Technical requirements for galvanized pipes

1. Brand name and chemical composition

The grade and chemical composition of galvanized steel pipe shall conform to the grade and chemical composition of black pipe steel specified in GB 3091.

2. Manufacturing method

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

3, thread and pipe joint

3.1 Galvanized steel pipe delivered with thread shall be made after galvanized. Threads shall conform to YB 822.

3.2 Steel pipe joints shall comply with YB 238; Malleable cast iron pipe fittings shall conform to YB 230.

4. Mechanical properties The mechanical properties of the steel pipe before galvanized should conform to the provisions of GB 3091.

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

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

7, hydraulic test Hydraulic test should be carried out in the black tube, eddy current testing can also be used instead of hydraulic test. Test pressure or eddy current test specimen size shall conform to GB 3092.

The mechanical property of steel is an important index to guarantee the final serviceability (mechanical property) of steel, which depends on the chemical composition and heat treatment system of steel. In the steel pipe standard, according to different use requirements, the tensile properties (tensile strength, yield strength or yield point, elongation) and hardness, toughness indicators, as well as user requirements of high, low temperature performance.

① Tensile strength (σb)

In the tensile process of the sample, the maximum force (Fb) under tensile break is derived from the original cross-sectional area (So) of the sample stress (σ), known as tensile strength (σb), unit is N/mm2 (MPa). It represents the maximum capacity of a metal material to resist damage under tensile force. The calculation formula is:

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)

For metal materials with yield phenomenon, the stress at which the specimen can continue to extend without increasing the force (keeping constant) during the tensile process is called 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 maximum stress of the specimen before the first drop in force due to yield; Lower yield point (σsl) : the minimum stress in the yield stage, excluding the initial instantaneous effect.

The calculation formula of yield point is:

Where: Fs-- yield force (constant) in the tensile process of the sample, N (Newton) So-- original cross-sectional area of the sample, mm2.

③ Elongation after fracture (σ)

In the tensile test, the percentage between the length increased by the standard distance and the length of the original standard distance after the specimen is broken is called elongation. It is expressed as σ, in %. The calculation formula is:

Where: L1-- the distance length of the sample after it is broken, mm; L0-- original range length of sample, mm.

(4) reduction rate of cross-section (ψ)

In tensile tests, the percentage of the maximum reduction of the cross-sectional area at the reduced diameter and the original cross-sectional area of the specimen after tensile fracture is called section shrinkage. ψ is expressed in %. The calculation formula is as follows:

Where: S0-- original cross-sectional area of 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 a metal material to resist the compression of a surface by a hard object is called hardness. According to the different test method and application range, 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)

A steel ball or hard alloy ball of a certain diameter is pressed into the surface of the specimen with a specified test force (F), and the test force is removed after a specified holding time. The indentation diameter (L) of the specimen surface is measured. The Brinell hardness value is the quotient of the test force divided by the surface area of the indentation ball. It is expressed in HBS (steel ball) and expressed in N/mm2(MPa).

Galvanized welded steel pipe for low pressure fluid transfer

(hot-dip galvanized part)

1. Uniformity of galvanized layer: steel tube sample shall not turn red (copper plating color) after 5 consecutive impregnations in copper sulfate solution.

2, surface quality: the surface of galvanized steel pipe should be complete galvanized layer, there should be no black spots and bubbles on the plating, allowing a small rough surface and local zinc tumor.

3, galvanized layer weight: according to the requirements of the customer, galvanized steel pipe can be measured zinc layer weight, its average should not be less than 500g/ square meter, including any sample should not be less than 480g/ square meter.