In this study, effects of the high temperature precipitation behavior and precipitate on the high temperature deformation stability were investigated by heat treatment and high temperature compression tests on incoloy 825 alloy.
The effects of mechanical properties and microstructural changes were investigated according to hot forging ratio and heat treatment temperature and aging time after hot forging.
The results obtained from this study were as follows : Cr23C6 carbide was precipitated under 950℃, but was not detected over 1,000℃. Most of the precipitate consist of Cr23C6 carbide. Strain-rate sensitivity was the highest in 0.01s-1 and the lowest in 10s-1. Strain-rate sensitivity was decreased sharply below 950℃. In the temperature between 850℃∼1,150℃, plastic instable area did not exist. It showed the lowest Ziegler Parameter value of 0.06. Ziegler Parameter was the lowest as 0.06 at 850℃ with 10s-1 of strain. The highest Ziegler Parameter value(0.43) was found in plastic deformation at 1,050℃ with 0.01s-1 of strain. It tends to have an higher resistance to the high temperature deformation under 950℃, due to the precipitate.
Hot forging was carried out at the forging ratio of 0%, 60% and 90% respectively in a range of 900℃∼1,140℃ and followed solution treatment was conducted at 1,000℃ for 1 hr. In all the specimens of hot forged of 0%, 60% and 90%, precipitates were not observed. The average grain size of 0% specimen is 82㎛ and that of 60% and 90% is 56㎛ and 31㎛, respectively. The range of grain size in the 0% specimen is uneven in 182㎛ to 31㎛, but the grain size of 90% specimen is uniform. With increasing hot forging ratio, the mechanical properties such as tensile strength, elongation, hardness increased and impact toughness increased by grain refinement.
With increasing solution treatment temperature, the grain size increased and the volume fraction of total precipitates decreased, and the precipitates disappeared at 1,000℃. With increasing aging time at 700℃, the volume fraction of precipitate increased and the precipitates size increased. Most of the precipitates consist Cr23C6 carbide, and a small amount of TiC carbide was also observed. With decreasing solution treatment temperature and increasing aging time, tensile strength and hardness increased, and the elongation and impact value decreased. With increasing aging time, the impact value decreased sharply by the increased of the precipitate size.