In general forged crankshafts are stronger then cast crankshafts. As always, and in particular for the Chevrolet 250 the past application of the design and materials is important. Per Leo Santucci' "Chevrolet Inline Six-Cylinder Power Manual" the forged steel crankshafts, '63 - 64', had narrower crank arms than the later cast units and six counterweights. The nodalur cast iron '64-'66 crankshafts had six counterweights. The nodular cast iron '67 and later crankshafts had 12 counterweights. So for the Chevrolet 250, in the previously built forged versus cast crankshaft comparison, other significant factors are important.
AISI 4340 is a strong, tough, steel. On of the standard super steels. In general, when you need something stronger and tougher than the standard chromly's, though technically it is a type of chromoly. Good for axles, shafts, landing gear, connecting rods, etc. It is tougher to machine than the lower strength alloys steels.
4340 (chromoly) Normalized Alloy Steel
Minimum Properties Ultimate Tensile Strength, psi 186,000
Yield Strength, psi 125,000
Elongation 12.2%
Rockwell Hardness B100
Chemistry Iron (Fe) 96%
Carbon (C) 0.37 - 0.43%
Chromium (Cr) 0.7 - 0.9%
Manganese (Mn) 0.7% max
Molybdenum (Mo) 0.2 - 0.3% max
Nickel (Ni) 1.83%
Phosphorus (P) 0.035% max
Sulphur (S) 0.04% max
Silicon (Si) 0.23%
In comparison to AISI 4130
4130 (Chromoly) Normalized Alloy Steel
Minimum Properties Ultimate Tensile Strength, psi 97,200
Yield Strength, psi 63,100
Elongation 25.5%
Rockwell Hardness B92
4130 (Chromoly) Annealed Alloy Steel
Minimum Properties Tensile Strength, psi 81,200
Yield Strength, psi 52,200
Elongation 28.2%
Rockwell Hardness B82
Chemistry Iron (Fe) 97.3 - 98.22%
Carbon (C) 0.28 - 0.33%
Chromium (Cr) 0.8 - 1.1%
Manganese (Mn) 0.4 - 0.6%
Molybdenum (Mo) 0.15 - 0.25%
Phosphorus (P) 0.035% max
Sulphur (S) 0.04% max
Silicon (Si) 0.15 - 0.35%
