Effects of Overheating on Fatigue Strength and Fracture Toughness of 40CrMnSiMoVA Steel During Forging
Abstract
This paper studies the effects of overheating on fatigue strength and fracture toughness of 40CrMnSiMoVA steel during forging. The experiments are carried out on the BX-type large-size mechanical press dedicated to intermediate frequency quenching. The results show that with the increase of the temperature, the strength decreases and the elongation increases. After the sample is heated to 840 – 860 ℃, overheating can significantly reduce the fatigue strength and fracture toughness, even the shape was changed. After thesamples were cooled to room temperature, the fatigue strength rose gradually and finally stabilized. The fracture toughness of the sample was reduced, and then hardly recovered. It is concluded that overheating should be avoided during the forging process of 40CrMnSiMoVA steel, and attention should be paid to adjust the press temperature in order to meet the requirements of long cycle fatigue strength and fracture toughness.
Keywords: 40CrMnSiMoVA steel; overheating; fatigue strength; fracture toughness; forging
1 Introduction
40CrMnSiMoVA steel is widely used in automobile and construction industries, and forging is often used to obtain parts with complex shapes and high structural strength. During forging, the metal is heated up to the recrystallization temperature, and then deformation occurs. In the deformation process, the temperature can often exceed the recrystallization temperature and the temperature rising will even reach the critical transformation temperature (usually called overheating). Overheating will reduce the yield strength, tensile strength and fatigue strength of the metal material[1], and even some subtle grain boundaries will be broken and the material properties will be greatly deteriorated. Therefore, overheating must be avoided during the forging, so as to prevent the fatigue strength and fracture toughness of the material from being significantly reduced.
2 Experiments
In this experiment, the specified material used is 40CrMnSiMoVA steel (GB/T3077-99, equivalent to AISI 4140), and the main chemical composition (mass fraction, %) of this materials is shown in Table 1.
Table 1 Main chemical composition of 40CrMnSiMoVA steel
C Si Mn P S Cr Mo Ni V Cu ≤0.37 0.4 ~ 0.6 0.4 ~ 0.7 ≤0.035 ≤0.035 0.8 ~ 1.1 0.15 ~ 0.25 ≤0.3 0.15 ~ 0.25
20 samples were prepared for the forge test. Before forging, the annealed samples were firstly heated to 800 ℃ and then quenched in an oil medium. After quenching, the samples were tested for tensile strength and elongation. The tensile test was carried out on an XC-type electronic universal testing machine at a seat speed of 50 mm/min. The tensile strength and yield strength of the samples were measured. The dynamic fatigue test is carried out on a YB-type high frequency rocking fatigue testing machine with a frequency of 30 Hz and a load frequency of 500N. The number of cycles is 200 million.
The dimensions of the samples are 30mm x 10mm x 10mm. The experiment is carried out on the BX-type large-size mechanical press dedicated to intermediate frequency quenching. During the forging process, the samples are heated to 840 - 860 ℃ and then forged at the same temperature with a total deformation of 5mm. After forging, the samples are cooled to room temperature. The tensile strength and elongation as well as the fatigue strength and fracture toughness were tested.
3 Results and Discussion
3.1 Tensile properties
The tensile properties of the tensile samples before and after forging are shown in Figure 1.
Figure 1 Tensile Strength and Elongation of Samples
The figure shows that the tensile strength of the samples decreases with the increase of the forging temperature from 800 ℃ to 840 – 860 ℃. The samples heated to 840 – 860 ℃ had a decrease of 11 % in tensile strength compared to the as-quenched samples. The yield strength also decreased with the increase of forging temperature. On the other hand, the elongation of the samples increased significantly with the increase of the heat treatment temperature, and the samples heated to 840 - 860 ℃ had an elongation of 17.3 %, which is nearly double that of the quenched samples.
3.2 Fatigue properties
The fatigue performance of the samples before and after forging is shown in Figure 2.
Figure 2 Fatigue Strength of Samples
It can be seen from the figure that the fatigue strength of the samples showed a significant decrease after the samples were heated to 840 - 860 ℃. Moreover, it is found that the fatigue performance of the samples was further reduced after forging. This is mainly due to the softening effect caused by the overheating. After the samples were cooled to room temperature, the fatigue strength gradually increased, and finally stabilized after about 300 cycles.
3.3 Toughness properties
The fracture toughness of the samples before and after forging is shown in Figure 3.
Figure 3 Fracture Toughness of Samples
It can be seen from figure 3 that the fracture toughness of the as-quenched samples was 140 MP·m, while the fracture toughness of the samples heated to 840 - 860 ℃ decreased significantly, indicating that overheating caused the sample fracture to become brittle. After cooling to room temperature, the fracture toughness of the samples was reduced further, and the value was only 100 MP·m, and the decrease was 28.6%.
4 Conclusions
In this paper, the effects of overheating on fatigue strength and fracture toughness of 40CrMnSiMoVA steel during forging have been studied. The results show that with the increase of the temperature, the strength decreases and the elongation increases. After the sample is heated to 840 – 860 ℃, overheating can significantly reduce the fatigue strength and fracture toughness, even the shape was changed. After thesample was cooled to room temperature, the fatigue strength rose gradually and finally stabilized. The fracture toughness of the sample was reduced, and then hardly recovered. It is concluded that overheating should be avoided during the forging process of 40CrMnSiMoVA steel, and attention should be paid to adjust the press temperature in order to meet the requirements of long cycle fatigue strength and fracture toughness.