Common Induction Hardening Problems and Resolutions30 September 2019
Just as a reminder, induction hardening technology is used to enhance the toughness and fatigue resistance of carbon steel parts. Components to be heat treated are placed inside a furnace, around which a copper-wound electrical coil runs. When this induction coil is energized, it generates a strong magnetic field, which “induces” workpiece heat. However, although considered a bleeding-edge technology, the induction hardening process doesn’t always go off without a hitch.
Troubleshooting Component Geometries
Of great benefit here, there’s no flame to stress a confined area on a carbon steel workpiece. Instead, a coil-generated alternating current sets up a powerful magnetic field, which induces thermal energy within the target steel part. However, that field assumes a relatively uniform shape. If the coils are circular, the magnetic flux takes on a spheroidal shape. Stretched out on one axis, the coil again influences the way the eddy currents flow. That’s all very well if the induction hardening process is heat treating a pin. For more complex parts profiles, cracks can appear if there’s a great amount of geometrical disparity between the coil and the workpiece. To correct this issue, specially shaped coils are fitted.
Low-Carbon Heat Treatment Limitations
The above instructions apply to medium and high-carbon steels. For alloys that have a less than 0.4% ratio of alloyed carbon, the case hardening strength will drop below 45-HRC. If that value satisfies a client’s heat treatment specs, that’s all well and good. If not, the supplied workpiece should contain more carbon. Alternatively, carburizing or carbon nitriding work will add surface carbon to the component. All the same, low-carbon steels will never be as case hardened as a medium or high-carbon alloy. If a tough and fatigue resistant surface is desired, the selection of a sub 0.4% carbon content steel will be enough to satisfy this process guideline. On the other hand, if hardness (Greater than 55-65 HRC) is the goal, then a steel family that contains more carbon should be sourced.
So, what do heat treatment engineers do when they receive an induction hardening order? They ascertain carbon content, check the geometry of the workpiece, and set up the copper coils so that a high degree of coupling parallelism is achieved. Incidentally, changes in flux settings can be made after checking a prototype. If it cracks, changes are required in the magnetic coils. To further narrow the nature of the fault, check to see whether the fractures occur as inter-granular cracks, subsurface discontinuities, or grain boundary cracks. Some kind of a radiographic or ultrasonic tester will be required at this point.
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