Galvanized steel refers to ordinary carbon construction steel that, after galvanization, can effectively prevent corrosion and rust, thereby extending its lifespan.
Galvanization is typically divided into two methods: electroplating and hot-dip galvanizing. This material is commonly used in exterior building walls, such as glass curtain walls, marble curtain walls, and aluminum curtain walls as columns and load-bearing materials.
It is also used in outdoor telecommunications towers and highways. These types of open-air construction steel materials are referred to as galvanized steel, further categorized into electroplated and hot-dip galvanized types.
1. Basic Introduction
Galvanized steel, known in Chinese as encompasses electroplating and hot-dip galvanizing. It’s a broad term that includes all types of galvanized steel plates.
Hot-Dip Galvanized Steel Pipe
The hot-dip galvanized pipe involves causing a reaction between molten metal and the iron matrix to produce an alloy layer, thereby combining the base and the coating. This process begins by acid washing the steel pipe to remove the iron oxide on its surface.
After acid washing, the pipe is cleaned in a tank of ammonium chloride or zinc chloride solution or a mixed solution of both, then dipped into a hot-dip galvanizing bath. The hot-dip galvanizing provides an evenly distributed coating with strong adhesion and long service life.
The base of the hot-dip galvanized steel pipe undergoes complex physical and chemical reactions with the molten plating solution to form a corrosion-resistant, tightly structured zinc-iron alloy layer. The alloy layer merges with the pure zinc layer and the steel pipe base, providing strong corrosion resistance.
Cold Galvanized Pipe
Cold galvanized pipe, also known as electro-galvanized, has a much smaller zinc coating of just 10-50g/m2 and its corrosion resistance is significantly lower than that of hot-dip galvanized pipes. Reputable galvanized pipe manufacturers typically don’t use electro-galvanizing (cold galvanizing) to ensure quality. It’s only used by small-scale, outdated equipment enterprises, although their prices are relatively cheaper.
Currently, the Ministry of Construction has officially phased out the technologically backward cold galvanized pipes, prohibiting their use in water and gas pipelines.
The zinc coating on cold galvanized steel pipes is an electroplated layer, with the zinc layer and the steel pipe base separate from each other. The zinc layer is thin and simply adheres to the steel pipe base, making it easy to flake off and therefore, its corrosion resistance is poor.
The use of cold galvanized steel pipes as water supply pipes is banned in newly constructed residences.
3. Weight Coefficients
|Nominal wall thickness
Note: The mechanical properties of steel are important indicators to ensure the final performance (mechanical properties) of the steel, which depend on the chemical composition of the steel and the heat treatment system.
In steel pipe standards, tensile properties (tensile strength, yield strength or yield point, elongation), hardness, toughness, and high/low-temperature performance are specified based on different usage requirements.
Steel grades: Q215A; Q215B; Q235A; Q235.
Test pressure values/Mpa: For D10.2-168.3mm, it’s 3Mpa; for D177.8-323.9mm, it’s 5Mpa.
4. Galvanized Pipe
Commonly referred to as galvanized pipe, the galvanized pipe is now used for gas and heating. The kind of iron pipe used is also a galvanized pipe. After a few years of use as a water pipe, a large amount of rust scale is produced inside the pipe.
The yellow water that flows out not only pollutes the sanitary ware, but also breeds bacteria on the unsmooth inner wall. The corrosion causes the content of heavy metals in the water to be too high, seriously endangering human health.
1. Production Process of the Galvanized Pipe
The production process of the galvanized pipe involves the following steps:
- a. Round steel preparation;
- b. Heating;
- c. Hot rolling piercing;
- d. Cutting off the head;
- e. Acid washing;
- f. Grinding;
- g. Lubrication;
- h. Cold rolling;
- i. Degreasing;
- j. Solid solution heat treatment;
- k. Straightening;
- l. Pipe cutting;
- m. Acid washing;
- n. Final inspection.
2. Technical Requirements That Should Be Met by the Galvanized Pipe
1. Grade and chemical composition
The grade and chemical composition of the steel for galvanized steel pipe should comply with the grade and chemical composition of the black pipe steel stipulated in GB 3092.
2. Manufacturing method
The manufacturing method of the black pipe (furnace welding or electric welding) is chosen by the manufacturer. Galvanizing is done by hot-dip galvanizing.
3. Threads and pipe joints
a. For galvanized steel pipes delivered with threads, the threads should be turned after galvanizing. The thread should comply with YB 822.
b. Steel pipe joints should comply with YB 238; malleable iron pipe joints should comply with YB 230.
4. Mechanical properties
The mechanical properties of the steel pipe before galvanizing should comply with GB 3092.
5. Uniformity of the galvanizing layer
Galvanized steel pipe should be tested for uniformity of the galvanizing layer. The steel pipe sample should not turn red (copper color) after being continuously immersed in a copper sulfate solution for 5 times.
