Look for ideas for material choices, 4140 vs 4340

[djwt]

I am looking for opinion n thebest route to follow in regards to a process and material choice. The part is a guide pin that I need to have a surface hardness of HRC 46, with surface roughness in the range of (Ra) 32 micro inch or better.

It wouldn't be a problem if I could find a shop to do contour grinding but I can't. So I am trying a couple of other options,

1. Use .5" 4140 HTSR, machine the correct profile and try to hold the surface finish on the profile using a high speed mill. Then nitiride the surface.

My concern is the long term where rate of the nitride layer and the dull finish that is assocaite with nitriding.

2. Use either .5" 4140 or 4340 annealed and machine the profile, then heat treat for a hardness of HRC46. During the machining process I would machining .005 under spec and hope the growth or distortion would not be more significant.

My concern with this option is that the growth maybe to great and the stress associate in the material would casuse to much distortion due to the profile cut out of the side of the bar.

I talked to the local heat treater and they mentioned that generally 4140 would be more stable than 4340. But this contradics what I have read online. Is this correct.

I have attached a picture of the part. Should I be concerned with the threads and heat treating?

 

[TVP]

The scalloped section definitely will distort during quenching, it's just a matter of how much. Threads are formed on billions of fasteners every day in the annealed condition and then subsequently heat treated, so the only concern is what type of thread quality do you really need? I would probably choose option 1 and work with a company that uses the Kolene QPQ process, which is a ferritic nitrocarburizing process rather than gas nitriding. The surface finish is excellent because the parts are polished (vibrated in media) after nitriding and before the oxidizing step, which produces a deep, cosmetic black color. Good wear resistance too. Here is a link for some additional info:

http://www.nitromet.com/A55B6F/nitr...burizing, with a Post Process Enhancement.pdf

 

[EdDanzer]

How close of tolerance are you trying to hold?

Austempering or marqunching 4340 can get close 46 RC and if done vertically should minimize distortion.

If you have very many having a plated form grinding wheel made could provide a grinding solution or you could try hard milling the part if you only have a few.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[swall]

If you go with 4340, you could quench in 400F salt instead of oil and minimize the distortion and cracking concerns. If the distortion is still a problem, then make a simple fixture (thick plate with drilled holes) to fit your parts. Oil quench with the parts in the fixture. Or, still going with 4340, you could look at vacuum heat treat with intensive gas quenching.

 

[kclim]

What about using 17-4 PH stainless? Machine to dimensions in the solution treated condition, and age harden to suit (H900 should get you up to 44 HRC, with minimal distortion).