Induction Hardening - Barberpoling effect 1

[MOB1]

I am interested in the so-called "barberpoling" effect produced on items which have been induction hardened. I understand that it is produced either by poor inductor design, non rotation of the part during hardening or quenching inadequacies.
My question relates to the effect on fatigue prioperties rather than the production of the effect.
Does anyone have any data on the effct on fatigue properties either through microstructural/hardness effects or residual stress effects?

 

[MikeHalloran]

I had occasion to watch induction hardening machines for hours at a time, several decades ago. The product was axle shafts.

- If the shaft didn't rotate at all, you'd get an axial stripe, not a barber pole.

- The inductors on those machines only had two turns (of copper tube) on the secondary, wound neatly as two nearly perfect incomplete circles, with a zigzag transition between them, opposite the transition to radial orientation for termination, brazed into big copper electrodes.

- Pretty much all of the shafts had some barber- pole striping on them, that I guess you could associate with the inter-winding transition and the coil ends. The coils didn't last long, so they were made by a process that valued interchangeability. I think the coils lasted a couple of days, in a plant that produced ~20,000 shafts a day using I think ~36 hardening spindles.

- I don't recall wondering why they didn't spin the shafts faster (about 1 rev/s); I guess the quench water would make more of a mess.

- Re the transition, if you wound the coils as a true helix, you'd probably get a nicer field, except at the bearing shoulder, which is critically important for fatigue.
There was a short dwell at that position after the power was turned on to develop some case into the hub. That area was sectioned and inspected in both directions regularly.

- I witnessed a fair number of destructive hardness tests, slicing a cross section, polishing and making a row of hardness tests on a radius to check the case depth. The guy who actually did the tests, for years, did not ever mention noticing a circumferential hardness variation, or any correlation of hardness values to the barber pole stripes. Doesn't mean it wasn't there, but you'd think in decades of doing it every 20 minutes or so, someone would pick up on it.

- What I did notice was that the case was pretty round when defined with a chemical etch, but not real sharply demarcated by hardness.

- I'm not sure how you'd correlate the presence or severity of barber pole stripes with fatigue life, but I'm guessing you'd carry and cut a lot of steel along the way. Sounds like a good exercise program...



Mike Halloran
Pembroke Pines, FL, USA

 

[redpicker]

The term "barberpoling" is not well enough defined to quantify its effect on microstructure or hardness, let alone fatigue. The pattern seen on the surface can result in a localized area that was too high in temperature, an area that was too low in temperature, or maybe just an area that saw a different heating rate. These differnet causes will have different effects, so you would really have to investigate the product to find an answer.

rp

 

[TVP]

As a follow-up to the comments from rp regarding barber-pole, etc., take a look at these articles from Dr. Valery Rudnev of Inductoheat, who is one of the world's foremost experts on induction heating:

http://www.inductoheat.com/pdf/Metallurgical_insights_7.pdf

http://www.inductoheat.com/pdf/Metallurgical_insights_6.pdf