Metal Dusting

HAYNES® HR-235® alloy has been tested alongside competitive materials in a controlled atmosphere, thermal cycling rig. The reaction gas was H2 + 68% CO + 6% H2O, the carbon activity of which was 2.9 at the reaction temperature. The cycling operation, which was controlled automatically, comprised 45 minutes at the reaction temperature of 1256°F (680°C), followed by a cooling period of 15 minutes, during which the samples rapidly reached a temperature of about 194°F (90°C). The samples were tested for 1,200 (one hour) cycles, with the following results. The formation of filamentary carbon deposits with metallic nanoparticles (coking) is an indicator of the onset of surface damage (pitting).

Alloy Approximate Number of Cycles to Coking Form of Coke
601 48 Grain boundary deposits, pits
602CA 48 Adherent coke, no metal visible
617 48 Numerous small pits, grain boundary deposits
693 24 Numerous small pits
696 100 Attack on grain boundaries
HR-235® 400 Grain boundary deposits, minor pits

HR-235 onset of coking