New Advancements in Superalloys, Two New Structural Alloys for Gas Turbine ApplicationsDr. Lee Pike - Haynes International
To begin let me tell a little about Haynes International. Haynes is headquartered in Kokomo, Indiana and has been involved with development of specialty alloys since its beginning in 1912 when its founder introduces Stellite. Since Stellite Haynes has developed many specialty alloys, of which the most currently popular is Hastelloy X developed in 1954. At its Kokomo facility bar, billet, wire, remelt materials, plate, sheet, strip and coil are produced. Alloys are vacuum induction melted and electroslag remelted. Its Arcadia, Louisiana facility produces seamless and welded pipe and tubing. A third facility at Mountain Home, North Carolina produces wire product.
Tonight I'm going to talk about two of the most recent developments: Haynes 282 alloy, and Haynes NS-163 alloy.
Haynes 282 Alloy is designed for high temperature service in aircraft and land based turbine engines. It provides extremely good high temperature strength, while maintaining creep strength, thermal stability, weldability, and fabricability at levels not matched by other hight temperature alloys. Its makeup is primarily Ni, with 1.5% Fe, 10% Co, 19% Cr, 8.5% Mo, 1.5% Al, 2.1% Ti, 0.06% C, and 0.005% B. The major contributors to its strength are Mo, Al, and Ti.
Product is produced for forming in the solution annealed condition. Maximum strength properties are obtained by a two-step aging process. Stage 1 is 2 hrs at 1850 F with air cooling and promotes grain boundary precipitate. Stage 2 is 8 hrs at 1450 F and promotes gamma prime strengthening with the grains, and causes grain boundary precipitates to become "blocky" or disconnected which greatly improves ductility of the product.
Competing high temperatures alloys suffer from the following problems:
Poor Cold Forming
Narrow temperature range for hot working
Poor Weldability
Heat Treated Alloy 282 is designed to have an intermediate amount of gamma prime (19 mole %). Higher gamma prime alloys have faster kinetics for gamma prime formation and are therefor difficult to form. 282 alloy is more sluggish, so the anneal is better preserved for cold working. It is also much less susceptible to "strain-age" cracking in welds. A test was developed for this which involves heating a tensile bar at a controlled rate to represent a typical post-weld heat treatment. The bar is then strained to failure and the elongation measured. 282 alloy provides more than double the elongation of Waspaloy, and Rene 41, and an extended high temperature capability over 263 alloy which is extremely ductile.
Fabrication of 282 Alloy by hot or cold forming is excellent. Solution annealed it has a .2% offset yield strength of about 50-55 ksi, and elongation of about 60%. It is readily hot worked between 1750 and 2150F. The higher end of the range is best for initial breakdown and finishing at the lower end. Of economic importance to Haynes is the fact that 282 alloy can bright anneal.
One of the major benefits of Haynes 282 is its resistance to creep. At 1400 F the alloy has actually been found to increase in strength after 1000 hrs compared to 100 hour exposure. Its creep resistance virtually matches Rene 41 alloy and is far superior to Waspaloy or 263 alloy in this regard. Factors which provide the excellent creep include:
optimal gamma prime fraction
Solid solution strengthening by W and Mo
A very stable microstructure
The alloy also maintains ductility > 20% after 1000 hr high temperature exposure due to no sigma phase formation.
The second alloy I want to introduce tonight is Haynes's newest Haynes NS-163 Alloy.
This is a nitride dispersion strengthened alloy only available in sheet form due to nitriding kinetics. Unlike other comparable alloys (such as Ma 956), NS-163 is produced by conventional melting and processing which keeps production costs comparatively quite low. The alloy is primarily Co, with 28% Cr, 21% Fe, 9% Ni, 1.25% Ti, 1% Nb. Initially it is vacuum induction melted, and hot and cold rolled by entirely conventional methods. It has a unique final heat treatment which affords extended stress rupture properties to higher temperatures. The three stage heat treatment involves:
48 hrs at 2000F in pure Nitrogen
1 hr at 2050F in pure Argon
2 hr at 2200F also in pure Argon
This has been found to create a very uniform dispersion of nitride particles (TiN and NbN). The product at heat treatment is restricted to sheet of 2.5 mm maximum thickness due to the limitation of initial nitrogen diffusion.
The result -- once heat treated the alloy has extremely high stress rupture strength, usually only achieved by Oxide Dispersion Strengthened alloys produced by powder metallurgy. At 1800F 100 hr creep strength is 9.7 ksi and at 1000 hr the creep strength is 7 ksi. The later figure compares to 2.6 ksi for 230 alloy and 2.4 ksi for 188 alloy.
Haynes 282 alloy was first introduced in 2005 and is currently in full production. Haynes NS-163 has just left the R&D phase, and one full size melt has been produced from which preliminary data is currently being gathered. The heat treatment has been successful with one outside vendor, Exotic Metal Heat Treat of Indianapolis. Data suggests that the alloy will be very serviceable at 2000F and over, but it will oxidize at temperatures above 1800F so protective coating will be necessary at higher temperatures. The 2.5 mm maximum thickness will also limit applications, but it is expected to find use in aircraft engine hot section ducting. Final specifications have yet to be developed.
Much more information about Haynes 282 and a preliminary data sheet on Haynes NS-163 is available at the Haynes International website www.haynesintl.com.