Abstract

In this work, a Cr35Ni45Nb alloy tube for use in a reformer furnace is subjected to high temperature (871 °C and 927 °C) creep regimes, and the microstructural changes (e.g. carbide precipitation and phase transformation) and consequent creep damage are investigated. The results of creep tests under closely similar applied stresses give different values of the Larson–Miller parameter (LMP) at different temperatures. As a result, the calculated creep rupture time at 871 °C based on the LMP value measured at 927 °C is approximately twice that which was actually measured at 871 °C. The microstructure of the as-cast tube is found to consist of an austenitic matrix with networks of MC (NbC) and eutectic (M₂₃C₆) carbides located both at grain boundaries and between dendrites. It is noted that the M₂₃C₆ carbide is not a primary eutectic carbide that can be observed in the as-cast Cr35Ni45Nb alloys. It can be argued that the primary eutectic M₇C₃ carbides were transformed into M₂₃C₆ due to the heat of spiral welding during the tube manufacturing process. After creep, the NbC carbides at both locations were mostly transformed to the G-phase (Nb₃Ni₂Si) and all the precipitates formed inside the austenitic matrix were composed of the M₂₃C₆ and G-phase. Creep cavities were initiated around the G-phase and grew cracks along the grain boundaries due to the formation of a Cr-depleted zone and the G-phase.

Graphic Abstract

中文翻译：

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Cr35Ni45Nb铸造合金重整管短期蠕变过程中的显微组织评估

摘要

在这项工作中，对重整炉中使用的Cr35Ni45Nb合金管进行了高温（871°C和927°C）蠕变状态的研究，并研究了微观结构变化（例如碳化物沉淀和相变）以及由此引起的蠕变破坏。在非常相似的外加应力下的蠕变测试结果在不同温度下给出了不同的Larson-Miller参数（LMP）值。结果，基于在927°C下测得的LMP值，在871°C下计算出的蠕变断裂时间大约是在871°C下实际测量的两倍。发现铸态管的微观结构由具有MC（NbC）和共晶（M ₂₃ C ₆）碳化物位于晶界和枝晶之间。注意，M ₂₃ C ₆碳化物不是在铸态Cr35Ni45Nb合金中可以观察到的主要共晶碳化物。可以说，在管材制造过程中，由于螺旋焊接的热量，初生共晶M ₇ C ₃碳化物转变为M ₂₃ C ₆。蠕变后，两个位置的NbC碳化物大部分都转变为G相（Nb ₃ Ni ₂ Si），并且在奥氏体基体内形成的所有析出物均由M ₂₃ C ₆组成。和G相。由于形成了Cr贫化区和G相，蠕变空穴在G相周围产生，并沿晶界生长裂纹。

图形摘要