4340 HT question. Mete? Kevin?

D

deker

Joined
Nov 14, 2005
Messages
3,496
I was visiting with a friend this weekend who has had some issues heat treating some power hammer dies in 4340. I took a look at a couple of sets that he's had crack fully through a die that measures almost 3"x3"x4". The quench used was motor oil (not ideal, but hey, it's 4340) and heating was done in a coal forge. There were a few less-radiused edges that seemed to be starting points for the cracks which makes sense, but the same radii have been used on many sets of successfully HTed dies.

He's only had 3 sets ever go bad, but it's disheartening for him to spend a day machining them and half a day in HT to have them fail.

My only guess here would be excessive heat prior to quench, but I haven't seen the inside of the cracks yet (I'm going to ask if we can try to break one next time I'm out to visit to look at the grain).

I'm just wondering what the pros think might be an issue.

Thanks,

-d
 
I have made alot fo hammers with alloy steel and it does need a long soak for thicker cross sections. The temper needs to be very thorough. For hammers I temper three times at 400 deg for 2 hrs. with a complete cool down in between each cycle.

One hammer did crack on quench and it was not forged, only machined. It probably had the crack right from the start.
 
4340 is a fine steel and very tough. For that application design should not have any sharp edges or corners ,that goes for any steel. I wouldn't recommend coal forge for HT as it should have controlled temperature and thorough soak to equalize the temperature. Quench should be in warm 130 F oil. Temper also should be thorough .
 
mete said:
4340 is a fine steel and very tough. For that application design should not have any sharp edges or corners ,that goes for any steel. I wouldn't recommend coal forge for HT as it should have controlled temperature and thorough soak to equalize the temperature. Quench should be in warm 130 F oil. Temper also should be thorough .
Click to expand...

What would an acceptable radius be? There weren't any sharp corners to speak of, but some of the corners were only radiused to about a 1/16" or so radius. Is that still too sharp?

They all cracked in the quench, not in use.

-d
 
I'll bet the corners were much hotter than the core. If you take a look at the fracture surface I'm sure you'll find coarse grain at the corners and finer grain in the center.If you can , get a photo of the fracture surface.
 
mete said:
I'll bet the corners were much hotter than the core. If you take a look at the fracture surface I'm sure you'll find coarse grain at the corners and finer grain in the center.If you can , get a photo of the fracture surface.
Click to expand...

Ill see if I can do that. It should be fun trying to finish the break in pieces of steel this big. Of course, that's what sledge hammers are for :)

-d
 
You must log in or register to reply here.
Back
Top