Hladna predelava zlitine Nimonic 263 v trakove Cold Working of Nimonic 263 Alloy into Strips A. Kveder*1, D. Gnidovec*2 UDK: 621.771.016:669.245 ASM/SLA: F23, 4—53, Nib, SGAh / \ Namen raziskave je bii ugotoviti sposobnost zlitine za vroče in hladno valjanje v trakove. Z metaiografskimi preiskavami in meritvami trdot smo ugotavljali spremembe struktur in mehanskih lastnosti. A. UVOD Zlitina Nimonic 263 je značilna superzlitina za uporabo v delih reaktivnih motorjev, ki se segrevajo do okoli 850 "C. Razvita je bila posebej za tiste dele, ki se izdelujejo iz pločevin, na primer za zgorevalne komore, vendar jo rabijo tudi za druge toplotno obremenjene dele v plinskih turbinah. Sestava zlitine Nimonic 263 je osnovana na niklju. vendar vsebuje tudi 20 % kobalta, ki zelo izboljša predelavnost, predvsem glede nevarnosti nastajanja razpok med vročim valjanjem. Ima pa, kot vse superzliti-ne, veliko odpornost proti deformacijam pri visokih temperaturah. Leta 1987 smo na instrumentiranem valjalnem stroju ugotovili, da ima zlitina Nimonic 263 pri temperaturah med 1000 in 1100°C za faktor 1,6 večjo predelovalno trdnost kot jeklo Prokron 11, ki sicer velja med jekli za zelo trdno pri visokih temperaturah. To in pa sorazmerno ozek temperaturni interval predelave (1150 do 950 °C) je glavni vzrok, da je zelo težko uporabiti za predelavo te zlitine iste predelovalne agregate kot za jekla. B. EKSPERIMENTALNI DEL 1. Material za preizkuse V Železarni Ravne so izdelali osnovno talino te zlitine (chg432 920), ki je bila nato po delih pretaljena po postopku EPŽ (chg 08574/0). EPŽ-blok je bil nato kovan na kovaškem stroju. Tabela 1: Sestava zlitine Element Predpisana sestava % (ppm) Sestava šarže 08574/0 % (ppm) C Si Mn P S Cr 0,04-0,08 maks.0,4 maks. 0,6 maks. 70 ppm 19,0-21.0 0,07 0,26 0,06 0,011 0,001 20,5 *1 Dr. Aleksander Kveder, dipl. ing. met., Metalurški inštitut Ljubljana, Lepi pot 11, 61000 Ljubljana *2 SZ — Metalurški inštitut Ljubljana The purpose of the investigation vvas to establish the alloy abiiity for hot and cold rolling into strips. By means of metallographic research and hardness measurements we estabiished the occurrence of changes in structure and in mechanical properties. A. INTRODUCTION Nimonic 263 alloy is a typical superalloy being used in jet engine parts vvhich are heated to around 850 "C. It vvas developed especially for parts vvhich are made from sheets e. g. for combustion chambers. but it is also used for other heated parts in gas turbines. The composition of Nimonic 263 alloy is based on nickel. but it also contains 20 % cobalt vvhich substantially improves its workability. especially vvith regard to heat checking dur-ing hot rolling. Like aH superalloys, it proves to have a great resistance to deformation at elevated tempera-tures. In 1987. using a rolling mili equipped vvith instru-ments. we estabiished that in a temperature range of 1000 to 1100 "C. Nimonic 263 ailoy has a resistance to deformation vvhich is 1.6 times greater than that of Prokron 11 steel vvhich is considered a very strong steel at high temperatures. This fact and the relatively narrovv interval of vvorking temperatures (1150 to 950 °C) are the main reasons vvhich make it very difficult to vvork such alloys on the same vvorking machines as used for steels. B. EXPERIMENTAL 1. Testing Material The prime charge of that alioy vvas produced in Železarna Ravne (chg 432 920) and it vvas remelted per partes by electroslag remeiting process (chg 08574/0). EPŽ ingot vvas then forged on a forging machine. The chemical composition of this material is shovvn in Table 1, together vvith the prescribed