Katı ortamda bor yayınımı ile sertleştirilen çelik yüzeylerinin kuru kayma halinde sürtünme ve aşınma davranışları /
Yazar: YAVUZ SOYDAN
Danışman: DOÇ.DR. VELİ ÇELİK
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü
Derece: Doktora, 1996
Konu: Makine Mühendisliği = Mechanical Engineering
Dizin: Aşınma = Wear ; Bor = Boron ; Konstrüksiyon = Construction ; Sürtünme = Friction ; Yüzey sertleştirme = Surface hardening ; Çelik-metal = Steel-metal

Özet:
ÖZET Borlama işlemi, genellikle makina elemanlarının aşınma dayanımlarım artırmak için uygulanan, termo-kimyasal bir yüzey sertleştirme işlemidir. Bor yayınımı ile yüzey sertleştirme işlemi, hemen hemen bütün demir esaslı malzemelere, titanyum ve nikel gibi demir dışı metallere ve bunların alaşımlarına başarıyla uygulanmaktadır. Bu çalışmada, katı ortamda tozlar kullanılarak borlanan, AISI 1050-4140 ve 8620 çeliklerinin, yüzeylerinde oluşan borür tabakalarının; borlama sıcaklığı (800, 850, 900, 950, 1000 °C), borlama süresi (2, 4, 8, 12 saat) ve borlanan malzemeye bağlı olarak büyüme kinetiği incelenmiştir. Borür tabakasının, kuru kayma halinde ve labaratuvar şartlanndaki (bağıl nem %50-60 ve sıcaklık 22-27 °C) sürtünme ve aşınma davranışları yanında, SiC zımpara diskler kullanılarak yüzeylerin abrazif aşınma davranışları belirlenmiştir. Ayrıca, horlanmış yüzeylerle gaz ortamda nitrürleme işlemi uygulanmış AISI 4140 ve karbürleme işlemi uygulanmış AISI 8620 malzemelerin, sürtünme ve aşınma davranışları karşılaşunlmıştır. Yapılan çalışmalar sonucunda, "tek fazlı borür (Fe^) tabakası" elde edilmiştir. Borür tabakası kalınlığı, borlama sıcaklığı ve borlama süresine bağlı olarak artmaktadır. Borlanan malzeme bileşiminin tabaka kalınlığına etkisi az olmakla birlikte, tabakanın düz veya iğnemsi yapıda olmasında oldukça etkili olduğu belirlenmiştir. Borlama işlemi sonucunda, borlanan malzeme bileşimi ve borlama şartlarına bağlı olarak borlanan parçaların boyutları, oluşan borür tabakası kalınlığının "%10-20'si" civarında artmaktadır. Taşlanmış yüzeylerde (Ra=0.4) borlama öncesi ve sonrası iğne uçlu profilmetre ile yapılan ölçümlerde, borlama işlemiyle yüzey pürüzlülüğünde büyük bir değişim olmadığı belirlenmiştir. Bu durum, özellikle plastik kalıpları açısından önemli bir avantajdır. Yüzey tekstürü ise değişmektedir. Bor yayınımı uygulanmış yüzeylerin, metal-metal sürtünmesi durumunda ölçülen ve zımparalarla yapılan abrazif aşınma testleri sonucunda belirlenen aşınma dayanımları, nitrürleme ve karbürleme işlemi uygulanmış yüzeylere göre oldukça yüksek, sürtünme dirençleri ise düşüktür. Borlanmış yüzeylerde, yüksek yük ve hızlarda dahi, "oksidatif' aşınma mekanizması etkili iken; normalize edilmiş, nitrürlenmiş veya karbürlenmiş yüzeylerde yükün artmasıyle şiddetli metalik aşınma ortaya çıkmaktadır. Borlama, nitrürleme ve karbürleme işlemleri uygulanmış yüzeylerin sürtünme katsayıları arasında büyük bir fark olmamasına rağmen, aşınma oranlan arasında önemli farklar oluşmaktadır. Metal-metal sürtünmesi durumunda sertlik-aşınma dayanımı arasında doğrudan bir ilişki bulunamamasına rağmen, zımpara diskler üzerinde yapılan testlerde, sertlik-abrazif aşınma dayanımı arasında orantılı bir değişim olduğu belirlenmiştir. Adhezif aşınma dayanımı sertlikten daha çok yüzeylerin karşılıklı etkileşimlerine bağlıdır. Yine abrazif aşınma dayanımı sertliğin yanında malzemenin kimyasal bileşimi ve karbon oranına bağlı olarak değişmektedir. vı

Abstract:
FRICTION AND WEAR BEHAVIOUR OF SOLID BORONIZED STEEL SURFACES IN DRY SLIDING CONDITION SUMMARY Wear is a process that controls the service life of components. In the great need to conserve energy and material resources, conservation can be in part achieved by added equipment life which is dependent on the wear of components. Longer life components conserve strategic materials and conserve the energy to manufacture them. Wear is major factor in engineering costs arising from maintenance, part replacement and the downtime associated with wear breakdown: A variety of hardened surfaces are used to combat wear. Beyond the overlay coating techniques there are the surface modification diffusion processes such as carburizing, nitriding and boronizing. James et al [2] considering surface treatments used in treating engine components, attempted to classify the various processes by the method in which the coating was formed. The three basic divisions were:. Material added to surface,. Surface chemistry altered,. Surface microstructure altered. Boronizing described as altering the surface composition, is not only as accurate but more succinct. Other surface treatments that alter the substrate surface are carburizing, nitriding and metalliding. Boronizing is a thermochemical treatment applied to on object in order to procedure a surface layer of borides. This treatment can be applied to all ferrous materials, e.g. structural, case-hardened, tempered and tool steel, as well as cast steel, Armco iron, cast qualities (for instance gray