Fracture failure Analysis of Automobile Electrical Tailgate spring after Electrophoresis
November 27, 2022
Fracture failure Analysis of Automobile Electrical Tailgate spring after Electrophoresis
The Writer:
1. Gongyi City Hengming Metal products Co., Ltd. Henan Gongyi 451253.
2.Henan Oil hardened and tempered Spring Steel Wire Engineering Research Center Henan Gongyi 451253.
3. Henan University of Technology Henan Zhengzhou 450001.
4. Henan Zhongyan Industry Co., Ltd. Henan Zhengzhou 450016
Oil hardened and tempered Spring Steel Wire used fo Automobile electrical tailgate spring. steel grade SAE9254-VHV, SWOSC-VHV, FDSiCrV, electrical tailgate spring steel wire is a kind of extra-high strength steel wire, and the steel wire is required to have a toughness of strength ≥2250Mpa and surface shrinkage ≤45% at the same time. The spring fatigue resistance is more than 80,000, and the heat shrinkage is between 3% and 5%. The material has good relaxation properties. EC (Eddy Current) flaw detection by rotary eddy current flaw detector (Circograph) and through-type eddy current flaw detector (Defectomat). The flaw detection depth is 40um.
Abstract: the ultra-high strength electric tail spring produced by a spring factory breaks during the pressing process after electrophoretic processing.
Through fracture morphology analysis and chemical formation. The fracture causes were comprehensively analyzed by means of analysis, metallographic structure
testing and microhardness testing. The results show that it is inferred that the electric tail spring is electrified. Hydrogen accumulates on the surface of the spring during
swimming, which leads to local embrittlement of the spring surface. when pressed and tested after electrophoresis, the spring surface produces stress concentration and
embrittlement on the surface. The source of the crack is formed and the fracture is extended.
Key words: electric tail spring; fracture; electrophoresis; hydrogen-induced fracture; failure analysis
1. Preface.
At present, electric trunk is used in medium and high-grade cars. The trunk is opened or closed by electric or remote control. The tail door strut is. The core component
to ensure the normal operation of the automobile electric tail door, the electric tail bomb. The spring is one of the key components of the tail door brace. If because of the
electric tail bomb, Electric tail caused by the decline of spring performance or sudden spring fracture.
If the door cannot be opened, it will cause a poor car experience for the user. And lose the convenience advantage of the electric trunk. Therefore, the tail door brace.
The spring has great requirements for the stability and anti-attenuation performance of force value. High, and the mechanical properties of the spring steel wire used are
also very stringent. At present, most of the raw materials used in electric tail springs on the market are imported. Mouth steel wire, domestic steel wire only occupies a small
market share. SAE9254HV ultra-high strength electric tail produced by a spring factory in China. The spring breaks in the process of pressing and testing after electrophoresis,
such as. As shown in figure 1. The raw materials of the spring are diameter 3.8mm and tensile strength. Ultra-high strength steel wire with strength ≥ 2200MPa. Steel wire production process flow. The process is: wire rod pickling, phosphating, drawing, heating and oil quenching. Warm tempering "non-destructive testing" packaging. Production process flow of electric tail door spring. Cheng is: selecting the end face of "wound spring" de-stress annealing "hot pressing" Smooth "low temperature setting" electrophoresis "press" and test flocking. In this paper, Fracture spring for chemical composition analysis, fracture morphology analysis, metallography.Tissue test and microhardness test were carried out to find out the cause of fracture. Some reasonable suggestions for improvement are put forward.
2 Physicochemical examination.
2.1 Macro-morphology of fracture.
Figure 2 shows the macroscopic morphology of spring fracture under stereoscopic microscope. The fracture surface shows a radial pattern, with clear crack propagation lines and radiation. The direction of convergence is the starting position of the crack [2Power3]. It is known that the crack originated at A. Near the surface, the crack propagates in the direction of the arrow in the figure until it is broken.See crack origin zone A, crack propagation zone B and termination zone C.
2.2 fracture morphology.
The fracture morphology and microcosmic shape of each region were observed under scanning electron microscope. As shown in figure 3, no defects such as inclusion and folding are found. From figure 3a. It can be seen that the crack origin zone is mainly intergranular fracture, and the grain opens along the grain boundary.
At first, the crack propagates inward along the radial direction, and the intergranular fracture zone. After that, the cross-sectional features gradually transition to dimples, and this area is the cross-sectional.
Stable extension area (see figure 3b). As can be seen from figure 3C, the terminating area is tough. The shape of the fossa and the number of dimples increased.
Most of the spring cross section. (expansion area + termination zone) all have dimples, indicating that the material is more plastic. good The grain boundary in this area has been obviously weakened and brittle. Open from the crack origin zone
2.3 Chemical composition of materials.
The samples were taken from the spring broken samples for chemical composition analysis. The results are shown in Table 1. As can be seen from Table 1, the chemical composition of spring steel is consistent with that of spring steel. SAE9254HV technical requirements.
