At present, domestic hot-dip galvanizing enterprises are facing a big problem, that is, the composition and types of steel products for steel structural parts provided by customers vary greatly, which puts forward higher requirements for hot-dip galvanizing operation. Different hot-dip galvanizing processes should be adopted for steel structures with different compositions and types. Proper galvanization operation is critical to achieving good galvanizing quality and low production cost. Hot dip galvanizing operation is mainly to control the temperature of zinc bath, immersion speed, immersion time, lifting speed, picking up zinc ash, drawing zinc slag and other operation steps effectively
Zinc bath temperature
Practice shows that hot dip galvanizing of most parts can achieve satisfactory results in the temperature range of 440 ~460 C, and the working temperature commonly used is 450 C. The lowest temperature of hot-dip galvanizing should be based on the free flow of zinc from the work piece when the work piece is removed. Generally speaking, the capacity of the zinc pot is large, the amount of zinc plating is low at a time, and the zinc droplet temperature can be lower. Low temperature of hot-dip galvanizing can reduce the effect of silicon on hot-dip galvanizing layer, reduce the formation of gray and super-thick layer, reduce the formation of zinc ash and zinc slag, and ensure the safety of the zinc pot fuel saving. When the zinc bath temperature is too high, the thickness of the coating increases. When the bath temperature rises from 450 to 470, the zinc slag produced by dipping steel for 30 s Doubles because the higher the temperature is, the faster the alloy layer is formed. But too low temperature will make the coating too thick, the galvanizing piece will not be smooth, this is because of low temperature makes zinc liquidity decreases. Therefore, in order to produce good quality and low zinc consumption products, we must pay attention to controlling the temperature of zinc bath.
Under the premise of ensuring the safety of the operator, the speed of zinc bath immersed in the work piece is as fast as possible. This requires that the processed parts should be as dry as possible, otherwise it will cause serious zinc splash, resulting in increased zinc consumption and unsafety. The speed of immersion also affects the uniformity of the coating, especially for long parts. If the tilt angle remains unchanged, the immersion time of the first end of the zinc bath in the zinc bath is different, the "first in, first out" method can be used to make up for it. Rapid immersion in zinc bath also helps to reduce distortion.
Zinc dipping time
When the product enters the zinc bath, the salt plating film on the surface will react violently with zinc. Generally speaking, the parts should be dipped into a zinc pot until the "boiling" phenomenon stops, that is, the plated parts should be removed immediately. The coating obtained in this way can meet the standard requirements in most cases. When the work piece is immersed in zinc bath for one or two minutes, the reaction between iron and zinc liquid on the surface of the work piece is rapid, forming a Fe-Zn alloy layer, but with the extension of soaking time in zinc bath, the growth rate of the alloy layer will gradually decrease.
Grilled zinc ash
Because a layer of zinc ash will be formed on the surface of zinc bath after the reaction between salt film and zinc, the zinc ash on the surface of zinc bath must be removed before the zinc bath can be removed from the zinc bath to obtain a smooth surface. Zinc dust can be used to gently remove the surface zinc dust to the ends of the zinc pot. Zinc ash grill can be made of wood or thin steel plate. If the work piece is removed, zinc dust will be adhered to the surface of the plated parts, which will affect the appearance quality of the plated parts. When picking up the zinc ash, we should pay attention not to exert too much force to make the zinc bath surface agitate greatly, so that the iron-zinc alloy protective layer formed on the inner wall of the zinc pot will fall off and the corrosion of the zinc pot will be accelerated.
Speed of lifting
The elevation speed of the plating piece from the zinc bath may have a great influence on the appearance and thickness of the coating. Generally speaking, for most parts, the suitable lifting speed is about 1.5m/min. But for different materials and different types of parts, the lifting speed should be adjusted accordingly. For the inactive steel with low silicon content, the extraction rate can be lower without affecting the production efficiency, so that the zinc liquid on the surface of the workpiece can be fully refluxed, resulting in a smoother, brighter and thinner plating. But for the active steel with high silicon content, it is necessary to put forward the plating piece quickly, but the lifting speed is too fast. The zinc solution on the surface of the plating piece solidifies on the plating piece without enough reflux to the zinc bath, forming burrs, droplets and scars, seriously affecting the surface quality of the coating and increasing the zinc consumption.
Therefore, it is advisable to adopt two-speed electric crane when galvanizing, and it is better to adopt the electric crane with CVT hoisting. This allows the parts to be quickly immersed and slowly removed. For longer parts, slow lifting speed will lead to longer operating time, in order to maintain a considerable production, it is necessary to use faster speed. However, the extraction speed of the parts must be slower than the free flow rate of zinc on the surface of the parts, so as to obtain a uniform pure zinc layer.
salvage of Zinc slag
Zinc slag will be formed continuously in hot dip galvanizing process. Zinc slag will sink to the bottom of the zinc pot in the static zinc bath. When the parts are dipped in zinc, it should not be stired as far as possible to avoid mixing a large number of zinc slag into the zinc bath, which will cause particles in the coating due to adhesion of zinc slag and affect the appearance quality of the parts and increase the zinc consumption. It is not suitable to use too shallow zinc pot to avoid stirring zinc slag in the process of zinc plating.
Attention should be paid to regularly salvaging zinc slag so as not to accumulate excessive zinc slag at the bottom of the zinc pot. On the one hand, the thick zinc slag layer is easy to stir up in the process of zinc plating; on the other hand, the pasty zinc slag has poor thermal conductivity. The thick zinc slag layer will cause the local overheating of the zinc pot wall, accelerate the corrosion of zinc to iron, thus affecting the service life of the zinc pot, and may lead to the perforation and leakage of zinc. Therefore, it is necessary to regularly remove the accumulated zinc slag from the galvanizing pot with a spade with holes. The interval time can be based on the actual situation of the factory, usually once a week or half a month.