Recrystallization of Ni-based Superalloy after Cold Deformation Rekristalizacija Ni-superzlitine po hladni deformaciji M. Torkar, B. Šuštaršič and F. Vodopivec, Inštitut za kovinske materiale in tehnologije, Ljubljana The strain hardening and isothermal recrystailization after cold deformation of Ni-based superalloy was investigated. Cold deformation belovv 10% and annealing temperature above 1050°C promote the grovvth of recrystallized grains. A cold deformation, not iovver than 10% and annealing betvveen 1000°C and 1050°C for 30 minutes, produce fine recrystallized grains. Key words: Ni-based superailoy, cold deformation, strain hardening exponent, statical recrystallization Izvršena je bila raziskava utrjevanja pri hladni deformaciji in poteka izotermne rekristalizacije po hladni deformaciji Ni-superzlitine. Rezultati kažejo, da končna hladna deformacija pod 10% in temperatura žarenja nad 1050°C pospešujeta nastanek velikih rekristaliziranih zrn. Hladna deformacija nad 10% in 30 minutno rekristalizacijsko žarjenje med 1000°C in 1050°C zagotavljata drobno zrnato rekristalizirano mikrostrukturo. Ključne besede: Ni-superzlitina, hladna deformacija, eksponent napetostnega utrjevanja, statična rekristalizacija. 1. Introduction Ni-based superallovs are used in manufacturing of turbine-tvpe machinerv. for rotors. vanes and combustion chambers, for exhaust valves in automotive industrv, in the tool industry for hot work dies, further in nuclear povver plants and for petrochemical equipment as well as in many other places, where a combination of good mechanical properties and corrosion resistance are de-manded. Despite of the material development, oriented to Fe-, Ni-, Ti-aluntinides and other intermetallics, the Ni-based superallovs remain the base material for use for the critical compo-nents (ref. 1). Ni-based superallovs vvith chromium and other alloying elements are strengthened bv precipitation hardening. The matrix is strengthened by precipitation of (Ni,/AlTi/) particles and the grain boundaries bv carbide particles, vvhich prevent the grain grovvth (ref. 2). As čast Ni-based superallovs have a limited hot workability. The hot vvorking is easier in a narrow temperature range above the creep range and belovv the solidus line, vvhere virtualy no precipitates are found in the microstructures. For most of the allovs vvith limited hot workability the use of hot extrusion is more suitable especiallv if it is performed at Iovver deformation rate (ref. 3). Electric slag remelting also improves the hot vvorkabilitv of the allov (ref. 4.51. The mastering of the hot vvorking in order to obtain the optimal properties, demands a striet control of the grain size and therefore it is important to control the process of grain grovvth and the grain si/e from the solidification to the final cold vvorking. During the hot deformation dynamic recovery and recrys-tallization occur and cause a much Iovver rate of strain hardening than is found at room temperature (ref. 6). After cold deforma- tion only static recrystallization occurs during the annealing. The aim of the research vvas to determine the strain hardening at cold deformation as vvell as to establish the influence of deformation grade and annealing temperature on the start of recrystallization and grain grovvth. 2. Experimental The allov vvith the follovving composition: 21 % Cr. 1.7% Co. 2.5% Ti. 1.7% Al, 0.62% Mn,(X72% Si, 0.74% Fe.0.05% C. bal. Ni, ali in vvt. % , vvas melted in induetion furnace. The ingots of 60 x 60 mm cross seetion vvere čast, electric slag remelted (ESR) into ingot of 100 mm diameter and forged to the bar of 15 mm diameter. Cvlindrical specimens vvith 13 mm of diameter and length of 10 mm vvere machined from the forged bar. solution annealed at 1150"C and vvater quenched. Some samples vvere continuously compressed vvith a maximal logaritmic deformation up to 0.9. The exponent of strain hardening (n) vvas calculated bv the method deseribed in ref. 7. Other samples vvere subjeeted to 3, 5, 10, 20. 30 and 50% of cold deformation vvith compression. In both cases a teflon foil vvas used as lubricant to diminish the friction, betvveen the tool-ing and the specimen. After the cold deformation the specimens vvere isothermal annealed 30 minutes at 900, 1000, 1050, 1100 and 1 150'C. vv ater quenched and submited to the examination in optical micro-scope. 3. Results The microstructure of solution annealed and vvater quenched specimen is shovvn on Figure 1. The strain hardening at cold deformation is shown on Figure 2. Factor k, represents the true yield stress, i.e. the true stress in the sample in the moment of the load action. It vvas established (ref. 7) that the curve of llow stress at compression test in the interval from 7 = 0.2 to 7= 1.0 can be approximated vvith the follovving parabolic function: ^f = ^ri.i) Y The exponent of strain hardening (11) can bc estimated from the equation: n = ln_kflu^ln kfui ln 0.2 using the values for kn)2 and krM, from figure 2. The calculated exponent of strain hardening vvas 11 = 0.44. The value for k, (N/mnr) can be calculated by tising the follovving equation: kf= 1770 x t"'44 The logaritmical deformation

