Effects of Trace Pb on Creep and Fracture of 25Cr35Ni-Nb Alloy

The centrifugal casting 25Cr35Ni-Nb ethylene pyrolysis furnace tubes with different contents of Pb were selected to study the effects of Pb content on the creep and fracture properties. Both the high temperature rupture test at 1100 °C /17 MPa and the slow straining test at 850 °C with different loading rates show that the increase of Pb content significantly degrades the high temperature creep properties of centrifugal casting 25Cr35Ni-Nb alloy furnace tubes. The fractographic observation and the chemical composition analysis show that Pb segregates to the grain boundary at high temperature. It shows that the segregation of Pb to grain boundary occurs even when the content of Pb is several ppm, which leads to the reduction of grain boundary surface energy at high temperature. The initiation of creep cavities becomes easier. The creep cavities coalesce together under the local stress, forming cracks and finally causing the early failure of furnace tubes.