How stable is 4340 for heat treat to 275-300ksi?

[Daniel M]

It's been a while since I've done one of these. I'm sure it'll shrink some.

 

Does anyone know if it will warp much?

[Jmick]

We heat treat 4340 all the time. It seems to be the one that we can count on to possibly move everywhere. We heat treat in a nitrogen rich enviroment to eliminate some of the movement and warp but it still happens. Just leave enough for finishing after.

[Guest CNC Apps Guy 1]

Can you HT first then machine?

[Daniel M]

I'd would machine after HT if stability is a BIG problem. The book says machinability goes down to 25% speed/feed and run time is the concern.

 

Heres the part: LWH dims are about 17" x 8.5 x 3.6

 

 

[Bruce Caulley]

With that part I would probably do a quick op to take the long corners off and whatever feature is needed for op1 fixturing and then HT. The rest would be no real drama machined in the hardened state.

 

Did a swaybrace for an F-111 from a pre-hardened block, and used a three flute tipped endmill with R2 cornered inserts as a plunger to get the material off. Ran sweet.

 

Bruce

[Ron Branch @ Verisurf]

On a part like that I would recommend you rough within .25 heat treat then finish machine. Cross sections become a concern and if you heat treated 1st you run the risk of spider cracking or soft spots. Do you have to nilaetch, cad, and paint the part? Do dimensions apply after plate or before? Does it get chrome in the bores?

[Daniel M]

Thanks guys.

 

I agree. right now the part has been water jet to about +.200 on the profile.

 

Plan is to use powdered metal fine pitch roughers and leave dovetails to locate after heat treat.