Salt Pot Case Hardening Defined

09 January 2019

Salt Case Hardening, the term sounds so exotic. The knowledgeable heat treatment professional knows exactly what’s happening when the technique is applied, of course. Meanwhile, for the novice engineers out there, it’s possible to infer the case hardening approach used here, perhaps by focusing on the saltiness of the procedure. Sit tight for the next few minutes as we turn that pot full of salt into a fiery mix.

What Is Salt Pot Case Hardening?

The word “Is” was almost placed in italics. Whatisa salt pot? One thing it’s not is a condiment, like the one that sits alongside a pepper pot on a dining room table. No, the salt pot in question here is a molten crucible full of chloride or cyanide salt. When a ferrous or non-ferrous workpiece is lowered into this hot “salt bath,” the inert mass of fusion-heated liquid imparts 90% of the heat source’s energy to the workpiece surface. That’s a more efficient and economical process, especially when compared to a 60% efficient atmospheric furnace. Using thermal conductivity, the energy is instantly transferred from an external heat source, gas or electric powered, and sent directly to the metal’s surface where it introduces a powerful case hardening effect.

Equipped With Process Flexibility

This salt bath doesn’t rely on corruptible convection currents or electromagnetic fields. Simple and uncomplicated, the fusion bath is loaded with an inert bath full of chloride salt, which is alkaline in nature. The mix takes on urea or dicyanodiamide, from which nitrogen and carbon are released. Using these free-floating chemical elements, salt pot hardening technology can operate as a nitride or carburizing system. Simply put, the pot adds carbon or nitrogen to its ingredients, which then diffuse their way into the case hardened surface. Maintained between 550°C and 950°C, the heat treatment technique is environmentally friendly, certain to generate uniform treatment, and it’s, therefore, a distortion-free solution. Incidentally, it’s the density of the material that discourages dimensional distortion.

The salt pots are available in different sizes and configurations. High-capacity models suit batch processing operations. As for their fuel source, gas or resistive electrical elements work well in this case. The actual pot material, hard and alkaline resistant, is typically made of what’s known as fire-box steel, which is further heat-strengthened by fitting a ceramic lining. Finally, then, let’s finish with a few advantages. Clearly, by immersing workpieces in salt, they’re protected from the effects of an oxidizing atmosphere. Also, undesirable surface scaling effects are no longer an issue, although a method of removing the cake-like salt coating afterwards is desirable.

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