Case Hardening trouble 5

[Kennethhp]

Hello all!
Im an intern at a heat treatment facility as a mechanical engineer. (non english speaking so sorry for typo-o's

I have a task which is causing alot of trouble in our case hardening department.

A costumer is producing different sizes axels in 20MnCr5.

When one certan size axel is treatet they simply dont get any carbon addition in the surface. The other parts in the oven at the same time have perfect result, but this particular axel doesnt.
I have tried to measurre hardness to see if there is any difference after treatment, but no change.

I got my hands on a piece of their raw material before they machined it, and tried to treat that part with great succes. It had the desired case.

So im thinking there is something in the machining of that particular size axel that leaves a protective "shell" so it wont be treated.

Does anyone have any suggestions or experience with similar problems ?

/Kenneth

 

[WKTaylor]

20MnCr5 = What material specification?

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can b

 

[arunmrao]

@wktaylor
AISI 5120 please.


"Even,if you are a minority of one, truth is the truth."

Mahatma Gandhi.

 

[mfgenggear]

yes aluminum oxide blast the parts before carburizing.

 

[Kennethhp]

@mfgenggear

Can you please explain ?

Im familiar with oxides, but i cant see how the oxide only appears after machining, and especially on certain diameter axel.

 

[mfgenggear]

yes there is some type of contamination on the surface acting as a mask. clean the parts with aluminum oxide. becareful with tight tolerances. blast lightly. try a sample part blast it carburize & harden it, then test it. make sure the material is what they say it is. do a spectra analysis if need be, to verify the correct material. ask them why they chose this material. normal carburize steels are AISI 8620 OR 9310, 4330 OR 4340 FOR HARDER CORE. BUT EXPENSIVE.

 

[Kennethhp]

@mfgenggear

I have done a spectra analysis and it is AISI 5120.

I will try and see if our costumer will supply a machined part we can try it on.

But im still confused why its not happening to the other diameter parts in the same material.

Im thinking it must be the machining of that particular diameter that is causing trouble since we can carburize their other parts and we can carburize the raw un machined material.

 

[Lyrl]

I've had this problem occur sporadically on certain parts over my heat treating career (~15 years) without satisfactory root cause determination.

It might be related to the surface finish.

Usually treating in an air draw at 700-1000° to develop an oxide layer prior to the carburizing process will cause the part to respond normally. I'm not sure if it is because the oxidation changes the surface finish, the temperature "burns off" some contaminant on the part, or if there's something about the iron oxide layer itself that helps.

 

[mfgenggear]

what about the carbon potential and the atmosphere controls. I would suggest hiring a metallurgical consultant specialist to help. some like maybe a setup problem.

 

[Maui]

Kenneth, you might want to check the processing layouts for each of these axle sizes and compare them. Is there anything in particular that stands out in terms of how this one axle size is manufactured versus the others? For example, do the other axles go through a cleaning operation before they get case carburized while this one size that is causing you problems doesn't?

Maui

 

[WKTaylor]

KennethHP...

I hate to be picky...

OK you are making the parts from AISI 5120 steel...

What is the 5120 steel bar material spec [IE: ASTM Axxx, AMSxxxx, etc]?
What is the core alloy heat treated to [EX: 180-KSI, etc]; and what is the heat treat specification You are performing the HT work per [EX: MIL-H-6875, AMS2759/7, etc]?
What is the Carburizing specification You are performing the carburizing to [EX: ASTM A534, AMS2762, AMS-S-6090, AMS2759/12, etc]

BTW... mfgenggear hit the nail on the head... 5120 is NOT an alloy steel that is normally carburized... that may be a big part of Your problem.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can b

 

[Maui]

Will, It's 20MnCr5. SAE 5120 is the closest US equivalent, but it isn't the same grade of steel.

Maui

 

[WKTaylor]

Maui... Steel can be miss-processed in many ways... Devil is in the details.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can b

 

[mfgenggear]

agreed will Taylor. and that's why I said for him to do a spectra analysis. but it should be to a spec.

 

[Maui]

Will, my point was that it isn't 5120.

 

[Kennethhp]

@mfgengear. We have full control over the process and the sensors are calibrated monthly. We have had our in house metallurical consultant in over to get help, but with no luck. He hasn’t seen this issue before.

@Maui. Exactly what im thinking. I cant help to think that there must be difference in the way they machine the axels since their smaller ones in the same material in same batch are fine. About the cleaning, yes we clean all the parts good before carburizing.
Unfortuneally the costumer is stubborn and is not so open around how they do things.
But I was hopeing someone had issues with machining before say, “hey this is deffently a potential cause”

@Wktaylor. As said im an intern with no special heat treatment education. Just a “regular” mechanical engineer, so the specs you are referring to doesn’t ring a bell yet. I will look into them. And to be clear I am not making the parts, just treating them 

As said I did a sprectra analasis. If you want I can get the complete material composition listed if it help anything.

 

[Maui]

Kenneth, how do the results from the spectral analysis that you performed compare to the composition that is listed on the material certification provided by the steel supplier?

Maui

 

[swall]

I have seen issues of "spotty case" on 8620 parts where part of the surface would not respond to carburizing. We traced it to the machining coolant the vendor was using.

 

[mfgenggear]

that is why the parts are all ways etched and cleaned by aluminum oxide blasting as I pointed out in an earlier post. the problem does not lie with the customer it lies with the in house procedure. verifying the material is an added safety.
don't throw stones until you have checked and double check you own stuff.

it has been very rare that our parts can not get the correct case depth and hardness.

 

[Kennethhp]

@Maui I will check up on that

@Swall. awesome! i was hopeing someone had this issue before, we just have to push through to our costumer and hope they will let us in on their procedures to see if they are using different coolants or machining techniques.

@Mfgenggear. But how do you handle tight tolerances if you in house blast the part ? wouldnt it be better to have the costumer change the coolant/machining, if this is the case, to the same coolant/machining as they use on their smaller axels ? I will talk to my supervisor to see if we can allow to try and etch and blast on one axel next time we get them in house, it sounds like the best option for now. The reason why we never thought of blasting is, that it has never been an issue we have seen here before.

And Thanks for all the replies by the way !