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

Area of Science:

• Materials Science
• Metallurgy
• Mechanical Engineering

Background:

• Inconel 690 (IN690) is a critical nickel-based superalloy used in high-temperature applications.
• Understanding thermal aging effects is crucial for predicting material performance and lifespan.
• Comparing different manufacturing routes, such as forging and powder metallurgy, is essential for optimizing material properties.

Purpose of the Study:

• To investigate and compare the effects of thermal aging on the microstructure and mechanical properties of forged IN690 and powder metallurgy with hot isostatic pressing (PM-HIP) IN690.
• To analyze the relationship between grain microstructure, hardness, and yield strength after aging at various temperatures and times.
• To evaluate the applicability of the Hall-Petch relationship in predicting the yield strength of aged IN690.

Main Methods:

• Isothermal aging of forged and PM-HIP IN690 at temperatures ranging from 400°C to 800°C for durations up to 1000 hours.
• Metallographic analysis to examine grain size and microstructure evolution.
• Nanoindentation testing to measure hardness and determine yield strength.

Main Results:

• PM-HIP IN690 maintained a consistent grain size across all aging conditions.
• Forged IN690 exhibited limited grain growth only at the highest aging temperature (800°C) and longest aging time (1000 h).
• Both material types showed comparable mechanical integrity; hardness and yield strength remained stable after 100 h of aging but increased after 1000 h of aging at all temperatures.
• The Hall-Petch relationship accurately predicted yield strength for specimens aged up to 100 h, but underestimated it for specimens aged 1000 h due to precipitate formation.

Conclusions:

• PM-HIP IN690 offers superior microstructural stability during thermal aging compared to forged IN690.
• Both manufacturing routes yield materials with comparable mechanical integrity, demonstrating resilience to thermal aging up to 1000 hours.
• Thermally induced precipitation significantly influences the yield strength of IN690 after extended aging, necessitating consideration beyond the standard Hall-Petch relationship for accurate mechanical property prediction.