Orta karbonlu DIN Ck45 çeliğinin elektrokimyasal yöntemle borlanması
Electrochemical boronizing of medium carbon DIN Ck45 steel
Yazar: ÇAĞIN BOLAT
Danışman: DOÇ. DR. TURGUT GÜLMEZ
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Malzeme ve İmalat Bilim Dalı
Derece: Yüksek Lisans, 2016
Konu: Mühendislik Bilimleri = Engineering Sciences

Özet:
Borlama işlemi, bor atomlarının malzeme yüzeyine difüzyonu ile gerçekleşen ve malzemenin yüzeyini ciddi miktarda güçlendiren bir termokimyasal yüzey sertleştirme işlemidir. Bu işlem oldukça verimli ve etkili bir uygulama olup; birçok mühendislik malzemesinin korozyon, aşınma, oksidasyon ve çizilmeye karşı olan dirençlerini artırır. Bu tez çalışması kapsamında,orta karbonlu DIN Ck45 çelik malzemenin elektrokimyasal borlama yöntemiyle yüzey modifikasyonu araştırılmıştır. 
Deneysel çalışmalar, eriyik susuz boraks (Na2B4O7) banyosu içerisinde farklı sıcaklıklarda(900-950-1000-1100˚C), değişen sürelerde(30-60-90-120 dk) ve farklı akım yoğunluklarında (0,05-0,1-0,2 A/cm2) yapılmıştır. İşlem sıcaklığı, süre ve akım yoğunluğu gibi parametrelerin proses üzerindeki etkileri detaylı olarak incelenmiştir. Bunlara ek olarak, susuz boraks temelli eriyik bileşimine %15 sodyum karbonat(Na2CO3) ilave edilerek bor sağlayıcı kaynağın kompozisyonu değiştirilmiş ve bu ortamın borür tabaka kalınlığına olan etkisi gözlemlenmiştir. Oluşan borür tabakaların karakteristik analizi için X ışınları kırınımı(XRD) yöntemi ve optik mikroskop görüntüleri kullanılmıştır. Borür tabakaların sertlik ölçümleri 1N yükleme altında mikro vickers test ekipmanı kullanılarak yapılmıştır. Uygulama sıcaklığındaki değişimlere bağlı olarak, borür tabakaların sertlik değerlerinin 1728 HV ile 1936 HV arasında olduğu belirlenmiştir. Elektrokimyasal yöntem kullanılarak elde edilen değerlerin, orta karbonlu DIN Ck45 çelik malzemenin standart sertlik mertebelerinden yaklaşık altı kat daha fazla olduğu farkedilmiştir. Sertlik ölçümleri, optik mikroskop görüntüleri ve XRD analiz sonuçlarına göre, tek fazlı Fe2B borür bileşiğinin malzeme yüzeyinde varlığı tespit edilmiştir. 
Deneysel çalışmaların sonucunda; işlem sıcaklığı, süre ve akım yoğunluğundaki artışın borür tabaka kalınlığını pozitif yönde etkilediğine ulaşılmıştır. Ayrıca geleneksel katı paket borlama ve pasta borlama işlemleri ile mukayese edildiğinde, elektrokimyasal borlama işleminin oldukça hızlı ve seri bir yöntem olduğu saptanmıştır ki bu durum günümüzdeki rekabetçi endüstri ortamında büyük avantaj sağlayabilmektedir. Borlama işleminin malzeme yüzeyi üzerinde ciddi dayanım artışları yaptığını görebilmek için farklı asidik ortamlarda korozyon testleri de yapılmıştır. Elektrokimyasal olarak borlanmış numuneler ile borlama işlemi yapılmamış ham numuneler, %15 hidroklorik asit(HCl), %15 sülfürik asit(H2SO4) ve %15 nitrik asit (HNO3) çözeltilerini ihtiva eden ortamlarda korozyon testine tabi tutulmuştur. Yapılan testler beş gün boyunca sürmüştür ve her iki tip numunedeki kütle kayıp miktarları hassas terazi yardımıyla günlük olarak ölçülmüştür. Korozyon testleri neticesinde, borlanmış numunelerin her üç korozif ortamda da borlanmamış numunelere oranla çok daha az kütlesel kayıba uğradıkları tespit edilmiştir.

Abstract:
Boronizing, or boriding, is a thermo-chemical diffusion process in which boron atoms can diffuse into the workpiece to form hard boride layers. It is mainly applied to ferrous materials such as structural steels, stainless steels, tempered, case-hardened and tool steels but occasionally the process also has been used in boronizing of non-ferrous materials and alloys. It is a promising surface modifcation technique which improves oxidation, corrosion, wear and scratch resistance of a wide range of engineering materials. Boronizing easily tops the performance of commonly used methods like carburizing and nitriding. The boronizing basically consists of two types of reaction. The first reaction takes place between boron yielding substance and the workpiece surface. The second reaction is diffusion controlled, the total thickness of boride layer growth at a particular temperature can be calculated.
In this thesis, we investigated eletrochemical boriding of medium carbon DIN Ck45 steel in molten borax (Na2B4O7) electrolyte at different temperatures (900-950-1000-1100°C), different process durations (30-60-90-120 min) and current densities (0,05-0,1-0,2A/cm2). We also analyzed effect of the salt bath composition on boride layers thickness by adding sodium carbonate (Na2CO3) to borax-based electrolyte. For examining the corrosion behaviour of borided substrates, we carried out immersion tests in HCl, H2SO4and HNO3(%15 vol.) acid solutions.
