Expansion of 4140 after hardening

[Michael Heppding]

Hi,

we have had issues with machining after heat treatment of the part attached. It is surface hardened to 3mm deep and we are machining the centre bore to final size after hardening.
Its all within tolerance until we bore the 6 through holes. After drilling of the 6 holes the Dia 60mm centre bore increases in size by 0.04mm.

Anyone knows what is happening within the material that makes the part increase in size?

kind regards Michael

 

[3DDave]

Presumably releasing internal strain. I've seen a part from rolled plate flip up like a pringles chip following release from a vacuum base fixture. Your amount seems like a lot - if my results are right about 20ksi of internal strain.

 

[EdStainless]

yes, stress relaxation. You should have a stress relief added after hardening, or rough the center home, drill the others and then go back and finish the center hole.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[mfgenggear]

I have and continue to mfg parts from 4140.
Was it rough machined and normalize, harden temper. To 30-36 HRC for machining.

Was it stress relieved after turn

Drill the holes u/s before Heat treat then hone them out after final heat treat.

Recommend Mar temper or Mar quench
If it's not to thick. Do a sample test.

If not pre-drilled
Also you should drill the holes before finishing the bore. If this method is continued.

Drilling the bolt holes from solid after heat treat causes inside hole to move because of heavy machining forces. It's going to move.