Hardenability curves for
five different steel alloys, each containing
0.4 wt% C. Approximate alloy compositions
(wt%) are as follows: 4340—1.85 Ni, 0.80 Cr,
and 0.25 Mo; 4140—1.0 Cr and 0.20 Mo; 8640—
0.55 Ni, 0.50 Cr, and 0.20 Mo; 5140—0.85 Cr;
and 1040 is an unalloyed steel.