Influences of microstructure on service properties of 5Cr21Mn9Ni4N heat resistant alloy

The plates of four diesel engine exhaust valves used in a truck failed when servicing for 2000 h, which are made of 5Cr21Mn9Ni4N (21-4N) heat resistant alloy. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction were performed on the plate material of the failed valves and a new valve. The results show that the plate material before service is composed of first carbides (Cr₂₃C₆) and γ austenite matrix. However, appearance of the lamellar structure and plate-like precipitates within the grain is the general metallurgical features of the plate material of the failed valves, which indicates that the austenite matrix decomposed in service. XRD and electron diffraction results demonstrate that the lamellar structure is mainly composed of Cr₂₃C₆ and γ austenite, coexisting CrN phase, which formed by the discontinuous precipitation mechanism. Electron diffraction shows that the convex lens-like CrN phases precipitate within the grain, which formed by the continuous precipitation mechanism. Fractographic studies indicate that the fatigue is the dominant failure mechanism of the three valves and burning-through occurred on the plate of another failed valve. Formation of the lamellar structure and plate-like precipitations within the grain is mainly responsible for the failure of the valves. Isothermal aging tests were conducted on the plate material from the new valve to study the material stability at high temperatures. The microstructure development during aging was observed by SEM and optical microscopy. The influence of the microstructure on the service properties was discussed and possible failure causes of the valves were assessed.