The Development of Improved Performance PM Udimet 720 Turbine Disks

Abstract

Udimet@ 720 is an important alloy because it exhibits an outstanding balance of strength, temperature, and defect tolerance characteristics (Ref. 1). In its cast and wrought alloy form, it is employed for turbine disk components utilized in a large number of civil and military propulsion systems. In its powder metal (PM) alloy form, which has been shown to be very cost competitive with its cast and wrought counterpart, Udimet 720 exhibits superior alloy homogeneity that provides an opportunity to develop the uniform and controlled microstructures desired for advanced designs (Ref. 2). Because it was recognized that Udimet 720 compositions developed for cast and wrought applications were not necessarily the best for PM processing, it was decided to study PM Udimet 720 chemistry, processing, and mechanical property relationships. The goal of this effort was to determine if an improved balance of performance characteristics could be developed for advanced turbine disk applications. For this work, four Udimet 720 chemistry modifications involving boron, zirconium, and hafnium additions were made to the baseline composition. Also represented in the program, for reference, was a contemporary baseline PM Udimet 720 composition. To produce materials for evaluation, powder representative of each of the five compositions was argon atomized and screened to a -150 mesh powder fraction and consolidated by hot isostatic pressing (HIP) technique into 30 lb billet preforms. The HIP processed billets were then isothermally forged at two different forging temperatures into pancakes. After forging, selected pancakes were subsolvus and supersolvus solution heat treated to achieve target grain sizes, respectively, of ASTM 11 and ASTM *@ Udimet is a registered trademark of Special Metals Corporation. 9. Cut-up evaluations were then performed on fully heat treated disks representative of each grain size and chemistry combination. This included tensile, creep rupture, fatigue crack growth (FCG), and low cycle fatigue (LCF) testing at temperatures and conditions of interest for gas turbine disk applications. Test results showed supersolvus processing to an ASTM 9 target grain size resulted in attractive reductions in 1200°F FCG rate relative to both the subsolvus processed ASTM 11 PM alloys and cast and wrought Udimet 720 disk product subsolvus processed. Significant improvements in both 1200°F LCF life and 1350°F creep rupture capability were also achieved for material processed to the ASTM 9 grain size. It was observed that the higher boron and boron/zirconium levels in the coarse grained PM alloys enhanced these properties. By comparison, tensile testing did not show trends that could be attributed to the chemistry modifications, but 0.2% tensile yield strengths for the ASTM 9 material were reduced on the order of 10 ksi relative to the ASTM 11 material. From this work it was found that selected chemistry modifications, when combined with appropriate grain size control, can yield an improved balance of FCG resistance, LCF capability, creep strength, and tensile properties relative to the baseline PM alloy as well as traditional cast and wrought Udimet 720 that is subsolvus processed. In particular, this work has shown promise for PM Udimet 720 alloy forms exhibiting boron and boron/zirconium levels that exceed baseline PM and cast and wrought Udimet 720 target levels. Based on these results, it should be possible to greatly extend the performance capabilities of PM Udimet 720 turbine disks. supml10ys 2ooo Edited by T.M. Pollock, RD. Kissinger, R.R. Bowman, K.A. Green, M. McLean, S. Olson, and J.J. Schirra TMS (The Minerals, Metals &Materials Society), 2CH33