Fracture toughness of medium strength steels (A533B, 2.25Cr-1Mo, electroslag welds, cast steels …) with different heat treatments was investigated by using big compact specimens.

The comparison of Charpy V and K_Ic transition curves shows that a very good correlation exists between the transition temperatures defined by these two tests. By defining TK_Ic as the temperature at which K_Ic = 100 Mpa√m and TK 28 as the temperature at which Charpy V-notch (CVN) = 28 J, the relationship is

TK_Ic = 9 + 1.37 TK 28 (°C)

The fact that the slope of this straight line is not one makes the existence of a direct correlation between K_Ic and CVN impossible. Nevertheless, this relationship may be used to predict accurately K_Ic from CVN. If one shifts the K_Ic versus temperature curves in order for TK_Ic and TK 28 to coincide, K_Ic and CVN are related by

K_Ic = 19 (KV)^½(MPa√m, J)

Predicting K_Ic versus temperature curve from the Charpy V transition curve can thus be achieved in several steps.