Microstructure Development and Thermal Response of Delta Processed Billet and Bar for Alloy 718

Abstract

Delta processing of alloy 718 billet and bar provides a means to achieve fine grain microstructures for input forging stock applied to the manufacture of rotating turbine engine components. For an optimum balance of properties in the finished part, however, it is necessary to understand the effect of thermomechanical processing on billet/bar delta phase and subsequent response to thermal exposure during heating for forging. This study investigated the effect of conversion process on delta phase present in 203 and 137 mm Ø billets, and 64 mm Ø bar. Analysis included characterization of delta phase at intermediate conversion preform, billet and bar product, and after thermal exposure in the 968°C -1024°C temperature range.

Influence of Precipitated Phase Formation on Recrystallization Behavior of Superalloy 718

“…26) Consequently, the ¤ solvus temperature varies from one location of the microstructure to another, according to Nb-rich and Nb-lean regions, respectively. 10) Given a large number of ¤ phase precipitates and a heat treatment temperature of 1000°C (i.e., lower than, yet close to the average ¤ solvus value), the ¤ phase fails to dissolve thoroughly into the matrix. Consequently, in the asforged specimens aged at 910°C for 5 h and then solution heat treated at 1000°C for 135 min, the grains in the Nb-lean regions of the matrix (i.e., the regions in which the ¤ phase is fully dissolved) become recrystallized and grow in size, while those in the Nb-rich regions (i.e., the regions in which residual ¤ phase remains) retain their original (fine) size and un-recrystallized structure ( Fig.…”

“…59) Moreover, Thermomechanical Process would achieve finer grain structure by aging and precipitation of a high volume fraction of acicular ¤ phase before hot forming and during the subsequent dynamic recrystallization process; the ¤ phase inhibits recrystallized grain growth and generate a grain finer than ASTM No. 10. 10) However, even slight deviations of the forging temperature from the designed value may significantly change the microstructure.…”

“…10. 10) However, even slight deviations of the forging temperature from the designed value may significantly change the microstructure. For instance, lowertemperature areas of the forging commonly retain unrecrystallized grains of the original billet.…”