Heat treatment science employs an equal opportunities approach to the field of metal strengthening. That’s another way of saying the techniques used here aren’t just used to harden ferrous workpieces. Indeed, full-service heat treatment facilities toughen soft non-ferrous components, too. Needless to say, though, a different set of processing standards are implemented when a non-ferrous workpiece run is ordered. Changing things up a little, a metal-by-metal list illustrates the different treatment variations.
Copper Alloys
Clients expect malleability when they receive their heat treated copper workpieces. Hindering this expectation, copper parts become brittle when they’re exposed to cold-worked fabrication practices. To minimize this effect, the orange-red metal is annealed at 650°C. Then, thanks to the alloy’s low melting point, copper workpieces can be precipitation hardened at a moderately high temperature. This action is generally sustained for three-hours.
Lightweight Aluminium
Here’s another relatively soft metal, one that’s not generally thought of as a heat treatment candidate. Nowadays, there’s a vast number of alloy-strengthened aluminium families on the market. Of note, manganese or magnesium alloyed aluminium don’t respond well to heat. Let’s drop those unresponsive types and turn instead to the copper and zinc amalgams. Taken as high as 410°C, the metal is held here for a few hours while it’s annealed. Interestingly, although the alloys respond well to solution heating and homogenizing temperatures, aluminium can also be naturally aged. At room temperature, in a warehouse environment, the metal stabilizes slowly after it has been quenched.
Exotic Brass Alloys
Annealed at approximately 590°C, the material strengthening options open up when the metal’s copper and zinc content is varied. This method of classifying different types of brass can be taken even further. For example, there are “Alpha” brasses, which are less than 37% zinc. For “Beta” alloys, the zinc content rises above 37% but stops at a 45% limit. Dependent on these metal ratios, a thermal treatment process is selected. Solution treatment procedures and normalizing operations are common, with the alloy reacting favourably to rapid quenching.
In ferrous metals, the mechanical and material properties correlate directly to the grain properties of a nominated metal. Those grain forms are easily influenced when the carbon content of the workpiece is manipulated. In non-ferrous heat treatment, this approach is much harder to emulate. To overcome such processing hurdles, we seek out recrystallization alterations and quench time controllability. Precipitation hardening and aging work well here, as do annealing operations. Lacking carbon as a process instigating master manipulator, furnace management profiles become essential, as do the quench times and mediums that regulate the recrystallization stage.
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.