Effect of Ultrasonic Impact and Nitriding Composite Strengthening on Microstructure and Fatigue Properties of 45 Steel

The surface of 45 steel was treated by ultrasonic impact and plasma nitriding at 540 ° C for 24 h. The microstructure of the surface layer, tensile properties and fatigue properties of the test steel after ultrasonic impact and after ultrasonic impact + nitriding composite strengthening were studied. The results show that after ultrasonic impact the number of small-angle grain boundaries in the surface layer of the test steel increased significantly and was mainly distributed in the pearlite region. The area between pearlite and ferrite formed large-angle grain boundaries. The average grain size of ultrasonic impact test steel in the range of 0–140 μm from the surface was 3.24 μm, which was significantly lower than that of the untreated test steel (8.12 μm). The maximum extreme density of the ultrasonic impact test steel was less than that of the untreated test steel, and the texture type changed from (001) texture to T-texture. The surface layer of the ultrasonic impact test steel was composed of a strong plastic deformation zone with a thickness of about 20 μm and a micro-deformation zone with a thickness of about 100 μm. After the composite strengthening, a typical nitrided layer was formed on surface of the strong plastic deformation zone. The nitrided layer consisted of a compound layer with a thickness of about 8 μm and a diffusion layer. The tensile strength of ultrasonic impact test steel (830 MPa) was similar to that of the composite strengthened test steel (833 MPa), and the percentage elongation after fracture (19.8%) was slightly greater than that of composite strengthened test steel (17.6%). The ultrasonic impact test steel underwent ductile fracture, while the composite strengthened test steel underwent mixed ductile and brittle fracture. The ultimate fatigue strength of the composite strengthened test steel was about 310 MPa, which was higher than that of the ultrasonic impact test steel (297 MPa), indicating that the ultrasonic impact+nitriding composite strengthening process could further improve the fatigue strength of 45 steel.