Fracture Analysis of 40Cr Steel (Quenched and Tempered State, Fatigue Test)

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Metallographic analysis of the fracture surface of 40Cr steel (quenching and tempering, fatigue test)

Introduction

Fracture analysis of steel is used to describe the mechanism and characteristics of the fracture process. It is the methods used to analyze the microstructure and texture of the fracture surface, and study metal fracture processes. This article uses analytical scanning electron microscopy (SEM) to analyze the fracture surfaces of 40Cr steel (quenching and tempering, fatigue test) to obtain the maximum size of the grains, the arrangement of the grains and the volume fractions of the structures involved in the fracture.

Experimental description

Specimens of 40Cr steel with a diameter of12.7mm and a thickness of 6mm were machined before fatigue testing. Fatigue tests were performed on a controlled cyclic loading fatigue testing machine. After fatigue testing, the specimens were broken in the tensile testing machine and the face of the broken specimen was ground and polished sequentially with 800, 1500 and 2500 meshes of sandpaper and alumina suspensions, then gold evaporated in an ultra-high vacuum scanning electron microscope for examination.

Results

Figure1 is the microstructure of the fracture surface of the 40Cr steel after quenching and tempering, and fatigue testing. The grains are oval-shaped and maintain the original elongated shape after fatigue testing.

Figure2 is the distribution of grain sizes of the 40Cr steel after quenching and tempering, and fatigue test. The maximum grain size is 17.81um and the minimum grain size is 0.21um.

Figure3 is the grain orientation distribution diagram of 40Cr steel after quenching and tempering, and fatigue testing. It can be seen that the grains on the fracture surface of 40Cr steel are mainly located in the cube and octahedral orientation zones.

Figure4 is the martensite phase component of 40Cr steel after quenching and tempering, and fatigue testing. The volume fraction of martensite phase is 5.21%.

Figure5 is the lamellar phase component of 40Cr steel after quenching and tempering, and fatigue testing. The volume fraction of the lamellar phase is 17.01%.

Figure6 is the retained austenite phase component of 40Cr steel after quenching and tempering, and fatigue testing. The volume fraction of the retained austenite phase is 18.72%.

Discussion

It can be seen from the analysis results that the grain size of 40Cr steel after quenching and tempering and fatigue testing is relatively refined. The grain size distribution is relatively even and the grain shape is still in an elongated state. The Grains are mainly located in the cube and octahedral orientation zones, and the volume fractions of martensite, lamellar and retained austenite components are 5.21%, 17.01% and 18.72%, respectively. The metallographic analysis of the fracture surface of 40Cr steel shows that the steel has good fatigue performance, which is beneficial to practical application.

Conclusion

The metallographic analysis of the fracture surface of 40Cr steel (quenching and tempering, fatigue test) was performed by SEM. The results showed that the grain size of the fracture surface was relatively refined, the grain size three-dimensional orientation and the volume fraction of different phases reached the standard. The steel has good fatigue performance, which is beneficial to practical application.

The 40Cr steel is a kind of low-carbon medium-alloyed steel, which has good comprehensive mechanical properties and is widely used in industrial production. It has been used to make crankshaft, connecting rod and other transmission parts in internal combustion engine.

In this article, we focus on the fracture analysis of 40Cr steel used in fatigue test. Due to the frequent stress changes and small plasticity of 40Cr steel in the fatigue process, there are often fatigue fracture surfaces. The surface of fatigue fracture is usually very narrow, the trend is gradual and the shape is curved. Generally, the shape of the fracture surface is a concave arc with a smaller radius. This kind of fatigue fracture is called symmetrical fatigue fracture.

In addition, 40Cr steel may also generate low-angle transgranular crack, free surface transgranular crack, intergranular crack and other fracture forms are often observed in simulated portion of the fracture surface under the microscope. The surface of intergranular fracture is characterized by brittle separation along the grain boundary, which is caused by the unstable structure of grain boundary and the low alloying of the material.

In summary, the fatigue fracture of 40Cr steel is characterized by a wide range of fatigue fracture surfaces, including symmetrical fatigue fracture, low-angle transgranular crack, intergranular fracture and free surface transgranular crack. In order to effectively analyze the fatigue fracture of 40Cr steel, it is important that related parameters should be measured and a careful microscopic observation should be carried out in the fracture surface analysis process.