| 5Cr21Mn9Ni4N heat-resistant alloy is usually used as plain material of exhaust valve the diesel. Based on analysis of the automotive component failure received for investigation during 2001-2002, it can be shown that the failure rate of the valve is 5.4‰, most of which are the exhaust valves. Fracture failure of the exhaust valve usually takes place at rod region of the valve, transition region between the rod and plate, and plate region. The failure of the valve may result in the engine damage to induce broke block a lot of serious accidents, which would produce a great economic lose to the businesses and customers.In order to solve the fracture failure of the valves and improve their service properties, a lot of failure cases of the exhaust valves were analysed. Various causes can result in the fracture failure of the exhaust valve. The interfering between valve and piston resulting from the bad coordination in valve train system can cause the fracture of the valve.The manufacture or design errors, such as less or larger gaps between valve and valve guide would lead to the fracture of the valve. Additionally, over-heating of the valve may make the valve fracture. This paper present four summarized cases studies of the exhaust valve failures. The analysis results show that the cracks grow in the form of transgranular intergranular fracture. The fatigue fracture is the main mechanism for the valves. Although a lot of external factors such as bending loads exerted to the rod of the valve or burning erosion by high temperature gases have something to do with the failure of the exhaust valve, the cellular precipitation consisting of M23C6+( resulting from un-stable microstructure of the valve material in serving at high temperature should be mainly responsible for the failure of the exhaust valves. Microstructure stability and service properties of 5Cr21Mn9Ni4N heat-resistant alloy at high temperature have been attended by scientists, manufacturers and customers. The microstructure of the alloy is closely related to its properties. The heat treatment technology should be one of the important ways to control or improve the microstructure of the alloy. The suitable heat treatment technology can obtain excellent microstructure to assure superior properties. Additionally, adding some alloy elements can improve the microstructure stability. The prior microstructure before using influences intensely on the service properties at high temperature and aging characteristics. So the graininess grade, the difference in graininess, especially cellular structure grade of the matrix for 21-4N alloy are formulated the routine examination. The cellular structure can seriously decrease the service properties to lead to the advanced failure of the exhaust valve. However, it should be noted that the matrix without cellular structure at ambient temperature might decompose to form cellular structure (M23C6+(), depending on the stability of the matrix, or prior microstructure. The cellular structure induced during servicing also influence on the service properties of the exhaust valve.21-4N alloys (H and J ) with different prior microstructure were aged isothermally and continuously at different temperature. The present phases during aging were studied by XRD and SEM, combined with TEM to show structure changes during aging. The influence of prior microstructure and different aging characteristic of the alloys on service properties were discussed.Four typical failed exhaust valves were analyzed to show the dependence between the prior microstructure, aging characteristics, and service properties more.The following conclusions were obtained:(1) The prior microstructure of the H specimens are composed of (-austenite matrix and granular M23C6 carbides. The grain size of the austenite matrix is very fine and well-distributed, whose graininess is 7-8 grade. The carbides are homogeneously distributed on the matrix in chain-like. The prior microstructure of the J specimens are also composed of (-austenite matrix and gra... |
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