A coil made of a nickel-chromium alloy (Material No. 2.4869) with approx. 80Ni and 20Cr had burned through after a brief period of operation as a heating element in a brazing furnace. The protective atmosphere consisted of an incompletely combusted coal gas. Furnace temperature reached 1150 deg C. This type of selective oxidation at which the easily oxidized chromium burns, while the nickel is not attacked, is caused by mildly oxidizing gases and is sometimes designated as green rot. Under these conditions, chromium-containing steels and alloys whose oxidation resistance is based upon formation of tight oxide layers are not stable.
A brazing-furnace muffle 34 cm (13 in.) wide, 26 cm (10 in.) high, and 198 cm (78 in.) long, was fabricated from nickel-base high-temperature alloy sheet and installed in a gas-fired furnace used for copper brazing of various assemblies. The operating temperature of the muffle was reported to have been closely controlled at the normal temperature of 1175 deg C (2150 deg F); a hydrogen atmosphere was used during brazing. After about five months of continuous operation, four or five holes developed on the floor of the muffle, and the muffle was removed from service. Analysis (visual inspection, x-ray spectrometry, and metallographic examination) supported the conclusion that the muffle failed by localized overheating in some areas to temperatures exceeding 1260 deg C (2300 deg F). The copper found near the holes had dripped to the floor from assemblies during brazing. The copper diffused into the nickel-base alloy and formed a grain-boundary phase that was molten at the operating temperature. The presence of this phase caused localized liquefaction and weakened the alloy sufficiently to allow formation of the holes. No recommendations were made.