Abstract
The combined oxidation-sulphidation behaviour at high temperature of two nickel-based and two cobalt-based cast alloys is analysed and discussed. These materials are evaluated in view of their possible application for building the components of the high temperature heating furnace up to 1380 °C. Typical service conditions combine extremely high temperatures with the simultaneous presence of sulphur-bearing impurities, due to the combustion of fuels derived from the coke production. The main requirements for this demanding application are simultaneous resistance to high thermal levels and to degradation caused by oxidation and sulphurization. The atmosphere chosen to simulate this harsh environment consists of atmospheric air enriched by 12% of sulphur. The testing procedure has been performed at three temperatures (1080 °C, 1180 °C and 1280 °C) and three times of exposure (24 h, 48 h and 96 h). Macroscopic observations have been coupled to microstructural investigations, performed by SEM-EDS. The analysis is mainly focused on the identification of the microstructural alterations experienced by each alloy and on the extent of internal attack caused by oxygen and sulphur. The combination of the macroscopic experimental data and the information from the microstructural observations alloys have been shown to point out the performances of alloy grade throughout the testing trials in order to define the chemical compositions associated to the moderate weight loss and limited depth of surface attack. The new chemical composition of a Ni-superalloy points out performance of corrosion resistance that are even better than the cobalt based alloys usually selected for such working conditions.
Highlights
Heat-resistant nickel-alloys and cobalt-alloys are commonly employed to build the component for the furnaces and devices for heat treatments and for performing several metal forming processes at high temperature [1e4]: they combine excellent heat resistance and superior mechanical performances at high temperature
The present study aims at assessing the response and durability of two Ni-based and two Co-based cast alloys, when exposed to a mixed oxidizing-sulphidizing atmosphere of fixed composition
A preliminary observation of the microstructure featuring the coupons has been performed on the as-received materials and the analysis has pointed out a typical dendritic as-cast microstructure (Fig. 1)
Summary
Heat-resistant nickel-alloys and cobalt-alloys are commonly employed to build the component for the furnaces and devices for heat treatments and for performing several metal forming processes at high temperature [1e4]: they combine excellent heat resistance and superior mechanical performances at high temperature These features make them very eligible candidate materials for hot-work components like furnace riders, conveyor belts, transportation rolls and other tools. Sulphur gas compounds are dangerous for the health of the equipment because they are featured by high diffusivity and they manage to form low-melting eutectics especially with nickel [7,8] damaging the alloy These issues acquire particular relevance in the field of materials selection, because both nickel and cobalt alloys are usually very expensive [9e11], and their adoption shall be justified and supported by a substantial increase in useful service life if compared to more conventional and cheaper heat-resistant steels. The field experience with similar service conditions has pointed out strong criticalities, even with alloys designed for maximized heat and oxidation resistance [10,12e18]
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