Abstract
Wrought Ni-Cr-Mo alloys of the C-family show a sensitivity to intercrystalline attack especially after exposure in the temperature range of 650°C to 950°C. Nevertheless, microstructural changes due to precipitation of intermetallic phases can occur up to a temperature level of 1050°C and this can affect the localized corrosion resistance. Thermal stability of wrought Alloy C-276 is a lot lower in comparison to Alloy 59. Sensitized at 870°C for only 1 hour, Alloy C-276 fails in the ASTM-G 28 B test due to rapid intercrystalline penetration and pitting whereas Alloy 59 can be aged up to 3 hours without any increase of the corrosion rate or any pitting attack. The same ranking applies during polythermal cooling cycles. Alloy C-276 requires a cooling rate of 150°C/min. between the solution annealing temperature and 600°C to avoid any sensitization whereas for Alloy 59 a relative slow cooling rate of 25°C/min. is acceptable. The critical pitting temperature of Alloy 59 when tested in the Green Death solution had been determined to be > 125°C. The temperature was not lowered during aging up to 3 hours at 1050°C or if a cooling speed of 25°C/min. was applied. However, cooling rates of 50°C/min. or less reduced the critical pitting temperature of Alloy C-276 from 115°C in the solution annealed and water quenched condition to only 105°C.
