Further hardening 4140 pre-hardened

[nsmith40]

I am making a splined engagement dog of about 2 inches in diameter. The stock material I have to use is 4140 pre-hardened to roughly 30 HRc.

The part will be machined then I would like the hardness to be increased to roughly 45 HRc and then finally internal splines will be cut using Wire EDM.

What processes will be required to achieve this increase in hardness?

 

[Metalguy]

The 2" dia. may make it hard to reach Rc45 near the center--IF that matters to you. Correct HT starts with ~1/2 hr. at 1550 deg F, followed by an oil quench or a polymer quench. The tempering temp. will depend on how hard it is after the quench.

Do you know what the actual chemistry of your heat is?

Gold is for the mistress - silver for the maid
Copper for the craftsman cunning in his trade.
"Good!" said the Baron, sitting in his hall
But iron - cold iron is the master of them all.
Rudyard Kipling

 

[nsmith40]

The part is 2 inches OD but has a 1 inch hole through the center, so it is relatively "thin" throughout.

The stock is actually 4142 hot rolled heat treated steel from Speedy Metals. Their documetation says it is pre-hardened to 27-32 rockwell C, but that's about all it says.


Is an anneal recommended or required before rehardening/quenching/tempering?

 

[dbooker630]

You don't have to anneal before rehardening. With a 1/2 inch wall you should not have a problem reaching an as-quenched hardness in the mid 50's HRC. Tempering temperature will be around 700F to achieve a final hardness of 45HRC. The warehouse still should provide you with a mill cert, however, as metalguy has suggested.

 

[Metalguy]

Good thing you have the center hole. A mill cert. won't be necessary-it should harden nicely.

Gold is for the mistress - silver for the maid
Copper for the craftsman cunning in his trade.
"Good!" said the Baron, sitting in his hall
But iron - cold iron is the master of them all.
Rudyard Kipling

 

[jsokal]

One thing though,..be sure not to finish machine your part before you do the second heat treatment,...you will have some distortion, and not knowing your tolerances it is something to keep in mind.

John

 

[dbooker630]

I agree that it will heat treat nicely but I always insist on a mill cert so I know whether it is a low or high side chemistry - just in case.

The concern I have would be EDM of the internal splines after quench and temper - ensuring that there is not any heat affect above your tempering temperature. Otherwise you should be OK

 

[nsmith40]

Thanks for the info.

What sort of trends can I expect for distortion? Will the part tend to grow or shrink in diameter? By about how much? Is twisting a possibility?

 

[TVP]

For a long, thin tube the cross-section will tend to go out-of-round upon quenching & tempering. Depending on how the part is actually quenched, bending may be a possibility. Long parts should be dropped longitudinally into the quenchant, not transverse.