Inconel-618E (54 Ni-23 Cr-6 W) was developed specifically to meet the requirements of the high-temperature gas-cooled reactor (HTGR). The alloy has good fabricability, weldability, and, above all, metallurgical stability. Alloy 618E is free of cobalt and aluminum. At temperatures of 954°C (1750°F) and higher, alloy 618E has stress rupture strength comparable to Inconel-617. At lower temperatures, the rupture strength of alloy 618E is somewhat lower than that of alloy 617. Alloy 617 remains the best-established alloy for HTGR service. However, alloy 618E does not have that material’s disadvantage of aluminum and cobalt content and may become a candidate for HTGR applications.