I think you will find that the leaf and coil springs these days are
something like 5160, which is a lot fancier than a simple high carbon
steel. It is a 0.60% Carbon steel with about 1% Manganese and about 1 %
Chromium steel.
You would want to oil qunech, not water quench when hardening.
Still a good choice for many cutting tools. If you find really old
car springs (30's or 40's and older) thay may be plain carbon steel).
I have been told that GM has used 5160 pretty much exclusively for many
many years.
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wrote:

Car leaf springs are high carbon steel as well as coil springs.