Salt Bath Nitriding
Our Melonite QPQ® process provides superior protection against wear and corrosion. The Melonite QPQ process is a multi-step process that provides a very uniform consistent nitride layer on your components. The first step of the process is a preheat to raise the component`s surface temperature to about 700 - 800°F in air. The product is then transferred to the MEL 1 / TF 1 tank containing the liquid Melonite salt to begin the nitrocarburizing process. The salt melt mainly consists of alkali cyanate and alkali carbonate. It is operated in a pot made from special material, and fitted with an aeration device. The active constituent in the MEL 1 / TF 1 bath is the alkali cyanate. The nitrocarburizing process step is conducted in the MEL 1 / TF 1 bath at 896-1166°F; the standard temperature is usually 1076°F.
Melonizing
During Melonizing® a nitrocarburized layer is formed consisting of the outer compound layer (ε-iron nitride) and the diffusion layer thereunder. The formation, microstructure and properties of the compound layer are determined by the base material. The compound layer consists of compounds of iron, nitrogen, carbon and oxygen. Due to its microstructure, the compound layer does not possess metallic properties. It is particularly resistant to wear, seizure and corrosion, as well as being stable almost to the temperature at which it was formed. Compared with plasma or gas nitro-carburizing, compound layers with the highest nitrogen content can be obtained by Melonizing®. Layers with a high nitrogen content give better protection against wear, and in particular corrosion, than those with a low content.
Depending on the material used, the compound layer will have a Vickers hardness of about 800 to 1500 HV. The graph above shows a comparison of the surface layers produced by various processes and their hardness. In the metallographic analysis of salt bath nitrocarburized components, that part of the total layer known as the compound layer is defined clearly from the diffusion layer thereunder as a slightly etched zone. During the diffusion of atomic nitrogen the compound layer is formed. The growing level of nitrogen results in the limit of solubility in the surface zone being exceeded, which causes the nitrides to precipitate and form a closed compound layer.
Obtainable Compound Layer ThicknessIn addition to the treatment parameters (temperature, duration, bath composition), the levels of carbon and alloying elements in the materials to be treated influence the thickness of layer obtainable. Although the growth of the layer is lower the higher the content of alloy, the hardness however increases to an equal extent.
The data shown in the graph above were determined in a MEL1 / TF1 bath at 1076°F. With the usual treating durations of 60-120 minutes, the compound layer obtained was 10-20 μm thick on most qualities of material.
Nitriding Depth & Hardness
The depth and hardness of the diffusion layer are largely determined by the material. The higher the alloying content in the steel, the lower the nitrogen penetration depth at equal treating duration. On the other hand, the hardness increases the higher the alloying content. In the case of unalloyed steels, the crystalline structure of the diffusion. In the metallographic analysis of salt bath nitrocarburized components, that part of the total layer known as the compound layer is defined clearly from the diffusion layer there under as a slightly etched zone. During the diffusion of atomic nitrogen the compound layer is formed. The growing level of nitrogen results in the limit of solubility in the surface zone being exceeded, which causes the nitrides to precipitate and form a In addition to the treatment parameters (temperature, duration, bath composition), the levels of carbon and alloying elements in the materials to be treated influence the thickness of layer obtainable.
Although the growth of the layer is lower the higher the content of alloy, the hardness however increases to an layer is influenced by the rate of cooling after nitrocarburizing. After rapid cooling in water, the diffused nitrogen remains in solution. If cooling is done slowly, or if a subsequent tempering is carried out, some of the nitrogen could precipitate into iron nitride needles in the outer region of the diffusion layer of un-alloyed steels. This precipitation improves the ductility of nitro-carburized components. Unlike unalloyed steels, part of the diffusion layer of high alloyed materials can be better identified metallographically from the core structure, due to the improved etchability. But the actual nitrogen penetration is also considerably deeper than the darker etched area visible metallographically. Cooling does not influence the formation of the diffusion layer to any noteworthy extent.