Strength of Carbuized part. 3

[NickE]

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

 

[TVP]

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.

 

[Carburize]

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).

 

[NickE]

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.

 

[swall]

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.

 

[NickE]

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

 

[NickE]

Swall- sealed envrionment, just temperature changes.

 

[Metalguy]

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

 

[Carburize]

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"

 

[Metalguy]

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!

 

[NickE]

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?

 

[TVP]

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).

 

[unclesyd]

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.

 

[NickE]

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

 

[swall]

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 ?

 

[Carburize]

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.

 

[metengr]

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.

 

[diamondjim]

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

 

[Carburize]

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

 

[diamondjim]

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.