Elektrokimyasal yöntem ile yüksek hız kesici takım çeliğinin borlanması
Electrochemical boronizing of high speed cutting tool steel
Yazar: ALİ GEZER
Danışman: DR. ÖĞR. ÜYESİ CANAN GAMZE GÜLERYÜZ PARASIZ
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Malzeme ve İmalat Bilim Dalı
Derece: Yüksek Lisans, 2021
Konu: Makine Mühendisliği = Mechanical Engineering

Özet:
Endüstrileşmede sürekli artan kalite ve verim makinaların ve bunlara ait teçhizatın performans ve ömürlerinde gelişmeleri zorunlu kılmıştır. Takım tezgahları ve bu tezgahlarda gerçekleştirilen işlemler de bu gelişme beklentilerinden nasibini almıştır. Özellikle CNC tezgahların çalıştığı yüksek kesme hızlarında çalışabilecek kesici takımların, mekanik ve ısıl özellikleri önemli bir sorun haline gelmeye başlamış, bu durum kesici takımların mekanik, kimyasal ve ısıl özelliklerini iyileştirmek için çeşitli alternatif yöntemler araştırılmaya başlanmasına vesile olmuştur.
Kesici takımlar, takım tezgahlarında talaşlı işlenen parça ile temas edip kesme işlemini gerçekleştirirken hem ısıl hem de mekanik büyük zorlanmalar ile karşı karşıya kalırlar. Bu nedenle gerek takımların genel mukavemetinin gerekse yüzeylerinin özellikleri büyük önem taşır. Takım ömrünü arttırmak amacıyla takım yüzeylerine kaplamalar uygulanması giderek yaygınlaşmıştır, termal bariyer oluşturan kaplamalar gibi farklı özellikleri olan kaplamalar sürekli geliştirilmeye devam etmektedir.
Kaplamalar alanındaki araştırmaların yanı sıra bor elementinin kullanımı ve uygulamaları da Dünya'da da ve Dünya rezervinin büyük bir kısmına sahip olan ülkemizde de yaygınlaşmaya başlamıştır. Borlama veya borürleme işlemi, malzemelerin bor sağlayan ortamlara maruz bırakılması ile gerçekleşmektedir ve sağladığı yüksek mekanik performans bu işlemi kesici takımlar açısından da çekici kılmıştır. Ne yazık ki bu performanslara ulaşabilmek genellikle uzun ısıl işlem süreleri gerektirmektedir. Son yıllarda ise termal işlemlere ek olarak ilave akım uygulamasının da yapıldığı elektrokimyasal borürleme işlemi, tutma sürelerini büyük ölçüde kısaltmayı başarmıştır.
Gerçekleştirilen tez çalışmasında bor elementi tuzları ile talaşlı imalatta kullanılan M2 yüksek hız çeliklerinin (1.3343) üzerinde elektrokimyasal borlama işlemi uygulanmıştır. Borlanan numune kesitleri üzerinde borür tabaka kalınlıkları ve mikro yapı özellikleri incelenmiş ve hem numune yüzeylerinde hem de yüzeyden içeri kısımlarına doğru olan dağılımları gözlemek için mikro Vickers sertlik ölçümleri gerçekleştirilmiştir. Kullanılan borürleme sıcaklıkları 950°C ve 1000°C olup başarılı borürleme sonuçlarına 1000°C'de ulaşılmıştır. Bu nedenle 1000°C için yapılan deneylerde iki farklı akım oranı kullanılmış ve akım değerlerindeki artışın tabaka kalınlığında artışa sebep olduğu gözlemlenmiştir. Benzer şekilde aynı sıcaklık için tutma süresindeki artış da borür tabaka kalınlıklarını arttırmaktadır.

Abstract:
As a result of increasing quality and efficiency expectations in industrilization, continuous improvements in performance and functional life of manufacturing machines and equipment have become a neccesity. Machine tools used and processes realized in these machines also take their share in high expectations. Especially, mechanical and thermal properties of cutting tools working in the high speed range of CNC machines have become improtant issues. This problem resulted in search for alternative methods to improve mechanical, chemical, thermal and corrosion properties of machining tools.
While they are in contact with the part to be machined, cutting tools work under harsh conditions where high temperatures and stresses are involved. Therefore, strength of the these tools and properties of their surfaces are always in concern. Application of coatings on tool surfaces in order to extend tool life have become widespread and development of various coatings with different properties (e.g. thermal barrier coatings) has been under continuous development.