composition. Table 1:Alloy composition Element Prescribed composition % (ppm) Charge (08574/0) composition % (ppm) C Si Mn P S Cr Ni Co 0,04-0,08 max.0,4 max. 0,6 max. 70 ppm 19,0-21.0 Bal. 18,5-21,0 0,07 0,26 0,06 0,011 0,001 20,5 50,4 19,3 Element Predpisana sestava % (ppm) Sestava šarže 08574/0 % (ppm) Ni ostalo 50,4 Co 18,5—21,0 19,3 Mo 5,6-6,1 6,05 Ti 1,9-2,4 2,2 Al 0,3-0,6 0,39 Ti + AI 2,4—2,8 2,59 Fe maks. 0,7 0,7 Cu maks. 0,2 0,01 B maks. 50 ppm 11 ppm Ag maks. 5 ppm 0,3 ppm Bi maks. 1 ppm pod 1,0 ppm Pb maks. 20 ppm 3,2 ppm Kot je videti iz tabele, je sestava zlitine v skladu s predpisano, še posebej je dobro, da so škodljivi oligoe-lementi daleč pod največjo dovoljeno vrednostjo. Za preizkuse valjanja smo imeli na razpolago dve gredici, debelin 25 in 45 mm, in več palic kvadratnega preseka 25 x 25 mm. 2. Vroče valjanje, trdote in strukture Gredice in palice smo pred valjanjem segrevali na 1150 °C, in sicer 1 do 1,5 ure. Palice smo nato valjali v eni vročini do debelin 4 do 5 mm, gredice pa v 3 vročinah do istih končnih debelin. Končne temperature valjanja so bile od 870 °C do 960 "C. Trdote vroče valjanih trakov so odvisne od končne temperature valjanja. Trak, ki smo ga valjali do 870 °C. je imel trdoto okoli 515 HV; trakovi, ki smo jih valjali do 915 "C, so imeli trdote okoli 450 HV, medtem ko so imeli trakovi, valjani do 950 °C, trdote okoli 365 HV. Tako visokih trdot (515 HV) ne dosežemo niti pri izlo-čevalnem utrjanju, niti pri zelo močnem hladnem valjanju. Očitno je visoka trdota po vročem valjanju do sorazmerno nizkih temperatur (870 °C) posledica seštevka utrditve zaradi valjanja pri temperaturah, pri katerih zlitina več ne rekristalizira, in izločevalne utrditve pri nizkih temperaturah valjanja in ohlajevanja po valjanju. Dva primera struktur po vročem valjanju prikazujeta sliki 1 in 2. Sodimo, da med vročim valjanjem zlitina rekristalizira le do okoli 1050° C, pri nižjih temperaturah pa se deformacijsko in izločevalno utrjuje. Slikal. Struktura zlitine Nimonic 263, valjane do temperature 870 °C v eni vročini Figure 1. Structure of Nimonic 263 aiioy, rolied to 870 °C in one heat Element Prescribed composition % (ppm) Charge (08574/0) composition % (ppm) Mo 5,6-6,1 6,05 Ti 1,9-2.4 2,2 Al 0,3-0,6 0,39 Ti + AI 2.4—2,8 2,59 Fe max. 0,7 0,7 Cu max. 0,2 0,01 B max. 50 ppm 11 ppm Ag max. 5 ppm 0,3 ppm Bi max. 1 ppm belovv 1,0 ppm Pb max. 20 ppm 3,2 ppm /4 s seen from the above Table the alloy composition is in compliance vvith the prescribed vaiues, it is espe-ciaily suitabie that the harmful impurities are far beiow the highnest aiiovved vaiue. To carry out rolling experiments two billets vvere used 25 and 45 mm thick and severa! bars of square crossection 25x 25 mm. 2. Hot Rolling, Hardness and Structures Before the billets and the bars vvere rolied. they vvere heated to 1150 "C for 1 to 1,5 hour. Bars vvere then rolied to a thickness of 4 to 5 mm in one heat vvhereas the billets vvere rolied to the same final thickness in three heats. Finish rolling temperatures vvere kept betvveen 870to 960 "C. The hardness vaiues of hot-rolled strips depend up-on finish rolling temperatures. The strip vvhich was rolied to a temperature of 870 "C had a hardness value of about 515 HV and the strips vvhich vvere rolied to a temperature of 915 "C had a hardness value of about 450 HV vvhereas the ones rolied to a temperature of 950 "C had a hardness value about 365 HV. Such high hardness vaiues (515 HVjcannot be achieved either by precipitation hardening, or by very in-tensive cold-rolling. Obviously high hardness after hot-rolling to relatively lovv temperatures (870 °C) results from