Lately, this writer has seen a number of instances of nitriding at elevated process temperatures (980-1020°F) as opposed to the more traditional temperatures of 935°F.
There are no hard or fast rules that categorically state what the nitride temperature should be, save that of saying that the process temperature should be 50°F below the tempering temperature of the steel being treated.
The objective of the nitriding process is to produce a nitrogen-rich surface that will induce:
- Good surface compressive stresses
- Good surface hardness in the region of 700 HVN (59 HRC)
- Good surface resistance to corrosion
- Consistent and repeatable surface metallurgy without affecting the core metallurgy by reducing the core hardness.
So, what is the ideal temperature? The higher the selected process temperature, the greater the risk of what are known as nitride networks. In other words, the steel now has an increased capacity for more surface nitrogen absorption because of its higher solubility limit for nitrogen. The networks are “fingers” that stretch into the formed case from the formed compound layer (white layer).
Figures 1 and 2 show the effect of higher-temperature processing, with the fingers extending into the nitride diffusion layer and the effect on the corners by having excess amounts of available nitrogen at the surface of the steel.
The selection of high nitriding temperatures allows the steel to absorb more nitrogen. In other words, the limit of solubility of nitrogen in iron increases and more nitrogen dissolves into the surface of the steel. When the steel reverts back to ambient temperature (i.e., cools down), the nitrogen begins to precipitate out of solution to form the networks.
While processing at higher temperatures offers the attraction of faster cycle times, it is fraught with problems.
The next problem that will occur is growth as a result of higher processing temperatures. Because more nitrogen is now available at the steel surface, more nitrogen is absorbed. Any time you introduce an element into the surface of the steel, you are displacing the surface. Hence, there is growth.
Another phenomena that will occur is that the compound-layer thickness will increase, depending on the steel chemistry and the process-gas chemistry. This will also influence the composition of the phases of the compound zone to either epsilon (carbon controlled) or gamma prime. So, the process temperature, the steel chemistry, the process-gas chemistry and the process temperature will determine the growth, case depth and surface metallurgy.
Selecting the higher process temperature will produce nothing other than the nitride networks and greater growth, as well as a thicker compound layer. This means longer grind times if it is the intention to remove the compound layer (white layer) by grinding. There is also now the serious risk of causing grind burns of the ground steel surface, which can lead to surface microcracks.
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