Through Induction Hardening of 52100 - Counterintuitive Delay

[sodeen]

Greetings,

Background: We currently use an induction hardening process to through-harden very thin (~.25 in) bearing races, normally 440C and polymer quenched.

Recently, we have started using this process on our 52100 bearings as well, to limited success. We have seen improvements in our hardness and in maintaining our roundness by increasing the "soak" time (from 80 to 160 seconds at 1630 degrees) and by introducting a delay before quench for thicker parts (~.5 in to 1.5 in).

I understand that increasing the soak time gives the carbon a greater chance to dissolve into the solution, but why would the delay (15 sec) before quench help? I thought that an almost-immediate quench was needed to get martensite out of 52100 steel?

 

[Metalguy]

52100 is a pretty deep hardening steel--look at how much C and Cr it has.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[redpicker]

The delay is to allow the temperature to equalize.

rp

 

[TVP]

I agree w/ redpicker, the delay is for the surface and core temperature to equalize/equilibrate.

 

[metman]

sodeen,

Quote "I thought that an almost-immediate quench was needed to get martensite out of 52100 steel?"

I don't have my atlas of TTT curves at hand but I would be interested to see the curve for 52100. Even though it is deep hardening I would expect 15 seconds delay to allow the cooling curve to intersect the nose of the TTT. However, the softer microstruocture products, unlike Martensite, require time to transform so that with a fast quench the steel sees very little time passing thru the temperature zone where these products tranform so that you still end up with maybe 90% Martensite.

 

[blacksmith37]

Looking at the USX IT /TTT handbook (1963): The "nose" is at 3 seconds and 1100F for a 1550F austenitize. However for a 0.5" dia. hardenability bar; the 1550 austenitize gives 60% mart at 1", while a 1950F aust gives 99% mart at 1".
As said , I suspect the hold time is needed to soak in temperature. Seems like longer time ,or higher temp is needed for austinetizing.

 

[metman]

blacksmith37
Thanks for the detailed info.

Your quote,
"Seems like longer time ,or higher temp is needed for austinetizing."

One consideration in doing this is that this will increase grain size, temperature having more effect than time and increased grain size will tend to reduce fatigue life. However, this could be a special application where loads are light enough so that bearing life based on Hertzian stress is not as important as surface wear.