Development history of galvanized pipe

Often said galvanized pipe, the use of galvanized pipe now gas, heating with the kind of iron pipe is also galvanized pipe, galvanized pipe as a water pipe, after several years of use, the pipe produces a large number of rust scale, the yellow water out not only pollution sanitary ware, but also mixed with not smooth inner wall of the bacteria, corrosion caused by the water heavy metal content is too high, seriously harmful to human health.

The production process of galvanized pipe has the following production steps

a. Round steel preparation; b. Heat; c. Hot rolling perforation; d. Cut the head; e. Pickling; f. Grinding; g. Lubricate; h. Cold rolling processing; i. Degreasing; j. Solution heat treatment; k. Straightening; l. Cut the tube; m. Pickling; n. Inspection of finished products.

Galvanized pipe should meet the technical requirements

1. Brand and chemical composition

The grade and chemical composition of galvanized steel pipe should conform to the grade and chemical composition of black pipe steel specified in GB 3092.

2. Manufacturing method

The black tube manufacturing method (furnace welding or electric welding) is chosen by the manufacturer. Galvanized by hot dip galvanizing method.

3. Threads and fittings

a: Galvanized steel pipe delivered with thread, thread should be turned after galvanized. The thread shall conform to YB 822.

b: Steel pipe fittings shall comply with the provisions of YB 238; Malleable cast iron pipe fittings shall comply with YB 230.

4, mechanical properties before galvanized steel pipe mechanical properties should comply with the provisions of GB 3092.

5, the uniformity of galvanized steel pipe should be tested for the uniformity of galvanized layer. The steel pipe sample shall not turn red (copper plating color) after continuous immersion for 5 times in copper sulfate solution.

6, cold bending test nominal diameter of galvanized steel pipe is not greater than 50mm should be cold bending test. The bending Angle is 90° and the bending radius is 8 times the outside diameter. Test without filler, the specimen weld should be placed on the outside or above the bending direction. After the test, there should be no cracks and zinc layer spalling on the sample.

7, water pressure test Water pressure test should be carried out in the black tube, eddy current testing can also be used instead of water pressure test. The test pressure or eddy current test specimen size shall comply with the provisions of GB 3092. The mechanical properties of steel are important indicators to ensure the final service performance of steel (mechanical properties), which depends on the chemical composition of steel and heat treatment system. In the steel pipe standard, according to different requirements, tensile properties (tensile strength, yield strength or yield point, elongation) and hardness, toughness indicators, as well as high and low temperature properties required by users.

① Tensile strength (σb) : during the tensile process of the sample, the maximum force (Fb) it is subjected to when it breaks, the stress (σ) obtained from the original cross-sectional area of the sample (So) is called the tensile strength (σb), and the unit is N/mm2 (MPa). It represents the maximum ability of a metal material to resist damage under tensile force. Where: Fb-- the maximum force borne by the specimen when it breaks, N (Newton); So-- original cross-sectional area of sample, mm2.

② Yield point (σs) : a metal material with yield phenomenon, the stress when the specimen can continue to extend without increasing the force during the tensile process (remaining constant), called the yield point. If the force drops, the upper and lower yield points should be distinguished. The unit of the yield point is N/mm2 (MPa). Upper yield point (σsu) : the maximum stress of the specimen before the first drop in force occurs. Lower yield point (σsl) : The minimum stress in the yield stage when initial transient effects are not considered. Where: Fs-- yield force (constant) of the specimen during tensile process, N (Newton) So-- original cross-sectional area of the specimen, mm2.

③ Elongation after breaking: (σ) In the tensile test, the percentage of the length increased by the standard distance of the specimen after breaking and the length of the original standard distance is called the elongation. The unit is %, expressed as σ. Where: L1-- the standard distance length of the specimen after pulling off, mm; L0-- original gauge length of sample, mm.

④ Section shrinkage: (ψ) In the tensile test, the percentage of the maximum reduction of the cross-sectional area at the reduced diameter and the original cross-sectional area after the specimen is pulled is called the section shrinkage. It is expressed in ψ and the unit is %. Where: S0-- the original cross-sectional area of the sample, mm2; S1-- The minimum cross-sectional area at the reduced diameter of the specimen after tensile break, mm2.

⑤ Hardness index: The ability of metal materials to resist hard objects to compress the surface, called hardness. According to the different test methods and application scope, the hardness can be divided into Brinell hardness, Rockwell hardness, Vickers hardness, Shore hardness, microhardness and high temperature hardness. There are three kinds of Brinell, Rockwell and Vickers hardness commonly used for pipes.

A, Brinell hardness (HB) : with a certain diameter of steel ball or carbide ball, to the specified test force (F) pressed into the surface of the pattern, after the specified holding time to remove the test force, measure the indentation diameter of the specimen surface (L). The Brinell hardness value is the quotient obtained by dividing the test force by the surface area of the indentation sphere. It is expressed in HBS (steel ball) and 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 the steel, but the worse its plasticity and toughness.

(2) sulfur; It is harmful debris in steel, and steel with high sulfur content is easy to be brittle when it is processed under pressure at high temperature, usually called hot brittleness.

(3) Phosphorus; Can make the plasticity and toughness of steel significantly decreased, especially at low temperatures more serious, this phenomenon is called cold brittleness. In high-quality steel, sulfur and phosphorus should be strictly controlled. However, on the other hand, the low carbon steel contains higher sulfur and phosphorus, which can make it easy to cut, and it is advantageous to improve the machinability of steel.

(4) Manganese; Can improve the strength of steel, can reduce and eliminate the adverse effects of sulfur, and can improve the hardenability of steel, high alloy steel with a high manganese content (high manganese steel) has good wear resistance and other physical properties.

(5) Silicon; It can improve the hardness of steel, but the plasticity and toughness decline, electrical steel contains a certain amount of silicon, can improve the soft magnetic properties.

(6) Tungsten; It can improve the red hardness and thermal strength of steel, and improve the wear resistance of steel.

(7) Chromium; It can improve the hardenability and wear resistance of steel, and improve the corrosion resistance and oxidation resistance of steel.

In order to improve the corrosion resistance of the steel pipe, the general steel pipe (black pipe) is galvanized. Galvanized steel pipe is divided into hot dip galvanized and electric steel zinc two, hot dip galvanized galvanized layer is thick, electric galvanized low cost, so there is galvanized steel pipe.