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1、 Principles for determining preparatory processing

 

Generally, structural steel parts are treated with quenching and tempering. The high-temperature tempering temperature should be about 20 degrees higher than the ammonia infiltration temperature. The insulation time should not be too long for workpieces with low impact toughness requirements. Normalizing treatment can be used, but the cooling speed of normalizing should be fast. For work with larger cross-sectional dimensions, normalizing 38CrMoA steel should not be used. Quenching and tempering should be used, otherwise needle like nitrides can easily appear in the nitriding layer. Tool and mold steel should be treated with quenching and tempering before nitriding, Annealing is not allowed for some complex and precise parts. Before nitriding, stress relief can be carried out one or more times and the temperature can be slowly increased

 

2、 Work preparation

 

1. Air tightness inspection

 

Before nitriding, air tightness checks should be conducted on the pipeline joints of the heating furnace, nitriding, and the entire nitriding system to ensure that ammonia gas does not leak and is unobstructed in the pipeline.

 

2. Polishing and cleaning of the workpiece working surface

 

The nitrided surface is required to undergo careful polishing and inspection. The nitrided surface should be free of oil stains, rust, sharp corners, burrs, bumps, and dirt that cannot be washed off. For non nitrided surfaces, protection should be checked and the coating should be intact.

 

3. Check for air leakage

 

Soak the phenolphthalein test paper and place it at the suspected leak location. If the test paper turns red, it indicates the leak phenomenon

 

3、 Regulations on process specifications

 

1. Ammonia infiltration temperature

 

1.1. Commonly used at 500-530C, it can ensure a large dispersion of nitrides and the highest hardness

 

1.2. As the ammonia infiltration temperature increases, the depth of the infiltration layer increases, while the hardness significantly decreases. Nitriding is greater than 550C, and the highest hardness of most steel grades is less than 1000HV

 

1.3. Using two-stage ammonia infiltration method, the temperature of the second stage is usually lower than 560C

 

2. Nitriding time

 

2.1 The ammonia permeation layer thickens with the extension of time, with a large initial growth rate and gradually slows down afterwards, with a general permeation rate of around 0.01mm/h

 

2.2 As the insulation time prolongs, the nitride accumulates and grows, and the hardness decreases. The higher the temperature, the longer the time, and the more severe the growth

 

3. Decomposition rate

 

When the decomposition rate is 20-40%, there are many active nitrogen atoms, and the surface of the part can absorb a large amount of nitrogen

 

3.2 If the decomposition rate exceeds 60%, the gas content in the atmosphere can reach over 52%, which will result in denitrification. At this time, not only does the number of active nitrogen atoms decrease, but also large amounts of hydrogen and nitrogen molecules stagnate near the surface of the part, making nitrogen atoms less easily absorbed by the surface, thereby reducing the nitrogen content on the surface of the part and thinning the depth of the ammonia layer.

 

4、 Ammonia infiltration operation

 

1. Heating up

 

Before heating up, ammonia gas should be used for exhaust. The ammonia gas flow is more than twice that of the normal flow. For those with strict deformation requirements, step heating can be used, or the heating speed can be reduced to less than 50 degrees.

 

2. Exhaust

 

During the exhaust process, the furnace and pipeline sealing can be checked using a pH test paper method; During the exhaust phase, nitrogen gas is continuously introduced to achieve a furnace pressure of 200-400Pa (U-shaped tube filled with oil). If the air is not cleared, the exhaust pipe cannot be ignited. During the heating phase, the ammonia decomposition rate can reach 70%, and the ammonia volume in the furnace gas is greater than 95%, which can reduce the ammonia flow rate

 

3. Insulation

 

When the temperature reaches the ammonia permeation temperature, adjust the ammonia flow to make the ammonia decomposition rate reach the range required by the process, control the furnace pressure between 200-600Pa, and measure the ammonia decomposition rate every 15-30 minutes

 

4. Cooling

 

After the end of nitriding, close the exhaust valve to reduce the ammonia flow, so that the ammonia decomposition rate reaches about 70%, and keep it for 2 hours to reduce the surface brittleness of the workpiece; Power off to cool down, maintain positive pressure inside the furnace and lower the furnace temperature to below 150 before discharging