Heat treatment for lathe spindle shaft

[mackeym]

I'm working on building a lathe spindle. I have suitable bearings and know about fits. For the bearing shaft and housing i plan on using 4140 steel. After rough machining, i want to do the following heat treatment prior to grinding:
1. Initial stress relieve and cool to room temperature
2. H/T to 44-48 Hrc
3. Cryo stablize
4. Stress relieve

Is there anything that is missing in the above process or special instructions i should ask for when i have it done? I got a reasonable quote from Metlab Potero to do this heat treat process for both the shaft and housing.

One more thing, i don't know what kind of residual stresses might be in the raw stock. Assuming a decent amount of stresses, approximately how much "grind stock" should be left prior to heat treating and grinding? thanks for any info.

[liveforever8281]

You've got it in one. That is the correct way to harden your spindle. 0.005"
on the dia should be ample for grinding. One thing to be carefull of is that when the part go's to heat treatment, make sure that they position your part in the oven in a way that will not cause you part to distort.

Regards Peter.

[Glacern]

One more thing, i don't know what kind of residual stresses might be in the raw stock.

Not much if you get the 4140 in hot rolled. Good luck!

[NC Cams]

I disagree, shightly.

After you machine and prior to H/T, subject the shaft to a 1200-1250 Deg F high temp stress relieve for a minimum of 1.5 to 2 hours. This does wonders for removing residual stresses that were in the part when you started machining (continuous cast bar is horrible for this). It also removes the stresses that get generated during machining. If possible, allow to furnace cool to ambient for best results.

Some bar moves around quite a bit during stress relieve and H/T. Try to get them to HANG the bar with the thing hanging from one end axially - do not support it simultaneously on the ends as it may "droop" and you'll never get it straight even when/if they hot straighten it. Any/all quenching should also be done by dropping it in axially as a side of bar quench will warp the shaft. They should EASILY be able to keep the shaft straight within 0.005"tir on centers - if not, find someone better (references given in and around Detroit upon request)

Some steel can get by with 0.005" grind stock - we leave more (up to 0.020" on OD) expecially on longer shafts (stuff does tend to pretzel up occasionaly, expecially if the section thickness varies). It is better to be looking AT material than to be looking FOR material, especially when you're grinding.

We prefer to do the heat treating in a neutral salt bath. Lacking that method, I'd try to get it done in a neutral atmosphere or vacuum furnace - yes you can sand/grit blast if/when it scales up but this can re-introduce stresses that you don't want/need.

There are two levels of cryo, a -40 and one that is like colder and slower than a simple nitrogen quench. Some guys do the "easy one" and special outfits do the latter - ask as it does make a difference in some materials (Hrc 60+) but probably not 4140 at 44-48...

Finally, after you H/T, get the thing black oxide and oil dipped ASAP. This is not essential but it does prevent rusting which does happen quite readily right after H/T. Besides, after you grind it, the part will look like jewelry.

[Oldmanandhistoy]

Hi all,

Hopefully some one won’t mind what might be seen as a dumb question and educate me a little. I understand why steel is heated to stress relieve it but I don’t understand why you would need to harden it. Take the spindle in this discussion; it does not rub on another surface so wear is not a problem? So why do you need to harden it, just a simple explanation will suffice if anyone can spare the time?

Thanks in advance,
John

[NC Cams]

As you harden steel above it's annealed point, you increase the tensile strength of the stuff.

The higher the tensile strength, the higher the loading you can appliy before fatigue starts to develop in the material.

Look at a hardness versus tensile diagram in a heat treat chart and the strength comparison becomes obvious.

Tensile strength is NOT the same as toughness which is NOT the same as rigidity.

Tensile strenth is the ultimate load that can be applied to steel in tension.

Toughness is the amount of energy that can be applied before tensile failure occurs.

Rigidity is essentially the same for ALL steels, regardless of type or heat treat and is more appropriately specified as "modulus of rigidity".

Wikipedia can give you more insight into EXACTLY what these terms mean and/or how they are expressed.

Metal fatigue is the natural degradation in the strenth that develops between the steel molecules at the atomic level. Once fatigue develops, cracks form between and thru the grain structure of the metal - as the crack consumes the remaining sound material, the part will fail catastrophically

[Oldmanandhistoy]

Thank you NC Cams, I obviously have a lot to learn.

John

[mackeym]

When hearing back from one heat treating company for the following heat treatment:
1. Stress relieve
2. H/T
3. Cryo
4. Stress relieve

They were concerned with the last stress relieve (it may undo some of the previous heat treating), and it was suggested to temper rather than stress relieve. They said that tempering is normally performed after cryo. I'm leaning toward a final temper rather than stress relieve. Anyone have any comments on this?

[RICHARD ZASTROW]

I always thought properly executed cryogenic treatment was a form of stress relief. We used to have our rotary dies (D-2 & D-5 Rc 58-62) cryogenicly treated after hardening. This was done in an enclosed heatreating oven in nitrogen atmosphere then as NC Cams mentioned in a cold nitrogen bath. Worked for us. The colder -300F or whatever may indeed require another stress relieving but I would imagine that to be a a long hold at some cold temperature rather than heating again. For a spindle or other longish part I agree also to hang it vertically. Just my 2 pfennigs.

[NC Cams]

Cryo is a way to turn any retained austenite into tempered martensite via "reverse heat treating". The problem/trick is that you have to stress relieve AFTER cryo to relieve residual stress that cryo generates.

Be careful, stress relieve and temper are similar operations but not done at identical temperatures or time cycles.

You harden and quench to make the stuff HARD. You then temper/soften to bring the tougness back to the part. The 4140 will harden to be glass brittle when you quench it but tempering at 250 to 450 (depending on final desired hardness) will bring it to the 44 HRc range being sought.

Now, when you cryo, to take it to -250 to -450 depending on process. After cryo you MUST stress relieve (250 - to 300 deg bake) to prevent cryo empbirttlement.

As long as your "stress relieve/temper" is at a LOWER temp than your temper to reach the Hrc44 value, the "stress relieve/temper" should not affect residual hardness. A 250 or so bake should be adequate for a post cryo stress relieve.

Finally, don't get TOO many folks involved into the heat treat process. The processes are/can be highly interactive. If you do some "my way" and some someone else's, there may be a conflict that results in a bizarre piec of metal when yo0're done.

The method we use and that was outlined above works quite well for highly stressed camshafts - it should work equally as well for a relatively mundane lathe applicaiton. If your heat treater has a better method, by all means listen to him - after all, if the thing gets screwed up in H/T, he's responsible to some degree, especially if he specs the process.....

[mackeym]

ok, sounds good. thanks everyone for the info.