Mechanical response and dislocation substructure of high strength Ni-Cr-Mo steel subjected to impact loading

This work focuses on investigating the mechanical response of high strength Ni-Cr-Mo steel (AISI 4340) subject to impact loading, and the relation between its properties and its dislocation substructure. The material was quenched (900°C/30 min.) in oil and then tempered at 550°C for 2 hours to obtain a better tempering martensite structure. The high-strain-rate properties are determined by a series of standard split Hopkinson compression tests at strain rate ranging from 102s-1 to 5x104s-l. The influences of the strain rate on the stress-strain characteristics, work hardening and deformation mechanisms are discussed. Parallel with this work, the deformed specimens are studied with transmission electron microscopy techniques to observe the development of a cell structure and measure the dislocation density and cell size. The relationships between dislocation density, flow stress, and strain rate have been described in terms of the dislocation structure observed.