Bazı kobalt esaslı alaşımlarda mikroyapı ve faz analizi
Microstructure and phase analysis in some cobalt-base alloys
- Tez No: 21724
- Danışmanlar: PROF. DR. E. SABRİ KAYALI
- Tez Türü: Yüksek Lisans
- Konular: Metalurji Mühendisliği, Metallurgical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1992
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 118
Özet
QZET Bazı kobalt esaslı aşınma dirençli alaşımların mikro- yapı ve faz analizlerinin yapıldığı bu çalışmada; sıcaklığın malzemelerin yapısına ve fazlarına etkisi de incelenmiştir. Deneysel çalışmalarda, dört tane kobalt esaslı alaşım kullanılmışdır. İlk alaşım Stellite 6 adıyla, Co-2BCr-4U 1.1C nominal kimyasal bileşimiyle bilinen aşınma dirençli, kobalt esaslı bir ticari alaşımdır. Stellite 6 alaşımına değişik oranlarda nikel(% 20), silisyum(%5) ve molibden (%6) ilavesiyle üç modifikasyon elde edilmiştir. Stellite 6 ve alaşım elementlerinin ilavesiyle elde edilen modifikasyonlarının sertlik değerleri 30-47 R arasında olup; bu malzemeler içerisinde en sert olanı Stelli te 6+Si(47R ) alaşımıdır. Stellite 6(44R ) ve Stellite 6+Mo(43R ) alaşımlarının sertlik değerleri birbirine çok yakın çıÜmışdır. Stellite 6+Ni alaşımı ise en düşük sertliğe sahiptir. Malzemelerin oda sıcaklığındaki mikroyapıları tesbit edildikten sonra X-ışınları ile faz analizleri yapılmış tır. Sıcaklığın mikroyapıya etkisini belirlemek amacıyla yüksek sıcaklık mikroskobunda numuneler 1000 C ' a- ısıtılmış ve bu ısıtma sırasında yapı devamlı gözlenerek çeşitli sıcaklıklarda fotoğrafları çekilmiştir. Soğutma sırasında da numunelerin yapıları sürekli gözlenerek fotoğrafları çekilmiştir. Numunelerin yapısında yüksek sıcaklık mikroskobunda yapılan ısıl-çevrim sırasında herhangi bir faz dönüşümü gözlenmemiştir. Yüksek sıcaklık mikroskobunda kullanılan küçük boyutlu numunelerde X-ışınları ile faz analizi yapılamadığından, yüksek sıcaklık mikroskobun da gelişen olayları benzeştirmek amacıyla daha büyük boyutlardaki numunelere ısıl-işlem uygulanmıştır. Bu dört malzemeye ait numuneler 1000 C'a ısıtılmış ve fırında yavaş soğutulmuştur. Isıl-işlem görmüş numunelerin X-ışınlarında faz analizi yapıldıktan sonra; orijinal numuneler ile ısıl-işlem görmüş numunelerin X-ışınları diyagramları karşılaştırılmıştır. Isıl-işlem görmüş numunelerin mikroyapıları belirlendikten sonra metalagrafik karşılaştırmalar yapılmıştır. Mikroyapı çalışmaları sonunda bu dört Stellite alaşımında genel olarak ötektik katılaşmış, dendritik yapı görülmüşdür. Stellite 6 alaşımına nikel ilavesi; dendritlerin yönlenmesine ve kabalaşmasına neden olmuştur. Stellite 6+Mo ve Stellite 6+Si alaşımlarında da yönlenmiş dendritler gözlenmiştir. Isıl-işlem görmüş numunelerde, her hangi bir yapısal dönüşüm gözlenmemiştir. X-ışınları karşılaştırması sonucunda, genel olarak, farklı fazlar bulunmamıştır; ancak, ısıl-işlem görmeş Stellite 6+Mo ve Stellite 6 + Si alaşımlarının X-ışınları diyagramlarında, göreceli olarak, farklı şiddetde pikler bulunmaktadır.
