What is Nitriding – Process, Advantages and Disadvantages
Introduction to Nitriding:
Nitriding is a case-hardening process of enriching the solid steel surface with nitrogen at a low temperature, normally in the range of 500-575°C (i.e., below A1), when the steel is ferritic.
There are two general types of nitriding processes:
1. For alloy steels containing strong nitride-forming elements. (Hard Nitriding)
2. For unalloyed low carbon steels. (Soft Nitriding)
Commonly, the definition of term ‘nitriding’ is synonym to gas-nitriding of nitriding (alloy) steels (also called nitralloys), i.e., it is understood as the enrichment of solid steel surface with nitrogen by heating it in an atmosphere of NH3 gas at a temperature normally in the range of 500-575°C for a prolonged period of 48 to 96 hours, depending upon the case-depth desired. Now-a-days, nitriding is also done in other mediums. The steels at the nitriding temperatures have microstructure consisting normally of ferrite and carbides.
As NH3 dissociates to give atomic-nitrogen at the steel surface, it gets absorbed there, and then diffuses inside. There takes place interaction between N and the alloy solute atoms, principally of Al, Cr, Mo, resulting in the formation of fine, closely-spaced and uniformly dispersed coherent-alloy-nitrides precipitate-particles in ferrite. Dislocations seem to be required for the nucleation of these nitride-particles, and thus, dislocations are created providing more sites for nucleation.
Main Reasons for Nitriding:
1. To obtain high surface hardness, wear resistance and antigalling properties. Hardness obtained is higher than obtained by carburising. Hardness HRC 62-67 or even 71 can be obtained.
2. To improve fatigue properties.
3. To improve corrosion resistance in atmosphere, water, steam, etc. (except stainless steels).
4. To have good high temperature (up to nitriding temperature ≈ 550°C) properties. As the case has high resistance to tempering, it retains high hardness at high temperatures.
5. No dangers of quench cracks and distortion i.e., high dimensional stability. No other heat treatment is required after nitriding.
Operations before Nitriding:
Normally several operations are done before nitriding:
1. Hardening and Tempering:
Nitriding steels are invariably hardened and tempered at high temperature to increase the strength and toughness of the core. The tempering temperature is at least 30° higher than the nitriding temperature, normally 600-675°C. Steel now has sorbitic structure which is also good for machining.
2. Final Machining:
All the machining operations are done which give the final size to the component, keeping a close tolerance of 0.03 mm to .05 mm on all areas as parts grow during nitriding. Sharp corners if possible should be rounded off. If heavy machining had been done, the parts may be stress-relieved by heating at 550-570°C for 2-4 hours. Final finish machining may be done then. No machining is done after nitriding.
3. Selective Nitriding:
The areas not to be nitrided are coated with a thin layer of tin (0.01-0.15 mm thick), copper or nickel or water glass. Surface tension keeps the molten tin sticking to the parts while being nitrided. Copper (electrolytically deposited) is also used quite often.
4. Then, nitriding is done as required.
5. Finish-lapping is done.
Process for Nitriding:
Iron-nitrogen equilibrium diagram (Fig. 8.35) can be used to study the nitriding process. At the commonly used nitriding temperature (below 590°), nitrogen dissolves in α-iron up to only 0.1% (called nitrogenous ferrite). When the nitrogen dissolved in a-iron exceeds 0.1%, next phase stable at the temperature, i.e. γ’- nitride (a solid solution) is formed. When nitrogen content exceeds about 6%, ε-nitride (a solid solution) is formed.
Advantages and Disadvantages of Nitriding :
Salt-bath nitriding is commonly restricted to 4 hours, because the density of pores increases with time. Gas nitriding is not restricted but normally a practically reasonable time of 90- 98 hours is not exceeded. Thus, when greater-depth than that can be obtainable with salt-bath nitriding is required, gas-nitriding is done.
Comparing for a case-depth of around 0.1 mm (at hardness of 400 VPN for a steel), salt bath takes much less time of around 2 hours than gas-nitriding.
3. Dimensional distortion is the same (for the same case depth) in both cases, but because salt-bath- nitrided components are water or oil-quenched, additional stresses result in changes in shape, which is extra than obtained in gas-nitriding.
4. Wear Resistance:
Salt-bath nitriding results in higher content of e-carbonitride than in gas-nitriding, unless hydrocarbon is added in nitriding atmosphere to result in same amount of ε -carbonitride. Higher content of ε -carbonitride provides more wear resistance and reduced risk of scuffing.
The toughness of the nitrided case by salt-bath nitriding is inferior to that obtained by gas-nitriding due to rapid quenching done after salt-bath nitriding.
Gas-nitriding is more clean job than salt bath nitriding particularly the related problems of using cyanide salts. There is always a danger even if all the precautions are observed with salt baths.
7. For smaller case depths, salt bath nitriding is cheaper as it takes lesser time.
1. The equipment is complex and needs closer control.
2. The parameters of the process have to be strictly controlled.
3. High initial equipment cost.
4. Different shapes and size parts cannot be ion-nitrided together.
5. Highly skilled personnel are required.
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