Small gear - 8620 core hardness concerns warranted?

[UCengno1]

Have run into a rash of gear failures on a small gear (.06 tooth thickness approx.) that for years did not have any issues.

Current spec is 89-91 R15N effective depth of .008-.014. Core hardness 35-45 Rc. Measure at pitch line.

The gears are currently always coming on the high end on the core hardness. I am checking to see if this was always the case. Regardless, is this the right investigative road to travel down?

Also, isn't the core hard hardness just a by-product of the temper before the carbururuzing process or are there significant controls with the process that dictate the final core hardness.

Not a metallurgist ... just a guy with way to many metallurgy issues. Thanks for any comment.

BCK

 

[dbooker630]

You will need to get a copy of the mill certification for the 8620 material. A gear of this size can easily core harden into the mid 40's HRC.

Gear tooth pitchline core hardness is associated with carbon content/overall chemistry, hardenability, temperature prior to quenching, quench media, quench temperature, and draw (tempering) temperature.

I would start with the material though. See if you can write an internal spec for a restricted hardenability 8620H with the near surface hardness properties (J1 and J2 jominy hardenability position) below 45 HRC.

 

[swall]

You first need to have a competent metallurgical engineer perform a failure analysis on the failed gears in order to find root cause. It may or may not have anything to do with your material or processing.

 

[dbooker630]

I went straight to the specs and overlooked the part of your post with regard to the investigation, swall is correct as far as the very first thing to do. If in fact material or heat treat is a factor you may wish to look into the specs.

But the met analysis will tell you if it was an overload, instantaneous, or fatigue type of fracture; it may also point out irregularities in machining or finishing. I've seen many gears that met specs yet did not perform in service. The metallurgist's conclusion will tell you what data you have to gather.