Gigacycle Fatigue Properties of Hydrogen Charged High Strength Steels

Gigacycle fatigue tests were conducted for hydrogen charged SCM440 low-alloy and SUJ2 bearing steels prepared with a double-melting method to improve their cleanliness. In case of the SCM440 steel, 200 and 550 degrees C tempered samples were prepared. The fatigue tests were conducted mostly using an ultrasonic fatigue testing machine. In case of the SUJ2 steel and the 200 degrees C tempered version of the SCM440 steel, fish-eye fracture occurred both in base and hydrogen charged steels, and the hydrogen charged steels showed large drops of fatigue strength. The 550 degrees C tempered version of the SCM440 steel revealed only surface fracture below 10(7) cycles and the fatigue strength was almost equal between the base and hydrogen charged steels, while the hydrogen charged steel showed fish-eye fracture above 10(8) cycles, decreasing the fatigue strength. These results meant that the effects of hydrogen were large on the fish-eye fracture properties, while they were small on the surface fracture properties. The degradation of fatigue strength was caused by diffusible hydrogen absorbed in the matrix. Although the SUJ2 steel revealed non-diffusible hydrogen trapped by insoluble carbides, the fatigue strength of the SUJ2 steel was almost equal to that of the 200 degrees C tempered version of the SCM440 steel. This meant that the non-diffusible hydrogen trapped by insoluble carbides had negligible effects on the fatigue strength.