For high strength steels in the upper shelf region, the equation is:

(K/sigma)^2 = 0.64 ((CVN/sigma)-0.01)

where

K = plane strain fracture toughness in MPa m^0.5
sigma = yield strength in MPa
CVN = Charpy V notch value in J

Greg,

    Yes there are correlations, but there is not one simple one.  There are different correlations for the upper shelf, the transition region, and the lower bound.  I could list a few equations, but I feel they would be of no help without you reading all the background text that goes with them.  Most of these correlations are investigated in depth in Chapter 5 of "Fracture and Fatigue Control in Structures: Applications in Fracture Mechanics" by John M. Barsom and Stanley T. Rolfe.

Andy

Greg,

Do you have information on the mechanical properties-- yield strength, tensile strength, impact, hardness, etc.?  I can give you some estimates for SAE 4340, which is similar to EN24 (817M40).  EN26 (826M40) should be tougher for a given strength/hardness level, due to its higher Ni content: 2.30-2.80 vs. 1.30-1.70.

I forgot to inquire about processing-- quenched and tempered?  What is the tempering temperature?  As-rolled?  Anything subsequent like shot peening, etc.?