[TheForge] 4140 vs 4340

[Grant Neil Sarver]

4140 and 4340 both have advantages. 4340 will harden deeper and in
larger cross-sections. When you get past around 1 1/2 inch thick 4140
won't get nearly as hard. "The Book" says that a 1" round 4140 quenched
in oil from 1550 F. will be about 55 Rockwell on the surface and around
50 Rc in the center. A 2" round 49 surface and 38 center and a 4" round
36 surface and 34 center. 4340 on the otherhand, in 4" section will give
53 surface and 47 center.

I prefer working with 4140 but use 4340 when I have to. 4140 is easier
to forge, easier to heat treat and gives wonderful results if the
thickness is under 1 1/2 to 2". 4340 is harder to forge, has heavy, ugly
scale (the nickel content causes this) that is a real bitch to remove,
more difficult to heat treat, but when I need a heavy sections I have to
use it.