Kaynak edilebilen kobalt ve demir esaslı alaşımların yüksek sıcaklıkdaki aşınma davranışları
High temperature wear behaviour of weldable cobalt and iron based alloys
- Tez No: 19298
- Danışmanlar: PROF.DR. E. SABRİ KAYALI
- Tez Türü: Doktora
- Konular: Makine Mühendisliği, Mechanical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1991
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 104
Özet
ÖZET Bu çalışmada kobalt ve demir esaslı alaşımların yüksek sıcaklıklardaki aşınma davranışları incelenmiş tir. İki elemanlı abrasiv aşınma metoduna göre yapılan deneylerde alüminyum oksit ve silisyum karbür taneli abrasivler kullanılmıştır. Deney malzemelerinden kobalt esaslı olanyarı, Co-28Cr-ith)-1,1 C nominal bileşimiyle stellite 6 olarak bilinen alaşıma belirli oranlarda nikel, silisyum ve molibden katılarak elde edilmiştir. Demir esaslı alaşımlardan bir tanesi düşük karbonlu olup mukayese malzemesi olarak kullanılmıştır. Diğerleri ise içerisinde Ni,Cr ve Co bulunduran demir esaslı sert yüzey kaplama alaşımlarıdır. Deneyler pim-disk esasına uygun olarak imal edilen bir aşınma deney cihazında yapılmıştır. Bu cihazda aşındırılacak malzemeleri istenilen sıcaklığa çıkartabilmek amacıyla bir elektrikli ısıtıcı monte edilmiştir. Deneyler oda sıcaklığı ile 1 D00°C arasındaki çeşitli sıcaklıklarda yapılmış ve numune sıcaklıkları termokupl ile kontrol edilmiştir. Üç farklı yükleme ağırlığı ile yapılan deneylerde kobalt esaslı alaşımlardan“stellite 6 + Ni”alaşımdın en yüksek aşınma direncine sahip olduğu bulunmuştur. Bunu sırasıyla stellite 6, stellite 6+Mo ve stellite+Si ' un takip ettiği belirlenmiştir. Malzemelerin aşınma direnci alüminyum oksit abrasiv kullanıldığı zaman düşük, SiC abrasiv kullanıldığı zaman ise daha yüksek değerlerde bulun muştur. Katkı elementleri içerisinde stellite 6'nın aşınma direncini en çok düşüren silisyum olduğu saptanmıştır. Molibden, aşınma direncinde az bir düşüşe sebep olmakla birlikte nikel, stellite 6'nın aşınma direncini yükseltmiştir. Demir esaslı alaşımlardan kobalt içerikli alaşımın manganez içerikli alaşımdan bütün sıcaklıklarda daha yüksek aşınma direncine sahip olduğu saptanmıştır. Fe 37 çeliğinin ise deneyde kullanılan bütün alaşımlardan daha düşük bir aşınma direncine sahip olduğu belirlenmiş tir. 600 C ye kadar devamlı düşen Fe 37 çeliğinin aşınma direnci 600 °C'den sonraki sıcaklıklar için yükselmiştir. Sıcaklığın, incelenen malzemelerde hacimsel kayıp üzerinde önemli bir etken olduğu anlaşılmıştır. Örneğin bu malzemelerde aşınma oranları arasındaki fark oda sıcaklığında çok küçük olmasına rağmen sıcaklığın yükselme si ile artarak 1000QC'de maksimuma ulaşmıştır.
Özet (Çeviri)
HIGH TEMPERATURE WEAR BEHAVIOUR OF WELDABLE COBALT AND IRON BASED ALLOYS SUMMARY Surface deterioration is important in engineering practice; it is often the major factor limiting the life and the performance of machine components. Wear may be defined as unintentional resulting from use or environment. It may be considered essentially a surface phenomenon. The displacement and detachment of metallic particles from a metallic surface may be caused by contact with another metal ( adhesive or metallic wear), a metallic or a nonmetallic abrasive (abrasion), or moving liquids or gases (erosion). LJear involving a single type is rare, and in most cases both abrasive and adhesive wear occur. Each form of wear is affected by a variety of conditions, including environment, type of loading relative speeds of mating parts, lubricant, temperature, hardness, surface finish, presence of foreign particles, and composition and compatibility of the mating parts involved. In adhesive wear, tiny projections produce friction by mechanical interference, with the relative motion of contacting surfaces increasing resistance to furter movement. If the driving force is sufficient to maintain movement, the interlocked particles are deformed. If they are of a brittle material, they may be torn off. This leads to the conclusion that wear resistance will be improved by preventing metal-to- metal contact and by increasing the hardness to resist initial indentation, increasing the toughness to resist tearing out of metallic particles, and increasing the surface smoothness to eliminate the projections. Abrasive wear occurs when hard particles slide or roll under pressure across a surface, or when a hard surface rubs across another surface. The Vllabrading particles from the harder abject tend to scratch or gauge the softer material. These hard particles may also penetrate the softer metal and cause the tearing off of metallic particles. The ease with the deformed metal may be torn off depends upon the toughness. Therefore, hardness and toughness, the same properties that influence adhesive year, also determine abrasive wear. Many materials and methods are available for protection against wear. The selectionof a particular material and process requires a thorough analysis of the actual service conditions, a knowledge of applicability and limitations of the particular material and process, and data concerning the cost involved. Hard facing is an important technique for the production of a hard wear-resistant surface layer on metals by welding. There are more than 150 different compositions of hardfacing materials commercially available, ranging from steels containing only about 2 percent total alloy content to nikel-base alloys and tungsten carbide. Cobalt-based hard facing alloys have been used extensively for high temperature applications involving metal-to-metal wear, abrasive wear and for erosion and corrosion