Influence of Overheating on Fatigue Strength and Fracture Toughness of 40CrMnSiMoVA Steel in Hot Forging
Abstract
This paper theoretically studied the effect of overheating on fatigue strength and fracture toughness of 40CrMnSiMoVA steel in hot forging. The experimental results show that when the overheating temperature is below the Ac3 point, the fatigue strength of the steel is significantly lower than that of normal forging temperature, and the fracture toughness decreases with the increase of overheating temperature. It is concluded that excessive overheating causes deformation softening of the steel, which reduces the fatigue strength and fracture toughness. With the accelerated cooling rate, the fatigue strength and fracture toughness are slightly improved.
Keywords: Hot forging; Overheating; 40CrMnSiMoVA steel; Fatigue strength; Fracture toughness
1 Introduction
Hot forging is a commonly used forming process. In order to improve the deformation properties of steel and its machinability and surface quality, it is necessary to use proper heating temperature. 40CrMnSiMoVA steel is widely used in automotive and aviation fields because of its excellent comprehensive performance. In the process of hot forging, due to insufficient control of heating temperature or other reasons, the actual temperature of the workpiece is higher than the normal forming temperature. This phenomenon is called “overheating”.
Studies have shown that the properties of forgings are significantly affected by the over~heating degree and heating rate. In particular, the fatigue of hot forging parts is the most critical property requirement. Therefore, it is of great significance to evaluate the influence of overheating on the fatigue properties of hot forgings. The fatigue strength and fracture toughness of hot forging materials are two important parameters that affect the performance and service life of parts. Therefore, the aim of this paper is to qualitatively analyze the influence of overheating on fatigue strength and fracture toughness of 40CrMnSiMoVA steel in hot forging.
2 Experiments
The samples in this experiment were 70mm × 70mm × 50mm die blocks of 40CrMnSiMoVA steel, which was specified and provided by suppliers.
2.1 Sample preparation
The samples were first normalizing and then machining. The process parameters of normalizing are as follows: heating temperature is 850°C, holding time is 1.5h, and cooling mode is air cooling. After normalizing, the structure of the samples was evenly tempered sorbite.
2.2 Hot forging
Hot forging tests were carried out by TFL-3000/30MN hydraulic forging machine. Forging temperature is monitored by ICX-2A infrared thermometer. During hot forging process, samples with different overheating degrees were produced. Forging temperature is 650~750°C. the specific experimental conditions are shown in Table 1.
Table 1 The experimental conditions
|Condition |Overheating degree| Forging temperature (°C)|
| :----|:----:|:----:|
|Normal forging|0| 650|
| Overheating A |50| 700|
| Overheating B | 100| 750|
2.3 Mechanical property test
Tensile test and impact test were respectively carried out by universal testing machine YAW-150 and impact testing machine GDTH-100 according to GB/T228 and GB/T229. The fracture toughness of the samples was measured by disc-type sample fracture toughness test machine WFW-50 (the test distance of the hammer pin is 6mm, and the average esposure height of the hammer pin is 1.8mm).
2.4 Fatigue test
The specimens of fatigue test were cut from the 40CrMnSiMoVA steel forged block by wire cutting machine, and the test was carried out on KW-750E bending fatigue testing machine. The test parameters were: loading frequency of 10 Hz, maximum load of 100KN, and the stress ratio R 1 ~ 0.1 and R= -1. The number of load cycles of the test was 4 million times. The test results of each group are shown in Table 2.
Table 2 Test results of different groups
|Group |Yield strength (MPa)| Stress amplitude (MPa)|Number of load cycles (million)|
|:----:|:----:|:----:|:----:|
| Normal forging| 770.77|120.33| 4.00|
| Overheating A |731.3| 123.8| 3.50|
| Overheating B |692.07|127.82|3.00|
3 Results and Discussion
3.1 Effect on mechanical properties
The tensile test results are shown in Fig.1. It can be seen from the figure that the yield strength of the three groups of materials is 770.77MPa, 731.3MPa and 692.07MPa, respectively. It shows that with the increase of overheating temperature, the strength of the material decreases and the plasticity decreases. The impact test results are shown in beginning Fig.2, which shows that the impact test results of three steel samples are above 55J, and the absolute impact energy drops with the increase of overheating temperature. These results confirm that overheating of 40CrMnSiMoVA steel reduces its mechanical properties and plasticity.
Fig.1 Yield strength of different groups
Fig.2 Impact test results of different groups
3.2 Effect on fatigue strength
The results of fatigue test on three groups of samples are shown in Fig.3.
Fig.3 Results of fatigue test
It can be seen from Fig.3 that when the samples are normal forging, the fatigue strength of the sample is 120.33MPa, and the number of load cycles is 4 million times; when the sample is overheated, the fatigue strength of the sample is 123.8MPa and 128.07MPa, respectively, and the number of load cycles is 3.5 million times and 3 million times, respectively. Compared with normal forging samples, the samples of the two overheating degrees have decreased fatigue strength. This is mainly because the excessive increase in temperature causes the structure of the material to soften, reducing the fatigue strength of the material.
3.3 Effect on fracture toughness
The experimental results of fracture toughnesses of the three groups of samples are as follows:
normal forging:1.7Mpa;
overheating A:1.6Mpa;
overheating B:1.3Mpa.
It can be seen that the fracture toughness decreased with the increase of overheating degree. The excessive increase of temperature destroyed the complexity and continuity of the lattice structure, caused the decrease of the strength and plasticity of the material and the weakening of the material organization.
4 Conclusions
(1) When the overheating temperature is lower than the Ac3 point, the fatigue strength of 40CrMnSiMoVA steel is significantly lower than that of normal forging temperature, and the fracture toughness decreases with the increase of overheating temperature.
(2) Excessive overheating causes deformation softening of steel. The fatigue strength and fracture toughness of the steel are greatly affected.
(3) With the accelerated cooling rate, the fatigue strength and fracture toughness of steel are slightly improved.
(4) In the hot forging process, the temperature of the forging should be controlled strictly, and the overheating temperature should be strictly controlled.