the combination of hardening due to rolling at temperatures at vvhich the alloy no longer recrystallizes and of precipitation hardening at lovv rolling temperatures and during cooling after rolling. Two examples of structures after hot-rolling are shovvn in Fig. 1 and 2. We estimate that during hot-roll- Slika 2. Struktura zlitine Nimonic 263, valjane do temperature 960 °C v treh vročinah Figure 2. Structure of Nimonic 263 alloy, rolied to 960 "C in three heats 3. Žarjenje vroče valjanih trakov Vzorce dveh trakov s trdotama 440 in 365 HV smo žarili pri temperaturah med 1150 in 800 °C. Trdote po teh žarjenjih so prikazane na sliki 3. Po žarjenju na 800 "C se trdota zaradi izločevalnega utrjanja celo poveča. Do 950 °C rekristalizira najbolj deformiran del strukture, do 1050° C pa še preostali del strukture. Obenem poteka tudi raztapljanje izločkov y', ki sicer utrjujejo zlitino in preprečujejo rekristalizacijo. Najnižjo trdoto pa doseže zlitina šele po žarjenju na 1150 "C. "O o o TD ing the aiioy recrystaiiizes only down to approximateiy 1050° C, at iower temperatures it hardens due to deformation and precipitation. 3. Annealing of Hot-Rolled Strips Sampies of two strips vvith a hardness of 440 and 365 H V vvere anneaied at temperatures betvveen 115° and 800 °C. Hardness vaiues after these annealings are shovvn in Fig. 3. After annealing at 800 "C. hardness even increases due to precipitation hardening. The most de-formed part of the structure recrystaiiizes up to 950 °C and the remaining part of the structure recrystailizes up to 1050° C. Simuitaneously, the dissoiution of V precipitates oc-curs. vvhich othervvise vvouid harden the alloy and pre-vent recrystallization. The alloy achieves the iovvest hardness oniy after annealing at 1150 °C. 4. Cold Rolling of Strips Hot-rolied strips vvere first anneaied in a vacuum at 1100 °C for 1 hour and then they vvere cold-rolied to a thickness of 3,6 mm in order to level their thickness ac-ross the vvidth because the strips vvere rather bovved — thinner at the edges and thicker in the middie — after hot rolling. After that the strips vvere anneaied again at 1100 °C for one hour in vacuum. One strip vvas then cold-rolied and sampies vvere cut off during the process to measure hardness. for metal-iographic research and to investigate annealing. The final aim vvas to produce 0,91 mm thick strips as required by the user of the alloy. Before cold-roiling, it vvas believed that at ieast one intermediate annealing vvouid be required for cold-rolling from 3,6 to 0,91 mm, but it succeded vvithout this and it could even be said that it vvorked out rather successfui-ly, especially vvith regard to the fact that the rolling mili vvhich vvas used vvas not very povverful. Thicknesses. de-formations and hardness vaiues are shovvn in Table 2. Table 2: Cold-rolling parameters and hardness 120 75 45 30 Čas žarjenja. min. Annealing tirne,min. True strain Conventional strain Thickness = d" do/d)- 100 % /:=[d-d)/d0\■ 100 % Hardness Slika 3. Žarjenje vroče valjanih trakov: odvisnost trdote od temperature žarjenja Figure 3. Annealing of hot-rolled strips. Reiationship betvveen hardness and annealing temperature 4. Hladno valjanje trakov Vroče valjane trakove smo najprej žarili v vakuumu na 1100°C 1 uro, nato pa jih hladno valjali na debelino 3,6 mm, zato da smo izravnali debeline trakov po širini. Po vročem valjanju so bili namreč trakovi precej bombi-rani, na robi tanjši, na sredini pa debelejši. Nato smo trakove ponovno žarili v vakuumu na 1100°C 1 uro. En trak smo nato hladno valjali in vmes odrezovali vzorce za merjenje trdot, metalografijo in preiskave