Besides increasing number of studies in the coatings area, usage and application of boron element in the world and in our country, Turkey, having the a largest percentage of boron reserve of the world, have started to be widespread. Boriding or boronizing is the process to exposure materials to boron-rich environments generally at high temperatures. This process has been attractive in the machine tools area because of outstanding mechanical performances provided. Unfortunately long process periods of heat treating are required to reach these high performances. More recently, electro-chemical boriding where application of current in addition to thermal process has achieved to shorten heat treating period substentially.
The advantages of electrochemical boriding are especially the high hardnesses that could be achieved (1200-2000 HV), also high resistance to corrosive, oxidative environments, application to steels and amount of lubricants to decrease when there is surface interaction. On the other hand, electrochemical boriding has disadvantages such as, high cost compared to nitriding, dimensional increase, low fatigue strength under cyclic loads. Kinetics of boronizing process is more complicated because of microstructural interactions.
When iron-based metals are boronized, nucleation that starts to form on the surface of the matrix material creates boride layers containing Fe2B and FeB phases, respectively. The boride layers have generally a columnar morphology.
Layer thicknesses in columnar structure are at least 15-20 μm due to their geometry. When layer thicknesses exceed 100 μm, boride layers of two phase (including Fe2B and FeB phases) begin to form. FeB phase formed on the matrix surface later has a harder structure than the initial Fe2B phase, but it is more brittle. For this reason, it cannot adhere well to the surface. It is important that the layer thickness does not exceed 100 μm in order to prevent two-phase-structure to form.
In this thesis study, initially thermochemical boriding at 950°C using a powder mixture of dehydrated borax and sodium carbonate as the salt bath for several periods of time have been applied on plain-carbon steel and M2 high speed steels (1.3343), used in machining, although increases in hardnesses have been observed, these increases were in the hardness level of M2 steel quenching area for that material, no additional improval in hardness levels has been achieved.
Later, salt bath boronizing test setup was modified to perform electrochemical boronizing process by adding a DC power supply and connecting cables. Similar salt mixtures were utilized. Specimens processed in the bath with a DC current level of current of 0.1-0.3 A/cm2 for a period of 1.5-2 hours at a temperature of 950-1000°C.
For electrochemical boriding, an electrical circuit is formed through the salt bath; here M2 steel specimens are used as cathode and the graphite crucible is the anode. The flow rate w so that the surface area of the graphite crucible, where the salt bath is in contact with, is six times the sample surface area.
In order to be able to measure the boride layer thickness and hardness from the cross-sectional views, the parts were sectioned in the wire electro-discharge machining (wire-EDM) machine and the mounting process was applied. Sample surfaces were grinded by sand papers and then they were polished using alumina paste.
The boride layer thickness, surface hardness and cross-sectional layer hardness distribution of the samples, whose surfaces were etched with nital solution preparation steps, were investigated. Boride thicknesses and microstructural properties have been evalutated on specimen cross-sections. Both hardnesses on specimen surfaces and distribution of hardnesses through specimen thicknesses have been measused by micro Vickers tests.
As a result of experimental studies; it was determined that the most important factor for the formation of a boride layer on the surface of DIN 1.3343 (AISI M2) steel is the temperature that the process is carried out. While no boride layer could be obtained for electrochemical boronizing at 950°C, boride layers were obtained for the process at 1000°C.
For 1000°C, when boriding treatment period has been varied, the boride layer thicknesses were increased. The current passed was one of the determining factors in the boride layer hardness. In the measurements made, a hardness range of 1200-2000 HV has been obtained. These surface layers were consisted of only single-phase Fe2B boride layer and their thicknesses were in the range of 40-100 μm
These results showed that boride layer hardnesses and layer thicknesses were obtained as a result of electrochemical boriding were compatible with literature studies.
In future research studies, electrochemical boronizing test setup can be improved to be more durable by using extra graphite bars as anodes instead of graphite crucible to perform experiments at higher temperatures or longer period of times. Besides, other boride layer properties can be evaluated by wear tests against various materials (hard materials such as ceramics or soft materials such as aluminum), corrosion tests at different environments, x-ray chemical analysis, fracture toughness tests and boriding process can be applied to actual tools for machining in order to observe their performance under real cutting conditions. The machining performance studies also may be combined with minimum quantity lubrication environments or patterning of the tools, which has been researched recently.