u n - i u rv ! / u (V ~kf = 17 70 K u V -------/. J / v / J / } ( 0 0,2 0,4 0,6 0,8 1,0 Log. deformation ^ Figure 2: The strain-stress relationship Slika 2: Odvisnost med deformacijo in silo Figure 1: Microstructure after solution annealing Slika I: Mikrostruktura po topilnem žarjenju The microstructure of the allov after cold deformation and annealing in temperatures range 900 to 1050°C for 30 minutes are shovvn in Figure 3, 4 and 5. Elongated grains on Figure 3 show the allov remain unre-crystallized after annealing at 900"C. At 1000"Č the recrvstal-lization occurs by at least 10% of cold deformation. The lovvest temperature at vvhich recrvstallization occurs at ali grades of deformation is about 1050" C. At the same annealing temperature the grain size of reervs-tallized grains decreases vvith the inereasing deformation. At higher temperature the recrvstallized grain starts to grow. Figure 6 shovvs the connections betvveen the grade of cold deformation and the temperature on the start and advance of recrvstallization. Both, higher annealing temperature and higher cold deformation promotes the start of recrvstallization. The grain size vvas measured from the micrographs. and rep-resented as ASTM number in Figure 7, as relationship betvveen the size of recrvstallized grains. the grade of cold deformation and the annealing temperature. A fine grained recrvstallized microstructure can be obtained by at least 10% grade of cold deformation at temperature betvveen 1000 and 1050 C and after 30 minutes of annealing. Higher annealing temperature promotes the grain grovvth of recrvstallized grains. O 1200 1100 t 1000 o. £ .v 900 800 o without » partial recrystallization • complete 3 5 10 20 30 40 Deformation 7. 50 60 Figure 6: Relationship betvv een the grade of eold deformation, the temperature of annealing and the start of recrvstallization. Slika 6: Povezava med stopnjo hladne deformacije, temperaturo žarjenja in pričetkom rekristalizacije. 3 o min Figure 3: Microstructure of the alloy after cold deformation 3 to 50% and annealing 30 minutes at 900"C. Slika 3: Mikrostruktura hladno deformirane (stopnja deformacije 3 do 50% ) zlitine po 30 minutnem žarjenju na 900"C. 30 min. 1000°C mm H 30% Figure 4: Microstructure of the alloy after cold deformation 3 to 50' ; and annealing 30 minutes at 1000 C. Slika 4: Mikrostruktura hladno deformirane (stopnja deformacije 3 do 50'/r) zlitine po 30 minutnem žarjenju na IQ00"C. Figure 5: Microstructure of the alloy after cold deformation 3 to 50% and annealing 30 minutes at 1050"C. Slika 5: Mikrostruktura hladno deformirane (stopnja deformacije 3 do 509f) zlitine po 30 minutnem žarjenju na 1()50"C. Figure 7: Effect of cold deformation and annealing temperature on recrystallized grain si/e after 30 minutes of annealing. Slika 7: Vpliv stopnje hladne deformacije in temperature žarjenja na velikost rekristaliziranih zrn po 30 minutnem žarjenju. 4. Conclusions Ni-based superalloy vvas cold deformed by compression lest and a hardening exponent n = 0.44 vvas obtained. The strain hardening can be calculated by the follovving equation: kr= 1770 x V'44 The investigation of the isothermal recrystallization of the superallov shovved that a small grade of deformation (belovv 10%) and a higher temperature (above 1050"C) of annealing promotes the recrystallized grain grovvth. The occurence of partial reerv stallization is limited to a relativen narrovv temperature range near 1050°C at deformation belovv 10% and near 1()00"C at deformation over 10%. Finer reervstallized grains of Ni-based superallov are obtained after a cold deformation not Iovver than 10% and 30 minutes of annealing betvveen 1000°C and 1050°C. At annealing above the 1050"C the reervstallized grains start to grovvth. 5. Aeknovvledgements The authors vvish to express their gratitude to the Ministry of Science and Technologv of Slovenia for the financial assistance of this research. 6. References 1 G.Sauthoff: Z. Metallkde., Bd.81 (1990). H.12. 855-861 2 N.L.Loh. K.Y.Sia: Journal of Materials Processing Technologv. 30, (1992). 45-65 3 CS.N.Maniar, D.A.Nail. H.D.Solomon: Optimization of Processing, Properties and Service Performance through Microstructural Control, ASTM Special Technical Publication 672. Philadelphia 1979 4 M.Torkar, A.Kveder. F.Vodopivec. A.Rodič, I.Kos: Poročilo MIL. Nr. 86-029, 1986. Ljubljana 5 A.Choudhurv: ISIJ International, Vol.32 (1992), No. 5. 563-574 6 H.J.McQueen, G.Gurevvit/. S.Fulop: High Temperature Technologv. Februarv 1983, 131-138 7 B.Ule: Poročilo MIL, Nr.85-033. 1985. Ljubljana.