Comparative Thermal Aging Effects on PM-HIP and Forged Inconel 690

Abstract

This study compares thermal aging effects in Inconel 690 (IN690) produced by forging and powder metallurgy with hot isostatic pressing (PM-HIP). Isothermal aging is carried out over 400-800°C for up to 1000 h and then metallography and nanoindentation are utilized to relate grain microstructure with hardness and yield strength. The PM-HIP IN690 maintains a constant grain size through all aging conditions, while the forged IN690 exhibits limited grain growth at the highest aging temperature and longest aging time. The PM-HIP IN690 exhibits comparable mechanical integrity as the forged material throughout aging: hardness and yield strength are unchanged with 100 h aging, but increase after 1000 h aging at all temperatures. In both the PM-HIP and forged IN690, the Hall-Petch relationship for Ni-based super-alloys predicts yield strength for 0-100 h aged specimens, but underestimates yield strength in the 1000 h aged specimens because of thermally induced precipitation.

Fig. 1.

Representative optical micrographs from PM-HIP IN690 (a) unaged, (b, c) aged at 400°C for 100 h and 1000 h, (d, e) aged at 600°C for 100 h and 1000 h, and (f, g) aged at 800°C for 100 h and 1000 h. Forged IN690 (h) unaged, (i, j) aged at 400°C for 100 h and 1000 h, (k, l) aged at 600°C for 100 h and 1000 h, and (m, n) aged at 800°C for 100 h and 1000 h. Scale bars are applicable to all PM-HIP or forged micrographs.

Fig. 2.

Effect of temperature on (a) grain size and (b) nanohardness for 100 h and 1000 h aging.

Fig. 3.

Hall-Petch relationship for Ni superalloys reasonably predicts yield strength for specimens aged 0–100 h, but underpredicts yield strength for 1000 h aging