Salt Bath Furnace

The furnace contains salt such as nitrates, nitrites, caustic soda, chlorides, carbonates, and cyanide. Mixtures of salt are selected to give a specific temperature range and a desired treatment (or lack of treatment) to the surface of the material being processed. When we want to change the physical properties of something without changing the surface of it, we use neutral salts such as chlorides and nitrates. We can change the surface conditions of a part by placing it in a salt bath furnace having ingredients that react chemically with the surface as in carburizing, cyaniding, nit riding, and aluminizing. Other salts are formulated to remove scale, paint, plastic, or organic contamination.

The heating medium of a salt bath is the salt itself. This is heated to the required temperature by either external heating, over the side submerged elements, or submerged electrodes. When a load is submerged into the salt it is heated rapidly and uniformly to the required temperature. The salt also doubles as a protective atmosphere during the heating cycle. When the load is withdrawn a thin covering film of salt will protect it.

Why Salt Bath Heating?

In salt bath heating processes, users completely submerge work pieces in a bath of molten salt. The benefits of choosing this process over radiation or convection heating processes include:

• Controlled Atmosphere: By immersing the work piece in a salt bath, you effectively remove its contact with the outside air eliminating scaling, oxidation, and decarburizing. The work piece is similarly benefitted by the thin film of liquid salt protecting the metal from oxidation when transferring to the quench medium.
• Uniform Heating: The electrodes that heat the salt cause electro dynamic circulation in the bath providing a natural agitation. This constant, vigorous stirring action of Ajax furnaces assures extremely uniform and precise temperature throughout the bath regardless of size or temperature. Even temperature distribution also prevents hotspots that will shorten the life of pots and cause excessive salt breakdown.
• Faster Heating: In salt bath processes, the entire work piece is uniformly surrounded by salts that hold a precise temperature forming a frozen ‘cocoon’ of salt, subjecting the work to an automatic preheat before melting. This modifies the work piece’s temperature by conduction at a quick, but safe rate, reducing heat shock and distortion.
• Thermal Efficiency: Internal heating of the bath does not have to be forced through pot walls allowing salt baths to recover quicker than fuel-fired furnaces. Salt also has a high specific heat and mass resulting in high heat storage capacity so that more work can be heated without a drop in temperature allowing for little recovery time between loads.