Fracture Surface Morphology of the Impact-Loaded Tempered Spring Steels

The objective of this study is to evaluate the surface morphology of 51CrV4 and 55Cr3 spring steels after undergoing tempering and testing at various temperatures, with a focus on dynamic fracture behavior. For this purpose, 51CrV4 and 55Cr3 spring steel samples were normalized at 870°C for 30 minutes for the same initial microstructure. Then, samples were austenitized at 870°C for 30 minutes and rapidly quenched in oil following tempering at 300°C-525°C range for 2 hours. Tensile tests at room temperature were performed to identify tensile properties, especially percent elongation values. Charpy V notched impact tests were carried out at 40, 0, room temperature, and +80°C testing temperatures to examine the fracture surface morphology of steels according to heat treatment procedures and testing (environmental) temperature. The fracture surfaces were examined by micro and macro analysis, respectively achieved by a digital camera and a scanning electron microscope (SEM). Mix mode (ductile and brittle) fracture (quasi-cleavage type) was detected for all quenched and tempered steels. Increasing tempering and testing temperatures resulted in more ductile fractures with many dimple formations and fewer secondary cracks.