resistance. The alloys are generally of the“Stellite”variety, containing large additions of chromium and other alloying elements. The first Stellite alloy was developed in 1900 by Elwood Haynes, with the nominal composition Co-28Cr-^ld-1,1C(ldt%),now referred to as Stellite 6. Since then, new alloys have been developed and there are at least 20 Stellite allays in common use. The alloys can be classified into two main groups; a)- The original Co-Cr-C-U allays, Which are used extensively for elevated temperature wear applications; b)- Alloys in which the Co,Cr, and Id rich matrix has been modified either by the addition of Mo, and Ni or Fe, C,Si and B for high temperature applications involving heavy impact wear conditions. vıııThe microstructures of Stellite alloys vary considerably uith composition. They may either be in the form of a hypoeutectic structure consisting of primary dendrites of a Co-rich solid solution surrounded by eutectic carbides, or of the hypereutectic type containing large idiomorphic primary chromium-rich carbides and an eutectic. Among the alloying elements, carbon is found to have a large influence on the microstructure, causing a change from hypoeutectic to a hypereutectic alloy. Carbide forming elements such as tungsten (which forms U,C type carbides) may reduce the amount of carbon required to reach the eutectic composition. Cr, Mo, and Id are found to partition into both the carbide and matrix phases. Solid solution strengthening is also improved significantly by the addition of these alloying elements, and this in turn increasses the abrasive wear resistance. The influence of the other alloying elements such as iron and nickel is explained through their effect on the stacking fault energy (SFE). The matrix in Co-based alloys can be either in the face centred cubic (fee), or hexagonal close packed(hcp) for depending upon the chemical composition, temperature and applied stress. It has been reported that the wear characteristics of the fee and hep phases axe significantly different so that any alloying element uhich stabilises a particular phase influences the wear resistance. Cobalt-based hardfacing alloys can bi deposited by a variety of melding techniques such as manual metal arc (MMA) melding, tungsten inert gas (TIG), and oxyacetylene processes. However, in recent years the use of laser cladding has found many applications in different areas. There is no simple proportionality between the abrasive wear resistance and hardness in multiphase alloys. The relationship must partly depend on the behaviour of individual phases. Although the carbides in cobalt-based hard facing alloys contribute to wear resistance primarily through their high hardness, the volume fraction, shape, morphology, and the strength of the carbide/matrix interface have also been found to play a significant role. Abrasive wear resistance is also strongly influenced by the type of abrasive particles. IXIn a microstructure consisting of a mixture of carbides in a soft matrix, it is aproximately the case that the hard carbide particles provide the resistance to abrasion whereas the tough matrix serves the role of binding the relatively brittle carbides. Both the matrix and carbide phases have to contain sufficient chromium to make the alloy corrosion and oxidation resistant. Since the matrix phase loses its strength more rapidly with an increase in temperature, it has to be solid solution strengthened for elevated tempe rature service (hence the tungsten addition). The size of the carbide particles becomes particularly important when compared with the width of wear grooves created by abrasion. If the size is com parable to the width of the grooves, then experiments suggest that the carbides may be removed completely into the chips which are created as a consequence of the micromachining processes associated with groove formation. It has also been reported that when the matrix-carbide interface is relatively weak, the detachment of fine carbides can leave behind a pitted surface with poor wear resistance. Since wear is not a simple phenomenon, wear resistance is represented by fewer standardized tests than other engineering properties. Therefore, equipment for wear testing must be designed to simulate actual service conditions. These tests should have proved reproducibility, should be able to rank various materials under consideration, and most important, should be validated by correlation with service data. One of the problems in studying wear is the very large number of variables which can affect wear mechanisms and wear rates. These include test geometry, load, rate factors, test duration, materials properties and environmental factors, including temperature. Data obtained with one set of conditions may be quite useless when considering other conditions, because different processes may become important and the wear rate can change by orders of magnitude with relatively small changes in experimental variables. This makes it difficult to use available wear data for reliable prediction of wear in engineering systems. This study centers on an investigatioon of elevated temperature abrasive wear resistances of cobalt based and iron based allays. Four cobalt based alloys (Stellite 6, Stellite 6+Ni, Stellite 6+ Mo and Stellite 6+ Si) and three iron based alloys ( Fe-Ni-Co-Cr-Mo alloy, Fe-Cr-Mn-U-C alloy and a low carbon stell(Fe 37) were used in this investigation, Stellite 6 is a commercially available cobalt based hard facing alloy. The other cobalt based alloys were produced with the additions of some