Heat treatment techniques use a two-step processing methodology. Logically, it begins with the workpiece entering a fiery environment, perhaps a gas-fired furnace. Here, the metal component is subjected to enough thermal energy to modify the metal’s base properties. This elevated temperature is intelligently maintained, but the time comes for the part to be cooled, which is when the metal quenching stage enters the treatment work. Lowered into a bath full of oil or water, the part cools rapidly.
Much effort is put into precisely heating a ferrous or non-ferrous component. The thermal treatment normalizes the part, hardens it, and removes work-incurred stress. Naturally, these astutely interred properties should remain intact and unaffected as the part cools. Unfortunately, the cooling stage represents yet another journey into the material modification realm. This is why we use metal quenching, as this accelerated cooling technique is regarded as a way of quickly cooling the hot part while retaining all heat-tailored properties.
Rapid cooling in water equals a climactic lockdown of the microcrystalline structure within the workpiece, but what if some material variations are desired? Quenching is used to harden metal parts, including steel. In stainless steel, for example, the austenite-induced state is rapidly cooled until it transforms into its martensite form, which is a brittle configuration, one that will require further tempering. Meanwhile, other material properties can be achieved by switching the cooling medium. Water is commonly used in these tanks, but oil and brine water are also employed, as is high-pressure nitrogen quenching.
Although it may sound counterintuitive, the controlled cooling of a hot metal part is every bit as important as the furnace stage, for this heat treatment method controls phase changes on the downward arc of the thermal cycle. Hardening is managed in this manner, as is the deadlocking of heated metal states. Of course, the quenching bath must be carefully used, for a substandard cooling station can introduce material cracks and parts deformation. Furthermore, a superior quenching station is a highly adaptable heat treatment asset, with oil and salt baths transforming the metal part into other, more desirable grains. These are the bainite and pearlite forms, the grains that emphasize certain carbon-rich lattices while minimizing others.
A well-supervised metal quenching station universally hardens a component and prepares it for tempering. Further fluid control elements, including oil and brine, enhance mastery over this rapid cooling phase, thus breathing a versatile state-altering mechanism into the process.
Work hardened metals are difficult to form. The machining and shaping work has impacted the material in such a way as to cause the metal’s grain to compact and shrink. The processed alloy is now tougher but it’s also brittle, next to impossible to deform without incurring a material weakening fracture. Fortunately, we can reset a metal’s ductile properties by annealing it, thus phase transforming the alloy and returning lost workability to its internal structure.
Every alloy suffers in some way from the stresses encountered in a machine workshop, but alloys of copper are particularly prone to these hardening effects. The grain of brass and bronze becomes smaller when it’s hammered and cut, which means the once malleable material no longer bends easily. Pure copper acts in the same manner, and so does a number of the most popular aluminium alloys. In fact, these non-ferrous metals are all renowned for their ductile properties, but those selfsame properties are lost or simply drastically attenuated unless we recruit an annealing stage.
We see examples of bronze parts cut into intricate shapes. Meanwhile, brass tubing angles around sharp corners, angles that mirror the dexterous twists and turns of a bronze product. Many engineering processes are used to create these curvilinear outlines, plus individual components require exact cutting in order to satisfy the scale of the project. A work hardened part is a likely result of this machining, which means no more bending or complex shaping can take place. The component requires annealing, a passage through a hot oven. The heat treatment process softens the metal, which restores ductility and much-needed malleability to the part.
Localized and complete heat treatment work anneals aluminium so that it can be formed into beautifully formed fences and geometrically detailed outlines. The lightweight metal is tough and more than a match for mild steel, but it requires this heat treatment stage to ensure its ductile characteristics are retained throughout the machining stage. Conversely, stainless steel favours a toughened material profile, but work hardening does reinforce an already formidable resistance factor. This heat treatment stage once again restores the robust alloy to its state of equilibrium.
An annealing station softens copper alloys. The result is a restoration of the metal’s predominant state, that of a supremely malleable alloy. Similarly, stainless metals are stress relieved and stabilized.