Small Parts Bolts, Nuts Galvanizing Equipment

High-temperature hot-dip galvanizing for fasteners

1. Advantages and disadvantages of hot-dip galvanizing process for fasteners

1. Advantages and disadvantages of electrogalvanized fasteners

There is no doubt that fasteners are widely used and widely used in mechanical basic workpieces. The barrel plating process is generally used in the electrogalvanizing production of fasteners.

Electrogalvanized fasteners have the following advantages:

(1) The galvanized layer is uniform and bright. After passivation, various colors can be obtained, and the decorative effect is good.

(2) Low cost and simple operation.

At the same time, electro-galvanized fasteners also have some defects, such as hydrogen embrittlement, especially high-strength (≥827MPa) fasteners are sensitive to hydrogen embrittlement, and incomplete hydrogen drive will cause failure of the fastener connection (fastening Parts cracking, sudden breakage, etc.). The electro-galvanized galvanized layer of fasteners is thin, and the thickness of the electro-galvanized layer is generally less than 10.00 μm, which cannot meet the requirements of outdoor anti-corrosion. In addition, environmental pollution is also serious.

2. Advantages and disadvantages of hot-dip galvanized fasteners

Electrogalvanizing of fasteners accounts for more than 60% of the total electroplating. When fasteners are used indoors or in other closed systems where the environment is not harsh, electrogalvanizing is the first choice. However, it is not the case outdoors. At this time, the fasteners must be treated with anti-corrosion treatment by hot-dip galvanizing.

Hot-dip galvanized layer is a metal cladding with low price and good corrosion resistance, especially suitable for mass production of standard parts and fasteners. Hot-dip galvanizing of fasteners has the following characteristics:

(1) A thick galvanized layer can be obtained with good corrosion resistance.

(2) The galvanized layer has good adhesion, and can be properly formed, welded and painted after plating.

Due to the small size and irregular shape of the fasteners, and some require threaded fit, there are also some obvious defects in hot-dip galvanizing:

(1) It is difficult to twist the thread after plating.

After hot-dip galvanizing, the remaining zinc residue in the thread is not easy to remove, and the thickness of the galvanized layer is uneven, which affects the fit of the threaded workpiece.

The external thread of the fastener should be processed or rolled according to the standard before hot-dip galvanizing, while the internal thread can be processed before or after hot-dip galvanizing.

However, in practical applications, customers often require that both internal and external threads have a galvanized layer, so people use various measures to solve the problem of galvanizing the threaded fittings, such as back tapping of threaded workpieces, reserving a large fitting gap, centrifugal throwing and other methods. Return attack is easy to damage the galvanized layer of the threaded part, and even expose the threaded iron matrix, causing corrosion of the fasteners. Deliberately increasing the nut aperture or reserving a fit clearance beyond the range can easily reduce the fit strength, which is not allowed for high-strength fit.

At present, it is more common to use mechanical centrifugal to remove excess zinc in China. In addition, although people consider the hot-dip galvanizing process itself, adjusting the heat

The galvanizing temperature and galvanizing time are used to improve the quality of the galvanized layer, but it does not fundamentally solve the problem.

(2) The high operating temperature of hot-dip galvanizing will reduce the mechanical strength of high-strength fasteners.

After hot-dip galvanizing for grade 8.8 bolts, the strength of some threads is lower than the standard requirements; the strength of bolts above grade 9.8 after hot-dip galvanizing basically cannot meet the requirements.

(3) Poor working environment and serious pollution.

The hot-dip galvanizing process of fasteners is carried out at high temperature. When the flux is dried and the workpiece is galvanized, strong irritating hydrogen chloride gas will be precipitated; the zinc pot has been at high temperature for many years, and zinc vapor is generated on the surface of the zinc liquid. It will cause the whole working environment to be bad. At present, many private enterprises still use coal-fired reverberatory heating furnaces for hot-dip galvanizing production, which emit a large amount of smoke containing sulfur dioxide, carbon dioxide, carbon monoxide and dust into the atmosphere, causing serious air pollution.

Although there are many defects in hot-dip galvanized fasteners, because of the thick galvanized layer, good bonding strength and long-term corrosion resistance of hot-dip galvanized, it has been highly praised in the electric power, communication, and transportation departments.

2. High-temperature hot-dip galvanizing process for bolts and nuts

Fasteners such as bolts and nuts are generally centrifugally used in the high-temperature hot-dip galvanizing process.

After galvanizing the small workpieces in the cage, when the galvanized layer is still in a molten state, the excess zinc liquid should be removed by centrifugation immediately, so that the workpieces can get a good glossy surface. This requires moving the workpiece from the zinc pot to the centrifuge as quickly as possible.

In addition, it is also very important that the centrifuge needs to be driven by a "motor with a large starting torque" in order to accelerate it to the highest speed within 2 to 3 seconds. The commonly used centrifuge adopts a speed of about 750r/min, and the excess zinc liquid can be thrown off in the first few seconds, and the extension of the centrifugation time has no obvious effect.

Proper attention must also be paid to the fitting tolerances of parts during production, as these workpieces are to be assembled after galvanizing, e.g. bolt holes, chain bolt holes, or those which are already assembled but still move freely after galvanizing .

The normal process procedure for galvanized bolts and nuts is that the bolts are galvanized after they are made into standard threads; the nuts are first galvanized as blanks, and then the internal threads are processed to expand the internal threads.

Immediately after centrifugation, the workpieces are poured into water, which coagulates the galvanized coating and prevents the workpieces from sticking together.

Our company mainly business scope is as follows:   

1) The design, manufacture, installation, commissioning and training of the hot dip galvanizing lines, redesign and improve of the old galvanizing plants.   

1. Jobbing galvanizing line for construction parts: such as steel tower , tube tower parts, high rails and lighting poles , etc. 

2. Galvanizing lines for steel tubes:suitable for 1/2"-8" steel tube.

3. Galvanizing lines for small parts:suitable for bolts, nuts and other small parts. 

2) The related chemical preprocessing of products and alloy   

Our main products are : chemical pretreatment tanks, galvanizing furnace, zinc kettle, utilisation of fuel gas from the fuel furnace, drying pit.   

If you have any questions, please contact our engineers for consultation, online.