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Strength of Carbuized part.
3

Strength of Carbuized part.

Strength of Carbuized part.

(OP)
In a very general way how would a case of 59HRc on 4130 steel effect the strength, impact strength, ductile to brittle transition, and fatigue life?

I haven't sectioned and mounted to determine core hardness yet, the case depth was given by the processor at 0.25-0.50mm and the part is 6mmx13mmx50mm.

Will I have residual compressive streses in the case?

Would impact at low temperatures cause the case to crack and "spall"?

The part is subject to repeated impacts and forces at the tip of ~90kg, the max Von-Mises stress (From FEA) is around 650MPa.

Temperature of operation is roughly from -25C-120C.

Thanks- the carburization was a mistake, how bad of sorts are we in now is why I'm asking.

nick

RE: Strength of Carbuized part.

NickE,

Can you provide some additional details on how the part was processed?  How did the carburization occur?  Was a tempering treatment performed after the carburizing cycle?  Times and temperatures for austenitization, quenching, and tempering would be useful.

RE: Strength of Carbuized part.

As TVP says - more details.
4130 is not a carburizing grade and to maintain a case hardness of 59Rc would require a low tempering temperature ( around 350F) after carburizing. This low temperature temper would seriously degrade the toughness properties of the 4130 core material compared to more conventional tempers after quenching ( around 1000F).

RE: Strength of Carbuized part.

(OP)
TVP & Carburize (I knew you would come through) - I'll have to call the heat treater today. Look for a followup. And the lack of toughness is what I'm worried about. I've posted about this part several times to get info for the material callout, I was disregarded when the print was sent out for prototypes.

Thanks, Watch this space for more details.

RE: Strength of Carbuized part.

NickE--One time we carburized some Timken 17-22-AS by accident. This grade is somewhat similar to 4130. The way we found out about the problem was field failures due to SCC. These were brake rotor segements for commercial aircraft brakes, and the segments would split in half after some time in service.So, this may have some "impact" on your toughness question.

RE: Strength of Carbuized part.

(OP)
Ok after a little bit of calling around:

1650F carburize case depth: .015"-.020"

1500F (oil quench) HUH This is pretty hot isnt it?

Temper at 350F.

i took 1 extra part an dhit it witha hammer at room temp, it didnt break. When chilled to ~-20F the part shattered with completely brittle failure all through.

Should I try to bring some toughness back by tempering at 550F for like 2 hours?

I really wanted ~45-48HRc to give me strenghs (UTS) above 200ksi and decent toughness....

Thanks for the help.

nick

RE: Strength of Carbuized part.

(OP)
Swall- sealed envrionment, just temperature changes.

RE: Strength of Carbuized part.

NickE,
*IF* you really require toughness I think those parts may be scrap.  What kind of loads do they see?

RE: Strength of Carbuized part.

The greatest danger I see is that if the part is deformed to any extent the case will crack and with the low toughness core the crack will run straight through the part just as your -20 experiment demonstrated.
I'm afraid the true answer to your question is the same response as I got from an Irish police officer once when I asked for directions to a place in central Ireland "If I was you I wouldn't start from here"

RE: Strength of Carbuized part.

That's a great line!  But NickE needs to remember (or discover) the experiment where a soft, tough Cu wire is given a thin Cr plate.  You can then snap the wire by bending it.

Granted a 6mm dia. probably wouldn't do that, but it makes you think about it!

RE: Strength of Carbuized part.

(OP)
I wish I didn't have to start from here, the parts however cost lots of money, and are at the customer already. I'm going to try and draw them back if given the chance. Any reccommendations as to time and temp?

RE: Strength of Carbuized part.

Carburize,

Not only did you provide sound advice, but that line is one in a million.  I laughed out loud when I read that!

NickE,

It is possible to temper this at a higher temperature to reduce the hardness and improve the toughness.  800 F (425 C) for 2 hours should reduce the core hardness to around 375 HB (~ 40 HRC) which is roughly equivalent to a tensile strength of 200 ksi.  The izod impact strength shows a moderate improvement from a low 14 J (10 ft-lb) to around 34 J (25 ft-lb) at this strength level.  For 45 HRC (~ 425 HB) you would need to temper around 600 F (315 C), which results in a tensile strength of ~ 214 ksi and izod of 14 J (10 ft-lb).

RE: Strength of Carbuized part.

Is the part complex, i.e. holes, threads, etc.?

I would turn back the clock by  annealing the part, Normalize, Austenize quench in oil or polymer based on the complexity, then tempering depending on the as quenched results.
Or
It may be feasible to just Normalize the part and Austenize, quench in oil or polymer based on the complexity, then temper based on the as quenched results.

