Classification and production process of galvanized pipes
2014-10-23
1. Overview of Galvanized Pipes To improve the corrosion resistance of steel pipes, general steel pipes are galvanized. Galvanized pipes (galvanized steel pipes) are divided into hot-dip galvanized and cold galvanized types. Hot-dip galvanized pipes have a thick zinc layer, uniform coating, strong adhesion, and long service life. Cold galvanized pipes have low cost and their surface is not very smooth; their corrosion resistance is significantly inferior to that of hot-dip galvanized pipes and has been gradually phased out. In most old buildings in China, galvanized pipes were used. Currently, the iron pipes commonly used for gas and heating are also galvanized pipes. As water pipes, galvanized pipes accumulate a large amount of rust over time, which not only pollutes sanitary ware but also breeds bacteria due to the rough inner walls. More seriously, corrosion leads to excessive heavy metal content in the water, severely harming human health. In the 1960s and 1970s, some developed countries began to develop new types of piping materials and gradually banned the use of galvanized pipes. The Ministry of Construction of China and three other ministries explicitly stated that starting from 2000, the use of galvanized pipes is prohibited; currently, new residential complexes rarely use galvanized pipes for cold water systems, although some complexes still use them for hot water systems. 2. Classification and Characteristics of Galvanized Pipes Galvanized pipes are classified into hot-dip galvanized steel pipes and cold-galvanized steel pipes, with cold-galvanized steel pipe being gradually phased out. 1. Hot-Dip Galvanized Pipe / Hot-Dip Galvanized Steel Pipe Hot-dip galvanizing involves a reaction between molten metal and iron substrate to produce an alloy layer that combines both the substrate and coating. The manufacturing process for hot-dip galvanized pipe begins with pickling the steel pipe to remove surface iron oxide; after pickling, it is cleaned in a solution of ammonium chloride or zinc chloride or a mixture thereof before being sent into a hot dip galvanizing tank. Hot-dip galvanized steel pipe undergoes complex physical and chemical reactions between the molten coating solution and the steel pipe substrate to form a dense zinc-iron alloy layer that is resistant to corrosion. The alloy layer integrates with both pure zinc layers and the steel pipe substrate; therefore, hot-dip galvanized steel pipe has advantages such as uniform coating thickness, strong adhesion, and long service life. 2. Cold-Galvanized Pipe / Cold-Galvanized Steel Pipe Cold galvanizing refers to electro-galvanizing with very little zinc applied (only 10-50g/m²), resulting in significantly poorer corrosion resistance compared to hot-dip galvanized pipe. Reputable manufacturers generally do not use electro-galvanizing (cold galvanizing) to ensure quality; only small-scale enterprises with outdated equipment tend to use it due to lower costs. The zinc layer on cold-galvanized steel pipe is independent from the steel substrate; because it is thinly applied directly onto the substrate without bonding well, it can easily peel off leading to poor corrosion resistance. Therefore, its use as water supply or gas supply piping has been prohibited in new residential buildings. 3. Production Process Flow for Galvanized Pipes The production process for galvanized steel pipe is considered proprietary information by manufacturers; thus only a general process flow can be provided: a) Round bar preparation; b) Heating; c) Hot rolling perforation; d) Cutting heads; e) Pickling; f) Grinding; g) Lubrication; h) Cold rolling processing; i) Degreasing; j) Solid solution heat treatment; k) Straightening; l) Cutting tubes; m) Pickling again; n) Finished product inspection. 4. Technical Requirements and Standards for Galvanized Pipes 1. Grades and Chemical Composition of Galvanized Pipes The grades and chemical composition of steels used for galvanized pipes should comply with GB-3092 standards applicable for black tube steels. 2. Manufacturing Methods for Galvanized Pipes (The manufacturing method—furnace welding or electric welding—is chosen by manufacturers.) Hot dip galvanization is commonly used. 3. Threads and Pipe Joints for Galvanized Pipes （1）带螺纹交货的镀锌钢管，螺纹应在镀锌后车制，螺纹应符合YB-822的规定； , , 公称壁厚（mm）：2.02.52.83.23.53.84.04.5重量系数c：1.0641.0511.0451.0401.0361.0341.0321.028 5、镀锌钢管的力学性能 钢管镀锌前的力学性能应符合GB-3092的规定。钢材力学性能是保证钢材最终使用性能（机械性能）的重要指标，它取决于钢的化学成分和热处理制度。在钢管标准中，根据不同的使用要求，规定了拉伸性能（抗拉强度、屈服强度或屈服点、伸长率）以及硬度、韧性指标，还有用户要求的高、低温性能等。 6、镀锌管镀锌层的均匀性 镀锌钢管应做镀锌层均匀性的试验。镀锌钢管试样在硫酸铜溶液中连续浸渍5次不得变红（镀铜色）。 7、镀锌管的冷弯曲试验 公称口径不大于50mm的镀锌钢管应做冷弯曲试验。弯曲角度为90°，弯曲半径为外径的8倍。试验时不带填充物，试样焊缝处应置于弯曲方向的外侧或上部。试验后，试样上不应有裂缝及锌层剥落同象。 8、镀锌管的水压试验 The water pressure test for galvanized steel pipes should be conducted on black pipes, and eddy current testing can also be used as a substitute for the water pressure test. The test pressure or the dimensions of the comparison sample for eddy current testing should comply with the provisions of GB-3092.
