Embrittlement of HP40Nb Heat-Resistant Alloy at Intermediate Operating Temperatures

Abstract

Sections of an ethane cracker furnace were constructed of a modified heat-resisting alloy, HP40Nb MA. The application included a section which was designed to operate at a maximum design temperature of only 740 °C. The selection of this alloy for this section is deemed conservative as the alloy is designed to operate at temperatures in excess of 1000 °C. An elbow from the colder section failed in a brittle manner in service after only 2 years in operation. A metallurgical investigation indicated the elbow material to possess significantly higher tensile strength than expected for a new material while tensile elongation indicated it to be severely embrittled. The embrittlement was proven to be the result of a combination of intradendritic precipitation of needle-like sigma phase and partial transformation of the primary Nb-carbides to G-phase. The sigma phase is expected to have resulted in strengthening of the matrix and embrittlement of the alloy. Strain localization of the interdendritic regions could not be accommodated by the brittle G-phase which resulted in embrittlement and interdendritic cracking. The precipitation of sigma phase is unexpected considering the high Ni-content of this alloy. Thermodynamic simulation, however, proved that the precipitation of sigma phase is indeed possible at temperatures between 680 and 725 °C when slightly elevated Si-content is present. From the perspective of elevated temperature strength and corrosion resistance, the use of HP40Nb MA may appear to be a conservative selection at the comparatively low operating temperature. Failure to recognize that the operating temperatures, while still very hot, are lower than recommended for this alloy therefore rendered the ostensible conservative selection as an incorrect selection.