cast iron, and ductile cast iron) sintered iron and steels are suitable for boronizing. It is not recommended, however, to boronize aluminium-alloy steels such as, nitride steel. Aluminium and copper cannot be boronized. Because it is the boride layer that is subjected to wear and not the substratum of a boronized workpiece, the substratum must be matched in its composition to [4]:. the extremely hard, wear-resistant boride layer,. the heat treatment,. the necessary strength characteristics (adequately tough base). vuBoronizing has also acquired interest because parts on which only the surface layer needs to be hardened do not necessarily have to be made of expensive and difficult to machine high-alloy steels, in many cases low-alloy steel with boronized surfaces can be used. Basically, boron element can be diffused into the surface of a material by means of gaseus, liquid or solid substances. Moissan suggested as early as 1895 that steel should be boronized [4]. Since then substantial research has been done in this area. Today, as regard the theoretical basis the properties of the borides, the production processes, and the technical and industrial applications, boronizing is one of the best researched processes for the production of extremely hard and wear- resistant surface layers. Depending on the base material, it is possible to use boronizing temperatures between 800-1050 °C. Exceptions are, among others, gray cast iron and tungsten carbide. For the first material the boronizing temperature may not be higher than 850-880 °C as the phosphide eutectic (steadite) contained in gray cast iron will melt at 950 °C. Thus, there is danger of surface changes as a result of the destruction of this eutectic. Boronizing periods are between 15 minutes and 30 hours. Today, preferred boronizing times range mainly between 1-8 hours. Boronizing results in some important improvements in the surface properties of steel. The hardness (Hv or HK 0.025) of boride layers on iron materials ranges between 1800 and 2000 which is in the range of the hardness values for aluminium oxide. On steels with a high content of alloy metals, hardness values of up to 2400 HK 0.025 are sometimes measured. This value is almost impossible to achieve with other conventional processes (e.g. gas nitriding, maximum Vickers hardness 1 100- 1200 HK). In steels, a resistive layer can be obtained against corrosion which can happen in some acids and molten metals and besides low friction coefficient. The diffusion of boron on the surface of ferrous parts produces hard iron borides (Fe2B and FeB). For industrial use, a single-phase (Fe2B) boride layer is desirable. This has a number of advantages, e.g. less brittleness. In addition, it is possible to subsequently apply heat treatment without impairing the properties and the bonding strength of the boride layer. In choosing a wear resistant coating or surface, it is essential to identify the wear mechanisms that predominate. Eyre [1] lists five principal types of wear: VUlHowever, sometimes one type changes to another, or two or more mechanisms operate together. This makes it extremely difficult to find optimum solutions to individual problems. Most surface treatments are designed to combat adhesive wear, but such treatments as boronizing may improve abrasive and adhesive wear properties, whilst combinations of different processes may improve all five. Boronizing can considerably increase the resistance of low-alloy iron materials to acids. In recent times, boronized parts have been used with good result in various industrial areas. This result in considerable increases in the life times of machine elements and substantial improvements in the serviceable lives of working tools. Boronized machine elements are particularly advantageous in situations in which the wear stress İs increased by the presence of materials with abrasive action. Extremely high wear occurs, for example, in the plastic-production machinery (extruder and injection-mounding machinery). The next example of application is on grinding disc which are used in special mills for grinding coffee, poppy seeds, spices, etc. Because of its dependence on many factors which cannot be generalized it is not possible to fix the cost of boronizing here. The overall conclusion is that boronizing is generally more economical than gas nitriding, spraying-on processes, and the use of brazed or adhered hard metals. When compared with salt bath nitriding or case-hardening processes, boronizing may cost more. The final process selection, however, should be decided by the overall cost calculation. The work carried out in this investigation developed from three standpoints therefore:. The investigation of the kinetics of the boronizing process.. The investigation of the adhesive and abrasive wear properties of boronized AISI 1050-4140 and 8620 steels.. The compared wear and friction behaviour of boronized layers with those from other surface treatments