Seamless steel pipe, its hot-dip galvanizing technology and the temperature of zinc liquid treatment

The hot-dip galvanizing technology of seamless steel pipes can usually be divided into two types: dip plating and blow plating

Dip plating: direct water cooling after immersion plating, average thickness of zinc layer is more than 70 microns, galvanizing cost is high, zinc usage is large, used in normal atmospheric environment for more than 50 years, there is an obvious trace of zinc flow, long seamless pipe can be Plated to 16 meters.　

Blow plating: blow outside after zinc dipping, and water cooled after blow inside. The average thickness of zinc layer is more than 30 microns, low cost, small zinc usage, more than 20 years in normal atmospheric environment, traces of zinc liquid are almost invisible, general zinc blowing production line It is 6-9 meters.

Due to the uneven wall thickness of the seamless pipe, effective force and tissue stress will be generated during heating, cooling and phase change. In addition, large parts are prone to residual stress after machining, and all these internal stresses must be eliminated. The usual heating temperature for stress relief annealing is 500-550℃, the holding time is 2-8h, and then the furnace is cooled. This process can eliminate 90-95% of the internal stress of the casting, but the cast iron structure does not change. If the temperature exceeds 550℃ or the holding time is too long, it will cause graphitization and reduce the strength and hardness of the casting.

When the casting is cooled, the surface layer and thin section often produce white mouth. The white mouth structure is hard and brittle, has poor processing performance and is easy to peel off. Therefore, the method of annealing must be used to eliminate the white mouth structure. During the high temperature holding period, the high cementite and eutectic cementite decompose into graphite, and the secondary cementite and eutectoid cementite also decompose during the subsequent cooling process, and the graphitization process occurs. Due to the decomposition of cementite, the hardness decreases and the machinability is improved.