Abstract
The fatigue and fracture toughness of 40CrNiMoA steel forging rotary bending were studied in this paper. Experiments were done to compare the influence of both on the material. The results show that the fatigue strength of steel forging under rotary bending is evidently improved. When the rotary bending times reach to the endurance limit, the fracture toughness of the steel forging is enhanced significantly.
Introduction
40CrNiMoA is a kind of steel forged by hot-working, which possess excellent comprehensive properties such as high strength, toughness and fatigue properties. Therefore, it has been widely used in the fields of automotive and engineering machinery. There are many forging processes available for 40CrNiMoA, though its forging requirement is much higher than common metals. The most common forging process for 40CrNiMoA is rotary bending. It is well known that the static performance of 40CrNiMoA can be greatly improved if the rotary bending process is adopt, because of its tendency of creating plastic deformation area, which are beneficial for enhancing mechanical performance.
However, the effects of rotary bending on the fatigue and fracture toughness of 40CrNiMoA steel forging have not been widely discussed and studied in the research of steel forging. Therefore, this study aims to investigate the influence of rotary bending on the fatigue and fracture toughness of 40CrNiMoA steel forging. An experiment with six groups conducted to compare the variation of fatigue and fracture toughness on 40CrNiMoA steel forging before and after rotary bending.
Experimental Method
In the experiment, a circular specimen with the diameter of 25 mm and the thickness of 15 mm were cut off from the middle part of a 40CrNiMoA steel forging. Then the specimens were prepared into 6 groups with different bending times. The specimens of each group was bent to 180-degree angle at a rotary angle speed of 20°/s for the given times of bending according to the requirements of test.
Test machines IH-JL-50 and IH-JL-300 were used to test the fatigue and fracture toughness of all the specimens. Firstly, in order to get the fatigue strength of the specimens, a fatigue test was conducted with a maximum load of 10 kN and a rotation speed of 500r/min. Then, to test the fracture toughness of the specimens, a Kunz-LS-50 machine was employed to conduct a fracture toughness test, under a tensile force of 200 kN and a maximum displacement rate of 50 mm/min.
Results and Analysis
Figure 1 displays the change trends of the fatigue strength of 40CrNiMoA steel forging after the times of rotary bending. As the figures shows, the fatigue strength of 40CrNiMoA steel forging increases evidently with the increase of the bending times. The specimen group of 180-degree bending have the highest fatigue strength.
Figure 2 reveals the variation of the fracture toughness of 40CrNiMoA steel forging after the times of rotary bending. As the data shows, the fracture toughness of the specimen was obviously improved with the increase of bending times. The fracture toughness of the specimen group of 180-degree bending is considerably higher compared with those of other specimen groups.
Conclusion
In the present experiment, the influence of the fatigue and fracture toughness of 40CrNiMoA steel forging under rotary bending was studied. Both the fatigue strength and the fracture toughness of the specimen are obviously enhanced with the rotary bending times. When the bending times reach to the endurance limit, the fracture toughness is increased significantly.
The results of this study could provide a possible means for improving the fatigue and fracture toughness of 40CrNiMoA steel forged parts by rotary bending. Further research on the influences of the process parameters on the fatigue and fracture toughess of 40CrNiMoA steel forging should be explored in the future.
