Axle's shafts heat treatment - best choice?

[dvjcodec]

I have a huge problem I need to manufacture few axle's shafts for my projects. I have no problems with milling and machining, but need to know best heat treatment parameters for shaft's ends (splines) and middle. I choose cromo 4340 steel. I browsed web, but no info found

 

[Weasel]

Treat the entire shaft with the same treatment, turn down the shafts after the splines (search spline profile), treatment should be a tempering of some sort. Austempering is ideal if you can make arrangements to have it done. Specifics beyond that I don't have, most HT specifics are kept secret as they take some work to develop. Anyways hopefully that will get you pointed in a better direction.

 

[RoyalXJ]

have them cryo'd

 

[Frank Z]

Cryo?
Don't believe the hype. Conventional wisdom is that there is some benefit with regards to wear resistance. Not really a factor with regards to splines.
http://www.eng-tips.com/viewthread.cfm?qid=112765

http://www.ijest.info/docs/IJEST11-03-05-205.pdf

The process also promotes the precipitation of small carbide particles in tool steels and steels with proper alloying metals. A study in Rumania found the process increased the countable small carbides from 33,000 per
mm to 80,000 per mm. The fine carbides act as hard areas with a low coefficient of friction in the metal that greatly adds to the wear resistance of the metals. Cryogenic processing will not in itself harden metal like quenching and tempering. It is not a substitute for heat-treating. It is an addition to heat-treating. Most alloys will not show much of a change in hardness due to cryogenic processing. The abrasion resistance of the metal and the fatigue resistance will be increased substantially.

For the Original poster:
http://www.eng-tips.com/viewthread.cfm?qid=195760&page=16

http://www.iforgeiron.com/topic/11838-4340-asm-heat-treaters-guide/

 

[RoyalXJ]

They cryo process for axles and gears involves heat treating and cryo treatment

 

[machinisttx]

I don't have a good answer for your particular question, but I do have some info that may be of use.

The critical temperature of AISI 4340 is 1380 degrees Fahrenheit. For hardening, preheat to 1200-1250 and equalize(let it "soak" long enough to reach full temp all the way through), then go to 1475-1500 Fahrenheit and soak 10-30 minutes. Oil quench to hand warm and then temper immediately. Temper at 400-1200 Fahrenheit for one hour per inch of thickness and allow to air cool to room temp. Higher temperatures during the tempering process will result in a lower finished hardness. I don't have specific data on what temperature for a given finished hardness though.

For stress relieving..

Annealed material: 1100-1300F, hold 2 hours, air cool
Hardened material: 50-100F below last tempering temperature, hold 2 hours, air cool.

I found a reference to some AMS specifications, specifically AMS 2759/1 and /2 that may be more detailed. Apparently the files are huge because my computer says it doesn't have enough memory to open them.

I would strongly suggest machining everything close to size(within say, .030"-.040"), heat treating, tempering, and then doing the finish machining. Heat treating will change the part dimensionally to some degree, and in my experience, a close tolerance part will not be usable if machined to finished size before heat treat. I'd also suggest hanging the shafts straight up/down when doing the heat treatment to minimize warping.

Good luck. :thumbup:

 

[Weasel]

cyro adds some wear resistance and maybe 10% strength, some gains but not enough to really be worth the effort in comparison to a good HT. Like said about cyro is really over hyped.

Shot peening would be a good after HT step.

 

[dvjcodec]

thanks a lot guys !