4340 cracking dhuring h/t
4340 cracking dhuring h/t
(OP)
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
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





RE: 4340 cracking dhuring h/t
Though not very common there is possibility of 4340 cracking during heat treatment.
What you have is probably a form of Die Cracking.
RE: 4340 cracking dhuring h/t
" All that is necessary for triumph of evil is that good men do nothing".
Edmund Burke
RE: 4340 cracking dhuring h/t
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.
RE: 4340 cracking dhuring h/t
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?
RE: 4340 cracking dhuring h/t
" All that is necessary for triumph of evil is that good men do nothing".
Edmund Burke
RE: 4340 cracking dhuring h/t
RE: 4340 cracking dhuring h/t
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 ?
RE: 4340 cracking dhuring h/t
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