Özet (Çeviri)
MICROSTRUCTURE AND PHASE ANALY5IS IN SOME COBALT-BASE ALLOYS SUMMARY A superalloy is an allay used at elevated temperatures, usually based on VIII- A group elements in periodic tables. The superalloys encounter mechanical stress at elevated temperatures and of course, they must have high surface stability. Superalloys are divided into three major class ; iron base superalloys, nickel-base superalloys and cobalt base superalloys. In addition, there is a major subgroup, iron-nickel-base superalloys have relatively large iron contents. T.heir metallurgical characterises are similar to the nickel-base. superalloys. First of all, the austenitic stainles steels had been discovered and developed. Here is important that the FCC austenitic stainless steel had been found as a fertile phase from the point of superalloys progressive. And then; Bedford, Pilling and Merica had added small amounts of titanium and aluminum to FCC BO/20 nickel-chromium alloy. They achieved significant creep strengthening. Scientists in England, U.S. A and Germany had built on the austenitic nickel- base solid solution containing chromium, carbides and fine particles. At that time, carbide strengthened austenitic cobalt-base superallyos had been found as a cast product. After addition of molybdenum, the other refractory elements were also used such as tungsten, tantalum, columbium. Carbon, of course has always been in superalloys. 5uperalloys consist of the austenitic FCC matrix phase plus a variety of secondary phases. The principal secondary phases are the csrbides MC, M“-C,, and M?C_ viin all superallays and the gamma prime ( V ), FCC ordered Ni,(Al,Ti) intermetallic compound in Ni- and Fe-Ni-base superallays. The superallays derive their strength from salid solution hardeners and precipitating phases. In some limited systems where sufficient Cb.(ar Cb +Ta) is present, gamma double prime( V) phase, body centered tetragonal (BCT), is a principal strangthener. In many systems, less desirable phases can be found. Amoung these phases are delta (5 ) orthorhambic Ni_Cb, sigma(a ) Laves and eta(rç) HCP Ni,Ti. Cobalt-base superallays are generally used at 650- 1150 C range and at temperatures about 1100 C, they are stronger- than nickel-base superallays. Addition of chromium, tungsten, nickel, molybdenum and the other Hlements strengthen superallays. Increasing role of use of cobalt-b-ase superallays over nickel-bas^ superallays have the fallowing reasons: 1)- Cobalt-base alloys exhibit higher melting temperatures and flatter stress rupture curves at high temperatures than nickel - and iron-base superallays. 2)- Cabalt-base superallays offer superior hat- carrasion resistance to contaminated gas atmospheres 3)- In general, cabalt-base superallays exhibit superior thermal fatigue resistance and weldability. The chemical constitution of cabalt-base allays is analogous to the general family of stainless steals, and the rale of allaying elements is virtually identical throughout these austenitic alloy systems. The key element chromium is added in the range of 20-30 uit. % to impart oxidation and hot corrosion resistance and same measure of solid solution strengthening. Where carbide precipitation strengthening is a desirable feature, chromium also plays a strong role through the formation of a series varying chromium-carbon ratio carbides. Since the cobalt-chromium binary system exhibits a stable sigma phase at about 58 at.% chromium, higher, chromium level must be avoided. Carbon is a critical element for highest creep, rupture strength levels since carbide strengthening is the primary precipitation hardening mechanism utilized Vllin cobalt-base allay systems. In addition, it has been known that a nonlinear increase in strength occurs over the range of about CL3-G.6 wt. % carbon. Conversely ductility decreases over this range. Carbides are al-so important in the simpler wrought alloys to control grain size during processing, heat treatment and subsequent service. The refractory elements tungsten and molybdenum are utilized as the major solid-solution strengtheners for bDth wrought and cast coba.lt/-base superalloys, while those of lower solubility/ such as tantalum, colombium, zirconium and hafnium are generally more effective in a cgrbide, forming role. To enhance the stability of high temperature austenitic FCC cobalt matrix, additions of up to 20 wt % fti-ckel or iron are used to suppress the transformations ta HCP cobalt at lower temperatures. The presence of these elements in wrought alloys lowers deformation resistance and benefits workability. Because higher levels decrease rupture strength, additions of the elements are generally limited to 10 wt. % in cast alloys. Pure cobalt exhibits an allotropic phase transformation from the high temperature FCC austenitic crystal structure to the low temperature HCP structure at 417 C, although the rate of formation of the HCP structure is low. The allotropic transformation in cobalt is often classified as martensitic. According to Giamei, the transformation essentially is athermal in* nature and exhibits reversability during temperature cycling. Upon cooling, the FCC to HCP transformation occurs at 390 C, heating causes a reversion to FCC at k3D C. And also the transformation occurs by shear. Tihe degree of transformation depends on alloy impurities and grain size of the starting material, where fine