Effect of Cooling Rate From Solution Heat Treatment on Waspaloy Microstructure and Properties

Abstract

Achievement of critical dimensions after wholesale removal of stock from large, ring rolled Waspaloy forgings and fabrications has been complicated by residual stresses induced via rapid quench from the solution heat treat cycle. Part movement during machining can be reduced significantly by substituting air cool for the oil quench commonly specified to achieve high mechanical strengths. However, excessive time in the sub-)/ solvus temperature range during slow cooling results in undesirable precipitate coarsening with an attendant degradation in strengths. Therefore, cooling rates must be achieved which strike a balance between manufacturing ease and Propew controlling microstructural features. This report provides microstructural and mechanical property data for flash welded, shaped Waspaloy bar stock subjected to various cooling rates from a 1018'C/4 hour solution cycle followed by a stabilize and age heat treat. Average cooling rates evaluated in this investigation ranged from 5.5 to 145"C/minute through the f precipitation temperature range of 982 through 760°C. Oil quench data from production hardware are also provided for reference. A significant improvement iu 538 and 760°C yield strengths, 760°C tensile strengths, and 704°C creep resistance were significantly improved by increasing cooling rates. Ductility simultaneously decreased at 760°C but did not exhibit dependence at 538'C. Minimum cooling rates of approximately 4OYYminute were necessary to approach the properties and microstructural features typical of oil-quenhed Waspaloy.

The Effects of Heat Treatment and Microstructure Variations on Disk Superalloy Properties at High Temperature

“…Yield strength and tensile strength increased with increasing cooling rate, as has been reported earlier for many disk superalloys [5,6] These relationships allowed consideration of mechanical property combinations for different heat treatment conditions. Tensile yield and ultimate strengths responded in similar manners to the heat treat parameters.…”

“…5 ) were included in these regression trials. Stepwise regression was then used to assess the significance of each term and their interaction terms:…”

“…Strength in this case was believed to be proportional to 1/d 0. 5 . The regression results indicated both terms significantly affected yield strength here, and included contributions of both γ S ′ and γ T ′.…”

“…Such capabilities would not only allow higher compressor exit temperatures, but also allow the full utilization of advanced combustor and airfoil concepts under development. It is well known that the tensile and creep properties of disk superalloys at elevated temperatures vary significantly with heat treatment, even for a constant grain size [5]. More recent work has shown that dwell fatigue crack growth resistance at high temperatures can also be highly sensitive to heat treatment [6].…”

“…However, to be useful, it is desirable that these models have resolution at least as good as that from more widely used process-property models [5].…”