Recrystallization of René N4 and N5

Abstract

Recrystallization of equiaxed, directionally solidified and single crystal cast nickel base superalloys can occur during some of the manufacturing and repair processes. While equiaxed, and to a lesser extent, directionally solidified cast nickel-based superalloys have grain boundary strengthening this is nonexistent for single crystal superalloys. Therefore, it is especially important to avoid or limit recrystallization during manufacture and repair of these nickel base single crystal gas turbine components. This study performed by TEServices, EPRI, MD&A and Sulzer concerns the effect of recrystallization when compressive stresses are introduced during manufacturing and repair processes (grit cleaning and shot peening) when followed by (full solution, partial solution and stress relieve) heat treatments. The objective of this paper is to obtain a better understanding the recrystallization behavior of two single crystal superalloys, René N4 and N5, during the repair processes (grit cleaning, shot peening and heat treatments) and determine how to minimize the depth of recrystallization. The impact of recrystallization on mechanical strength (stress rupture and fatigue) on René N4 material was limitedly tested. This study concludes that for both single crystal alloys, René N4 and N5, the less compressive stresses that are introduced during manufacturing and repair, and the lower temperature of exposure during follow up heat treatments, results in a lower recrystallization layer depth.