The Effect of Dual Microstructure Heat Treatment on an Advanced Nickel-Base Disk Alloy

Abstract

Five heat treat options for an advanced nickel-base disk alloy, LSHR, have been investigated. These included two conventional solution heat treat cycles, subsolvus/oil quench and supersolvus/fan cool, which yield fine grain and coarse grain microstructure disks respectively, as well as three advanced dual microstructure heat treat (DMHT) options. The DMHT options produce disks with a fine grain bore and a coarse grain rim. Based on an overall evaluation of the mechanical property data, it was evident that the three DMHT options achieved a desirable balance of properties in comparison to the conventional solution heat treatments for the LSHR alloy. However, one of the DMHT option, SUB/DMHT, produced the best set of properties, largely based on dwell crack growth data. Further evaluation of the SUB/DMHT option in spin pit experiments on a generic disk shape demonstrated the advantages and reliability of a dual grain structure at the component level.

P/M Alloy 10: A 700�C Capable Nickel-Based Superalloy for Turbine Disk Applications

“…This maintains a fine grain size while solutioning a sizable fraction of the γ'. 2 However, operating the disk rim at 700°C will require a microstructure optimized for time dependent properties, requiring a coarser grain size resulting from solution heat treating above the γ' solvus (i.e., supersolvus). Another option is to fabricate dual microstructure forgings, characterized by a fine grain bore and a coarse grain rim, such that the interface is located in a relatively benign region of the machined disk.…”

Fatigue Failure Modes of the Grain Size Transition Zone in a Dual Microstructure Disk

“…Segments of the extrusion billet were machined to cylinders approximately 15 cm diameter and 20 cm long, then isothermally forged into flat disks approximately 31 cm diameter and 6 cm thick. A contoured disk was then machined with an outer diameter of 30 cm, maximum bore thickness of 5 cm, and rim thickness of 3.8 cm [7]. The disks were heat treated by Ladish Company, Inc.…”

“…The monotonic and fatigue properties of these DMHT disks were previously compared for bore and rim locations [7]. This discussion will therefore remain focused on the fatigue properties of the grain size transition zone.…”

“…However, disks heated above the γ′ solvus in "supersolvus" solution heat treatments lose constraining coarse "primary" γ′ particles, allowing grains to grow near 30 µm -70 µm in diameter. Such coarse grains can give lower strength and cyclic fatigue resistance, but can improve time-dependent properties such as creep and dwell fatigue cracking at application temperatures of 600 °C -700 °C, often required in disk rim regions for high performance gas turbine engines [5][6][7][8].…”

“…Based on these perceived benefits, specialized heat treatment methods have been developed for producing tailored grain microstructures in the bore and rim portions of nickel-based superalloy disks [5][6][7][8]. These methods can limit the bore to subsolvus temperatures producing fine grain sizes near 5 µm -10 µm in diameter, while allowing the rim to reach supersolvus temperatures to produce grain sizes of 30 µm -80 µm in diameter.…”