To some extent, worked alloys have memory. Stress can be absorbed and stored in the metal. Under these circumstances, heat treatment facilities call upon the services of a stress relieving process, a technique that’s designed to reset that solid-form memory. In a nutshell, the grain of the metal is homogenized. Thermal relaxation takes over at this point, then the workpiece is ready for more processing or a post-processing finish operation.

Challenging Stress Relief Actions 

Playing devil’s advocate for a moment, what happens when a heat treatment run skips this crucial material-homogenizing stage? Formed or machined, cold-worked or welded, the alloy piece is saturated with residual mechanical stress. It’s about to be passed back to another processing stage for more machining, with those wildly interacting intragranular still in place. The part deforms, it warps, and it loses dimensional form. Even if those internal forces don’t make themselves known during a secondary heat treatment operation, the workpiece is still packing that trapped energy. Pressed into service by some fly by night heat treatment functionary, the metal part will fail. Prematurely and drastically, it will release those stresses and fail.

Initiating a Heat Treatment Reset 

It takes that kind of reverse logic to really gain an insight into the process. Stress relieving technology sits firmly as a looping branch of the thermal conditioning tree. If the workpiece absorbs stress during the initial machining stage, these forces are thermally relaxed until the part is mechanically equalized once more. Passed through this processing loop and into a heat station that introduces a thermal gradient, more heat-induced stress is encountered. Back into the process sub-branch, that newly added stress is mitigated. The end result, now that the part has left all of the machining stations and heat treatment work behind, is a dimensionally stable, geometrically accurate component. This being the case, the finished metal workpiece is now less susceptible to stress corrosion cracking, post-processing dimensional variations, and grain irregularities.

A number of thermal relaxation temperatures work well here, then there are material normalizing techniques and annealing procedures to further regulate the work. Of the fundamental stress relieving techniques, two types of stress relief dominate. First of all, a 150°C to 200°C method operates as a peak force releasing process. That relatively low-temp system offers significant relief without affecting other treatment work. Using this method, parts lose most internal stress while maintaining their case hardened exterior shell. Alternatively, using a thermal spread of between 600°C to 680°C, the furnace will fully homogenize the alloy grain.

 

On one side of this comparative guide, induction hardening technology employs a strong electromagnetic field as the heat treatment engine. On the other side, a similarly potent case hardening station is busily armouring the surface of a newly arrived workpiece. Both methods harden exterior surfaces. Case in point, flame hardening equipment is incredibly selective, but is this focused blaze more capable than an induction hardening station?

Aggressive Energies Catalyze Case Hardened Components 

On the fire and chemical side of things, a focused oxy-gas flare selectively surface hardens the teeth on a gear wheel. Further down that line, a gas carburizing furnace cooks a component while it simultaneously feeds a small percentage of carbon into the sealed heating enclosure. In short, these case hardening techniques employ furnaces and gases, plus a preordained quantity of surface armouring carbon. Induction hardening equipment is more, shall we say, production friendly, and here’s why.

Compared to Production-Friendly Induction Technology 

From the start, there are obvious benefits. The process doesn’t take place inside a sealed furnace, nor does it require a specially conditioned atmosphere. Look closer, look at the nature of this beast. It’s an electrical coil, a series of twisted conductors that induce a powerful magnetic field. As that field interacts with a nearby workpiece, heat is induced. That means this electrical equipment doesn’t touch the component while it’s being heat treated. Furthermore, the entire operation is finished in seconds. Incorporated as part of a production line, one that’s processing hundreds of vehicle axles, the entire project is repeatedly heat treated in hours, not days.

Comparing Case Hardening Benefits 

Geometrical profiles can stymy certain induction hardening stations. For example, that flame hardening equipment makes short work of a targeted project, perhaps one that accommodates the radial outlines of gear wheels. That circular induction unit, perfect for cylindrical extrusions, can’t exactly handle the radial teeth on a heavy gear. However, there are induction hardening rigs that do suit this rim-turning requirement. Mounted on an arm, a stationary induction hardening head quickly handles those rim protrusions. Still, that little case study does demonstrate a need for selecting the correct equipment form factor.

Meanwhile, furnace stations case harden all workpiece types, no matter their size or shape. However, it can take many hours to case harden a number of components in these cooking chambers. Induction hardening equipment, on the other hand, incorporates seamlessly into an existing component processing line. Finally, it takes great diffusion mastery to control the case hardening depth in those gas-delivered equipment lines. As for the induction hardening station, a change to the frequency controller will deepen the case hardening effect.