Carbon High Strength Spring Steel Corrosion Fatigue Failure Behavior

High strength is an eternal theme in steel materials research and development. After strength and hardness increases, the susceptibility of gap increase. Because many spring worked in poor working environment, Corrosion occurred from time to time, destroyed the integrity of the spring surface, Fatigue properties dramatically reduce. So how to improve of the corrosion fatigue properties for the high-strength spring steel is became an urgent problem.In this thesis, the corrosion fatigue fracture behaviors of newly developed 2000 MPa grade medium-carbon spring steel U and emphasis researched into different alloy elements Ti and Ni content effect on corrosion fatigue properties and corrosion resistance of U were studied by rotating-bending fatigue test,scanning electron microscopy(SEM)and salt spray corrosion test etc. Conventional spring steel 60Si2MnA was also used for comparison.The U steel has higher fatigue strength (s_-1(N_f=10⁷ cyc)=910 MPa than that of conventional spring steel 60Si2MnA and s_-1(N_f=10⁷ cyc)=780 MPa) , the S-N curves display continuous declining type, the fatigue failure mainly initiated from specimen surface at high stress amplitude and the fatigue failures mainly initiated from near surface inclusion at low stress amplitude; 60Si2MnA steel all fatigue failures initiated from specimen surface both at high and lower stress amplitude. The corrosion fatigue failure of Two tested steel both from surface corrosion pits, so, control corrosion pit generated and growth is very important for improve the corrosion fatigue properties of spring steel.The steel U which with different Ti and Ni content was studied by using accelerated corrosion test,electrochemical test and corrosion fatigue test. The results revealed that corrosion weight loss and corrosion pit depth after different cycles of corrosion all reduced, and the pitting potential in 5 percent NaCl aqueous solution increased. It indicate that addition Ti and Ni is beneficial to improvement of corrosion resistance for tested steels. Furthermore, the corrosion fatigue strength was gradually enhanced along with the content of Ti and Ni increase. It is concluded that Ti and Ni can increase corrosion fatigue property of the tested steels via improve corrosion resistance.