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What is the process of vacuum nitriding?
Vacuum nitriding process
Different from gas nitriding, vacuum nitriding does not use the one-stage, two-stage, and three-stage nitriding methods of traditional gas nitriding, but uses pulse nitriding for nitriding. Vacuum treatment has the effects of less oxidation, degassing and degreasing, but if the gas pressure is too low, the NH3 content is low, and it cannot provide enough active nitrogen atoms required for nitriding; if the gas pressure is too high, the meaning of vacuum treatment is lost. Vacuum pulse nitriding takes advantage of both vacuum treatment and gas nitriding, so that the workpiece has good nitriding performance. The specific method is as follows: first, evacuate the furnace to a vacuum of 0.1Pa, then heat the workpiece to the nitriding temperature (generally 520~560℃), keep it warm for 30~60min (depending on the amount of furnace loading) to make the workpiece evenly heated and the surface purified and degassed, then fill it with NH3 to 50~70kPa, keep it for 2~5min, then start the vacuum pump, quickly pump out the N2, H2 and residual NH3 in the furnace, pump out the gas and reduce the pressure to 5~10kPa, then fill it with NH3 to 50~70kPa, and repeat “filling-evacuation” several times until the depth of the nitriding layer meets the use requirements. Finally, cool down the furnace to 200℃ and take it out of the furnace.
Factors affecting nitriding performance
Factors affecting vacuum pulse nitriding include: initial vacuum, ammonia nitriding temperature, nitriding time, pulse interval, furnace pressure, ammonia flow rate and cooling method.
Initial vacuum degree
The initial vacuum degree is to purify the surface of the workpiece, remove the oxides, grease and adsorbed gases on the surface of the workpiece, and it may not be necessary to evacuate to a vacuum degree of 0.1Pa. References believe that when heated to above 500℃ at 1.33Pa, Fe2O3 and FeO on the steel surface will be converted into metastable oxides and evaporate, and then be evacuated. Other gases and adhesions adsorbed on the steel surface (such as cleaning agents, etc.) will also be desorbed and discharged out of the furnace.
Cooling method
The cooling method is mostly based on gas nitriding, cooling to 200℃ with the furnace, and then air cooling after exiting the furnace. As for exiting the furnace at 200℃, it is to prevent oxidation of nitrided parts caused by high temperature exiting the furnace.
When studying the influence of vacuum nitriding process parameters on the performance of the nitriding layer of Q235, it was found through orthogonal test analysis that the pulse interval had no significant effect on the performance of the nitriding layer.
Vacuum nitriding time
Nitriding temperature and nitriding time are two main parameters that have a significant impact on nitriding depth and surface hardness. The influence of nitriding temperature and nitriding time on the thickness of 38CrMoAl gas nitriding: In the same nitriding time, the higher the nitriding temperature, the greater the depth of the nitriding layer. At the same nitriding temperature, the depth of the nitriding layer increases continuously with the extension of the nitriding time. In the early stage of nitriding, the depth of the nitriding layer increases rapidly, and in the later stage it increases slowly, the economic efficiency of nitriding deteriorates, and it may also lead to the growth and coarsening of the compound layer, the reduction of hardness, and the reduction of resistance, corrosion resistance, and fatigue strength.
Vacuum nitriding temperature
The influence of nitriding temperature and time on the surface hardness of 38CrMoAl gas ammonia nitriding: When the nitriding temperature is not less than 500℃, the surface hardness increases with the increase of nitriding time, and then the surface hardness reaches a peak value, and then the surface hardness decreases with the further extension of nitriding time. At different temperatures, the higher the temperature, the shorter the time to reach the peak value of the surface hardness, but the greater the reduction in the surface hardness afterwards. The study also shows that the increase in nitriding temperature has no obvious effect on the peak value that the surface hardness can reach. However, when the nitriding temperature is 600℃, its peak value is much lower than the peak value of other ammonia nitriding temperatures.
Increasing the furnace pressure can shorten the nitriding time, while reducing the furnace pressure can prolong the nitriding time. Moreover, in the low pressure range of positive pressure greater than 53kPa, the change of furnace pressure has no obvious effect on the nitriding time, while when it is less than 27kPa, the nitriding speed drops suddenly.
Ammonia flow rate is one of the main parameters affecting the properties of the nitriding layer. Studies have shown that: as the ammonia flow rate increases, the nitriding hardness and ammonia depth increase. Studies have also found that the sequence of ammonia flow rate has a decisive influence on whether a white bright layer is produced and the brittle toughness of the white bright layer. Ammonia flow rate that is high first and then low helps to obtain a white bright layer with good toughness, and also helps to obtain a nitriding layer without a white bright layer.
Vacuum nitriding furnace