Mechanism of failure of UNS R20501 high temperature tube support pipes after 10 years of service

摘要

Microstructural analysis of failed support pipes made of cast UNS R20501 alloy in a reboiling heater unit was performed to understand its physical metallurgy and the origin of the pipe failures. By design, these support pipes are exposed to a temperature gradient from near ambient (furnace wall) to about 800 °C (furnace temperature). It was found that all the failures had occurred at locations where the pipes were exposed to around 470-550 °C. Analysis of different regions of failed pipes indicated that the failure was associated with a high degree of local hardening causing poor ductility. Detailed microstructural studies revealed the hardening and low ductility was caused by the precipitation of ultrafine cr-Cr precipitates during service. In addition, the austenite phase had also undergone an ordering reaction resulting in the formation of Ni2Cr domains. It was concluded that the combination of α-Cr precipitation and Ni2Cr ordering resulting in local hardening caused the failure of the pipes. The results of the analysis suggest that this alloy may not be suitable for service in the range of 470 ～560 °C in the as-cast condition.