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Vacuum Heat Treatment Furnace

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Nitriding Heat Treatment Technology Process

1. Surface cleaning of parts before nitriding

Most parts can be nitrided immediately after degreasing by gas degreasing. Some parts also need to be cleaned with gasoline, but if the final processing method before nitriding uses polishing, grinding, polishing, etc., it may produce a surface layer that hinders nitriding, resulting in uneven or uneven nitriding layer after nitriding. Defects such as bending occurred. At this time, one of the following two methods should be used to remove the surface layer. The first method uses gas to remove oil before nitriding. Then use alumina powder to abrassive the surface. The second method is to apply phosphate coating to the surface.

2. Exhaust air from nitriding furnace

Put the processed parts in the nitriding furnace and seal the furnace cover to heat up, but before heating to 150℃, the air in the furnace must be exhausted.

The main function of the furnace is to prevent explosive gas from contacting with air when ammonia is decomposed, and to prevent the surface of the processed object and the support from oxidizing. The gas used is ammonia and nitrogen.　　The essentials for removing the air in the furnace are as follows:

① After the parts to be processed are installed, the furnace cover is sealed, and anhydrous ammonia gas is started to flow, and the flow rate may be as large as possible.

②Set the automatic temperature control of the heating furnace to 150°C and start heating (note that the furnace temperature cannot be higher than 150°C).

③When the air in the furnace is removed to less than 10%, or the exhaust gas contains more than 90% of NH3, then the furnace temperature is increased to the nitriding temperature.

3. Decomposition rate of ammonia

Nitriding is carried out by contacting other alloying elements with nascent nitrogen, but the production of nascent nitrogen is that the steel itself becomes a catalyst when ammonia gas comes into contact with the heated steel, which promotes the decomposition of ammonia.

Although nitriding can be done under ammonia with various decomposition rates, the decomposition rate is generally 15-30%, and the thickness required for nitriding is maintained for at least 4-10 hours, and the treatment temperature is kept at about 520℃. .

4. Cooling

Most of the industrial nitriding furnaces have a heat exchange unit, in order to quickly cool the heating furnace and the processed parts after the nitriding work is completed. That is, after the nitriding is completed, the heating power is turned off to reduce the furnace temperature by about 50°C, and then the ammonia flow is doubled and the heat exchanger is started. At this time, pay attention to observe whether there are bubbles overflowing in the glass bottle connected to the exhaust pipe to confirm the positive pressure in the furnace. After waiting for the ammonia gas introduced into the furnace to stabilize, the flow of ammonia can be reduced until the positive pressure in the furnace is maintained. When the furnace temperature drops below 150°C, the furnace cover can be opened after introducing air or nitrogen using the method of removing the gas in the furnace as described above.