Effects of Al Additions and Reheat Treatments on Microstructures of Modified Nickel-Based Superalloy, Grade Inconel 738, by Vacuum Arc Melting Process

Abstract

This research has an aim to modify and improve the microstructures and hardness property of cast nickel-based superalloy, IN-738 by modified chemical composition of various Al additions as well as different reheat treatment conditions. IN-738 was modified by adding aluminium in 1, 2 and 3% by weight and casting with vacuum arc melting process. Then all casted alloys were performed with 5 different solutioning temperatures of 1125°C, 1145°C, 1165°C, 1185°C and 1205°C for 6 hours and all following with the same precipitation aging temperature of 845°C for 24 hours. The various obtained microstructures of the casting and reheat treatments were observed and investigated by SEM to evaluate the average area fraction and size of γ’ precipitated particles. The mechanical property was evaluated by Vickers hardness. From the obtained results, it was found that fine microstructure, which would be the most appropriate for using at elevated temperature, was obtained by the Al addition with 1%wt. following with solutioning temperature range between 1125°C – 1145°C/6 hours and precipitation aging at 845°C/24 hours. The microstructures received consist of high area fraction of γ’ particles in cubic shape with the proper size. However, the maximum hardness value of 683 HV was obtained from the alloy with 3%wt. Al addition following with solutioning temperature of 1205°C.