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Pit Furnace Gas Nitriding Process Operation

1. Preliminary Treatment Determination Principle

General structural steel parts are quenched and tempered. The high-temperature tempering temperature should be 20-40ºC higher than the nitriding temperature, and the holding time should not be too long.

Normalizing can be used for parts with low impact toughness value, but the normalizing cooling speed should be fast, and the parts with larger cross-sectional dimensions should not be normalized.

38CrMoAl steel should be quenched and tempered, otherwise needle-like nitrides are easy to appear in the infiltration layer. Before nitriding, the tool and die steel must be quenched and tempered. Annealing is not allowed.

For slender, thin-walled, complex and precise parts, one or more stress relief treatments must be carried out before nitriding. The stress relief temperature is generally lower than the tempering and tempering temperature and higher than the nitriding temperature. If the deformation after treatment is out of tolerance , After correction, the stress is removed again according to the original process until the deformation is fully qualified.

2. Ready To Work

2.1 Equipment Preparation

Before nitriding, perform a comprehensive inspection of temperature control instruments, liquid ammonia bottles, flow meters, ammonia decomposition testers, and various piping systems to ensure the normal operation and use of the equipment. The temperature difference in the furnace should be less than 10ºC.

The desiccant is usually calcium oxide, calcium chloride, etc., and roasted calcium oxide is commonly used. If it fails, it should be replaced or dried.

Liquid ammonia with a volume fraction of water for the nitriding medium less than 0.2% is better.

According to the shape, size, quality and technical requirements of the parts, prepare the spreader, and carefully check whether it is in good condition and reliable before use, and clean it.

2.2 Part Preparation

The surface roughness of nitriding parts is Ra1.6μm or less, and the surface must not have defects such as roughing, bumps and rust. Parts that cannot be processed in time must be protected by oil to avoid rust.

For easily distorted parts, the amount of distortion should be predicted and recorded.

The material and pre-heat treatment of the sample are the same as those of the parts. The size of the sample is ф (20~30)mm×(6~10)mm. There are 3~5 samples, and the numbers are placed in different representative positions.

Clean the parts carefully, and when they are hoisted into the furnace, clean them with clean gasoline to ensure the cleanliness of the parts.

Put the cleaned parts into the nitriding furnace. The elongated rods that are easily deformed must be hung vertically, and the spacing must be even, do not block the exhaust holes, and the parts must not exceed the effective heating zone.

3. Determination Of process Specifications

3.1 Nitriding Temperature

500~530ºC is commonly used, which can ensure that the nitride has a large dispersion and the highest hardness.

With the increase of the nitriding temperature, the depth of the nitriding layer increases, but the hardness decreases significantly. When nitriding is greater than 550ºC, the hardness of most steel grades is lower than 1000HV.

When using two-stage or three-stage nitriding, the temperature of the second stage is often lower than 560ºC.

3.2 Nitriding Time

The nitriding layer thickens with the extension of nitriding time. The initial growth rate is large, and then it will gradually slow down, following the parabolic law, and the general seepage velocity is about 0.01mm/h.

With the extension of the holding time, the nitrides will accumulate and grow, and the hardness will decrease. The higher the temperature, the longer the time, the stronger the growth.

3.3 Ammonia Decomposition Rate

It is better to control the ammonia decomposition rate at 25%-40%, the active nitrogen atoms are the most, and the surface of the parts absorbs a lot of nitrogen.

The ammonia decomposition rate exceeds 60%, and the volume fraction of hydrogen in the atmosphere reaches 52%, which produces denitrification. A large number of nitrogen and hydrogen molecules stagnate on the surface of the part, so that nitrogen atoms are not absorbed and the infiltration layer is thinned.

One-stage or two-stage nitriding, the ammonia decomposition rate of strong penetration is 18%~30%, and the ammonia decomposition rate during diffusion is 40%~60%, which is ideal.

4. Nitriding Operation

Temperature rise: Ammonia exhaust gas should be vented before heating. The ammonia flow rate is more than 2 times the normal flow rate. For those with strict requirements on deformation, a stepped temperature rise is used. The temperature is kept at 200~250ºC and 400~450ºC for 1~2h, and the temperature can also be increased. The speed is less than 50ºC/h. When it is close to the holding time, the amount of ammonia should be increased to control the decomposition rate at the lower limit of the process requirements.

Exhaust: Generally, the temperature is increased while exhausting, and the temperature is 150~200ºC. During the exhaust process, pH test paper (the test paper will be soaked with water, it will turn blue when exposed to ammonia) or a glass rod (the glass rod will be exposed to salt and ammonia. The gas has white smoke exhausted) Method to check the sealing of the furnace tank and pipeline. In the exhaust stage, continuously pass in ammonia gas to make the furnace gas pressure reach 200~400Pa. Since the ammonia gas is easily soluble in water, the U-shape of the ammonia decomposition rate is measured The pipe is usually filled with oil. In the nitriding tank, the air is unclear and the exhaust pipe cannot be ignited. The ammonia decomposition rate can reach 70% during the heating stage. When the volume fraction of ammonia in the furnace gas is greater than 95%, the ammonia flow can be reduced and the furnace can be kept positive. Pressure.

Insulation: When the temperature reaches the specified temperature of the process, adjust the ammonia flow to control the ammonia decomposition rate at 18%~35%, and start to calculate the holding time. At this time, the furnace pressure is 200~600Pa. During the nitriding process, adjust ammonia The ammonia decomposition rate is controlled by the flow method. Generally, the ammonia decomposition rate is measured every 15~30min. During the heat preservation process, the deviation of the furnace temperature should be less than 5ºC. At the end of the heat preservation, the sample should be taken out of the sampling hole and necessary Analysis of.

Denitrification and cooling: After the entire nitriding stage, close the exhaust valve, reduce the ammonia flow, and keep it warm for 2 hours, so that the ammonia decomposition rate is greater than 70%, so that the nitrogen concentration on the surface of the part is reduced, and the surface brittleness is reduced. After that, the power is cut off and the temperature is lowered, and a small amount of ammonia gas is continued to be introduced into the furnace to maintain a positive pressure in the furnace. The ammonia supply can be stopped when the furnace temperature drops below 150ºC. The parts should avoid collisions when they are released.