Layers and Considerations
The Plasma nitriding process produces a hard outer skin on the material being nitrided. The hardness achieved on the surface decreases with depth until the core hardness is reached. The graph in Fig. 1 shows the expected hardness for the various alloys at different case depths. The slope is more gradual for low alloy steels and very sharp for highly alloyed steels. The nitride forming elements in the steel's composition are the primary factors controlling the hardness and the case depth. The low alloy steel will provide a deeper case depth but a lower overall hardness.
The micrograph in Fig. 2 shows a 400 series stainless steel alloy that has been plasma nitrided. The high alloy content of the stainless steel creates a high surface hardness and a sharp transition zone between the nitrided surface and the core material. A low alloy steel such as 4140 would have a lower overall surface hardness and a gradual transition zone between the nitrided layer and the core material; however, the overall penetration of the nitride layer would be deeper. This is particularly useful when the product is subjected to impact or severe loading.
Fig. 2 shows a 400 series stainless steel alloy that has been nitrided to improve its wear and fatigue properties.
Fig. 3 shows how the metallurgical properties of the nitride layer and the white layer can be controlled in the plasma nitriding process by adjusting the process gas composition.
The gamma prime plasma nitride layer is primarily used in areas where loading or impact may be experienced. The gamma prime is more ductile than the epsilon layer. The "white layer" or compound layer will only build to between 0.0001 and 0.0004". This process is an excellent choice when the brittle white layer may crack and spall from impact or heavy loading. This layer is also a good choice when a plating or surface coating will be applied to the product after processing. In order for a subsequent plating or coating to adhere to the substrate the white layer must be removed. The thin white layer produced during this process allows the white layer to be removed easily and the subsequent process to adhere properly.
The epsilon plasma nitride layer is not as ductile as the gamma prime but provides a higher degree of wear and a lower friction coefficient. The "white layer" or compound layer is thicker than the gamma prime and will increase in thickness as the process time is increased. The compound layer generally ranges from 0.0002" to 0.0012". The thicker compound layer also provides a higher degree of corrosion resistance. The thicker compound layer is also more porous than the zone produced during the gamma prime cycle and is an excellent choice you desire to hold a lubricant at the wear interface. Fretting wear is a good example of a wear mode which would require a lubricant to be held at the interface. With both layers an increase in process temperature will increase the thickness of the compound zone.
Stress relieving considerations
It is important to stress relieve your product prior to plasma nitride processing. Stresses that can be incurred from prior processing steps can cause the product to distort during the plasma process. NCT recommends products to be stress relieved at 1050 Fahrenheit.
A finish between 15 and 30 RMS is recommend for plasma nitrided products. Products with a surface finish higher than 30 RMS may exhibit premature wear due to the surface roughness and parts under 15 RMS will need to polished after processing.
Considerations when choosing your case depth
Thin walled sections should be avoided or a shallow case depth specified. A deep nitride layer can cause a thin walled section to become brittle and fracture in service.
Appearance of the nitrided product
The overall appearance of a nitrided product is a chalky gray color. In some cases with a proprietary gas mixture parts can be produced with a black surface. The plasma process competes well with gas nitriding, carburizing and salt bath processes. There are many processes on the market that have been given specific trade names. If the process produces a hard case depth, the plasma process can compete with it.