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Gas Nitriding Heat Treatment Process

Soft nitriding heat treatment, in order to shorten the nitriding period, and the nitriding process is not limited by the steel.In recent years, two new nitriding processes, soft nitriding and ion nitriding, have been developed on the basis of original nitriding.Soft nitriding is essentially nitriding based low temperature nitriding, steel surface nitrogen atom infiltration at the same time, there are a small amount of carbon atom infiltration, the treatment results compared with the general gas nitriding, permeability layer hardness is lower than nitriding, brittleness is small, so it is called soft nitriding.

1.Soft nitriding methods are divided into three categories: gas soft nitriding, liquid soft nitriding and solid soft nitriding.Gas soft nitriding is the most widely used in domestic production at present.Gas soft nitriding is carried out in the atmosphere containing active nitrogen and carbon atoms at low temperature nitrogen and carburizing, commonly used co-osmosis media are urea, formamide, ammonia and triethanolamine, they take place in the soft nitriding temperature thermal decomposition reaction, the production of active nitrogen and carbon atoms.The active nitrogen and carbon atoms are absorbed by the surface of the workpiece, and penetrate into the surface of the workpiece through diffusion, thus obtaining nitrogen-dominated nitrocarburizing layer.The gas soft nitriding temperature is usually 560-570℃, because the hardness value of the nitriding layer is the highest at this temperature.Nitriding time is usually 2-3 hours, because over 2.5 hours, the depth of nitriding layer increases slowly with time.

2. Soft nitride layer structure and characteristics of soft nitriding: steel after soft nitride, the surface of the outermost layer can be obtained a few microns to dozens of microns of white layer, which is composed of ε phase, γ ‘phase and nitrogen containing cementite Fe3(C, N).The secondary layer is the diffusion layer, which is mainly composed of γ phase and ε phase.Soft nitriding has the following characteristics :(1) the processing temperature is low, the time is short, the workpiece deformation is small.(2), is not limited by the type of steel, carbon steel, low alloy steel, die steel, stainless steel, cast iron and iron based powder without metallurgical materials can be soft nitriding treatment.The surface hardness of the workpiece after soft nitriding is related to the nitriding process and materials.

3. It can significantly improve the fatigue strength, wear resistance and corrosion resistance of the workpiece.It also has the properties of abrasion resistance and bite resistance under dry friction condition.

4.As there is no brittle ξ phase in the soft nitriding layer, the nitriding layer is hard and has certain toughness, which is not easy to peel off.Therefore, in the current production of soft nitriding has been widely used in mold, measuring tools, cutting tools (such as: high-speed steel tool), crankshaft, gear, cylinder liner, mechanical structure parts and other wear-resistant workpiece processing.

Gas nitriding treatment process:
1. Clean the surface of the parts before nitriding
Most parts can be immediately nitriding after oil removal by gas deoiling method.Some parts also need to be cleaned with gasoline is better, but in the final processing method before nitriding if the use of polishing, grinding, polishing, etc., that may hinder the nitriding surface layer, resulting in nitriding, nitriding layer is not uniform or bending defects.In this case, one of the following two methods should be used to remove the surface layer.The first method is to remove oil with gas before nitriding.Then, the surface was abrassive cleaning with aluminum oxide powder.In the second method, the surface is treated with phosphate coating.

2. Elimination of air in nitriding furnace
The treated parts are placed in the nitriding furnace, and the furnace cover can be heated after sealing, but before heating to 150℃, air must be removed from the furnace.
The main function of the furnace is to prevent ammonia decomposition and air contact and explosive gas, and to prevent the surface of the treated material and support oxidation.The gases used are ammonia and nitrogen.
The essentials of removing the air in the furnace are as follows:
(1) After the processed parts are installed, the furnace cover is sealed, and the anhydrous ammonia gas is started. The flow rate is as much as possible.
(2) Set the automatic temperature control of the heating furnace at 150℃ and start heating (note that the furnace temperature should not be higher than 150℃).
③ When the air in the furnace is discharged to less than 10%, or the discharged gas contains more than 90% NH3, the furnace temperature shall be raised to the nitriding temperature.

3. The decomposition rate of ammonia
Nitriding is carried out by the contact of other alloying elements with primary nitrogen, but the production of primary nitrogen is promoted by the decomposition of ammonia due to the contact between ammonia gas and steel in heating which itself becomes a catalyst.
Although in a variety of decomposition rate of ammonia, can be nitriding, but generally use 15 ~ 30% of the decomposition rate, and according to the required thickness of nitriding at least 4 ~ 10 hours, the treatment temperature is maintained at about 520℃.

4. Cooling
Most industrial nitriding furnaces are equipped with heat exchangers for rapid cooling of the furnace and the treated parts after the nitriding work is completed.That is, after the nitriding is completed, the heating power supply is turned off to reduce the furnace temperature by about 50℃, and then the heat switch is started after the ammonia flow is doubled.At this time, it is necessary to observe whether there is any bubble overflow in the glass bottle connected to the exhaust pipe to confirm the positive pressure in the furnace.After waiting for the ammonia to be stabilized in the furnace, the flow of ammonia can be reduced to maintain the positive pressure in the furnace.When the furnace temperature drops below 150℃, the furnace cover can be opened after air or nitrogen is introduced by the method of removing gas from the furnace as described above.

Editing by Francis Lee

Horizontal Type Gas Nitriding Furnace

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