ABSTRACT

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 a-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 a-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.

INTRODUCTION

As the result of its excellent resistance to fuel ash corrosion, 50 %Ni-48 %Cr- 1.5 %Nb (UNS R20501) cast austenitic alloy has been widely used as a supporting pipe or hanger of heater tubes for high temperature units in refineries and coal power plants. Several investigations have established the alloy’s improved resistance to molten salt corrosion compared to HK or HP heat resistant alloys, particularly in vanadium rich environments.1,2 However, there is insufficient information in the literature about damage mechanisms when the alloy is used in the service. G. Caironi et al. first mentioned an issue on the use of UNS R20501 alloy at high temperatures.3 They found ductility loss of the alloy when exposed to 700 °C due to precipitation of a-Cr phase. The authors explored annealing pretreatments at 1000 °C to obtain a desired morphology of the precipitates prior to aging at 700 °C. It was revealed that preheat treatment had an beneficial effect of reducing ductility loss during service. There have been few other investigations on the precipitation in Ni-Cr binary alloys and these studies focused on the crystallographic orientation relationship between a-Cr precipitates and austenite.4-6 A few studies also made an effort to investigate Ni2Cr ordering in a 35 %Cr-65 %Ni alloy and its effect on the mechanical property.7,8 It was shown that an ordering reaction occurred when the alloy exposed to around 500 °C and resulted in loss of ductility.

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