How to harden slender shaft without warpage?

[Yamunan]

We had to harden a En-31 shaft with 2" dia and 50" length to achieve a hardness of over 60 HRC.
The shaft had a 2 degree taper for 20" length and a 4 mm pitch Thread for the remaining length. We are not much into heat treatment and upon some little research, we chose induction hardening to avoid warpage since our shaft being too thin for its length. The shaft was hardened all along the taper and heat treater suggested that he would just blacken out the thread portion to some 30 HRC so that the thread wouldn't get too brittle and skin harden the taper portion for 60 HRC.
The application needed some grinding to be done on its taper portion. We had a stock of 0.5 mm for heat treatment + grinding.

After the shaft was hardened, there was a 4 mm bend in the middle. The bend remover suggested that it would break if we cold work it to remove bend. So we had to use a gas torch(neutral flame) to heat red hot and a little stroke on hydraulic ram almost straightened the shaft to run out of 0.3 mm.

Grinding was done and the shaft was well within the specified tolerances. But then , the final hardness test revealed that it is only 28 HRC hard and the part was rejected.

So, Is there any way to rework the part to achieve the earlier 60 HRC? Or if i want to start from scratch, how much allowance must be given for heat distortion? How to prevent bending of such slender shafts during vertical hardening? Any guidelines on where & how to set up support jigs to prevent such distortions?

 

[HuFlungDung]

I don't rightly know, but maybe a preliminary anneal should have been carried out on the part in the semifinished state, in order to see if it had some natural stress relief. You could then straighten it in the annealed condition and anneal it again, and see if it stays put, then try the final hardening process.

I cannot envision how you would clamp it to a jig to hold it for induction hardening. I think maybe if done in a furnace, it would be possible to build various fixture blocks to hold it in a quench press for the quench, but, that could be expensive to do for a single part.

 

[dave powelson]

I don't rightly know, but maybe a preliminary anneal should have been carried out on the part in the semifinished state, in order to see if it had some natural stress relief. You could then straighten it in the annealed condition and anneal it again, and see if it stays put, then try the final hardening process.

I cannot envision how you would clamp it to a jig to hold it for induction hardening. I think maybe if done in a furnace, it would be possible to build various fixture blocks to hold it in a quench press for the quench, but, that could be expensive to do for a single part.

To add to the above---multiple stress reliefs with straightness checks and straightening done after each stress relief
is needed. You are dealing with imparted stress from the mill production, plus stresses induced in prior processing....all
kinds of unknowns.

This approach was successful in multiple production volumes, multiple types of tool steel, multiple types of parts (10K+ parts/order) for long, skinny tool steel nail driver anvils requiring finished hardness in the Rc 60's and straightness w/in .005 total after finish grinding (including grinding a flat along the main point length of 4" +. Straightening via flat, stepped blank dies in flat die roll thread machines. All grinding performed in center less grinders with fixturing and form dressing.
-more finish size grinding done after final HT, blanks were verified w/in final cleanup limits prior to vertical hanging for HT.

-for lengths not practical for roll thread blank dieing.---straightening presses that turn the part with indicators along length are
available and can be used in a one-off or production environment.

The above outline I've given is obviously a fairly intensive R@D effort, making test runs of parts and trying.
To expect 'Magic Results' on a one-off production item is.....uuuhhhh.... 'whistling thru your britches'.

[It never ceases to amaze me that companies have the chutzpah to quote parts new to them, without
contemplation of 'just how?' are the final customer requirements to be met---then raise the question after
the fact....rather than prior to the fact. Must be the overriding influence of New Age Management--an Elon
Musk kinda mentality.
At the minimum, at least do some sample testing of critical questions PRIOR TO QUOTING. I did this repeatedly
to avoid embara$$ing and expen$ive conundrums.]

 

[SeaMoss]

Not sure why you went with induction hardening in the first place, seems not so good to me. For long shafts my heat treater used to hang them, and quench end-on. That helped a bunch. Your part is kinda long but still could do that, just use a tall skinny barrel for the quench. And keep something nearby to smother the fire

Can't say I have have done any shafts quite as long as yours but for smaller sizes, maybe 3' long, a straightening press worked fine. No reheat. There are other methods too - banging the shaft on one spot with a blunt cold chisel and such - but I am not experienced with that, just seen it done. But if you need it hard, annealing it doesn't seem like a great idea

 

[dian]

why not plasma nitride it?

 

[dgfoster]

Not sure why you went with induction hardening in the first place, seems not so good to me. For long shafts my heat treater used to hang them, and quench end-on. That helped a bunch. Your part is kinda long but still could do that, just use a tall skinny barrel for the quench. And keep something nearby to smother the fire

Can't say I have have done any shafts quite as long as yours but for smaller sizes, maybe 3' long, a straightening press worked fine. No reheat. There are other methods too - banging the shaft on one spot with a blunt cold chisel and such - but I am not experienced with that, just seen it done. But if you need it hard, annealing it doesn't seem like a great idea

That is how Monarch hardened their lead screws. They were hung vertically and slowly dropped through and induction coil which was a few inches above a large oil quench bath. Part moved through the coil and into the fairly large bath maybe five or ten inches a minute. The good-sized bath helped prevent the oil from reaching ignition temps. Something like this thoughthis is a water quench: Induction hardening king pin - YouTube

 

[HuFlungDung]

FWIW, I've never had good luck straightening something that hard. I can get it partway straighter, but the ultimate push required to get complete straightness takes the part so far past center that it always fractures. For example induction hardened chromed shafts will break, hardened rear vehicle axles will break....leads me to believe the piece is toast if the bend is significant.

 

[triumph406]

. The shaft was hardened all along the taper and heat treater suggested that he would just blacken out the thread portion to some 30 HRC so that the thread wouldn't get too brittle and skin harden the taper portion for 60 HRC.
The application needed some grinding to be done on its taper portion. We had a stock of 0.5 mm for heat treatment + grinding.

I would think that the 0.5mm stock left on the part for grinding would be marginal if the part was carburised and then heated/quenched. In other words the part wasn't carburised to an adequate depth.

 

[Erich]

No carburizing required. EN31 is 1% Carbon and approx. 1% Chrome . Closest us grade of steel is 52100 aka bearing steel. It will through harden with no issues.

 

[TDegenhart]

Since this appears to be a one shot deal not worthy of extensive R&D just remake the shaft with enough stock to grind to size and shape.

 

[mkd]

Wow, seriously knowledgeable responses!
I'll just add, i worked in a shop that did post heat straightening to some Hydraulic type shafts for aeroplanes. Couple of v-blocks in a hand pump press, followed by grind.
.16" over 50" is significant. Bet the heat treater just threw it in the oven horizontal. maybe you could bend it back if it's gradual.

 

[TDegenhart]

Since the OP didn't say how the parts are used, attempting to straighten them could be a disaster. At the least they may be need to Magnafluxed or dye penetrate tested. The other potential problem is latent tensile stresses that later could lead to fatigue cracks.

Tom

 

[eKretz]

Since this appears to be a one shot deal not worthy of extensive R&D just remake the shaft with enough stock to grind to size and shape.

Bingo. I saw the bit about 0.5mm stock left for finish and could have done a forehead slap. Nowhere near enough for a part like that described.

If your going to redo this part, get a heat treat guy who knows what they're doing, and leave some more room for movement. Also absolutely stress relieve the part after it's roughed out.