grains and high impurities inhibit the transformation. Low temperature deformation can induce the martensitic transformation in cobalt-base alloys depending on the stacking fault energy of the allatrope and the deformation temperature. As a rule, the martensitic transformation can be induced at room temperature in alloys where the stacking fault energy is less than 15 mO/m2. Nickel, iron manganese and carbon are favored to stabilize the FCC structure with high stacking fault energy. Conversely, chromium and tungsten,.which are major alloying additions for corrosion resistance and strength, are strong HCP stabilizers with low stacking fault energy. However, vixicobalt-base superallays are designed to have a stabilized FCC matrix at all temperatures and any reversion of the matrix to HCP crystal structure is undesirable. The equilibrium matrix composition of most cobalt-base superallays can be 50 wt. % Co with the balance of the composition consisting of half chromium plus significant amounts of nickel, tungsten and the other refractory elements. It must be needed to control silicon. The strengthening mechanism utilized in cobalt-base superallays are principally a careful balance of refractory element solid solution hardening and carbide precipitation In cobalt-base allays, a number of different carbides can form, depending on the chemical composition of the alloy. The carbides can be divided into two groups representing chromium -rich and refractory element-rich carbides. The chromium-rich carbides are FUC-.M-C, and M23^6 carnidES the others are MgC and MC carbides. Because the Cr content ih cobalt -base allays is usually high (e.g. greater than Z0 %),M”C, and M^C- are seldom found, or if present, uilll decompose upon aging. Cr content favors the formation of M“,Cg type carbides and they are the mast common. The exact chemical campositon of the M”,Cg carbide depends an the specific alley. The addition of Zr, 1Mb, Ti and Ta favor the formation of M“C3 and MC type carbides. Additions of Mo and Id favor the formation of M--C type carbides. From the formation of structure, there are two diffe rent types intermetalilc phase in cobalt-base superallays One is geometrically close-packed (GCP) phases, the other one is tapolgically close-packed phases. Geometrically close-packed phases have the form A,B, where A is the smaller atom and the phase is an ordered, coherent precipitate within the FCC austenitic matrix, In the case of nickel- base superallays the gamma prime CV) phase is the principal, strengthening agent' and is represented by the general formula Ni,(Al,Ti). The generation af GCP phases within cobalt-base superallays is substantially mare difficult since the chemical and crystallographic stability is affected by a lattice mismatch that is rarely less than 1%. In addition, the cobalt-aluminum binary phase diagram.does not exhibit a comparable Ca-Al phase, although the Ca,Ti phase exhibit in the cobalt-titanium system. During the late 1950s., two commercials, cobalt-base superallays, 0-1570 and 3-1950 had been strengthened by an ordered, coherent Ni^Ti precipitate, and certain high Id content cobalt-base superallays had been developed by NASA reportedly were hardened by precipitation of Co, hi. Considerable study had IXbeen expended in cobalt-chramium-tantalum system. In summary, a usable cabalt-base superallays having the characterstics of V like in nickel-base superallays has not yet been developed. Tapalagically close-packed phases that have been observed in cabalt-base superallays are sigma(cr ), mu(/Le), Laves and Pi(7r) semicarbide phase. Lavas is common in L^-506 and had been found occasionally in S-816 and HS-18S. TCP phases can cause a loss of strength and ductility at service temperatures as well as a severe loss of law- temperature ductilty. According ta production farm, cabalt-base superallays are generally divided into two groups; wrought and cast cabalt-base superallay. Wrought cobalt-base superallays can be subdivided in to three simple groups an the basis of use; a)- Allays for use at temperatures from 650 to 1150 C including Haynes 25, Haynes 188, Haynes 556, S-816 etc. b)- Fastener alloys MP-35IM and MP-159, for use ta about 650aC. c)- Wear-resistant allays, Stellite 6B. All wrought cabalt-base allays have FCC crystal structures, however, allays MP-35N and MP-159 develop controlled amounts of HCP structure during thermomechanical processing recommended before service applications. Stellite 68 heat treated between 650-1050°C and Haynes 25 exposed for 10QD h or more at temperatures near 650 C may partly transform to a HCP structure. S-816 arginally was used extensively in turbochargers and gas turbine wheels, blades and vanes, but it has been largely replaced by higher-strength, lower-density nickel base superallays. Haynes 25 has been widely used for hot sections of gas turbines, far nuclear-reactar components, far surgical implants. Haynes 188 is an alloy specially designed for sheet-metal components' such as combustars and transition dutes. Haynes 188, like Haynes 25, MP-35 IM and MP-159, can be work hardened ta relatively high hardness and tensile strength.MP-35N and MP-159 had been specificaly designed ta be work hardened and both alloys have high strength and ductility in the uork-hardened conditions. The combination of high strength and ductility in these alloys is attributed to the formation. of small