Failure Analysis of 21CrMoV5-7 Steel Nut Impact Test

21CrMoV5-7 steel is a quenched and tempered steel, heat-resistant structural steel, German material grade, its code is 1.7709. The steel has good hardenability and can be obtained with good strength, plasticity and toughness after quenching and tempering treatment. Commonly used in the manufacture of fasteners such as bolts and nuts, the maximum operating temperature is 540 °C. As an important structural component, the failure of the nut will cause great damage to the product. In the impact test of 21CrMoV5-7 steel nut, it is found that the impact absorption energy is much lower than the standard value. After the service, it will cause great harm to the product.

The raw material status of the batch of nuts is tempering state, and the processing flow is: blanking → polishing → car hexagonal → thread thread → surface treatment. Due to the urgent needs of the manufacturers, the raw materials were purchased and directly put into production, and the relevant acceptance tests were not carried out. In the subsequent product processing process, the re-inspection of raw materials is gradually carried out. When the impact test is performed, the impact absorption energy is far from the standard value. The test results are shown in Table 1. At this point, the nut production has reached the surface treatment process.

Physical and chemical test

(1) Chemical composition analysis For the batch of nuts, one piece was taken for inductively coupled plasma emission spectrometer (ICP) chemical composition analysis, and the results are shown in Table 2. It can be seen from the data in Table 2 that the sulfur element content is close to the upper limit and the silicon element content is more, but in general, it meets the requirements of raw material standards.

(2) Microstructure analysis The lateral and longitudinal microstructure analysis of the batch of nut impact specimens is shown in Fig. 1. Figure 1 shows the microstructure characteristics of the impact specimen. It can be seen from the figure: Figure 1a shows the lateral 100× inclusion morphology, the inclusions are mainly granular oxide inclusions, and the size is small, according to GB/T10561. -2005 "Determination of non-metallic inclusions in steel" can be rated as fine 2; Figure 1b is 100 × longitudinal inclusion characteristics, non-metallic inclusions, mainly sulfide inclusions, according to GB/T10561-2005, Can be rated as fine 2 grade; Figure 1c is the transverse 200 × microstructure morphology, the microstructure is mainly pearlite and ferrite, from the perspective of microstructure, the material state is normalized; Figure 1d is 500 × transverse microstructure, ferrite is massive, the proportion is about 70%, the proportion of pearlite is small; Figure 1e is 100 × longitudinal microstructure, the tissue shows more obvious banded tissue characteristics Fig. 1f is a 500× longitudinal microstructure. It can be seen from the figure that the proportion of ferrite is significantly larger than that of the transverse microstructure, accounting for about 80%, which further indicates the segregation of the material.

The microstructure of the impact sample is a mechanically mixed structure of ferrite and pearlite, and the heat treatment state of the raw material is normalized.

(3) Mechanical property analysis The Vickers hardness test was performed on the batch of nuts, and the results are shown in Table 3.

It can be seen from Table 3 that the basic deviation of the data is not large, and the difference between the maximum value and the minimum value is 8HV5. According to ISO18265-2013 "Metal Material Hardness Conversion Table", the tensile strength is converted to 740~770MPa, and the actual measured resistance is The tensile strength of 750 MPa is not much different. From the hardness data analysis, the microstructure of the batch material is not annealed, and the common annealed tensile strength is about 550 MPa, and the hardness is about 170 HV5.

2. Comprehensive analysis

The 21CrMoV5-7 material is quenched at 890~940°C, and the quenching cooling medium is oil. After tempering at 680~720°C, the impact energy is about 160J. The normal quenched and tempered microstructure and grain are shown in Fig. 2.

Figure 2 shows the microstructure and grain size after quenching and tempering. It can be seen from the figure that the structure in Figure 2a is tempered sorbite, and during the tempering, the supersaturated carbides are from the martensite matrix. Precipitation, formation of ferrite and alloy carbide (spherical), alloy carbide dispersion distribution, high toughness and strength; tempering grains in Figure 2b is small, according to GB/T6394-2002 "metal average grain Degree determination method, its grain size can be rated 8th.

The sub-eutectic steel normalizing fire generally chooses Ac3 above 100~150 °C. After a certain holding time, it is air-cooled and the cooling rate is slow. The normal 21CrMoV5-7 high temperature tempering temperature is 680~720°C, and the normalizing temperature is equivalent to the tempering of higher temperature. When the steel is tempered at a higher temperature, slowly passing 450~650°C will cause slow cooling and brittleness. The phenomenon of tempering, that is, the second type of temper brittleness occurs. The second type of temper brittleness can significantly reduce the room temperature impact toughness aK, the cold-brittle transition temperature is 50% FATT, and the fracture is generally characterized by intergranular fracture.

In summary, the batch nut raw material impact test failed, the main reason is that the material state is normalized rather than tempered. During the normalizing cooling process, slow cooling and embrittlement occur, resulting in the second type of temper brittleness; due to the high content of impurity elements such as P, S, Si, etc., and interaction with elements such as Ni, Cr, Mn, when impurities When the content of elements is certain, the higher the content of Mn, Cr and other elements, the more serious the embrittlement; and because the content of non-metallic inclusions is more and linear, which together, the room temperature impact performance of the batch is unqualified.

3. Conclusion

The main reason for the unsatisfactory impact performance of the raw materials of the batch of nuts is that the raw materials are not quenched and tempered, and the second type of temper brittleness is caused during the normalizing and slow cooling process, which seriously reduces the room temperature impact toughness. This raises two suggestions:

(1) Strengthen the quality control of raw materials entering the factory. For special-purpose materials, they should be tested and then put into production, so as to avoid scrapping the products due to raw materials.

(2) The batch of nut and the remaining material are re-quenched and tempered at high temperature, and the room temperature impact performance is acceptable before being released.

About the author: Xu Yongchun, Xu Wei, Mu Yanbing, Deng Lijuan, Shihang, Henan Aerospace Precision Manufacturing Co., Ltd.