alloying elements (Ni, Mo and Si) to the composition of stellite 6. One of the aims of this study mas to determine the effects of these alloying elements on the abrasion wear resistance of stellite 6 alloy at elevated temperatures. A low carbon stell (Fe 37) was used for comparison of the wear resistances of the materials studied. The other two iron based alloys are known as wear resistant materials. To determine microstructures of the materials studied, samples for metallographic examination were prepared in the standard manner. Metallographic examinations showed that the microstructures of the stellite alloys were generally in dendritic, forms. Uickers hardness measurements were carried out on the samples. The hardness of the cobalt based alloys were found between 3B3 and 567 HV at room temperature. The hardest material was stellite 6 alloy and the addition of alloying elements in this alloy reduced the hardness in the decreasing order of stellite 6+Si, stellite 6+Mo and stellite 6 + Ni. The hardness of the iron based alloys were in the range of 120-242 HV. The hardest iron based material was Fe-Cr-Mn-U-C olloy and the softest material was Fe 37 steel. Compression tests were performed at room tem perature to determine the strain hardening behaviours of these materials. The compression specimens were machined with a diameter of 3 mm and a length to diameter aspect ratio of 2. From the true stress- true strain curves obtained from compression tests, the strain hardening exponents of the materials were determined. Strain hardening exponents of iron based alloys were higher than of cobalt based alloys. The strain hardening exponent values of stellite and iron based alloys were found between 0,12-0,22 and 0,29- 0.31 respectively. Since density is an effective parameter used in the calculations of wear resistances of materials, the densities of the studied materials were measured with picnometer. It was found that the densities of the iron based and stellite alloys were between XI7,7-B,5 g/cm3 and B, 0-8,3 g/cm3 respectively. To measure the wear resistances of the materials, a two body pin-on-disc wear tester uas constructed. Abrasion wear tests were performed on 3 mm diameter cylindirical specimens, impinging vertically on a rotating disc coated with the appropriate abrasives. The effects of two kinds of fixed abrasives were studied using the rotating coated disc test: BO mesh SiC and BO mesh A1“03. The abrasive papers were changed after e a c h u s e. elec test ther wide 1000 test to a From alio appl Howe resi stud To tri te mo ra °C. c bra th ys ica ver sta ied perform cal heate mperature couple. nge of te The wea onditions sive wear e wear te were supe tions com, the ste nee among wear r att of t The w mpera r tes. Th was sts, rior pared llite the tests ached he sp ear t tures ts we e wei preci it to a for h to t 6+ S stell at to ecim est fro re p ght sely s f o igh he i i al ite elevated the test en was c were car m room t erf ormed loss of measure und that temperat ron base loy had alloys a temper specim ontroll ried ou emperat in air the spe d after the st ure wea d alloy the low t all t atures, an en and the ed with a t over a ure to under dry çimen due each test, ellite r s studied, est wear emperatures The worn surfaces of the abrasives and the materials tested at 1 000 C were examined in a scanning electron microscope. It was observed that the charac teristics of the worn surfaces were different for low and high wear resistant materials. It was also observed that some of the abrasive particles were fractured and some of them removed from the binder. The following coonclusions were obtained from the experiments. 1- No correlation was found between hardness and wear resistance of the materials studied at room temperature. 2- Room temperature abrasive wear resistances of the materials increased with increasing strain hardening exponents of the materials. XII3- The wear resistance of the materials decreased with increasing temperature for all of the materials studied. k- The wear resistances of the materials were close to each other at room temperature. However the abrasive wear resistances of the stellite alloys were higher than of the iron based alloys at elevated temperatures. 5- The addition of silicon (5%) to the composition of stellite 6 alloy did not signlif icantly change the hardness of the material, but it significantly reduced the wear resistance of this alloy. 6- Molybdenum and nickel additions to stellite 6 did not significantly alter the wear resistance of this alloy. However, the wear resistance of stellite 6 increased witn Ni addition and decreased with Mo addition slightly. 7- The iron based alloy containing cobalt (Fe-Ni- No-Cr-Mo) had highest wear resistance among the iron based alloys studied. 8- The low carbon stell (Fe 37) had lowest hardness and lowest wear resistance between all of materials studied. The wear resistance of this steel showed a minimum around 600 C than increased with increasing temperature probably due to c< 3>yphase transformation which occurs in this material. 9- The wear resistances of all materials with A1”0, abrasive were lower than of the materials tested with SiC abrasive at all test temperatures between room temperature and 1000 C probably due to the removal of SiC particles from binder during wear tests. 10- Scanning electron microscopy examinations showed that the appearances of worn surfaces for low wear resistant materials were smooth and high wear resistant materials were rough generally. xm
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