žarjenja. Končni cilj so bili trakovi, debeline 0,91 mm, ker take potrebuje določen uporabnik te zlitine. Pred hladnim valjanjem smo menili, da bo za hladno valjanje od 3,6 do 0,91 mm potrebno najmanj enkratno vmesno žarjenje, vendar je šlo brez tega. lahko bi rekli celo precej gladko, posebno glede na to, da naš valjalni stroj ni zelo močan. Debeline, deformacije in trdote so navedene v tabeli 2. mm intermediate cumulative intermediate cumulative HV 1000 g 3,6 — _ _ _ 214 3,17 12,72 12,72 11.94 11,94 324 2.9 8.9 21,62 8,52 19,4 360 2,6 10,92 32,54 10,34 27,8 395 2,3 12,26 44,8 11,54 36,1 414 1.97 15,49 60,3 14.35 45,3 449 1,63 18,95 79,23 17,26 54,7 460 1,38 16,65 95.88 15,34 61,7 476 1,12 20,88 116,76 18,84 69,0 490 1,03 8,38 125.14 8,03 71,4 495 0,91 12,39 137,52 11,65 74,7 510 REMARKS on the Table: — intermediate deformation does not mean only one pass but more especially for smaller thicknesses. — in true strains. each addition of intermediate vaiues is equal to cumulative ones, vvhereas this is not true of conventional strains. Hardness vaiues vvhich vvere measured after deter-mined intermediate strains vvere set out in a diagram of hardness — deformation and a hardening curve vvas obtained as illustrated in Fig. 4. The initial hardness of the alloy is 214 HV as before rolling it vvas anneaied at 150 L 750 800 850 900 950 1000 1050 1100 1150 1200 Temperatura žarjenja - Annealing temperature. °C Nimonic 263-vroče valjan trak hot rolled strip o 440 HV • 355 HV Tabela 2: Parametri hladnega valjanja in trdote Logaritmična Debelina deformacija mm

= 137.52 7.) / j 0 20 40 60 80 100 120 140 Logaritmična deformacija - True strain -P = [In (d0/d)D-100 7. 10 20 30 40 50 60 70 75 Tehnična deformacija - Conventional strain. e= [(d0-d)/d0J 1007. Stika 4. Utrjevanje ztitine Nimonic 263 pri hladnem valjanju traku Figure 4. Hardening of Nimonic 263 alloy by cold-rolling of a strip 1100 "C and not at 1150 "C vvhich is the appropriate temperature for the soiution treatment. From reference data we learned that a temperature of 1100 "C is sufficient for recrystallization annealing although even a lovver hardness is achieved at a temperature of 1150 °C. As shovvn in Table 2 and diagram in Fig. 4 the alloy vvas cold-rolled beyond 500 HV (50 HRc. tensile strength according to comparative tables around 1670 N/mrrf). The alloy has therefore a really very great deformabiiity. To establish hardening properties a log-log plot of hardness vs. deformation vvas set out as shovvn in Fig. 5. The alloy is obviously behaving in accordance vvith the conventional hardening equation: HV=a- Slika 5. Utrjevanje zlitine Nimonic 263 pri hladnem valjanju traku (logar-itmične koordinate) Figure 5. Hardening of Nimonic 263 alloy by cold-rolling of a strip (logarithmic coordinates) 5. Rekristalizacija hladno valjanih trakov Vzorce s parcialnimi in končnimi deformacijami smo žarili na temperaturah 1150, 1050, 950 in 850 °C različne čase: 5, 10, 20 in 60 minut. Stopnjo omehčanja in rekri-stalizacije smo ugotavljali z meritvami trdote in metalo-grafskimi pregledi. « 300 15Q750 800 Temperatura žarjenja - Annealing temperature. °C Slika 6. Rekristalizacijska žarjenja hladno valjanega traku Figure 6. Recrystallizing anriealings of a cold-rolled strip Slika 8. Rekristalizirana struktura po žarjenju 60 minut na 1150 "C. Predhodna hladna deformacija 12,7 % Figure 8. Recrystaiiized structure after 60 minutes annealing at 1150 °C. Preiiminary cold deformation 12,7% trdnost po primerjalnih tabelah okoli 1670 N/mm2). Zlitina je torej zares zelo plastična. Za določitev utrjevalne lastnosti smo iste podatke vrisali v logaritmični