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Case Study of Valve Spring and Bogie Spring-fatigue Fracture

2020-02-28 09:45:47
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1. Fatigue fracture of spring

Generally, the fatigue fracture is composed of three parts: fatigue source, crack propagation zone and final transient fracture zone. The source of fatigue is sometimes very clear and sometimes unclear. The crack propagation area and the final transient fracture area are the main components.

Features of crack growth area: The surface is relatively smooth, which is the result of slow crack growth, crack surface contact and friction. It is a brittle fracture feature, and the crack propagation direction is perpendicular to the direction of maximum tensile stress. You can usually find the beach-like, shell-like or annual ring-shaped patterns on the fracture with the naked eye. The fatigue source of spring fracture can be determined according to the phenomenon that the crack propagation direction is perpendicular to the beach-like stripes and the feature of the smallest radius of curvature.

Characteristics of transient fracture zone: After the fatigue cracks continue to expand to a certain extent, the effective load bearing area continues to decrease, and the corresponding working stress gradually increases. When the stress exceeds the spring's fracture stress, the spring will instantly fracture. Its characteristic is that the fracture is relatively rough and uneven.

Microscopic characteristics of fatigue fracture: Most of the fatigue cracks start to form between grain boundaries, phase boundaries, inclusions and brittle carbides, and then gradually expand inward. The microscopic characteristics of the fatigue fracture are mainly manifested in the crack propagation zone. The main micro-characteristic of the extended zone is the fatigue band. Fatigue strips have the following characteristics: ① the shape of the strips is undulating or rippled; ② each strip represents a cyclic load; ③ the position of the crack front line can be determined by the strip; .

2.Case Studies

(1) Broken valve spring

① Accident description: Valve spring is an important part of automobile engine. It not only controls the opening and closing of engine valves, but also is an extremely important safety component. Therefore, automobile manufacturers attach great importance to the quality of engine valve spring. / T 10591-2007 requires its bench fatigue test not to break 23 million times. A certain type of car experienced abnormal engine noise and idle vibration when traveling 199km after leaving the factory. After inspection, it was found that a valve spring was broken.

② Cause analysis: It can be seen from observation that the corresponding spring surface at the fracture source has an approximately elliptical pit with a long axis length of 0.82 mm and a short axis length of 0.43 mm. Can be found from the following figure:

There are no signs of shot blasting in the pits on the surface of the spring break, and there are obvious scratch marks, indicating that the pits are not inherited from the pit defects on the surface of the oil-quenched and tempered spring steel wire, but are generated when the spring is well-prepared of. At the same time, the figure also tells us that there is a covering on the surface of the spring at the source of the fracture. The covering originates from the side of the pit, so it can be concluded that the covering is the metal material at the pit location. It can be inferred that the accidental factor caused the spring to be scraped by the external force, forming the above-mentioned pit, and generating concentrated stress when the impact scratched the bottom of the pit. The valve spring is subjected to alternating stress during service. The maximum tensile stress is located on the subsurface of the spring, and the stress concentration at the bottom of the pit and the maximum tensile stress are superimposed on each other. As a result, the valve spring starts early under the action of shear force and forms under the pit. The source of fatigue fractures expands rapidly, causing early fatigue fractures.

Since the cause of the fracture was improper use, we artificially started the fracture directly from the crack propagation stage. As explained in the textbook, the initial stage of fatigue cracks accounts for 90% of the entire fatigue fracture process, so the fatigue life of valve springs is only about 10%. It is not difficult to explain that the valve spring will break after a short period of use. The main factor affecting the crack growth at this time is the stress intensity factor K:

From the formula we can see that β is a dimensionless quantity, so the value of K is determined by a. With the gradual expansion of the crack, the value of K increases continuously. When K = Kmax, the spring will break.

③Preventive measures: By analyzing the chemical composition of the spring, its chemical composition meets the requirements of national standards.

However, there are loopholes in the post-processing process, and the process of shot peening is lacking. The textbook tells us that shot peening can effectively remove residual stresses to relieve fatigue damage and extend the service life of springs. Because if the manufacturer can increase the shot peening process during the processing, the service life of the valve spring can be effectively extended. However, in this case, dents on the spring surface due to scratching are the main cause of fatigue fracture, so the shot peening process can only appropriately alleviate this problem. To fundamentally solve the problem of fatigue and fracture in the case, non-destructive testing can be used to pick out defective parts in advance, thereby improving product safety and reducing economic losses. The visual rule is the most economical non-destructive test method. If you want to improve the accuracy of the test on this basis, magnetic particle testing and ultrasonic testing can also be adopted.

