Case hardening of 42CrMo ... by mistake

[UCengno1]

I have a part from a non-domestic source that is being fabricated from 42CrMo in lieu of 4140. We recently discovered that the parts were being case hardened instead of thru-hardened. The core ran around 42 Rc, within specification, but there was a .009" thick case that was around 60 Rc when no case should exist. First question is whether it is possible to carburize 42CrMo, even by mistake, and what would you end up with. Comments are most welcome.

BCK

 

[dbooker630]

A few questions for clarification toward a better answer for you...
What is the thickness of the part? If the case does not dominate the cross section there may be some salvage options such as tempering at a higher temperature as long as the core does not fall below your spec.
Do you know what the surface carbon content of the case is, and how much of the case is above .80%C?
What hardness scale was used to obtain the 60HRC value? If it was not a direct HRC perhaps the carbon penetration is shallow at worst.

As far as heat treating process selection, 42CrMo is similar to 4140 as far as alloying is concerned, in general neither one is carburized.

 

[PeterCharles]

1) Yes, you can carburise this steel. The nominal carbon is 0.4% so heating in a carbon rich atmosphere will increase the surface carbon content, and quenching will
2) The surface conditions will depend on the time, temperature and carbon potential in the furnace.
3) 0.009" seems low for a carburising cycle. We used to experience a thin case occasionally when through hardening in out pit furnace even with a neutral atmosphere. Since the furnace was usually used for carburising we thought it might be residual carbon coming out of the refractory lining (if this is possible). Alternatively the neutral atmosphere was not as neutral as we thought!
4) If you want to get rid of the case all I could suggest is local heating to temper it out.

 

[dbooker630]

Yes it can be carburized, I agree, but least in the automotive field it is either through hardened or through hardened then induction hardened.

But when surface enhancement is desired in medium-carbon grades such as this one, I would recommend gas or salt bath nitriding instead.

 

[swall]

One thing you should check, since this part was from a non domestic source, would be the possibility that the part was carbonitrided, rather than carburized. If that is the case (no pun intended), your part would be slightly more brittle and your salvage options more limited.

 

[UCengno1]

Many thanks. It appears that the part was not induction hardened as we have a continuous case even on an internal bore. I will follow up on the potential for carbo-nitriding as this was discovererd through an analysis of a failed part.

BCK

 

[NickE]

I once messed up and didnt follow a batch of prototypes through the whole process. They were 4130 I posted a thread about my experiences with carburized Alloy Steel. (it was not fun, but we got the business we needed)

thread330-123117

Nick
I love materials science!

 

[UCengno1]

NickE,

Thanks for the information. I am concerned that we may be seeing something similar with our part. They did get a good temper on the core, approx 42 Rc. Did the attempt at case hardening compromise the toughness even with the adequate core hardness? Comments are appreciated?