Effect of Heat-treatment on Microstructure and Tensile Properties in Cast Alloy 718

Abstract

The effect of various types of heat-treatment on the mechanical properties of cast Alloy 718 has been investigated. Cast Alloy 718 bars were subjected to 'standard heat-treatment'_(SHT), 'HIP (Hot Isostatic Pressing) heat-treatment'_(HHT), and 'HIP-simulated heat-treatment'_(HS). In the absence of long time high temperature heat-treatment, a small amount of Laves phase remained in the 'SHT' specimen, and needle shaped δ precipitated in the vicinity of the Laves phase. Due to the formation of the Laves and δ phases in the 'SHT' specimen, it exhibited lower tensile properties than those of the others_specimens. On the other hand, the Laves phase was completely dissolved into the matrix after 'HHT' and 'HS' treatments. It is known that isostatic pressure reduces the self-diffusion coefficient, because of the lower self-diffusivity under HIP conditions in the interdendritic region, Nb segregation and the high amount of γ" precipitation that occurs. Due to the higher fraction of coarse γ" phases, the 'HHT' treated Alloy 718 showed excellent tensile strength.

Estimation of homogenisation time for superalloys based on a new diffusional model

“…Therefore, the segregation of these elements has adverse effects on the performance of the material in the industry through impairing the precipitation processes and solution hardening effect. As a result, the homogenisation treatment of the cast ingot is an inevitable practice [15][16][17][18]. Since this process needs holding the workpiece for a long time at elevated temperatures, optimisation of holding time is demanded by the industry from the standpoints of grain growth inhibition and saving the processing time.…”