The electrochemical boronizing experiments by molten salt eletrolysis were conducted in a electrically resistance furnace consisting of a graphite crucible utilized as an anode and a steel cathode material. During electrolysis, current was supplied digitally from a direct current source. In all boronizing experiments, before putting the cathode steel material into the borax-based electrolyte, the cathode was polished in order to degrease the surface and attain defined surface roughness. The cathode was withdrawn from the slurry salt bath and left in air to cool then the current was closed down. Wastes of elctrolyte accumulated on cathode material surface were cleaned in hot boiling water. After this cleaning process, borided workieces were cut into two pieces and one of them was used for optic microscope and hardness tests. Boride layers characteristics were carefully examined optic microscope (OM) and X-ray diffraction (XRD). Metallographic studies and XRD analysis revealed that single-type Fe2B layers were formed on the steel substrate surface. In addition to these works, optic microscope views showed that saw-tooth morphology, which is very unique property of boride layers, was formed on steel surface. Hardness of borides created on steel substrate was measured by micro vickers test equipment under the load of 1N. Depending on eletrochemical boronizing temperature and time it has found that the hardness of boride layer ranged from 1728 HV to 1936 HV that is nearly 6 times more than substrate material hardness. After the hardness tests of electrochemical borided medium carbon steels, we noticed that hardness values reduced sharply from boride layer to substrate material and they went down as distance from the surface increased.
Experimental results clearly indicated that thickness of boride layer went up as the process temperature, duration and current density escalated. At 1100°C and in borax based salt bath, we obtained approximately 145 μm thick Fe2B layer while the thickness of boride layer on steel surface at 900°C was almost 48 μm. Morever, in the light of experimental data at different temperatures, some theoritical calculations were made about boride layers growth. Experiments conducted in different process durations(30-60-90-120 min) at 950°C and 0,1 A/cm2 current density demonstrated that it was very possible to obtain 38 μm thick Fe2B layer on medium carbon DIN Ck45 steel surface only in 30 minutes. At the present time, there are lots of boronizing processes available for the treatment of ferrous (structural steels, cast irons, sintered steels etc.) and non-ferrous(titanium, cobalt, nikel, molybdenum alloys etc.) materials. The most prominent types of the boronizing treatment are pack boriding, paste boriding, liquid boriding and gaseous boriding but all of the boriding processes mentioned require long operation time to form hard boride layers on substrate surface. In this study, we easefully observed that hard boride layers were acquired in a short time on workpiece surface by electrochemical boronizing method. The dependence of boride layer formation rate on process duration was neither linear nor logarithmic in the long run, indeed it had a parabolic character. This is a predictable situation, since boronizing is a diffusion controlled surface hardening treatment like other diffusion-based nitriding and carburizing processes whose growth rates abides a parabolic law. The parabolic growth rate constants were calculated for different process temperatures and were found to be 0,29×10-6 ms-2, 0,50×10-6 ms-2, 0,78×10-6 ms-2 and 0,85×10-6 ms-2 respectively. In addition to that results, we explored that ifthe current density increased, thickness of boride layers also scaled up. For instance; in molten Na2B4O7 electrolyte, at 950°C, it can be achieved 47, 84 and 92 μm thick Fe2B by applying to system 0,05, 0,1 and 0,2 A/cm2 current densities respectively. In order to improve boron diffusion in the electrochemical boronizing process, concentration of the molten salt bath was changed proportionately and boronizing experiments were carried out in this new bath concentration. Thanks to adding %15 Na2CO3 to the borax based mixture, we created longer boride layers on medium carbon steel surface. In molten salt bath containing %85 Na2B4O7 and %15 Na2CO3, we formed very dense and homogenously 43, 63,79 and 92 μm thick layers at 950°C and 0,1 A/cm2 current density in different proess durations (30-60-90-120 min). It was easely noticed that almost 45 μm thick Fe2B layer was formed on workpiece surface only in 30 minutes and diffusion of boron atoms was triggered by adding of sodium carbonate. 
If we look from the viewpoint of corrosion behaviour, it is true to say that electrochemical borided steels are more endurable than non-borided steels especially in HCl and H2SO4 acid solutions. In the experiments, three different media consisting of %15 HCl, %15 H2SO4 and %15 HNO3 was prepared for immersion tests. During five days, borided and non-borided steels were tested in three diffrent acid solutions and their weight loss were measured for each day with precision balance. According to results of corrosion tests, even though electrochemical borided steels were pretty resistant aganist corrosive effect of HCl acid solution (nearly 8 times higher than non-borided steels) and that of H2SO4 media (nearly 3 times more than non-borided steels), non-borided steels lost their weight seriously. However, when it came to corrosion behaviour in HNO3 acid solution, it was understood that difference between electrochemical borided specimens and other specimens, which was not subject to boronizing, was not very significant but borided specimens still lost less weight than the others.
In conclusion, electrochemical boriding of medium carbon DIN Ck45 steel was investigated profoundly and the parameters affecting the process were viewed deeply. According to results of the experiments, process temperature, boronizing time and current density influenced the thickness of boride layers positively. Compared to other traditional boriding methods, it was observed that electrochemical boronizing was more rapid than the others. In addition to that, electrochemical borided steels were more resistant than non-borided steels in acid solutions, particularly in HCl and H2SO4.