Investigation On High Temperature Aging Characteristics Of 21-4N And 21-4N (Nb, V) Heat-Resistant Steel

5Cr21Mn9Ni4N (21-4N) heat-resistant steel is usually used as plate of the diesel exhaust valve. It is widely used in automotive industry. Its service properties at high temperature have been attended by scientists, manufacturer, and customers. The aging characteristics at high temperature and service properties of the alloy are related to the prior microstructure.Especially, the presence of cellular structure in the matrix can seriously decrease the service properties of the valve at high temperature. However, It should be noted that because the exhaust valve services at temperature(700-900℃), the matrix without cellular structure at ambient temperature might decompose to form cellular structure (M₂₃C₆+γ+CrN), even a phase, depending on the stability of the matrix. The cellular structure. The cellular structure induced during servicing also influence on the service properties of the exhaust valve. The appearance of the cellular structure is one of the causes of the premature failure for the valve.In order to investigate the main factors related to the matrix stability of 21-4N series alloys, according to feedback of failed valve in the recent years, in present study, isothermal aging tests were conducted in the standard 21-4N steel, the advanced 21-4N (Nb,V) steel(specimen 1 and 2) and two kinds of prior microstructure of 21-4N(specimen 3 and 4). The present phases and changes in microstructure during aging were analysed qualitatively and quantitatively by XRD and SEM and OPM to study the influence of the prior microstructure and alloy composition on the aging characteristics. The microstructure of the cellular structure was observed by TEM. The tensile properties of the specimens with different amount of the cellular structure at room temperature and high temperature(800℃) were determined to indicate the dependence of the cellular structure on the mechanical properties. The fracture morphology was observed by SEM to show fracture mechanism.Four typical failed valves were analyzed to further demonstrate the dependence matrix stability or the cellular structure on aging characteristics and service properties.The following conclusions were obtained: (1) Addtion of Nb and V can increase the matrix stability of the series of 21-4N...