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Vacuum Nitriding Process

1. Advantages of Vacuum Nitriding

Compared with the traditional gas nitriding process, vacuum nitriding has the following characteristics:

(1) The nitriding speed is fast. Taking 38CriMoAl steel as an example, vacuum nitriding can obtain a nitriding layer of about 0.3 mm in only 10 hours at 530 °C, while traditional gas nitriding requires more than 20 hours. Relevant studies have shown that vacuum pulse nitriding at 550 °C for 10 hours has a similar depth of nitride layer compared with traditional gas nitriding at 540 °C for 33 hours. Because the nitrogen potential of vacuum nitriding is about 40% higher than that of ordinary nitriding, and vacuum purification improves the surface activity of the workpiece, thereby improving the adsorption capacity of the workpiece surface.

(2) The hardness of the seepage layer is high. Some data show that the hardness of vacuum nitriding at 530℃ is 200HV higher than that of gas nitriding at 540℃. The reason is that the air not only purifies the surface of the workpiece (active and clean), but also purifies the atmosphere in the furnace, making the white compound layer more dense.

(3) The consumption of ammonia gas is small. A vacuum furnace with a volume of 1m³ needs less than 2kg of liquid ammonia to obtain a 0.3-0.5mm infiltration layer, while the traditional gas nitriding process requires more than 1kg/h of consumption.

(4) Nitriding can be carried out on sharp corners, sharp edges, slits, blind holes, micro holes and compacted surfaces to obtain a uniform and dense nitriding layer and effectively prevent the sharp corner benefits of nitriding. Due to the pulsed gas supply, the reactive nitrogen atoms can reach the parts that cannot be reached by gas nitriding. Comparison between vacuum nitriding and ion nitriding: In addition to nitriding sharp corners, sharp edges, slits, blind holes, micro holes and compacted surfaces, the temperature uniformity and the surface quality of the workpiece are better than those of ion nitriding. It is easier to control the quality of the infiltrated layer, and ion nitriding often has many small pits on the surface due to ion bombardment.

2. Vacuum Nitriding Technological Process

After the workpiece is put into the vacuum furnace, the air is pumped to a vacuum of 0.1Pa, and the temperature is increased by heating. After the workpiece reaches the required nitriding temperature (generally 520 – 560℃), keep it for a period of time (30 – 50min, depending on the amount of furnace installed ), purify the surface of the workpiece and diathermy, and then stop the vacuuming, pass nitrogen gas (ammonia, etc.) into the furnace, raise the furnace pressure to 50 – 70Pa, keep it for a certain period of time (2 – 5min), and then vacuumize To 5-10Pa, and keep it for a certain time, stop pumping, and then introduce ammonia gas to 50-70Pa, repeat “inflation-extraction” for many times until the infiltration layer meets the requirements, and finally cool down to a certain temperature (200 – 300 °C) released. The furnace temperature can remain constant or vary throughout the nitriding process.