Effect of Thermomechanical Processing on the Properties of Inconel 706 Alloy

Abstract

Variation in grain size is often observed in heavy sections of superalloy forgings because regions of the workpiece may undergo different deformation histories during upset-type forging operations; however, uniform, fine grain size is critical to achieving the desired balance of mechanical properties in forged products. Discussed in this paper is the effect of thermomechanical processing (TMP) approaches on the microstructure and key mechanical properties of Inconel@ alloy 706 (IN-706). IN-706 is a precipitation strengthened nickel-iron based superalloy utilized in gas turbines, particularly large turbine components in land-based power generation engines. To investigate TMP, forging preforms were machined from as-received ingot, billetized to refine the coarse ingot grain structure and then finished forged to the desired size prior to solution heat treatment aging. Resulting pancakes were solution heat treated (SHT) at 1790 1810°F (977 988°C) for various times at temperature and cooled to room temperature at two cooling rates. Two-step aging of the pancakes was conducted at 1350°F (732°C) and 1150°F (621°C). Grain size and mechanical properties, including Charpy V-Notch (CVN), of solution treated and aged (STA) pancakes were evaluated and rationalized on the basis of TMP history. Microstructure in the as-finish forged pancakes exhibited an duplex grain morphology, with grain size in the range of ASTM 6.1 to 9.4. SHT was observed to eliminate the duplex grain structure and provide a more uniform final grain size of ASTM 3.3 to 5.1, depending upon the SHT time and temperature. The duplex as-forged microstructure could be leveled after relatively short exposures at SHT temperatures. Sluggish grain coarsening characteristics were observed, thus longer SHT times at temperature did not unduly coarsen grain size. The duplex asforged microstructure was observed to be a function of microstructural and compositional inhomogenieties in the forgings. UTS, YS, Elongation and RA properties remained constant. However, CVN values were observed to vary with time-at-temperature in SHT. This CVN behavior for IN-706 is then rationalized on the basis of microstructural observations. Superalloys 718,625,706 and Various Derivatives Edited by E.A. Loria The Minerals, Metals & Materials Society, 1997