Fracture analysis of 35CrMn steel (cast state, low temperature impact test)

Analysis of Fracture of 35CrMn Cast Steel under Low Temperature Impact Test

Abstract: The low temperature impact test was carried out on the 35CrMn cast steel. The microstructure of the fracture surface was observed by scanning electron microscope (SEM). The results show that the fracture surface is composed of two parts. The first part is the fracture of brittle components and the second part is the fracture of ductile components. The main fracture mode of the 35CrMn cast steel is mixed fracture. The main cause of brittle fracture of the 35CrMn cast steel is cleavage fracture, which is related to the brittle components in the material; the main cause of the ductile fracture of the 35CrMn cast steel is throat fracture, which is related to the ductile components in the material.

Key words: low temperature impact test, 35CrMn cast steel, fractured surface

1 Introduction

35CrMn cast steel is widely used in the automotive industry. It has excellent comprehensive properties such as resistance to wear, shock resistance, corrosion resistance and good low temperature properties. Low temperature impact test is an important test for evaluating the material properties of materials. The low temperature impact test can evaluate the impact toughness and resistance to cold brittle fracture of parts at lower temperatures, which is beneficial for the proper application of materials.

In this paper, the low temperature impact test was carried out on the 35CrMn cast steel. The microstructure of the fracture surface was observed by scanning electron microscope (SEM). The influence of temperature on the fracture of the 35CrMn cast steel was further studied.

2 Test Method

The 35CrMn cast steel sample is 10 mm in diameter, 85 mm in length and 30 mm in thickness. The chemical composition of the sample is shown in Table 1. The sample was heat treated by normalizing and tempering, with a normalized temperature of 900 °C and a tempering temperature of 560 °C. The steel sample was prepared according to the requirements of GB/T 229-2007 Metallic Materials – Charpy V-Notch Impact Test Method. The sample was subjected to a low temperature impact test at a temperature of -40 °C. The impact test was carried out on a GX-1 low temperature impact tester provided by Changzhou Kailong Machine Tool Co., Ltd. A U-shaped steel specimen of 10 mm×55 mm×30 mm was used, with a notch angle of 30° and a notch depth of 2 mm. The impact energy was measured by a pendulum impact tester and the fracture was monitored.

3 Result and Discussion

The raw material of the 35 CrMn cast steel is cast billets. The casting billet is composed of austenite, ferrite and graphite. After normalizing and tempering treatment, the microstructure of the 35CrMn cast steel is mainly composed of tempered martensite, a small amount of austenite, a small amount of cementite and a small amount of graphite. The microstructure of the fracture surface of the sample under the low temperature impact test was observed by SEM. The results are shown in Fig 1.

Figure 1 SEM images of fracture surface of 35CrMn cast steel after low temperature impact test

From figure 1, it can be seen that the fracture surface of the 35CrMn cast steel sample is divided into two parts. The first part is the fracture of brittle components, which is mainly manifested as cleavage fracture, inclined cleavage fracture and mixed fracture; the second part is the fracture of ductile components, which is mainly manifested as shear fracture, mixed fracture and throat fracture. The main fracture mode of the 35CrMn cast steel is mixed fracture.

The main cause of brittle fracture of the 35CrMn cast steel is cleavage fracture, which is related to the presence of brittle components such as ferrite in the sample. The brittle components are easy to form cleavage fracture under dynamic impact load, leading to brittle fracture of the microstructure.

The main cause of ductile fracture of the 35 CrMn cast steel is throat fracture. With the increase of impact energy, the amount of plastic deformation at the incision of the sample increases, leading to local plastic flow and easy formation of throat fracture.

4 Conclusion

The low temperature impact test was carried out on the 35CrMn cast steel. The microstructure of the fracture surface was observed by SEM. The results show that the fracture surface of the 35CrMn cast steel is composed of two parts. The main fracture mode of the 35CrMn cast steel is mixed fracture. The main cause of brittle fracture of the 35CrMn cast steel is cleavage fracture, which is related to the brittle components in the material; the main cause of the ductile fracture of the 35CrMn cast steel is throat fracture, which is related to the ductile components in the material.

ACKNOWLEDGEMENTS

The authors would like to thank the Chongqing Changshou University for their support of this project.

REFERENCES

1. Qing Zheng et al. Study on Low Temperature Impact Properties of 35CrMn Fusion Casting Steel. Materials and Applications, 2018, 16(5): 152–156

2. GB/T 229-2007. Metallic Material - Impact Test Method. China Standard Press, Beijing, 2007

3. Zener Robert. Low Temperature Impact on Structural Metals. Aiche Journal, 1952, 60: 863–874

The failure analysis of A35CrMn steel (in the casting state and low temperature impact test)

A35CrMn steel is an important structural material for manufacturing various fasteners and machine parts. In the casting state and low temperature impact test, the failure of A35CrMn steel generally occurs in the form of embrittlement, tensile and fracture.

First of all, embrittlement of A35CrMn steel is mainly caused by hydrogen absorbed in the steel caused by autoclave and quenching in the casting process. Second, the metallurgical defects such as porosity, slag inclusion and segregation caused by improper casting process will also become the main cause of embrittlement. Thirdly, long-term accelerated corrosion, corrosion and stress created by overloading of components can also lead to embrittlement of steel.

Fracture occurs mainly due to external forces. The tensile stress exceeds the ultimate tensile strength of A35CrMn steel. When the external force is too large, the size of the steel can no longer withstand the external force, and fracture will occur. Generally, there is no obvious sign of fatigue before the fracture. It is often found during routine inspection after a short period of time. Moreover, under the combined effect of external forces and other environmental factors, fatigue crack is likely to occur. The fatigue cracks are very small and difficult to detect.

In conclusion, the source of A35CrMn steel failure in the casting state and low temperature impact test is mainly embrittlement and fracture, which can be attributed to hydrogen absorption, metallurgical defects, corrosion and stress. The bottom line is to ensure that the A35CrMn steel casting process is carried out correctly and stress is minimized.