platelets of HCP structure, in the work-hardened FCC matrix. The lost cobalt-base superalloy is Stellite 6B. The alloy is characterized by high hot hardness and relatively good resistance to oxidation. The latter property is relatively high chromium content (about 30%). Hot hardness is obtained through formation of complex carbides Stellite 6B is widely used for erosion shields in steam turbines, wear pads in gas turbines and bend in tube systems. Cobalt-base casting alloys exhibit superior wear and heat resisting properties. In addition, all offer excellent corrosion resistance in contaminated gas atmospheres. Compared to the wrought cobalt-base superalİDys, cobalt-base casting alloys are characterized by higher contents of high-melting metals such as chromium, tungsten, tantalum etc. and by higher carbon contents. The high temperature strength of the heat resisting grades is derived from solid-solution hardening by chromium and the others as well as by carbide precipitation. This makes for excellent strength properties up to 815DC. The wear resisting grades are characterized by the following: a)- Good resistance to wear, heat and oxidation b)- Good resistance to impact and thermal shock c)- High hot hardness and toughness d)- Good high temperature dimensional stability A unique application of cast cobalt-base superalİDys is the use of ASTM F-75 for medical implant devices. It has the combination high strength, corrosion and wear resistance necessitated by the human body's environment. In general, cast cobalt-base superalloys exhibit good casting properties, among which are good fludity, low alloy losses, freedom from dissolved gas defects. XIThere are many cabalt-base wear resistant allays. Same are Stellite 1, 6,12 and 21. Stelllte allays are derivates from original cobalt-chromium-tungsten allays developed by Elwaad Haynes. There are aver 20 ^Stellite allays commercially available today. They are divided into two major groups: D-Co-Cr-W-C and 5)-Ca-Cr-U/Mo-l\li/ Fe-C with Si, B modifications there of for special hardfacing applications. The traditional Co-Cr-üJ-C Stellite allays are quaternary allays consisting of 30 % chromium with varying amounts of tungsten (4 to 7 %) and carbon (1 to 3.2 %) and are used primarily for unlubricated or elevated temperature were applications. This study centers an an investigation about microstructural and phase analysis of four cobalt-base wear resistant alloys. The cobalt-base allays used in this study are Stellite 6, Stellite S+Ne, Stellite 6+Mo and Stellite 6+Si. Stellite 6 is a commercially available cobalt-base wear resistant alloy. The others were produced with additions of some allaying elements such as Ni.Mo and Si to the composition of Stellite 6. Rockwell hadness measurements were carried out on the samples. The hardness of cabalt-base alloys were found between 30-A-7 HRc at room temperature. The hardest material was Stelilte 6+Si (^7 HRc). Hardness of the Stellite 6 (kk HRc) and Stellite 6 + Ma(43 HRc) were close to each other. Stellite 6+Ni had the lowest hardness (30 HRc) among these materials. One of the aims of this study were to determine the effect of these alloying elements an the structural change and phase transformation during thermal cycle. Thermal cycle applied to these materials was heating up to 1000 C in a high temperature optical microscope then cooling slowly to roam temperature. At the end af thermal cycle any reversion of the FCC matrix to HCP crystal structure was not observed in all alloys depending an heating time. To determine micrastructure af the allays studied, samples for metallographic examination were prepared in the standard manner. Metallographic examination showed that the micrastructure of the Stellite alloys were generally in dendritic farms. Eutectic solidification was observed in all alloys. Stellite alloy had coarse primary and secondary dendrites and some interdendritic carbides distributed irregularly in the structure of this material. Stellite 6+l\li, Stellite 6+Mo and Stellite 6+Si alloys had directionally solidified dendritic structures. xnSame inderdendritic carbides were alsa observed in these allays. Addition of nickel (20 %) ta Stellite 6 makes coarse dendrites. Phase determinations af the Stellite allays were done by X-ray diffraction analysis. Fiam the results af X-rays analysis, these materials generally contain FCC Co-rich matrix and Cr- rich M”3Cg carbides. Individually, present phases are FCC Ca, Cr-rich M"_Cfi{ Ca^ld. Co7Wg,Ca2Si in Stellite S;FCC Ca, Cr-rich ^23C6 in stellitB S+Ni; FCC Ca, Cr-rich M23Cs in- Stellite 6+Ma;FCC Ca, Cr-rich M23Cg, Ca2Si and Co3üi in Stellite 6+Si. The Stellite allays were heat-ibreated at 1000DC and then cooled slowly in an air furnace to determine microstructural change and phase transf ormatio.during this thermal cycle. Then, X-ray diffraction analysis were used to determine phase change. Generally, different phases were not found in these heat-treated allays except Stellite 6 + Si In this alloy, Ca_Ld phase was not found in heat-treated conditions. From the comparison of X-ray diffraction patterns af original and heat-treated samples for Stellite -6+Ma and Stellite 6+Si alloys, 'it was observed that the inten sities of peaks were different. The applied heat-treatment generally did not affect the. dendritic forms af these allays. xnx
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