koordinati za trdoto in deformacijo. To je prikazano na sliki 5. Vidimo, da se zlitina dobro ravna po klasični utrjevalni enačbi: HV = a- C "O 250 o 200 o "O 150 Ni Rc — 1 ht nonic 263 ztopno žar )0°C vakuu ajeno v N2 jenje - SoLu n/2uri - 2 h - rapid coc i ion treatment: ours / hitro )ling in N2 _ 800°C i—O— Tri Hc jota po ra; lrdness aft i to p nem ža er solution i rjenju (180 treatment (1 i HV) 80 HV) D 2 4 6 8 10 12 14 Čas izločevalnega žarjenja-Ageing tirne, h 16 Slika 9. Trdota zlitine Nimonic 263 v odvisnosti od časa žarjenja na 800 °C Figure 9. Relationship between hardness of Nimonic 263 alloy and annealing tirne at 800 "C greater deformation (125,14 %) and short annealing tirne (5 minutes) crystal grains are relatively small (Fig. 7) vvhereas at smaller deformation (12%) and longer annealing tirne (60 minutes) grain growth and tercial rec-rystallization occur (Fig. 8) Also a temperature of 1050 "C. recrystallization is comp/ete in aH cases. Hovvever. crystal grains are essen-tially smaller and tercial recrystallization no longer oc-curs. Hardness is somevvhat higher due to smaller crys-tal grains (Fig. 6). At a temperature of 950 "C recrystallization is stili oc-curing but it is not complete, this is reflected in too high a hardness (250 to 275 HV). At stili lovver temperature (850 °C) the alloy no longer recrystallizes. vvith the ex-ception of a very small scale of recrystallization ovving to previous higher deformations. At this temperature. V precipitation occurs, completely preventing recrystalliza-tion. 6. Precipitation Hardening Semi-finished products of this alloy are normally de-livered to the end-user in a solution treated state (1150"C/air cooling or other cooling media), vvhereas finished products are precipitation hardened since it is the only way to give the alloy the adequate properties i. e. high strength and resistance to creep at high tem-peratures (800 "C and more). According to standards. this alloy hardens at a temperature of 800 "C for 8 hours. The kinetics hardening vvas established in such a way that the sample vvere annealed at 800 "C for differ-ent periods. Hardening is illustrated in Fig. 9 The first hardening occurs a little over the first half hour, after that it regresses somevvhat and the alloy hardens again for about 4 hours. from then on the state of the alloy re-mains nearly unchanged. The significance of the first ef-fect is not knovvn. it is probably a matter of unstable pre-cipitates vvhich are abie to precipitate rapidly and to transform themselves into stable precipitates. The hardness of the precipitation hardened alloy is relatively low as it hardly reaches 290 HV (the standard required is at least 275 HV hardness). Hovvever. as it has aiready been stated. the main advantages of this alloy shovv up only vvhen used at elevated temperatures. C. CONCLUSIONS 1. Nimonic 263 alioy previousiy forged into a billet (or slab) can very well be hot-rolled into a strip. VVithout exception, even if rolling vvas carried out beiovv 950 °C no larger tears occurred at the edges. After hot-rolling. the strip is very hard and it can even achieve a hardness of 500 HV vvhen it is rotied beiovv 950 "C. 2. Softening (recrystailization) of hot-rolled strips must be carried out at least at 1050 "C, at the best at 1150 "C, hovvever for a very short tirne in order to avoid crystal grain growth. At this temperature the lovvest hardness is also achieved (180 to 190 HV) vvhich is a good state for further cold-rolling. 3. Cold-rolling established that the alloy is very de-formabie. Despite the use of a less povverful rolling mili the total deformation of