We normally considered 4130 as water quenched material unless we were going to weld on it.  I believe that you can achieve a polymer quench even with the carburized case.  

RE: Strength of Carbuized part.

(OP)
Nope its pretty simple, Starts as a flat shape kinda like a lollypop, the width crossection is a trapezoid, it is formed on a 90R. There is one conical hole for a M6bolt.

carburize- If I normalize/austenitize this part I should get more diffusion of the carbon into the core and if I use an air furnace I should get some de-carb. Would this effectively remove som of the bad effects of the high carbon case?

The customer has been informed of the problem (well not exactly what the problem is, just that there may be one) and I may need to make some decisions in the next few days...


nick

RE: Strength of Carbuized part.

NickE--Is this the first time this part was ever made, or is there a satisfactory history of usage on the 4130 (non carburized) part ?

RE: Strength of Carbuized part.

Austenitizing in an oxidizing atmosphere and then quenching is probably not a good idea. The resulting carbon gradient will take a "camel hump" profile and will generate a very odd residual stress profile with tensile stress at the surface leading to even higher likelyhood of cracking.
I would be most tempted just to try raising the tempering temperature as TVP has indicated, accepting that you are going to lose some surface hardness but with the benefit of a tougher core.
 

RE: Strength of Carbuized part.

Nicke;
Here is my 2 cents worth with no specific recommendation but some advice;

In watching this post blossom with excellent options and opinions, I would like to say that a customer on the receiving end of this "nonconformance", better be involved every step of the way to resolve this problem - including taking their suggestions if they have the engineering expertise.

Having been on the receiving end - as a customer - , I have never appreciated a vendor "engineer" their way out of a material nonconformance without our involvement on the front end. Instead, I always appreciated working directly with a vendor or supplier each step of the way to resolve material nonconformance’s. Sometimes, an alternative option worked out and other times I had to scrap components ($1M) to protect my company’s interest because the rationale for using this material was not technically justified or would place the component in an unacceptable risk of failure in service.

In this particular case, I would make sure that the customer has been presented with all detailed heat treatment options (tempering, re-heat treatment) including scraping the material, if necessary. Not knowing the specifics of this part - if it pertains to equipment reliability, personnel safety of the customer, etc, NickE needs to be absolutely sure that he has performed a detailed engineering analysis of each option to validate product performance.

RE: Strength of Carbuized part.

Does anyone else think it might be
better with a 8600 type steel?

RE: Strength of Carbuized part.

86xx won't make the core strength that is being sought but other carburizing grades such as 17CrNiMo6 might

RE: Strength of Carbuized part.

Thanks Carburize, I knew the 8600 steels were
used in Alaska for the train couplers which
see high impact loads and just thought that
NickE was more concerned with the latter.

RE: Strength of Carbuized part.

(OP)
Swall- First time the part has existed.

Carburize- Any tensile stresses in the surface, or near, would be very bad for the operation of this part.

metengr- thanks.

I think that tempering down will likely be the only solution as opposed to anything else. The customer will be well informed from my end.

8600 isnt available in coil is it?

nick


Nick
I love materials science!

RE: Strength of Carbuized part.

What is the thickness tolerance for the part.  If you decided to go all the way back to the beginning as suggested by unclesyd, you might be concerned about losing thickness due to oxidation, decarburization, etc.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: Strength of Carbuized part.

(OP)
cory- thickness isnt going to be toomuch of an issue.

RE: Strength of Carbuized part.

CoryPad,
If there is no atmosphere in the furnace you can always wrap the part in brown paper, bury in spent pitch coke or CI shavings, and proceed with no concern for carb or decarb. Or you could drop in a mothball every once and a while

NickE,
I've been trying to see if my old Bethlehem Data Book is still around to see if you could compromise between the 4130 core and the 41xx in the case by judicious quenching and tempering based on the value of xx.  My small book only goes to 4152.  I still looking and thinking about the problem and ways and means to remediate your problem.

meteng,
We use hundreds of pressure containing parts made from 31xx and 41xx in our process and have worked with many a vendor and our shop  in the reclaiming or modification of components by re-heat treating without detrimental effects.  My only worry is my lack of knowledge on the effect on the low temperature impact properties on the material in his application.  A redeeming thought is that his material is fairly forgiving based on the treatment received in daily use in the cold country.     
I like the reminder of mutual understanding in any process of altering the original conception of the part and processes, it doesn't happen enough.  

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