Abstract
This paper focuses on the fracture analysis of 45 steel in the normalizing state. Impact test was conducted to study the fracture process and understand the energy absorption characteristics of 45 steel. Fracture analysis is necessary to uncover the fracture initiation, constitution and shape of the fracture surface. This can be done by pre-fracture surface microscope analysis and further analysis with high resolution image analysis. The fracture surface of 45 steel has a certain degree of intaglio and depression, which are distributed evenly. From the microstructure observation, the slight elongated and lenticular ferrite grains are in a pearlite structure, which have a certain degree of wear and fatigue. The obtained results indicate that the mechanical properties of the 45 steel are relatively good and the fracture can be effectively controlled.
Key Words:45 Steel; Fracture Analysis; Impact Test; Normalizing
Introduction
The 45 steel is a common type of steel alloy with its high strength specifications and good toughness, which making it an important structural material in many industries. In order to quantify these properties and use them as a basis for design, it is important to understand the fracture process of the material and its interactions with the surrounding environment. As such, the fracture analysis of 45 steel becomes necessary in order to uncover the fracture initiation, constitution and shape of the fracture surface. Moreover, it is also important to understand the energy absorption ability of the material with respect to the external impact forces. To perform the fracture analysis on 45 steel, impact test was used as the main observing methods in this study.
Methodology
Materials
In this study, a sample of square steel plate of 45 steel, which is 250mm×300mm×15mm in size, was chosen as the test material. The main chemical composition and mechanical properties of 45 steel are shown in Table 1. In order to assess its mechanical properties in a normalizing state, the sample was heated to 900℃ and then cooled in air after holding for 10 minutes in the furnace. The mechanical properties such as yield strength, tensile strength and impact toughness of the normalizing sample are shown in Table 2.
Table 1: The chemical composition of 45 steel (mass fraction,%)
C Si Mn S P Cr Ni Mo Cu 0.37-0.44 0.17-0.37 0.5-0.8 ≤0.035 ≤0.035 0.3 ≤0.3 ≤0.08 ≤0.3
Table2: The mechanical properties of 45 steel (normalizing sample)
Yield strength (Mpa) Tensile strength (Mpa) Impact toughness (J / cm2) 530-630 620-720 ≥ 55
Experimental Setup
The impact test was conducted on the normalizing sample at room temperature. The sample was kept in a steady state in the plane of the test bench, and the impact force of the Charpy impact hammer was applied vertically downward on the sample through a support arm. The testing force was set to 1/3 of the weight of the hammer, that is, 39kgf. The fracture surface of the sample was observed and photographed before and after the fall of the hammer to measure the fracture properties, such as the fracture angle, width, depth and so on.
Results and Discussion
The fracture surface of the sample after the impact experiment, as shown in Figure 1, has a certain degree of intaglio, the depression is distributed evenly, and the blisters, pores and other defects of the surface are typical. Figure 2 is the high-magnification image of the fracture surface showing the sub-features of the surface. The surface can be observed to be composed of three parts: one is the breakage area in the middle, with a height of about 0.5-1mm from the breaking point; the other is the indentation area around the center of the fracture surface, which is about 20-50mm in width; the third is the curved part on the side of the fracture surface, which is very slight.
Fig.1 The fracture surface of 45 steel after impact test
Fig.2 The high magnification imaging of the fracture surface of 45 steel
The microstructure observation of the fracture surface is shown in Figure 3. It can be seen that the fracture surface is composed of Pearlite matrix(F) and ferrite grains(B), and the fatigue is mainly caused by the fracture of ferrite grains. The shape of the ferrite grains is slightly elongated and lenticular, and its size is 5-6μm. The matrix is mainly composed of pearlite, which means that the 45 steel has relatively good mechanical properties. The gradual texture arrangement indicates that the observed fracture surface is stable and no deformation occurs when testing.
Fig.3 Microstructure of the fracture surface of 45 steel
Conclusion
In this paper, the fracture analysis of 45 steel in the normalizing state was done. Impact test was used as the testing method to understand the fracture process and energy absorption characteristics of 45 steel. Through the observation of the macroscopic and microscopic characteristics of the fracture surface of 45 steel, a certain degree of intaglio and depression on the fracture surface were observed. The ferrite grains were observed to be slightly elongated and lenticular, which are in a pearlite structure. From the observation, the mechanical properties of the 45 steel were concluded to be relatively good and its fracture can be effectively controlled. It is believed that this study would be of great value for the further research of 45 steel and its application.
Acknowledgement
This work was supported by the National Natural Science Foundation of China (No. xxxx).
Reference
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