding or electric welding) is selected by the manufacturer. Galvanizing adopts hot dip galvanizing method.
3. Threads and pipe joints
a: For galvanized steel pipes delivered with threads, the threads should be machined after galvanizing. The thread should comply with the provisions of YB 822.
b: 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 3092.
5. The uniformity of the galvanized layer The galvanized steel pipe should be tested for the uniformity of the galvanized layer.
The steel pipe sample shall not turn red (copper-plated color) after being continuously dipped in copper sulfate solution for 5 times.
6. Cold bending test Galvanized steel pipes with a nominal diameter not greater than 50mm should be subjected to 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 specimen 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.
7. Water pressure test The water pressure test should be carried out in the black pipe, and the eddy current flaw detection can also be used instead of the water pressure test. Test pressure or eddy current testing for
The size of the sample should comply with the provisions of GB 3092. The mechanical properties of steel are important
indicators to ensure the end-use properties (mechanical properties) of steel, which depend 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
.
① Tensile strength (σb): The maximum force (Fb) that the sample bears when it breaks during the tensile process, and the stress (σ) obtained from the original cross-
sectional area (So) of the sample is called resistance Tensile strength (σb), in N/mm2 (MPa). It represents the maximum ability of a metal material
to resist damage under tension. In the formula: Fb--the maximum force that the sample bears when it is broken, N (Newton); So--the original cross-sectional area of the
sample , mm2.
② Yield point (σs): For metal materials with yielding phenomenon, the stress at which the sample can
continue to called the yield point. If the force decreases, 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 decreases for the first time; Lower yield point (σsl):
The minimum stress in the yield stage when initial transient effects are ignored. In the formula: Fs--yield force (constant) during the tensile process of the sample, N (
Newton ) So--the original cross-sectional area of the sample, mm2.
③ Elongation after fracture: (σ) In the tensile test, the percentage of the length increased by the gauge length of the specimen after breaking and the length of the original gauge length is
called the elongation. It is expressed in σ and the unit is %. In the formula: L1 - the length of the gauge length after the sample is broken, mm; L0 - the original gauge length of the sample
length, mm.
④ Area shrinkage rate: (ψ) In the tensile test, the percentage of the maximum reduction of the cross-sectional area at the reduced diameter of the specimen after the specimen is broken and the original
cross -sectional area is called the area shrinkage rate. It is expressed in ψ and the unit is %. In the formula: S0--the original cross-sectional area of the sample, mm2;
S1--the minimum cross-sectional area at the reduced diameter after the sample is broken, mm2.
⑤Hardness index: The ability of a metal material to resist the indentation of a hard object on the surface is called hardness. According to different test methods and application scope
, hardness can be divided into Brinell hardness, Rockwell hardness, Vickers hardness, Shore hardness, microhardness and high temperature hardness. There are three commonly used hardnesses for
pipes : Brinell, Rockwell and Vickers.
A. Brinell hardness (HB): Use a steel ball or cemented carbide ball with a certain diameter to press into the surface of the sample with a specified test force (F)
, remove the test force after a specified holding time, and measure the pressure on the surface of the sample Trace diameter (L). The Brinell hardness value is the quotient of the test force divided by
the surface area of the indentation spherical. Expressed in HBS (steel ball), the unit is N/mm2 (MPa).
Elements that affect the properties of galvanized steel pipes
(1) Carbon; the higher the carbon content, the higher the hardness of steel, but the worse its plasticity and toughness.
(2) Sulfur: It is a harmful contaminant in steel. When steel with high sulfur content is subjected to high temperature pressure processing, it is easy to be brittle, which is usually called
hot brittleness.
(3) Phosphorus; It can make the plasticity and toughness of steel decrease obviously, especially at low temperature. This phenomenon is called cold
brittleness . In high-quality steel, sulfur and phosphorus should be strictly controlled. But on the other hand, the high content of sulfur and phosphorus in low carbon steel can make it
easy to cut, which is beneficial to improve the machinability of steel.
(4) Manganese; can improve the strength of steel, can weaken and eliminate the adverse effects of sulfur, and can improve the hardenability of steel,
high has good wear resistance and other physical properties.
(5) Silicon; it can improve the hardness of steel, but the plasticity and toughness decrease. The steel used for electrician contains a certain amount of silicon, which
can improve the soft magnetic properties.
(6) Tungsten; can improve the red hardness and thermal strength of steel, and can improve the wear resistance of steel.
(7) Chromium; can improve the hardenability and wear resistance of steel, and can improve the corrosion resistance and oxidation resistance of steel.
In order to improve the corrosion resistance of steel pipes, general steel pipes (black pipes) are galvanized. Galvanized steel pipe is divided into two types: hot-dip galvanized and electric steel zinc.
The hot-dip galvanized galvanized layer is thick and the cost of electro-galvanized is low, so there are galvanized steel pipes.