I fabricated my own broadfork but it's been almost 10 years and i'm sorry to say, I've retained little of the metal info. I did find the receipt from the tine stock I used and it's "1/2 inch 86L20 steel" from Pacific Machinery and tool steel co. in Portland, OR.
While I do use the fork and it works well, the tines will bend if really stressed (rocks, new ground, etc). If I was making it again, i'd use slightly harder steel, but not too hard or they'll crack under stress. I also would have made it a little less burly, it's weight makes it tough to use for a long period of time.
Hope this helps a bit.
-Mark

This post was edited by madroneb on Sat, Mar 1, 14 at 21:08

Thanks Mark, I went ahead and bought a fork, mostly because I couldn't readily make one of the quality and functionality I perceive this one to have. Hope my perception isn't off.

Probably make another , wider with lots of tines for better working of beds that are already pretty soft and for harvesting root crops. I don't anticipate too high of forces on the tines and the metal you suggested would probably work for that.

Here is a link that might be useful: broadfork