6. Cold bending test
Galvanized steel pipes with a nominal diameter of no more than 50mm should undergo a cold bending test. The bending angle is 90°, and the bending radius is 8 times the outer diameter.
The test is conducted without filler, and the weld of the sample should be placed on the outside or the top of the bending direction. After the test, there should be no cracks and peeling of the zinc layer on the sample.
7. Hydrostatic test
The hydrostatic test should be carried out on the black pipe, or it can be replaced with eddy current testing.
The test pressure or eddy current testing comparison sample size should comply with GB 3092. The mechanical properties of the steel are an important indicator to ensure the final use performance (mechanical performance) of the steel, which depends on the chemical composition of the steel and the heat treatment system.
In the steel pipe standard, according to different usage requirements, tensile properties (tensile strength, yield strength or yield point, elongation), hardness, toughness indicators, as well as high and low temperature performance required by users, etc. are stipulated.
①. Tensile strength (σb):
It refers to the maximum stress (σ) that a test sample can withstand at the point of fracture during the tensile process, obtained by dividing the maximum force (Fb) by the original cross-sectional area (So) of the sample.
It is denoted as σb, with the unit being N/mm2 (MPa). This value indicates the maximum capacity of a metal material to resist destruction under tensile force.
Here, Fb represents the maximum force the sample can withstand at fracture, in Newtons (N); So represents the original cross-sectional area of the sample, in mm2.
②. Yield point (σs):
For metal materials that exhibit yield phenomena, the stress when the sample can continue to elongate while the force remains constant during the tensile process is called the yield point. If the force decreases, the upper and lower yield points should be distinguished. The unit of the yield point is N/mm2 (MPa).
The upper yield point (σsu) is the maximum stress before the sample starts to yield; the lower yield point (σsl) is the minimum stress during the yield phase when the initial transient effect is not considered.
Here, Fs represents the yield force (constant) during the tensile process of the sample, in Newtons (N); So is the original cross-sectional area of the sample, in mm2.
③. Elongation after fracture (σ):
In a tensile test, the percentage of the increase in length of the gauge length of the sample after fracture to the original gauge length is known as the elongation. It is denoted by σ, with the unit being %. Here, L1 is the gauge length of the sample after fracture, in mm; L0 is the original gauge length of the sample, in mm.
④. Reduction of area (ψ):
In a tensile test, the percentage of the maximum reduction in cross-sectional area at the reduced diameter of the sample after fracture to the original cross-sectional area is known as the reduction of area. It is denoted by ψ, with the unit being %.
Here, S0 is the original cross-sectional area of the sample, in mm2; S1 is the least cross-sectional area at the reduced diameter of the sample after fracture, in mm2.
⑤. Hardness Index:
The ability of a metal material to resist the indentation of a hard object on its surface is known as hardness. Based on the test method and application range, hardness can be divided into Brinell hardness, Rockwell hardness, Vickers hardness, Shore hardness, microhardness, and high-temperature hardness.
For pipes, the commonly used ones are Brinell, Rockwell, and Vickers hardness. The Brinell hardness (HB) is obtained by pressing a sphere of a certain diameter made of steel or hard alloy into the surface of the sample with a specified test force (F), removing the test force after a specified holding time, and measuring the diameter (L) of the indentation on the sample surface.
The Brinell hardness value is the quotient of the test force divided by the spherical surface area of the indentation. It is denoted by HBS (steel ball), with the unit being N/mm2(MPa).
3. Elements influencing the performance of galvanized steel pipes:
1. Carbon: The higher the carbon content, the harder the steel, but its plasticity and toughness decrease.
2. Sulfur: A harmful impurity in steel, high sulfur content can lead to brittle fractures during high-temperature pressure processing, often referred to as hot brittleness.
3. Phosphorus: It significantly decreases the plasticity and toughness of steel, especially at low temperatures, a phenomenon known as cold brittleness. In high-quality steel, sulfur and phosphorus need to be strictly controlled. However, on the other hand, high sulfur and phosphorus content in low carbon steel makes it easy to cut, which is beneficial for improving the machinability of the steel.
4. Manganese: It enhances the strength of steel, mitigates and eliminates the adverse effects of sulfur, and improves the hardenability of steel. High alloy steel with high manganese content (high manganese steel) has good wear resistance and other physical properties.
5. Silicon: It increases the hardness of steel but reduces its plasticity and toughness. Steel used for electrical purposes contains a certain amount of silicon to improve its soft magnetic properties.
6. Tungsten: It enhances the red hardness and thermal strength of steel, and improves its wear resistance.
7. Chromium: It enhances the hardenability and wear resistance of steel, and improves its corrosion resistance and oxidation resistance.
To improve the corrosion resistance of steel pipes, general steel pipes (black pipes) are galvanized. Galvanized steel pipes can be divided into hot-dip galvanized and electro-galvanized types. Hot-dip galvanized pipes have a thicker galvanized layer, while electro-galvanized pipes have lower costs. Hence, we have galvanized steel pipes.
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