2.4 metallographic analysis.
Samples are taken along the transverse position near the fracture of the spring. After sample, sandpaper grinding and mechanical polishing, 4% volume fraction of nitric acid is used.
Corrosion in alcohol solution, the matrix structure was observed under metallographic microscope. Tempered troosite, there is no obvious decarburization at the edge of the sample, and the microstructure is uniform. Often, as shown in figure 4. Therefore, decarburization and abnormal microstructure of steel wire can be excluded. The influence of factors
2.5 microhardness testing.
Samples were taken near the fracture surface of the spring to test the microhardness, and the results. As can be seen from figure 5, the edge is hard and the heart is hard.
The degree is slightly lower, and the hardness decreases slightly from the edge to the center, which is consistent with steel wire quenching. Hardness law of spring steel after tempering and tempering; maximum hardness 639HV. The minimum value is 615HV, the maximum difference is 24HV, and the edge and heart are hard. There is little difference in value, and the hardness distribution is generally uniform, which is also from the side. The surface proves the overall uniformity of the organization.
(3) fracture cause analysis.
From the above test results, the chemical composition and matrix of steel wire. The microstructure, depth and hardness of decarburized layer were not abnormal. From the microcosmic
of the fracture.
According to the morphology, there are dimples in most of the fracture surface, excluding the toughness of the material.The possibility that lack of sex leads to fracture.
The observation of stereoscopic microscope shows that the source of the crack originated from the bullet. Near a surface at the edge of the spring, which extends radially and eventually leads. Cause the spring to break (see figure 2). Microscopic fracture entry in the crack source region. Through observation, it is found that the source zone of the fracture crack near the surface of the material appears along the. The fracture characteristics of crystal fracture; the fracture in this area is rock sugar-like intergranular fracture. There are intergranular cracks between grains, with typical microstructures of hydrogen-induced fracture. According to the morphological characteristics [6-9], the analysis may be caused by hydrogen embrittlement.
Check the steel wire addition. Process and spring processing technology, found that pickling of wire rod and spring electricity. Hydrogen will be generated in the two stages of swimming, if it is caused by pickling, locally. After hydrogen embrittlement, the steel wire will be produced in the process of drawing and spring winding. Fracture, so the pickling of wire rod is excluded.
Combined electric tail spring generation. The production process, electrophoresis is the electrophoretic coating at the yin and yang poles, applied to. Under the action of voltage, the charged coating ions move to the cathode and interact with. The alkaline action on the surface of the cathode forms an insoluble substance, which is deposited on the.
Spring surface. Electrolyte solution is sodium hydroxide alkaline environment, hydrogen and oxygen. The electrode reaction formula of cathode and anode of sodium electrolysis cell is as follows. Hydrogen evolution is inevitable in the electrophoretic process of electric tail spring. In the process of hydrogen permeation, part of the hydrogen released is overflowed in the form of hydrogen molecules. The surface, the other part adsorbed on the surface of the spring matrix and infiltrated into the steel wire, will. Enter and distribute in various reversible hydrogen traps by diffusion. These defects include dislocations, vacancies, low-angle grain boundaries and so on. Enter. The H entering into the metal sample is around its own stress field and hydrogen trap. Under the interaction of the stress field, H gathers and knot at the trap. The synthesis of H2 results in the existence of hydrogen pressure at the grain boundary at the hydrogen trap [125013]. Hydrogen pressure. The existence of the trap forms a stress gradient with other areas, which will induce. The surrounding H continues to spread to the hydrogen trap, and then around the hydrogen trap. To form a hole. With the continuous increase of hydrogen concentration in the cavity, the hydrogen pressure.
It will gradually increase when it exceeds the local fracture strength of the material. Lead to the nucleation and propagation of cracks, the formation of hydrogen-induced cracks, resulting in materials.
Because the strength of the electric tail spring is very high, the tensile strength exceeds. 2200MPa, and the higher the strength of the material, the more sensitive to hydrogen, so. Finally,
it causes the spring to crack at the hydrogen-induced microcrack on the surface, and in. It expands rapidly under the action of stress and finally becomes unstable and breaks. To sum up, the electric tail spring has local surface brittleness in the process of electrophoresis. The formation of microcracks is the main cause of compression and fracture.
4 concluding remarks. The electric tail spring is caused by the accumulation of hydrogen on the surface during electrophoresis.Local brittleness occurs on the surface, and when pressed and tested after electrophoresis, the spring surface. It will produce stress concentration, form a crack source at the surface embrittlement, and expand.
The spread is broken. In view of the above situation, the electrophoretic processing technology is proposed. Make improvement measures and suggestions.
1) during spring electrophoresis, under the premise of ensuring the quality of the electrophoresis layer.
The electrophoretic time should be shortened as much as possible to prevent local hydrogen leakage corrosion. Local embrittlement on the surface of the spring.
2) effective hydrogen removal process should be adopted as far as possible after electrophoresis, such as heating hydrogen removal.
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