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8620 Case Carburized

8620 Case Carburized

8620 Case Carburized

Hi all,

Back in March I sent out for a few sets of prototype bevel gears to be made. For cost, capability (I've spent the last decade doing spurs and none of my local shops wanted anything to do with 2 sets of 88 deg bevels) and timing reasons, I went with a shop I had never worked with before in Canada. I had originally specified the material to be 4140, but they said all they had available was 8620, so I revised the print to case carburize 8620 to 50 HRC. I've never worked with 8620, but the analysis said the parts should be sufficient for the prototype life and stresses.

The 3 weeks promise time got to around 8 weeks and so I asked for one of the sets to be sent to us without heat treat so I could do an assembly fit up, I would then swap in the hardened gears when they came in.

Despite calling and emailing ever since, and believing the shop had ghosted me and my last gear set I finally got an email last week that the parts had been lost at the heat treaters but were finally back from heat treat and were on their way.

When I received them they were covered in brown, black and yellow scale and the surface finish as compared to the non-heat treated gears was abysmal. I have spec'd case carburizing before and have never seen this type scaling.

That got me wondering if this is unique to 8620? Or perhaps since they were lost they were treated while rusting or were mishandled? Or have I been lucky with the shops I've worked at and they either pickle or post HT grind? I want to update my drawings and specs going forward to avoid this, but I will admit my knowledge on the heat treat process is still a work in progress and I would prefer to not leave it to a note saying to remove scale unless that is best practice. (I did have a note on the drawing for final surface finish requirements that was clearly ignored). My note reads: "MATERIAL: 8620 STEEL CASE CARBURIZED 48-50 HRC 0.05"-0.06" DEPTH." With a finish call-out on the drawing of 32 microinches. Which has been a typical kind of note that I've seen and made at both industrial and aerospace companies in the past.

Thanks for your help folks! I've been reading as much as I can get my hands on on gear HT processing, just not able to find this case quite yet.

RE: 8620 Case Carburized

Used to work in an automotive gearbox plant.
All the gears were from 8620.
They were gas carburised and the only finishing required was bore, thrust face and sychromesh cone grinding.

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RE: 8620 Case Carburized

Please advise the agma Q quality.
Backlash required.
Unfortunate that mfg promised gears that take minimum of 10-12 weeks.to do in 3 weeks. It takes time ⌚ to buy material, mfg, heat, not. Testing and inspection. A proper heat for post finishing would have bead blast the gears clean then fi is grind all the critical dimensions.
With 4140 parts if the AGMA quality is liberal. Could have been finished before heat. And heat. Treated in inert or gas quench. Now permitted by the AMS specification.
Carburized part 99 % of the time require post processing to remove distotion. Remove oxidized surface. Grind to proper back lash with Matin part. And there is more

RE: 8620 Case Carburized

Please advise application if possible

RE: 8620 Case Carburized

Hi mfgengear, thank you for the help!

I agree on the timing - whole reason I went with them and why I switched to 8620 was they had bars in that size in hand. Oh well live and learn, the first gear was in roughly 9 weeks but untreated. The 23 weeks ones and being lost at heat treat is way unacceptable. I've done gearing long enough that I know if there is a mistake I am the first person I should blame. Wondering if I missed some sort of additional call-out on my drawing with the heat treat requirement or if they just got tired of me bugging them. Would the post operations you mentioned need to be called out? Before I would have thought that was part of the process. My guess is in the past when I had specified case carb that those operations took place to meet the final specs. Not sure though.

To your questions: AGMA Q6, I believe about 0.004" backlash (I'm not in the office so it's off memory). The application is a prototype ebike gearbox. Was a proof of concept and fairly minimal life and stress requirements to hit deadline and show concept to investors so I wasn't too worried about the lack of heat treat except after the investors I was supposed to be able to do some testing on. It all went well and is already collecting dust and on to the next design, but I would have liked to have had those hardened gears back when I had time to test them on the assembly! Oh well...

RE: 8620 Case Carburized

Hi ranorris87
yes so the gears are very liberal tolerance. should have been a no brainer. continue with 4140, through harden.
look up gas quench or mar temper to reduce distortion. and may want to core harden to 40 HRc the induction harden to 50 Hrc minimum. reduce distortion. bar can be heat treated to core hardness then machine then induction harden, and its economical, carburizing steel is expensive, carburizing is expensive, and post machining is required and expensive.

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