Variation of the fracture mode in temper embrittled 2.25 Cr-1 Mo steel

Temper embrittled 2.25 Cr-1 Mo steel was tested by slow bending of notched specimens at various temperatures, and the fracture mode was examined by SEM fractography. Comparison of the local fracture mode with the load-displacement curves showed that intergranular fracture occurred most prominently in the region where cracking initiated, but that the fracture mode tended to change to cleavage as the cracking propagated and accelerated. When the area fraction of intergranular fracture was plotted as a function of test temperature, a maximum appeared, and the temperature of this maximum tended to increase with specimen hardness. It is argued that the gap between the cleavage fracture stress (σ _F ^CL ) and that of intergranular fracture (σ _F ^IG ) was greatest at some particular temperature, allowing a maximum amount of grain boundary fracture. However, the gap (σ _F ^CL -σ _F ^IG ) diminished as cracking accelerated, and the fracture mode tended to switch to cleavage. The contrast in behavior between temper embrittled CrMo and NiCr steels is discussed.