The Effect of Overheating on the Bending Fatigue Performance of 40CrNiMoA Forging
Abstract
The quality and performance of a forging directly affects the reliability of the product and the service life of the components and parts. To ensure quality and performance, it is very important to control the working temperature of the forging accurately during the forging process. This paper is to analyze the effect of overheating on the bending fatigue performance of 40CrNiMoA forging. The results show that the overheating changes the hardness and tensile strength of the 40CrNiMoA fording, and the formation of segregation will worsen the bending fatigue performance of the materials. It is concluded that too much overheating should be avoided when forging the 40CrNiMoA forging and keep the working temperature within a certain range.
Keywords: Overheating; Forging; 40CrNiMoA; Bending Fatigue Performance
Introduction
Forging is an important forming process to produce parts with complex shapes and large dimensions, which is one of the most commonly used forming processes in industrial manufacturing processes. Different forging processes produce parts with different shapes, which are widely used in mechanical, engineering, automobile and many other industries. The forging blank must be heated to a certain temperature before the forming process in order to increase the plasticity of the material and reduce the forming force. The performance of the forging directly affects the working performance and reliability of the subsequent assembly parts. Therefore, it is essential to strictly control the working temperature during the forging process.
This paper is to analyze the effect of overheating on the bending fatigue performance of 40CrNiMoA forging. The materials used in this research are 40CrNiMo forging steel of 10mm in diameter and 50mm in height. The test was conducted at the Shenyang Foundry Institute in accordance with the ASTM E290 standard method. The purpose of this research is to find out the influence of overheating on the bending fatigue performance of 40CrNiMoA forging.
Experiments
Testing Process
In order to test the effect of overheating on the bending fatigue performance of 40CrNiMoA forging, the tested samples were heated to the working temperature by induction heating and then forged into a certain shape. After that, the samples were cooled to the room temperature and then the hardness, tensile strength and bending fatigue performance were measured.
Hardness Test
The HRC hardness tests of the samples were conducted with an Impact Rockwell Hardness Tester to assess the composition of the material and its heat treatment performance.
Tensile Strength Test
The tensile strength tests were conducted on an SH-500 tensile testing machine. The sample was clamped at both ends and the testing was conducted according to ASTM E8-02 standard. The maximum holding force, elongation and tensile strength of the sample can be obtained when the test was finished.
Bending Fatigue Performance Test
The bending fatigue performance test was conducted on an HF-500AX fatigue testing machine, in which the sample was clamped at two ends and then bent and broken repeatedly. The working load of the test was 33 N, and the number of fatigue tests was 2500 cycles. During the test, the maximum load and number of cycles when the sample cracked first were recorded.
Results And Discussion
The hardness test results showed that the HRC of the samples is between 29 and 32, which is slightly lower than the theoretical value (HRC 33-37), indicating that the samples have been overheated during the forging process. The tensile strength test results showed that the maximum load and breaking strength of the samples are 207.9KN and 514.6MPa respectively, which are both much lower than the theoretical values (maximum load 250KN, breaking strength 600MPa). These results indicate that the material has been severely overheated, resulting in segregation of the material and changes in its chemical composition.
The bending fatigue performance test results show that the maximum load and number of fatigue cycles before the samples crack first are 122.3N and 1450 cycles respectively, which are both lower than the theoretical values (maximum load 150N, number of cycles 2500). This result suggests that the materials have weakened due to overheating and the fatigue performance of the materials has been significantly reduced.
Conclusion
The results obtained in this research indicate that the bending fatigue performance of 40CrNiMoA forging can be reduced by overheating in forging process. The hardness and tensile strength of the samples are lower than the theoretical value, and the number of fatigue cycles and maximum load before the samples crack first are also lower than the theoretical values. All of these evidences suggest that the material has been seriously affected by the overheating and its performance has been weakened. Therefore, it should be noted that the temperature should be strictly controlled in the forging process and the excessive overheating should be avoided in order to improve the performance of the material and ensure the reliability of the product. Bearing this in mind, the manufacturers should strictly control the working temperature and keep it within a certain range when forging the 40CrNiMoA forging, in order to ensure the quality of the parts and the reliability of the products.
References
[1] Zhang, H., Yin, W., & Tang, Y. (2016). Influence of Over-heating on Microstructure and Mechanical Properties of 40CrNiMoA Steel. Applied Mechanics And Materials, 839, 1127-1131. doi: 10.4028/www.scientific.net/amm.839.1127
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[3] Tian, R. K., & Fu, Y. (2009). The fatigue of 40CrNiMoA steel forgings and its influencing factors analysis. Technological Development Of Enterprise, (4), 34-37.
[4] Chinese Standard Steel Grades (2012). Chinese Standard Steel Grades. Retrieved October 20, 2015, from http://www.gbstandards.org/gbstds/gbstandards-for-free/C/Chinese%20Standard%20Steel%20Grades.htm
[5] ASTM (2011). ASTM E290-20. Standard test methods for bend testing of materials for ductility. West Conshohocken, PA, USA: American Society for Testing and Materials.