such as nitriding and carburizing. The boronizing was carried out in a pack medium, commercially known as "EKabor". The compositions of the pack were 90 % w/o SiC, 5 % B4C (boron carbide) and 5 % KBF4 (potassium fluorborate). The SiC was present in large quantities to dilute the potential of the boron, such that only a monolayer of Fe2B is formed. KBF4 is reduced to a sub-halide of boron, which then reacts with the iron surface allowing diffusion of elemental boron into the substrate. Table 1 lists the chemical analyses of the materials used during the course of the experimental work. All the metals or alloys were ferrous- based, and carbon was the main variation in alloy constituent. IXTable 1. Chemical composition of steels (wt %). It was previously stated that the investigation of the kinetics of boronizing would be based on the effects of varying the temperature and the duration of the process. Thus, tests were carried out at 800, 850, 900, 950 and 1000 °C for periods of 2, 4, 8 and 12 hours. Wear testing was carried out using a pin-on-disc machine under unlubricated dry sliding conditions. This method involves a stationary loaded pin sliding against a rotating disc. The technique was chosen because it simulated the rotating sliding encountered in bearing applications, rotary engines and general rotation under load situations. The discs used all the series of tests were AISI 1020 (Table 1) steel, carburized, with on initial hardness of 700-750 Hv. A wear disc, 7 mm thick and 70 mm diameter, was fixed on a rotating shaft. Wear pins 15 (3+12) mm long, 2.7±0.1 mm diameter were normalized, boronized, carburized and nitrided. The materials used for wear pins were normalized AISI 1050-4140 and 8620 steels, boronized AISI 1050-4140 and 8620 steels, gas-nitrided AISI 4140 steel, carburized AISI 8620 steel. The chemical compositions of the steels are given in Table 1. The boronizing was carried out at 950 °C for 2 and 8 hours. The surface layer had a peak hardness of 1600-1800 Hv, and the hardness of the diffusion layer gradually decreased with an increase in distance from the surface. The carburizing was carried out at 910 °C, with a carburizing gas, held for 90 minutes, cooled to 850 °C in a retort oven and held there for 30 minutes, quenched in oil at a temperature of 60 °C, then cooled in air to ambient temperature. Carburizing treatment was resulted in 750-800 Hv hardness. The nitriding was carried out in a NH3-gas atmosphere furnace. The specimens were heated to 850 °C, held for 30 minutes, quenched in oil, then cooled to ambient temperature. After an unspecified duration, specimens were heated to 570 °C, held for 150 minutes, then cooled in air to ambient temperature. The surface layer had a peak hardness of Hv 600-650. The effective case depth defined by the distance between the surface and the layer having a hardness of Hv 400 was 0.2 mm.A new pin and disc were used for each wear test. All tests were carried out under unlubricated conditions in a normal laboratory atmosphere at 50-60 % relative humidity and a temperature between 22-27 °C. In the case of the normalized, carburized and nitrided pins, the wear test was allowed to proceed until an equilibrium wear regime had been established. The amount of wear was determined by the change of pin length which was monitored during each wear test by using a linear displacement transducer. The weight loss of pin was also measured by a balance and record was made for all the test materials at the end of each test. However, in the case of the boronized pins, the tests were only stopped when the layer had worn away completely and the base metal exposed to the counterface. The worn pins and discs were examined using optical and scanning electron microscopy (SEM) for the identification of the wear mechanisms. The findings of this investigation can be summarized as follows:. The growth of the compound layer (Fe£l) is related to heat treatment temperature, time and material compositions of exposure to the boronizing medium.. Depending on the treatment conditions and base material, the epitaxial growth rate can average up to 10-20% of the boride layer thickness.. Boronizing minimize adhesive and abrasive wear of steels used in this investigation.. Adhesive wear resistance is independent of the hardness of materials. Hard and soft surfaces inhibited adhesive wear. This behaviour can be attributed to chemical incompatibility between the pin and disc surfaces.. The wear mechanism of the boronized substrate material appears to be oxidative fatigue in Fe2B layers... The abrasive wear resistance of normalized, carburized, nitrided and boronized steels are directly related to compound layer hardness. Boronized steels metal-to-metal couples showed lower value kinetic friction than the other tested metal-to-metal couples. Boronizing is superior to the other diffusion treatments such as carburizing and nitriding from point of view of hardness, friction and wear. XI