The process of induction hardening is for the purpose of hardening the surface of components made from steel or other carbon-alloy metals. Components or parts include a wide assortment of axles, pins, rollers and shafts. As much as 15-mm hardness depths can be achieved. Parts are placed in a special furnace that utilizes a coil to perform its function. When this coil is energised, it induces the parts with thermal energy via a strong magnetic field. While this process enhances the fatigue-resistance and toughness of various types of parts, the following issues do exist with this method.
Material Limitations
Typically, induction hardening is utilised on a wide variety of steels and carbon alloys. However, it is important to note that medium-carbon steels ensure the best results in hardening. Issues can arise in metals with a high carbon content, which will be discussed further later. Decarburized surfaces and surfaces with a low-carbon content will not harden properly with this process.
The Shapes of the Components Can Negate the Utilisation of Induction Hardening
Since the components must fit into the furnace without interfering with the induction coil, certain shapes may not be compatible with the shape of the coil or for that fact, the tooling of the furnace. Even though, there is a range of sizes and shapes of coils readily available for this use, some projects may require custom coils, which may cost more than the project merits. As a result, the shape and size of the components must be fully analysed to discover their compatibility with the available equipment for induction hardening.
Cracking Can Occur during the Induction Hardening Process
Another common issue that can happen with the induction hardening process is cracking. This can happen immediately during the process or as a delayed reaction days after the process. Top reasons for cracking include:
• Overheating during the process
• A high-carbon content in the steel or other carbon alloy
• Rapid heating and quenching also increases the risk for cracking in comparison to other heat-treating methods
• Geometric irregularities may interfere the proper operation of the induction coil
Distortions May Occur
The risk of distortion is higher with induction heating that with gas or ion nitriding, but it can be lower than the traditional heat treatment if it is applied to only a specific area.
Luckily, all these issues are preventable. Companies must analyse the materials closely to ensure that they contain the right contents for the ideal results. Also, the components need to be compatible with the shape and size of the induction furnace, coil and tooling. Proper monitoring is another necessity to keep problems from occurring due to overheating. When the process is performed correctly in all these areas, the results will be satisfactory.
Ferrous metals, those that are rich in iron (Fe), often require normalizing. Why should this be the case? Is the grain size really that inconsistent and material-coarse after an iron workpiece has been heat-treated? Actually, yes, the grain size will alter in an iron workpiece if it’s continually processed. This effect worsens when the material is alloyed. Restoring uniformity, the workpiece’s atomic structure does require normalizing.
Carbon Steels Retain Memories
That’s not quite true. In place of memories, it’s work stress that gets locked within the microcrystalline lattices of a ferrous metal part. The internal stresses are there because of a welding operation, or a forging service, or because of a cold-work process. Carbon content, the amount of alloying carbon that diffuses into an iron workpiece, seems to be the primary offender here, for low carbon steels don’t typically require normalizing. Anyway, the internal stress, those trapped “memories” of welding and forging, are swept away by the slow cooking and air cooling treatment. With those internalized atomic tensions leeched away, workpieces won’t distort or deform when exposed to more heat treatment work.
Incorporating Grain Refined Toughness
Here’s another problem that occurs in carbon and iron alloys. As the grains are worked or exposed to supplementary thermal treatment operations, they become coarse. The alloy crystals change in size and adopt an irregular form. The loss of grain uniformity causes a matching loss of workpiece toughness. By taking the part into a normalizing furnace and then into an air cooling room, the grains homogenize. They become smaller, finer, and they produce harder workpiece structures. Of some concern, the process is usually conducted in an air-charged furnace atmosphere, so parts scale and decarburized contaminants can form during the process. That’s not exactly surprising, not when the parts are heated to 890°C while being exposed to air. Subsequent machining work or surface finishing operations may be required to remove the scale.
In practice, normalizing services are executed faster than comparable annealing operations. Ferrous metal workpieces may require subsequent post-processing, but the tasks required to restore a presentable surface finish can be carried out relatively fast. As for the machining risks, coarse-grained materials won’t cut as smoothly as normalized, fine-grained parts. Again, the benefits far outweigh any possible processing downside. In skipping the normalizing stage, ferrous metals are weaker and possibly loaded with deformability potential because of the internal stresses still contained within their crystalline structures. Therefore, certain ferrous alloys, including tool and carbon steels, require that extra degree of hardness, which comes only from normalizing. By the way, if ductility is preferred over hardness, an annealing service should be used in place of the normalizing process.
A second heat treatment effector exists in an industrial furnace, one that’s sometimes forgotten by the average layperson. Besides the heat source, the gas-fuelled or electrically energized thermal energy flowing evenly around a subject workpiece, there’s the atmosphere inside that sealed chamber to consider. Sometimes, the atmosphere is totally taken out of the heat treatment formula, so the process takes place in a vacuum. At other times, that atmosphere becomes an essential process agitator.
Reviewing Atmospheric Effectors
In a regular heat treatment operation, the air itself functions as a heat load or thermal conductor. The currents convect the thermal energies from the walls of the furnace to the workpiece. Radiated heat sources function differently, without the need for air. Importantly, air can be pumped out of a vacuum-sealed chamber to add more control to the process. Alternatively, a regular atmosphere can be replaced by a second gaseous medium. This medium facilitates the formation of different surface protection finishes. Better than a coating, the gas actually impregnates the outer casing of the alloy and transforms this surface layer into a mechanically and chemically desired finish. Essentially, just like a controlled oxidation operation, the atmospherically pressurized gaseous compounds chemically alter a finite percentage of a workpiece’s surface casing.
The Demand for Controllable Metallurgical Outcomes
Even compared to twenty years ago, heat treatment technology has advanced at an unprecedented rate. Vacuum furnaces are one result of this evolutionary jump, then there are the gas-pumped furnaces. These machines pump in carbon to apply a carburizing transformative finish, which improves wear performance. Nitrogen is another gaseous medium of interest. Nitrogen atmospheres augment the annealing process. Inert argon gas environments also act as an annealing improvement agent. Even carbon dioxide, thanks to an additional oxygen atom in its chemical makeup, has become a popular supplementary furnace atmosphere. Instead of air, which only contains a small percentage of oxygen, CO2 packs a stronger oxygenated punch, so it has become something of an oxidization gas standard.
The above passages of text have barely scraped the surface of what’s possible. Modern heat treatment facilities are now working with a whole palette of different gaseous compounds. Hydrogen gas, which obviously isn’t inert, performs as a reducing agent. It purifies iron and copper oxides. Like carbon monoxide and carbon dioxide, hydrocarbons are also in use here, usually as carbon-rich compounds that divulge their chemical loads at contrasting alloy treatment temperatures. However, and this point is of critical importance, such refined chemical reactions can become corrupted. It’s, therefore, best to use clean gasses, free-flowing atmospheres that have been entirely divested of pollutants, especially water.