The instantaneous fracture zone of fatigue crack is the area where the material is unstable and suddenly fractured. Due to the continuous expansion of fatigue crack, the effective stress area of the material is continuously reduced. When the stress of the fatigue crack extending to the net section reaches the fracture stress of the material, the material will suddenly fracture. The instantaneous fracture zone is the area formed by the final unstable and rapid expansion of fatigue crack. The figure shows the morphology of instantaneous fracture zone of fatigue fracture of low alloy HT250.

Figure (a) and (b) show the morphology of 50x and 100x of low alloy HT250 fatigue fracture instantaneous fracture zone. It can be seen from the figure that the low alloy HT250 fatigue instantaneous fracture zone has gray color, large roughness, uneven section, large drop and a large number of tear edges (shown by solid line arrow). This is because there are certain “impurities” in cast low alloy HT250 The existence of may weaken the bonding force between grain boundaries and pull up along the grain boundary during the transient fracture of crack instability. A large number of quasi cleavage planes can be found in figure (c), indicating that the fatigue fracture of low alloy HT250 has the characteristics of quasi cleavage fracture. Figure (d) shows that the fatigue fracture of low alloy HT250 has the characteristics of intergranular fracture (shown in the curve). Figure (E) shows the cleavage step and cleavage tongue on the fatigue fracture (shown in solid frame). Due to the complex grain boundary structure between grains, when the crack propagates along the grain boundary, the stress at each position is different, and many microcracks will be formed. The microcracks begin to nucleate and expand here, and finally form cleavage step and cleavage tongue. In conclusion, the fatigue fracture mechanism of low alloy HT250 used in this test is the composite fracture mechanism of quasi cleavage and intergranular fracture.