17-4 PH shaft?

[brashear]

Hi,
I am designing and making an engine and as such several parts need to be hardened to reduce wear and increase strength. I don't have the capabilities to grind after heat-treating. One way I want to get around this is to use 17-4 PH aged to H900 (482 deg C for 1 hr). My questions are as follows:

-Does aging it to H900 cause any appreciable distortion in the part that would warrant grinding after heat treatment?

-How is the toughness and fatigue strength of H900 17-4PH compared to similar strength conventional heat treated steels? This engine will be used for maybe 100 hours.

I plan to use this material to make the valves, crankshaft, and cams. Thanks.

 

[metman]

You cannot expect to achieve precision dimenions even though 17-4 allows hardening by aging without quench.

But you can readily CNC or manual machine in the solution treated (annealed) condition to net size and age harden for a test using a facsimile geometry of the most severe geometrical change (probably a couple lobes of the crank) and measure to see how much distortion you get. It just might work.

But don't spin yer wheels until you get some confirmation from other posters here.

I don't have the spec's on toughness and fatigue here at home but suspect numbers to very good. Nickel content helps a bunch(psuedo technical term).

 

[mfgenggear]

out source the grinding, it will be a mistake not grinding bearing diameters.

theres lots of good shops out there will happily take your work.

machine all finished extreamly close dimensions after age.
rough or semi finish machine prior to age.

 

[brashear]

the problem with that is my cams are extremely small (1/8" lift on 1/4" shaft) with an concave shape for a section of it. Do cam grinders use CNC grinders with very small grinding wheels?

Maybe annealed 17-4PH for the cam is good enough as I'm using roller followers and not expecting a long life...

 

[tbuelna]

brashear,

17-4PH at H900 cond will likely not yield adequate surface contact fatigue capacity with the high localized stress a roller follower will produce, even at very low loads.

If you cannot grind after hardening, you might consider using a nitriding steel like 4140, 4340 or nitralloy. The nitriding will give a hard case and will produce very little distortion. The nitrided case will be much thinner than a carburized case, but should still give much better life with a roller follower than 17-4PH H900. The only finish operation the nitrided cam surface will require is a careful hand polishing to improve the surface roughness and remove the white layer.

With regards to cam grinding, most conventional radial cam grinders utilize large diameter wheels in order to reduce the amount of dressing required. If your cam profile is partially concave, the grinding wheel would need to be slightly smaller than the minimum rad. of curvature in the concave portion. If this requires a very small (<1 inch)diameter grinding wheel, then your only likely option would be a jig grinding process. But even jig grinding may not be possible, depending on the arrangement of your camshaft.

Good luck.
Terry

 

[unclesyd]

In addition to the suggestion by tbeuelna to change material and nitride you can still make the part from 17/4 H900 and nitride it. The is a process that gives an excellant, in fact one of the best, wear surfaces on 17/4 called QPQ.

I'm giving you the information on Nitromet who is part of the Kolene corporation who I worked with on numerous projects.

Go to this website and drill down through Technical Resources>Brochures. There several papers describing the process.

http://www.nitromet.com/

 

[brashear]

Thanks for the advice everyone.

I had actually considered nitriding, but didn't think it was feasible because I have literally 5 very small parts to do (it's for personal use). How much does this process usually cost?

BTW for the shaft that's going to be acting as the inner race for the roller bearing, it's a one-way locking type. I am only transmitting max 1Nm with a 3/4" bore, which is much lower than what it's rated for.

As for the cam, I guess I will do a contact stress analysis based on the Hertz procedure and see what I end up with.

For the valve, I don't think hardness matters except for accelerated wear, which I can deal with.

 

[metman]

This engine will be used for maybe 100 hours.

It seems like surface hardening is overkill for such a short term run but yes do the contact stress analysis to confirm if needed. I agree with grinding for bearing fits.

Steel Selection by Kern and Suess has some good info on case hardening calculations.

 

[tbuelna]

brashear,

For nitriding, you'll pay for a lot charge plus time in the furnace. You should be able to get your parts gas nitrided for somewhere around $300 to $500. It will cost much less if the parts can be run with someone else's parts.

To put your cam loads in perspective: 100 hours at 1500 valvetrain load cycles per minute (or 3000 crank rpm) equals 9x10^6 cycles. 17-4PH H900 would only be good for about 60KSI in shear at that number of cycles, and sub-surface shear stress is usually the limiting factor in hertzian contact.

Regards,
Terry

 

[unclesyd]

Gleaned this information from another thread from a post by RPstress and QTAFF about using Ferrium C61 but from the Questek literature it looks like C69 would be the one for your application.

http://www.questek.com/producthead.shtml

http://www.latrobesteel.com/assets/documents/datasheets/Ferrium_c61.pdf

tbuelna,
I would advise brasher to check around with shops that do the QPQ process as I've gotten some small lots and one offs done at very reasonable prices, less than $100.

 

[mfgenggear]

Brashear
17-4PH using bodycotes malcomize process works very well for nitriding, parts are very stable there is slight movement if parts are not stressed relieved properly prior to Nitride.

I still recommend grinding after nitride but you may obtain limited service by not grinding. any bearing journal should be ground because it's easy to do. make sure to add centers to your shafts. this way it's easier to inspect & grind.

with CNC grinding or some one out there that does cam ginding or jig grinding should be able to do the job.

I have ground special cams on standard cylindrical grinders by adding extra lengths on the ends & held with caps that had off set centers.

 

[unclesyd]

I have never had to regrind any parts treated with the QPQ process with the exception of some collets for our in house pin grinders.

I agree that the malcomizing process is fine for this application and as for the regrinding of the my parts it's been too long ago to recall without my notes.