Analysis and prevention of vibration failure of self-locking nut
In the process of design, manufacture, use and maintenance of aeroengine, the safety and anti loose problem of screw connection is becoming increasingly prominent. The self-locking nut, especially the high temperature self-locking nut, has become the most widely used thread joint in aviation industry because of its reliable anti vibration and anti loose and reusable functions. The self-locking nut is expanded at the end after the nut is tightened by non-circular closure. The thread is compressed to each other by the return force of the closure, which produces the locking torque, which plays the role of coupling and anti loosening. High temperature alloy, especially GH2132, has become an irreplaceable key material in the construction of modern national defense and national economy because of its good high temperature strength, anti-oxidation, corrosion resistance, excellent fatigue resistance and creep resistance, as well as excellent fracture performance and structural stability.
Most of the previous researches on self-locking nuts focus on the material related properties, and few researches on the failure of the manufactured parts, especially the failure cases of the self-locking nuts are rarely reported. This study takes the crack of a self-locking nut in the vibration test as the research object. By means of stereoscopic inspection, fracture micro analysis, energy spectrum analysis, metallographic structure analysis and microhardness test, the causes of crack formation of self-locking nut are determined, and corresponding preventive measures are put forward.
Test process and results
Table of Contents
- Test process and results
- Analysis of the causes of cracks
- Conclusion and preventive measures
Appearance inspection and micro analysis
(1) Appearance inspection
The results of the visual microscope inspection showed that the crack distribution in the closing part was regular: the cracks appeared in the closing part of the short axis and penetrated into the internal thread; The crack is in a fine straight line, starting at the top of the straight mouth and extending along the axial direction. As shown in Figure 1.
Figure 1 crack appearance at the end
(2) Micro analysis
Metallographic observation of the sample after the interruption shows that the crack starts at the bottom of the internal thread and extends to the outside (see Fig. 2). The results showed that the structure near the bottom of thread was loose and massive, and the fatigue arc was obvious in the source and expansion area. The intergranular crack and cleavage characteristics appeared on the section, and no material defects were found on the cross section (see Fig. 3).
Figure 2 metallography of crack at the bottom of thread tooth
Figure 3 scanning morphology of screw root by electron microscope
Energy spectrum component analysis
The energy spectrum components of crack area and artificial interruption area are analyzed, and the results are shown in Figure 4. Besides the main elements of matrix material, there are a lot of Ag elements in the crack area, and a large amount of Ag elements exist in the root of the thread in area a in Figure 3.
Figure 4 Analysis of energy spectrum components in crack region
Microhardness analysis is carried out on the crack of the broken sample and the non closed part from the top to the bottom of the thread. The microhardness change curve at different distances from the surface is shown in Fig. 5. The normal hardness range of the product is 281-357hv. It can be seen from the hardness curve that the hardness value of non closed part is normal, and the microhardness value of the closing part is obviously higher than that of the non closed part. For the end of the mouth, the microhardness within 0.4mm from the inner and outer surface is significantly higher than the core hardness value.
Figure 5 microhardness measurement curve
Metallographic structure analysis
As shown in Fig. 6 and Fig. 7, the microstructure of self-locking nut is free of overheating and over burning, and the grain size is uniform, and there is no coarse or fine grain band structure. In addition, the non-metallic inclusions in the structure are normal and there is no impurity beyond the material standard.
Fig. 6 metallographic structure of nut
Fig. 7 non metallic inclusions in gold phase of nuts
Analysis of the causes of cracks
Material and heat treatment analysis
The metallographic structure and non-metallic inclusion analysis of the nut show that the structure of the material is free of overheating and over burning, and the grain size is uniform, and there is no coarse or fine grain band structure. The microhardness of non – closing part also meets the requirements of relevant technical conditions. At the same time, no material defects were found at the fracture of the nut, that is, the failure of the nut has no direct relationship with the material and metallurgical quality of the product.
Analysis of crack properties and formation time
Through the fracture analysis of the crack area, it can be seen that the crack starts from the root of the thread tooth, the micro morphology of the crack is obvious fatigue strip and fiber shape, and the intergranular crack and cleavage feature appear on the section, and the crack property can be judged as fatigue fracture.
The color of the root of the thread in the artificial interruption area is deep, which is different from other areas. The energy spectrum analysis shows that there are a lot of Ag elements in the crack area besides the main elements of the matrix. Combined with the main processing procedures of self-locking nut (centerless grinding → blanking → ordinary car processing → hot heading → sand blasting → solid solution → ordinary car processing → thread tapping → NC car processing → closing → aging → fluorescence detection → sand blasting → silver plating → final inspection), it can be concluded that Ag element comes from surface treatment process, that is, microcracks have been produced before silver plating.
From the analysis of the crack forming part of the nut, the cracks all appear in the end of the short shaft and run through to the internal thread. At the same time, the crack is in a fine straight line, starting from the top of the straight mouth and extending along the axial direction. In addition, microhardness analysis shows that the microhardness of the end is significantly higher than that of the non closed part. Especially, the microhardness of the inner and outer surface of the closing part is higher than that of the center part in 0.4mm. This is because the tensile stress on the outer surface of the long axis is also stressed on the inner surface of the short axis during the closing. In addition, the thread bottom of the thread at the end of the closure is relatively sharp, which is easy to form the stress concentration and produce micro cracks. After vibration test, the microcracks expand and finally fatigue fracture occurs. Therefore, it can be judged that the microcracks are produced in the closing process.
Conclusion and preventive measures
- 1) The crack of the self-locking nut of superalloy is produced in the closing process before the surface aging treatment. Because the self-locking nut is closed by numerical control closing machine, the unit area of the deformation part of the nut is stressed greatly. In addition, the thickness of the nut wall is thin, the thread bottom is relatively sharp, and the stress concentration is easy to occur, which causes the microcracks after closing.
- 2) The measures to prevent crack in the closure of self-locking nut are to change the way of closing, from two-point to elliptical, to compress the outer circle of nut in a package way, so as to avoid the large stress concentration in some parts. In addition, the diameter of the outer circle at the nut end should be increased appropriately, that is, the wall thickness of the nut should be increased. Through this process improvement, the crack failure of nut is eliminated effectively.
Author: xujiaqiang, fujianjian
Source: China Nut Supplier: www.epowermetals.com
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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