[JamesCH95]

Here's a Datasheet on AISI 01 Steel: http://www.act-on.ca/acton/Steel/Ima.../AISI%2001.pdf

It outlines different heat-treating procedures. Usually one would send parts out to a heat treating shop.

Look at this hardness-to-strength conversion chart for steels to chose the hardness you would like to achieve through heat-treating: http://www.engineersedge.com/hardness_conversion.htm
You'll pick a range for the heat-treater to aim for. My experience with 4130/4340 steel (which is commonly used in high-performance driveshafts) has been that Rockwell C 25-30 is plenty strong with good ductility. If you harden the part too much it will crack before it yields much, and I would much rather have a part yield and still be functional than crack and be useless. If the shaft is experiencing frequent impact loading, a more ductile or less-stiff material would handle those loads better.

@Tristan: why did you (your team) go from 1018 steel to the somewhat weaker 12L14 steel? EDIT: I guess it depends on the grade of each steel, one could be stronger than the other, depending on heat-treatment and cold working.