An investigation into the mechanism of cleavage fracture in a dual phase steel

The cleavage fracture behavior in notched and precracked specimens of a dual phase steel was studied as a function of grain size and test temperature. It was found that critical cleavage fracture stress sigma(F) is roughly proportional to the reciprocal square root of bainite packet size, but the dependency is weaker in comparison with other types of steels. The sigma(F) is relatively temperature independent within the test temperature range. The cleavage fracture toughness K-IC decreased with increasing grain size, but rose again after passing a critical grain size, leading to a minimum in K-IC-grain size dependence. A strong temperature dependence of K-IC was observed at higher test temperature near the transition temperature of cleavage-dimple fracture. To explain the observed behaviors, a detailed fracture process observation was conducted by using acoustic emission monitoring, cross section observation and fractography on interrupted test specimens. It was found that the cleavage fracture mechanism could change with the type of test specimen and test temperature. The grain size and temperature dependencies of yield stress, plastiscity effect, and changes in fracture mechanisms all contributed to observed behaviors. A comparison was made with major available theories and a modeling methodology is suggested to better explain the experimental phenomena.