Induction Heating: Everything You Wanted to Know, But Were Afraid to Ask (November 2016)

November 29th, 2016

Question of the Month: For a few years, we scan hardened relatively straight shafts using a 30 kHz machine. Our new component has a 4.8-mm radial step (sharp shoulder) that melted when we tried to scan harden. We heard that a single-shot inductor could reduce shoulder overheating and still provide sufficient hardening, so we asked a local coil builder to make one for us. Corner overheating is reduced, but now we have a 4-6 HRC surface hardness variation around the circumference of the as-quenched component. We didn’t have this with scan hardening. How can we fix it?

Answer: An appropriate inductor/quench design and suitable process recipe should not produce such a wide hardness variation along the shaft circumference. A number of possible causes exist, with two of the most common discussed here.

  1. Heat-related factor. With scan coils, induced eddy current flow is circumferential, producing very uniform temperature distribution at any instant of the heat time. Only minor heat variation occurs in a flux fringing region of coil terminals. Part rotation makes this impact negligible (practically speaking), normally providing very uniform temperature distribution (assuming an absence of slots, keyways, holes, and similar geometrical discontinuities). Unlike scanning inductors, the majority of single-shot inductors produce a predominantly axial eddy current flow rather than circumferential. Therefore, if the heating time is relatively short, the coil heating face is narrow, and rotation speed is insufficient, there might be a noticeable, non-uniform circumferential temperature pattern at the time quenching is applied. Solution: Increasing rotation speed during single-shot heating might solve this problem. The use of a serpentine-type inductor is another option.
  2. Quench-related factor. MIQ coils are often used with scan hardening, normally providing sufficiently uniform circumferential quenching. Quench rings/barrels/followers used in scan hardening also produce reasonably uniform quenching. With a single-shot inductor, there might be obstructions for providing 360o access for spray quenching. In some cases, quenching is done from one side, covering only 90o-120o of the austenitized surface. This could result in appreciable circumferential hardness deviation. Solution: Redesign the quenching device, making an attempt to cover as much surface as possible. In addition, try to change the rotation speed during quenching (for example, a 25% increase and then a 25% decrease) and compare results. Both insufficient rotation and excessive rotation could result in circumferential quench nonuniformity.

Dr. Valery Rudnev, FASM
Director, Science & Technology
Inductoheat Inc

Beginning in July 2016, the Professor Induction column started a new article series called Induction Heating: Everything You Wanted to Know, But Were Afraid to Ask. The most commonly asked questions related to different aspects of induction heating and heat treating will be reviewed and explained. All are welcome to send questions to Dr. Rudnev at rudnev@inductoheat.com. Selected questions will be answered in this column without identifying the writer unless specific permission is granted.

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