Study On Very High Cycle Fatigue Properties Of High Strength Steels For Torsion Bar Spring

45CrNiMoVA steel has ultra-high strength of over 2000MPa after quenching and low-temperature tempering,and it is often used as torsion bar spring,but its early failure due to fatigue and hydrogen embrittlement sometimes occurs.To this end,a new high-strength steel 40Si2Ni2CrMoV?N1?for torsion bar springs with higher fatigue performance and lower hydrogen embrittlement sensitivity has been developed in recent years.In order to ensure the high reliability and long life?>10⁷?requirements of high-strength torsion bar springs,this work is aimed to compare the very high cycle fatigue properties of 45CrNiMoVA steel and N1 steel with and without by using ultrasonic fatigue test,electrochemical hydrogen charging method,TDS measurement and SEM fracture observation,and the mechanism of very high cycle fatigue crack initiation and propagation was also discussed.45CrNiMoVA steel and N1 steel showed very high cycle fatigue characteristics after10⁷ cycles,and the fatigue limits at 10⁹ cycles were 682 MPa and 692 MPa,respectively.The fatigue life of N1 steel was significantly longer than that of 45CrNiMoVA steel at the same stress level.The very high cycle fatigue of the experimental steels initiated at the large-sized non-metallic inclusions inside,and the crack grew very slowly around the inclusions to form the GBF zone.The very high cycle fatigue life mainly depends on the crack initiation in the GBF zone,and the crack initiation life accounts for 80%-99.9%of the total life.Compared with the 45CrNiMoVA steel,the crack initiation rate in GBF zone of N1 steel is lower,so its fatigue life is higher.The very high cycle fatigue life of 45CrNiMoVA steel and N1 steel after hydrogen charging decreased significantly.With the same hydrogen content,the very high cycle fatigue life of N1 steel was still higher than that of 45CrNiMoVA steel.After hydrogen charging,the very high cycle fatigue cracking mode was not changed,but the morphology of the crack initiation zone around the inclusions was changed,and the GBF zone characteristics became shallow or even disappeared.Hydrogen reduces the stress intensity factor threshold?K_GBF of crack initiation in the GBF zone of the test steels,and accelerates its initiation rate,resulting in a significant reduction in fatigue life.Compared with 45CrNiMoVA steel,the diffusion coefficient of hydrogen in N1 steel is relatively lower,and the crack initiation rate in GBF region is lower,so its fatigue life under hydrogen conditions is also longer.