回火温度对2.25Cr-1Mo-0.25V钢粒状贝氏体显微组织和力学性能的影响

EFFECTS OF TEMPERING TEMPERATURE ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF GRANULAR BAINITE IN2.25Cr-1Mo-0.25V STEEL

利用纳米压痕仪,OM,SEM,TEM,XRD,EPMA等设备研究了加氢反应器用2.25Cr-1Mo-0.25V钢正火态粒状贝氏体组织及力学性能随回火温度的变化.结果表明,2.25Cr-1Mo-0.25V钢正火后得到由贝氏体铁素体、马氏体和残余奥氏体岛(M-A岛)组成的粒状贝氏体组织.纳米压痕测量结果表明,由于M-A岛中富集C,其硬度显著高于贝氏体铁素体.在回火过程中,M-A岛分解和贝氏体铁素体软化的综合作用导致了2.25Cr-1Mo-0.25V钢在-18℃的冲击功随着回火温度的升高先增加后减少.除了铁素体基体回复再结晶软化效应外,粒状贝氏体组织中硬相M-A岛回火转变程度以及析出碳化物形貌、尺寸和分布是影响2.25Cr-1Mo-0.25V钢冲击韧性的关键因素.

2.25Cr-1Mo-0.25V steel is the most popular material used for pressure-vessel applied at elevated- temperature in hydrogen environment. For higher process efficiencies in future coal-conversion plants, chemical processing plants, and petrochemical-refining plants, much thicker cross-section component are necessary for con- structing much larger pressure-vessel for these plants. Because of the thick cross-section, the cooling rate in the central region of the component is insufficient to obtain low bainite during quenching treatment, and a large amount of granular bainite appears in the central region. Previous studies have shown that good impact toughness can be achieved by appropriate tempering for 2.25Cr- 1Mo-0.25V steel with low bainite microstructure. However, the impact toughness of 2.25Cr-lMo-0.25V steel with granular bainite after tempering always cannot satisfy the de- manding requirement due to the unclear understanding of the evolution of microstructure and mechanical proper- ties during tempering. In this work, the influence of tempering on the microstructure and mechanical properties of 2.25Cr-1Mo-0.25V steel with granular bainite microstructure was investigated by OM, XRD, SEM, TEM and EP- MA. The results show that the normalized 2.25Cr-1Mo-0.25V steel with granular bainite microstructure is com- posed of bainite ferrite and island of martensite and austenite （M-A island）. Nanoindentation test indicates that M- A island is much harder than that of metrix bainite ferrite, because of the high concentration of carbon in M-A islands. The synergistic effect of the decomposition of M-A islands and softening of bainite ferrite determined that Cbarpy absorbed energy at -18 ~C increases first and then decreases with the increasement of tempering tempera- ture. The degree of decomposition of M-A islands and the morphology, size and distribution of carbides in granular bainite, coupled with the softening effect ofbainite ferrite recrystallization are the key factors determining low-tem- perature impact toughness of 2.25Cr-lMo-0.25V steel.