Abstract：In order to gain insight into the mechanical behaviors of China’s high-speed EMU (electric multiple unit) axles made of 30NiCrMoV12 and EA4T，both the in-serving and over-served axles were examined in their chemical composition，conventional tensile strength，fatigue，impact toughness，fracture toughness，the threshold value and rate of fatigue crack growth and the metallographic structure. The final results are shown as follows：(1) The 30NiCrMoV12 axles had 10 times higher Ni element content，2 times higher Mo and V elements content，and slightly higher C element content than the EA4T axle. The conventional tensile strength，yield strength and fatigue strength were increased by 34%，54%，and 30%，respectively. (2) Compared with the EA4T axle，the conventional impact energy of the 30NiCrMoV12 axle at room temperature decreased by 12%; however，its fracture toughness increased by 34%. The EA4T axle had a 12% higher threshold value of fatigue crack growth than the 30NiCrMoV12 axle，and as for the two types of the old axis，this feature are quite similar. (3) When the stress intensity factor was less than 50 MPa·m1/2，the crack growth rate was greater than the EA4T axle; However， when it was larger than 50 MPa·m1/2，it was on the contrary. (4) The grain size of the full section microstructure of the 30NiCrMoV12 axle was fine bainite and tempered martensite with good hardenability and manufacturing performance. However，the section of the EA4T axle was composed of uniform bainite and tempered martensite within the depth of 30 mm from surface，and the ferrite gradually appeared with depth increasing. When the depth increased to 60 mm，the microstructure consisted of pearlite and ferrite，meanwhile the volume fraction of ferrite was significantly higher than that of pearlite.
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