4340 too hard

[Shaw123]

Dear all,

4340 steel
I’m having difficulties achieving hardness properties from a hot press forged section.
Forging from 1100C (2012F) with a greater the 4:1 reduction.
Following forging I’m annealing at 650C (1202) for 13hrs and furnace cooling at 50C per hour.
I need to achieve a hardness of no greater than 229HB but unfortunately its coming out up to 269HB.

Any ideas?

Thanks

 

[unclesyd]

Need a little more information on your thermal treatment and physical properties of the forging.
What is the thickness cross section?

How are you cooling from the forging temperature?
In the same vain what is your start and stop temperature?

The annealing temperature for 4340 is normally 1490F with 20F/hr furnace cooling rate to 650F, then air cooled.

Your soak time seems to be a little excessive.

 

[Shaw123]

Thank you for you help.

Section is 98 x 70mm.
The material is forged from 2012F down to size in one heat and floor cooled. I’m not sure of the stop temperature but we always get to final size in one heat.

 

[redpicker]

How are you cooling from forging temperatures?

Are you are cooling to below 400F (200C) prior to your annealing cycle? The annealing cycle you have described is a sub-critical annealing cycle and you can lower the hardness by increasing the annealng temperature up to 1275 F (690C). This still might not be enough to keep the hardness below 229 HB.

Is there some reason you're using the sub-critical cycle? If not, I would recommend a full anneal, as unclesyd has suggested, since this should definately knock the hardness down and it may also help refine the grain size, which would help a subsequent heat treatment.

rp

 

[TVP]

I agree with the others-- you are unlikely to attain the desired hardness using a subcritical anneal with 4340. The prior microstructure is just too inhomogeneous. Intercrtical annealing is required when the parts are previously hot forged.

 

[dbooker630]

If you must subcritical anneal, try raising the temp to 1250F (677C), hold for four hours, furnace cool to below 800F (427C) then air cool to ambient. Hardness should fall between 207-229 HB.

Normalize and temper may also work to achieve the same range. I have used both on the 4340 in my shop.