Effect of Ti on Charpy Fracture Energy and Other Mechanical Properties of ASTM A 710 Grade B Cu-Precipitation-Strengthened Steel

Addition of titanium (Ti) to ASTM A710 Grade B Cu-precipitation-strengthened steel significantly increases the impact absorbed fracture energy and reduces the ductile-to-brittle transition temperature. The effect of Ti correlates with the reduction of the amount of pearlite in the ferritic microstructure. A thorough study of the mechanical properties of Ti-modified A 710B steel is presented. Introduction The ductile to brittle transformation in steels depends on interplay of the fracture stress and the flow stress. In steels the flow stress depends on temperature and strain rate because the motion of screw dislocations is function of temperature and strain rate. In this work the fracture stress has been assumed essentially independent of temperature and strain rate. At high temperatures and low strain rates thermal energy is sufficient to give plastic flow at stresses below the fracture stress, but this is not so at low temperatures and strain rates; thus there is ductile to brittle transformation temperature as shown in Figure 1. The stress (Peierls stress) to move a long dislocation segment from a deep crystallographic energy valley would be very large. J. Weertman proposed many years ago that a high Peierls energy dislocation would likely move by first forming a double kink [1]. In the BCC metals the kink edges would be in the edge orientation and thus very mobile. Later he suggested that a misfit center would interact with a dislocation to help pull it from its Peierls energy valley [2]. This is depicted in Figure 2. Such a misfit center will both give increase in yield stress at elevated temperatures where thermal energy is sufficient to nucleate double kinks along the dislocation line but decrease the yield stress at low temperatures where thermal energy is small. The misfit center helps the applied stress displace the dislocation escape from the deep energy valley. This behavior is depicted in Figure 3.