Here's a short overview of the steel groups used in high-tensile applications such as axle shafts.
This is the carbon steel group.
Most standard axles are made of 1040 grade, generallz heat treated to 40 Rockwell
in the center and 55-60 Rockwell in the outer layer of about .180 inch. They
have about 1-1.65% manganese.
The manganese steel group which
contains between 1.45 and 2.05% of manganese. Many aftermarket manufacturer
use this as the higher manganese content allows for more flexibility during
the heat treating process.
These steels contain a small amount
of silicon but otherwise are similar to a standard type. Silicone adds a measure
of hardness. Silicone additions are in the .15-.30% range.
The nickel-chromium-molybdenum group. They contain between .65-.95% chromium, .2-.3% molybdenum and 1.55-2.0% nickel. Chromium increases hardness and elastic limits as well as corrosion resistance. Molybdenum and nickel also increase hardness.