Material for halfshafts?

It is what I used for customers and my own DSR and have had no problems when I was running it

 

For aerospace materials you should download a copy of Mil Handbook 5. It is available on the web and it costs nothing. It has all of the properties of most popular aerospace materials and it is something that anybody doing race car engineering shoudn't be without.

For conversion of Rockwell hardness to yield strength, here is a link to a DTIC report that provides a conversion from Rockwell C to yield strength....

http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA016166&Location=U2&doc=GetTRDoc.pdf

The 4340 tha that you are using has a yield strenght of about 130 ksi at Rc 30. I would prefer to use it in a quenched and tempered condition since that way you know what you have. Pre-heat treat isn't a good idea unless it is specified as being quenched and tempered. The low alloy steels can be heat treated to pretty high strength, and you should think about what you will be doing with the car. A DSR is going to have a lot more downforce than a FF. If you have more power then you will put more load into the half shafts.

Do a quick torsional stress analysis, first gear ratio x the max engine torque, x axle ratio to get the max torque you can expect at the shaft. Calcualte the stress using stress =tr/j where t= the torque, r is half the diameter, and J is the section modulus. Remember that torsional yield strength is about 1/3 of tensile. Look at what kind of safety factor you want because there is going to be a fatigue aspect and that should give you a good idea if you are in the right ball park.

 

Ask this company. http://driveshaftshop.com/