The nitriding process is over 100 years old, and yet we are still puzzled by the formation of the surface metallurgy (the compound layer, aka the white layer). The surface metallurgy will always form to have two integrated phases within the immediate surface layer.
The phases are based on the use of ammonia as the nitrogen source and with a gaseous dissociation of approximately 30%. This gas ratio will always produce the compound layer of a mixed phase zone. This zone (on the immediate surface) will be approximately 50% epsilon nitride and approximately 50% gamma-prime phases. The formation of the two phases is primarily controlled by steel chemistry, carbon potential of the steel and gas dissociation rate.
While the two phases coexist, they can be identified as separate and individual phases. The epsilon phase can be enhanced via the steel’s carbon percentage or by the simple addition of a hydrocarbon process gas (such as acetylene, propane, methane and other hydrocarbon gases).
Epsilon phase is generally used where sliding wear is a problem. The carbon content of the steel or the addition of an additive hydrocarbon gas will initiate the formation of epsilon phase. Epsilon will produce a very high surface hardness (sometimes in excess of 1000 HV).
The high hardness value does not have high impact values and should only be used for potentially high wear resistance. The epsilon phase does not have any high-impact characteristic values. Generally, the formed case will shatter under high impacts, such as the impact loading encountered in a forging die.
Consider a gear. If the tooth has a predominant surface metallurgical case of epsilon, there is a significant probability the surface of the nitrided gear tooth will shatter into small fragments. These fragments can be the source/result of a premature failure, which could cause a catastrophic gear-tooth failure.
The gamma-prime phase will produce a much lower surface hardness value that has more surface flexibility combined with a lower surface hardness value, which could vary from 700 HV up to 850 HV. While this sort of hardness value will reduce the impactive risk, it will not be able to withstand high wear resistance.
So, it is a question of “which phase best suits my nitrided component.” This will be the subject of at least two more blogs on the compound layer and the use of the nitride procedure.Please stay tuned.