Comparative qualities of steels

[Doggitter]

I just know I'm going to get grief for this, but I know not where else to ask.

I have a reasonable beginner's knowledge of how hot rolled, 8620 and 4140 behave when working them, and somewhat of their strengths. I also have a desire to make a small differential carrier for a project. Carrier for me means the housing the ring gear bolts to, and that the spider and side gears are inside. We use a lot of 8620 and 4220 at work and I would easily be able to get a rem of 8620, and especially 4220 to do this job. Can someone give a comparison between the 4220 and 8620, using 4140 or hot rolled as examples also?
I have access to the numbers, I'm looking more for " X is tougher than Y, but wears better... yadayada", or whatever descriptions may help me? Maybe there is a clear cut top choice for this job? Or perhaps the qualities requirements are so meager compared to the steels at hand it's a meaningless question? I don't know. The original was cast, from an import car in the 90's.

I hope I'm not making a mess of the question. And, thank you.

[Smrtman5]

My understanding is:

8620 has a high nickle content (compared to other steels) and is a carburizing or nitriding alloy. Therefore a high case hardness is obtainable, which is good for wear resistance.

4140 is a fully heat treatable alloy, therefore it can be hardened all the way through. It is also more resistant to shock loading. 4340 is even better than 4140 for shock loading applications. (Yeild of ~100,000psi @40Rc, i believe)

Not familiar with 4220 alloy.

$.02

[Perfect Circle]

I would go with 4140 IMO its just a good all around steel for the parts that you want to make.
Good Luck~!

http://www.suppliersonline.com/propertypages/8620.asp
http://www.metalsuppliersonline.com/.../metals/25.asp

[RICHARD ZASTROW]

Are there any surfaces (bearing surfaces etc.) that actually require hardening? If not, I'd look at toughness and resistance to distortion

If it's just a rigid part, I like to use 4140 pre-hardened steel. It's machineable at 28-32 Rc and stable.

Dick Z

[Doggitter]

There are two bearing fits for tapered roller sets, and surfaces for the friction discs to wear on, neither were hardened on the original. No welding will need to be done, nothing except machining. It runs in gear oil so all surfaces should be lubed under use also.