What material to use for geared shaft

[Sunset Machine]

It doesn't sound like something that has been hardened. Take a file to things and see, and look for signs of grinding. If it's hardened you want, then try the 414x HT/PH/"half-hard" series. Something easier to work might be Stressproof. It doesn't seem to resist twisting very well, but it might be just the ticket for this.

http://www.speedymetals.com/information/Material63.html
http://www.speedymetals.com/information/Material37.html
http://www.speedymetals.com/information/Material32.html

I wouldn't want to turn an expensive bar of 1-1/2" whatever down to 3/4". f350ca is right-on with his suggestion of a weldment if you can make it work.