The Reason for the Difference in Spangle Size Between Thin-Walled and Thick-Walled Tubes Under the Same Process and Operating Conditions

Under the same hot-dip galvanizing and cooling conditions, the spangles formed on thin-walled and thick-walled tubes after hot-dip galvanizing are different. The former exhibits larger spangles, while the latter has smaller ones. It is known that the cooling rate of the zinc liquid on the surface of a steel tube is related to the wall thickness of the tube's substrate. When using the "flux method" for hot-dip galvanizing, the temperature of the steel tube entering the zinc liquid (150-230°C) is lower than that of the zinc liquid (470-510°C). Therefore, thin-walled tubes absorb less heat, while thick-walled tubes absorb more. However, under the same process conditions, after immersing the steel tube in the zinc liquid, when the temperature of the thin-walled tube becomes uniform inside and out, the center of the thick-walled tube may still be below the galvanizing temperature at the surface. As a result, once removed from the zinc liquid, the zinc liquid on the thin-walled tube gradually solidifies due to air cooling alone, whereas the zinc liquid on the thick-walled tube, in addition to air cooling, also needs to dissipate heat towards the lower temperature at the tube's center, accelerating the solidification process. Therefore, the slow cooling of thin-walled tubes results in large spangles, while the faster cooling of thick-walled tubes leads to the formation of small spangles.