Determining the Role of O2 and H2O Atmosphere Impurities on the Oxidation of IN718 During High Temperature Inert Gas Heat Treatment

The influence of trace levels of O₂ and H₂O, contamination in an inert gas heat treatment atmosphere on the oxidation behvaiour of IN718 was investigated. Heat treatments consisted of holding IN718 at 1050 °C for 2 h in a combined thermogravimetric balance and gas chromatography-mass spectrometer (GCMS). Furnace atmospheres explored included 22–703 ppm O₂ and H₂O concentrations of 23–387 ppm. The GCMS measurements were able to quantify the O₂ and H₂O concentrations during heat treatment and revealed that oxidation became measurable at approximately 800 °C. The oxidation rate was parabolic during the 1050 °C isotherm, increasing linearly with an increase in either O₂ or H₂O concentration up to a value of 480 ppm. Beyond 480 ppm the oxidation remained constant and equivalent to that reported in air. A two layer surface oxide structure consisting of Cr₂O₃ and TiNbO₄ formed when the O₂, and H₂O content increased beyond 33 and 23 ppm respectively. Dry O₂ conditions (i.e. H₂O of approximately 25 ppm), caused spalling of the Cr₂O₃ oxide surface during cooling when the O₂ ppm was 124 ppm or above. In higher H₂O concentrations the Cr₂O₃ layer showed good adherence to the base metal and no cracking during cooling. The use of a He–5% H₂ carrier gas did not alter the oxidation rate significantly, but did increase the H₂O concentration, thus preventing oxide spalling during cooling.