1. General structural steel parts are treated with quenching and tempering. The high temperature tempering temperature should be 20~40oC higher than the nitriding temperature. The holding time should not be too long.
2. For parts with low requirements on the impact toughness value, normalizing can be used, but the normalizing cooling speed should be fast, and the parts with larger sectional dimensions should not be used for normalizing.
3. 38CrMoAl steel should be quenched and tempered, otherwise needle-like nitrides are likely to appear in the layer. Before the nitriding of the work and die steel, it must be quenched and tempered, and no annealing is allowed.
4. For slender, thin-walled, complex and precision parts, one or more stress-relieving treatments must be carried out before nitriding. The stress-relieving temperature is generally lower than the tempering and tempering temperature, higher than the nitriding temperature, such as deformation after treatment. After correcting, re-stress the original process again until the deformation is fully qualified.
1) Before nitriding, comprehensively check the temperature control instrument, liquid ammonia bottle, flow meter, ammonia decomposition tester and various pipeline systems to ensure the normal operation and use of the equipment. The temperature difference in the furnace should be less than 10oC.
2) Desiccant is commonly used in calcium oxide, calcium chloride, etc., commonly used calcined calcium oxide, if it fails, it should be replaced or dried.
3) Liquid ammonia with a volume fraction of water used for nitriding medium less than 0.2% is good
4) According to the shape, size, quality and technical requirements of the parts, prepare the spreader. Before use, carefully check whether it is intact and reliable.
1) The surface roughness of the nitriding parts is less than 1.6μm, the surface must be free of defects such as napping, bumping and rusting. Parts that cannot be treated in time must be protected by oil to avoid rusting.
2) Distorted parts should be predicted and recorded3) The material and pre-heat treatment of the sample are the same as the parts. The sample size is ф(20~30)mm×(6~10)mm, the sample is 3~5 pieces, and the number is placed in a representative different position.
3) Liquid ammonia with a volume fraction of water used for nitriding medium less than 0.2% is good
4) Carefully clean the parts and hoist them into the furnace, then clean them with clean gasoline to ensure the cleanliness of the parts.
5) Install the cleaned parts into the nitriding furnace. For the easy-deformed slender rods, they must be hung vertically, the spacing should be even, do not block the venting holes, and the parts must not exceed the range of the effective heating zone.
1) Commonly used 500~530oC, can ensure the nitride has a large dispersion, the highest hardness
2) As the nitriding temperature increases, the depth of the layer increases, and the hardness decreases remarkably. If the nitriding is greater than 550oC, the highest hardness of most steel grades is lower than 1000HV.
3) When using two or three stages of nitriding, the temperature of the second stage is often lower than 560oC.
1) The nitriding layer thickens as the nitriding time increases. The initial growth rate is large, and it gradually becomes slower. It follows the parabolic law. The general seepage speed is around 0.01mm/h.
2) As the holding time is prolonged, the nitride aggregates grow and the hardness decreases. The higher the temperature, the longer the time, the greater the growth
1) The ammonia decomposition rate is controlled at 25%~40%, the active nitrogen atom is the most, and the surface of the part absorbs a large amount of nitrogen.
2) The ammonia decomposition rate exceeds 60%, the volume fraction of hydrogen in the atmosphere reaches 52%, and denitrification occurs. A large amount of nitrogen molecules and hydrogen molecules are stagnated on the surface of the part, so that the nitrogen atoms are not absorbed, and the layer is thinned.
3) One-stage or two-stage nitriding, the decomposition rate of strong osmotic ammonia is 18%~30%, and the ammonia decomposition rate during diffusion is 40%~60%.
1. Temperature rise: Before the temperature rise, the ammonia gas should be exhausted. The ammonia gas flow rate is more than twice of the normal use flow rate. The deformation requirements are strict. The step temperature is raised. The heat is kept at 200~250oC and 400~450oC for 1~2h. The speed is less than 50oC/h. When the holding time is close, the ammonia amount should be increased to control the decomposition rate to the lower limit of the process.
2. Exhaust: Generally, the exhaust gas is exhausted at a temperature of 150~200oC, and the pH test paper can be used during the exhaust process (the test paper is wetted with water, the ammonia gas turns blue) or the glass rod (the glass rod is saturated with ammonia). Gas has white smoke exhausted) Method to check furnace tank and pipe sealing During the exhaust phase, ammonia gas is continuously introduced to make the furnace gas pressure reach 200-400 Pa. Since the ammonia gas is easily soluble in water, the U-shaped tube for measuring the ammonia decomposition rate is usually filled with oil. In the nitriding tank, the air is not clear, the exhaust pipe cannot be ignited, the ammonia decomposition rate can reach 70% in the heating stage, and the ammonia flow rate in the furnace gas is greater than 95%, the ammonia flow can be reduced, and the furnace is kept in positive Pressure
3. Insulation: When the temperature reaches the specified temperature of the process, adjust the ammonia flow rate, control the ammonia decomposition rate to 18%~35%, and start to calculate the holding time. At this time, the furnace pressure is 200~600Pa. In the nitriding process, the ammonia is adjusted. The flow rate method is used to control the ammonia decomposition rate. Generally, the ammonia decomposition rate is measured every 15~30 minutes. During the heat preservation process, the upper and lower deviations of the furnace temperature should be less than 5oC. At the end of the heat preservation, the sample should be taken out from the sampling hole to make necessary Analysis
4. Denitrification cooling: After the whole stage of nitriding, close the exhaust valve, reduce the ammonia flow, keep the temperature for 2h, make the ammonia decomposition rate more than 70%, reduce the surface nitrogen concentration of the parts and reduce the surface brittleness. After that, the power is cooled down, and a small amount of ammonia gas is continuously introduced into the furnace to keep the furnace at a positive pressure. When the furnace temperature drops below 150oC, the ammonia supply can be stopped, and the parts should be prevented from colliding.
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