④Maintenance: This kind of problem is caused by artificial unconscious crack damage, so the engine does not appear abnormal and it is difficult to find the cracks. When the engine really fails, the spring is usually broken and cannot be repaired, so there is no effective way Repair the spring before it breaks.

(2) Analysis of spring breakage of k2 bogie

① Accident description: The process of turning k2 spring is: raw material incoming inspection → blanking → flattening → heating and coiling → quenching → tempering → primary process compression → end face grinding → shot blasting → magnetic particle inspection → secondary process Compression → Final inspection → Dip paint → Drying → Storage → Delivery. During the spring manufacturing process, due to various reasons, some springs were scrapped due to failure during the manufacturing process. Early fracture occurred during the inspection and random inspection process, and especially serious was the fracture after loading. In our tests, 11 manufacturers producing K2 springs, 4 of which were broken during the quality inspection process, the number of spring fatigue tests was 3 million, and the number of 4 fatigue test breaks were 680,000, 79 10,000 times, 2.29 million times, 2.49 million times.

②Cause analysis: Unlike the previous case, the failure of the spring in this case was not due to surface defects caused by humans. Before testing, we can roughly guess that it is because of a loophole in the processing process that caused the spring to fail the standard inspection. From the textbook, we know that the residual stress has a great impact on the fatigue performance of the component. At the same time, the process of heat treatment will introduce residual stress, so we should focus on checking the problems in the heat treatment process. According to the information introduced in the literature: the spring heat treatment process is 850 ° C ± 10 ° C, the No. 10 mechanical oil is cooled, and the tempering process is 515 ° C × 65min. By observing the microstructure of the transverse metallographic sample of the fracture spring, it was found that the spring has a deep fully decarburized + semi-decarburized layer with a depth of 0.45mm.

Remarks: The left of the figure is the total decarburization and the semi-decarburization. The right is the heart tissue

Then we started to analyze the fracture structure. The fracture can be clearly divided into three areas: the white bright area, the wood grain fracture area, and the other rapid fracture areas. The white bright area can be determined to be a fatigue area, the source area of which is located on the surface of the inner ring of the spring, and the maximum stress point when it is pressed down. Micro-cracks propagating inward from the surface can be seen in the source area, and fatigue cracks begin at the micro-cracks. The extension area is flat, and obvious fatigue groove lines can be seen. On the other hand, inclusions can be found in the wood grain-like fracture area. This shows that the wood grain fracture area is related to debris. It can be known from the processing process and fracture analysis that the cause of spring fracture is not caused by a single process, and each of the main links in the processing process has different degrees of influence. The quenching and heating time during the heat treatment of the spring is too long, causing serious decarburization on the surface; the defects existing on the surface of the raw material have not been completely removed. Since the maximum stress of the spring occurs on the surface of the spring, the surface quality of the spring has a great impact on the fatigue strength. Defects caused by rolling, drawing and coiling are often the main causes of fatigue fracture of springs. Therefore, we can conclude that the reason for the spring fracture is the influence of the material itself (too many inclusions), and there is a deep decarburization layer on the spring surface.

③Precautions: 1) Spring material: first review the raw materials before processing, and then process after the inspection is qualified, and then perform the blanking operation after the surface quality inspection has no defects. 2) Manufacturing method: In the process of spring coiling and heat treatment, a controlled atmosphere furnace should be used as far as possible to achieve less oxidation-free heating. 3) In addition, shot peening can be added in the process. This process can reduce harmful residual tensile stress, and can also convert residual tensile stress into favorable participating compressive stress, thereby improving the fatigue strength of the spring. 4) After shot blasting, clean up the steel shots in the spring to avoid contact fatigue fracture of the spring. 5) For the surface of the spring, we can detect non-conforming products in advance by non-destructive testing methods, such as ultrasonic testing or eddy current testing.

④Maintenance: For components that have suffered fatigue damage, I have not found an effective method to repair the spring from the root cause, but we can extend the fatigue life of the spring by other means. Since the spring is a steel material part, we can extend its fatigue life by repair annealing. According to data, repair at 550 ° C can extend its fatigue life up to twice. During the annealing process, the strain energy stored in the spring will be released as a driving force, which will change the microscopic state of the structure to a more stable state. In particular, the transformation of carbides delays the occurrence of carbide interface cracks. Of course, the effect of annealing repair is also related to the degree of fatigue damage. As the degree of damage increases, the conditions and number of microcracks increase, and microcracks are unhealable, and the repair effect will naturally decline. In addition, we can also increase the surface hardness and isostatic residual stress by means of a thermostatic isostatic method at medium temperature to delay the initiation of microcracks or to close the microcracks that have been generated. Reduce the effective driving force at the crack tip and slow down the propagation rate of the microcrack. Thereby increasing fatigue life.

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