4340 cracking dhuring h/t

[GTMule]

Hey everyone,

We've got a rather large 4340 part that is cracking at some point during heat treatment. All the work is being done out of house, and I assume the heat treater never noticed the cracks (though I can't IMAGINE how not)

Print speced 180-200ksi

Link below with some pictures. For reference the smaller tapped hole is 1/2-20 and the thickness there is about 5". As you can see the crack extends through the hole and WAY (5-10") around the corner both ways. We did a total of 3 parts and one end on two of the parts are cracked. (so, 2 of 6 total ends)

Any ideas?

http://picasaweb.google.com/boulware/Cracks

 

[unclesyd]

Can you comeback with the exact heat treatment specified and the one used for the parts?

Though not very common there is possibility of 4340 cracking during heat treatment.

What you have is probably a form of Die Cracking.

 

[arunmrao]

Not surprised to see a 4340 cracking. I have lost many a parts this way. However, as Unclesyd has requested please come back with more details of heat treatment cycle and the hardness expected,quench media etc.

" All that is necessary for triumph of evil is that good men do nothing".
Edmund Burke

 

[dbooker630]

I have had issues with cracks in 4340, that initiate from holes such as this. Root cause in my case, after removing as much of the quench aggressiveness as possible, was the surface finish of the hole.
In addition to verifying the heat treat recipe see if you have a way of checking the hole surface and thread roots. Then open up the fracture to see if your case is similar to mine.
Also find out how cool these parts get after quenching then how quickly these parts are tempered. You may not want to allow the 4340 to fully transform through the Mf temperature.

 

[GTMule]

The drawing specified:

Heat treat per ASM-6875 TO 180-200ksi, Rc 40-42, IIRC.

The problem is that the machine shop that is making the part got it heat treated out of house, so he doesn't know the exact process. It's one of those "job shop" parts, so he'll have to get it right eventually to get paid. I don't really even have the ability to suggest changes to his heat treater, since if it cracked as well, it'd make us (my company) potentially responsible.

Not sure how much I helped. Should he just try to machine that hole after h/t?

 

[arunmrao]

Holes are always problematic while quenching. In this case as I saw just one hole,. Normally I plug the hole with some insulating material before I heat treat. Was the part stress relieved after machining.

" All that is necessary for triumph of evil is that good men do nothing".
Edmund Burke

 

[GTMule]

insulating material like.....a bolt? or?

 

[stanislasdz]

GTMULE,

1- Are you sure that your part was OK before quenching ?

2- Did impose a fixed position of the part in the quench media ?

3- Can you send us the exactly the composition of the all elements ?

 

[redpicker]

Cracking in 4340 is not all that uncommon., particularly at the higher carbon contents.

Those look to be some very large parts (I'm guessing 4" by 6" by 12"). I'd hate to try and guess how they heat treated these.

You should be specifying a protective atmosphere. You should make sure that an oil quench is specified (some shops may be using polymer, which has a greater cracking tendency). I would pack the holes with ceramic wool (particularly knowing the part has cracked in the past). I would also make sure the parts were not quenched for longer than 20 minutes and that they were charged into the temper as soon as possible after quenching (no longer than 30 minutes). The cracks occur when the material is below 400F, so minimizing the amount of time spent below this temperature will help.

rp