Analysis of factors affecting cracks in cylinder head bolts
The metallographic examination sample is taken from the vicinity of the fracture, the grinding surface is consistent with the axis of the bolt, and the metallographic structure of the material is as shown. (a) represents a secondary crack caused by a thread root machining tool mark and 45b with the bolt axis, and a plurality of elongated metallurgical inclusions that are parallel to the bolt axis. According to their morphology and distribution characteristics, it can be concluded that these inclusions are sulfides. (b) indicates that the metallographic surface is polished and immersed in a 3% nitric acid alcohol solution to form a tempered sorbite + slender sulphide inclusion. It can be concluded that the failed bolt has undergone normal quenching and tempering treatment.
The microscopic topography of the fracture (b) The typical microscopic morphology of the fracture in the source region of the fracture indicated by the arrow. (a) indicates that the thread surface is corroded by the working environment, and the pit hole associated with the material defect (inclusion), (b) indicates the presence of corrosion products, and (c) indicates that the fracture surface starts from the surface and has a grain-like orientation. Secondary crack, (d) represents fatigue striation and local quasi-cleavage pattern. Observation and analysis of the microscopic morphology show that the failure of the bolt is broken, and the local stress corrosion fracture leads to the final corrosion fatigue fracture.
Stress corrosion cracking occurs when the oxide film on the metal surface is destroyed by the combination of stress and corrosive medium. The service conditions of the cylinder head bolts meet the necessary conditions for stress corrosion cracking, namely stress and working environment medium. The problem is whether the metal surface oxide film will be damaged under service conditions. According to the metallographic examination and mechanical properties measurement, although the composition and heat treatment state of the bolt material are not abnormal, the metallurgical quality is inferior, and there are a large number of fine needle-like strips, sulfide inclusions parallel to the axis of the bolt, and the mechanical properties are lower than the specified values. In particular, the Rs is 10% lower by 13%. At the same time, the root of the thread is rough, the pocket of the knife mark is clearly visible, and there is a large stress concentration. In addition, bolt breakage occurs at the root of the 12th thread. We know that when the bolt is subjected to force, the force assigned to each thread is not the same, and the force applied to the 12th thread is the largest. Therefore, it can be concluded that under the service conditions, at the root of the 12th thread with the highest force, local plastic deformation occurs due to stress concentration, metallurgical defects and low Rs, resulting in new discontinuous metal. The surface destroys the integrity of the oxide film. In addition, some sulfide inclusions are exposed on the thread surface and the fracture surface, and their electrochemical characteristics are far from the metal, so it is impossible to maintain a complete oxide film on the metal surface.
After the oxide film on the metal surface is destroyed, it is an anode relative to the unbroken passivated surface, which induces cracks under the action of the medium and stress in the working environment. When the stress corrosion crack increases to a certain extent, the actual control system developed successfully is as shown. Control system block diagram After the system has been debugged, a large number of tests have been carried out. Here are a few test results: in order to set the depth of the soil to be 15cm, the speed of the vehicle is 4km / h, when the test road is a beach, the depth error curve of the plow; for setting the depth of the soil is 20cm, the speed is 3km / h When the test pavement is a soil-grade secondary hardness pavement, the plowing soil depth error curve. Conclusion The actual test shows that the designed controller can basically guarantee the requirement that the depth of the mine clearance system is less than 1cm. This shows that the system not only has better static and dynamic precision, but also has strong robustness.
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