Bor yayınımıyla çeliklerde yüzey sertleştirme

Abstract

Bor ve bileşikleri, çelik sanayinde cüruf yapıca alaşım elementi ye yüzey sertleşti rici olarak kullanılmakta dır. Türkiye'de bol miktarda bulunan bor minerallerinin meta lürjide değerlendirilmesine yönelik olan bu çalışmada özellik le bor bileşikleriyle çeliklerde yüzey sertleştirme `BORLAKA` işlemi üzerinde durulmuştur. Borla yüzeyi sertleştirilmiş çelik grubu malzemeler, borlu tabakanın çok sert (*v 2000vikers), aşınmaya dayanıklı ve sürtünme katsayısının,çok düşük olması nedeniyle endüstri de bir çok kullanım alanı bulmuşlardır. Borlu malzemelerin ko- rozyon direnci, HCl asitde ve Al, Zn, Pb metallerinin sıvı banyolarında oldukça yüksektir. Bor ila yüzey sertleştirme» esas olarak borun yüksek sıcaklıkta celine yayınımı olayıdır. Bor kaynağının fiziksel durumu sıvı, gaz veya katı olabilir. Borlama banyosu ana bile şeni herhangi bir bor bileşiğidir. (B-C, tîaJB.O-, BJHL gibi). Bor yayınımı genel olarak çeliğin borlama ortamında 850-1000 °C sıcaklıkta 2-6 saat bekletilmesiyle gerçekleşmektedir. Borlu tabakanın özellikleri, Borlanan çeliğin bile şimi başta olmak üzere, borlama sıcaklığı ve süresi ile ila ve ısıl işlemlere bağlıdır. Borlamanm endüstriyel uygulaması için tek fazlı (Fe`B) yapı veren ve bor yayındırma potansiyeli yüksek olan borlama ortamları gereklidir. Son senelerde uygulamaya ko nulan bu tür banyo bileşimleri genellikle patentlerle korun maktadır. Ülkemizde mevcut bor kaynaklarıyla borlama banyo-- II - sunun tesbiti bu çalışmanın ilk aşamasını oluşturmaktadır. Deneysel çalışmalarda, borlama metodu olarak `sıvı ortam borlaması` ve borlama ana bileşeni olarak da `boraks` seçilmiştir. Tez, `Ülkemiz kaynaklarıyla borlama banyosu tes biti` ve `elde edilen borlu tabakanın özellikleri` çerçevesin de sunulmuştur. ' Deneysel çalışmaların birinci bölümünde, ana banyo bileşeni olan `Boraks`a, `Ferro-silis!' ı ve `Ferro-silis+kalsi- ne borik asit` ilavelerinin borlu tabaka faz yapısı ve kalın lığına etkisi araştırılmıştır. Deneyler, 940 °C sıcaklıklı banyolarda AISI 1042 çeliğinin 5 saat, süreyle borlanmasıyla gerçekleştirilmiştir. `Boraks+Ferro-silis` bileşimli banyo larla tek fazlı borür tabakası (FeJB) elde edilmiş ve banyo daki ıFerro-si lis miktarı arttıkça borlu tabaka kalınlığının artığı gözlenmiştir. `Boraks+Ferro-silis` sistemine kalsine borik asit ilavesi ise, borür tabaka kalınlığının 3-4 misil artırmıştır. `BorakstFerro-siıis+kalsine borik asit` bileşim li banyolarla elde edilen borür tabakası faz yapısının sis temdeki kalsine borik asit miktarıyla ilgili olduğu» %18'den fazla kalsine borik asit ilavesinin borür tabakasında' Fe2B yanında FeB fazını da oluşturduğu tesbit edilmiştir. Borlu tabaka faz yapısı ve çalışma koşulları bir arada` gözönüne alındığında, borak'sa ilave edilen Ferro-si lis` in %25' i ve kalsine borik asidin de %16'i geçmemesi öne rilmiştir. Deneysel çalışmaların ikinci kısmında borlu taba kanın Özellikleri araştırılmıştır. Borlu tabakanın borür ta bakası ve geçiş zonundan oluştuğu tesbit edilmiştir. Geçiş zönunun varlığı, otoradyograf i metodundan faydalanılarak- Ill aydınlığa kavuşturulmuştur. 5B10.ı.0n,» 2«K + Li* +2,4 MeV reaksiyonuna dayanan otoradyograf i metodunda o( partiküllerini tesbit için selüloz asetat butirat (SAB) kullanılmıştır. Nöt ron ışınlamasından sonra SAB malzemesi, 60 °C sıcaklıktaki % 28'lik KOH banyosunda 10-25 dakika süreyle dağlanmıştır. Bu metodla geçiş zonundaki borun mikroskobik dağılımı SAB üzeri ne çıkarılmıştır. Alaşımlı ve yüksek karbonlu çeliklerde optik mikroskopla tesbit edilemeyen geçiş zonu» otoradyografi meto duyla tesbit edilebilmiştir. Yine bu metodla bor yayınımının, malzeme haddeleme yönüne paralel bir yönlenme gösterdiği de gözlenmiştir. AISI 1042 çeliğinde elde edilen geçiş zonu ile ilgi li diğer bir bulgu ise, borun bu zonda çeliklerde alaşım ele menti olarak gösterdiği etkileri göstermesi olmuştur. Deneysel çalışmaların son bölümünde borlu tabaka nın kinetiği araştırılmıştır. Bu seri çalışmalarda farklı iki banyo kullanılarak borür tabakası ve geçiş zonu kinetiği ay rı ayrı incelenmiştir. Farklı bileşimdeki banyoların borür tabakası büyüme kinetiği açısından büyük farklılıklar göster diği belirlenmiştir. Önerilen optimum banyo kompozisyonunda AISI 1042 çeliğinin borlanmasıyla elde edilen borür tabakasın, ait aktivasyon enerjisi değerinin 31 k.kal/mol K olduğu be lirlenmiştir. `Geçiş zonu tabaka kalınlığı zaman` bağıntısının ise logoritmik eksenlerde doğrusal (lineer) olduğu bulunmuş tur.

Boron and its compounds can bu used as an alloying element, slag maker and case hardener in the steel industry. Aiml of this work primarily depends on the utilization of the Turkish boron mineral resources from metallurgical aspect, particularly boronizing of steel. Boronizing results in some important improvements in the surface properties of steel. Among the advantages of boronizing, the increas in various engineering properties such as surface hardness up to around 2000 HV and a high resistance to frictional wear, oxidation, and corrosion can be particu larly mentioned. Boronizing is a diffusion process analogous to carburizing. However, unlike carburizing (in which there is a gradual transition) (in composition between the carbon-rich surface and the substrate) in boronizing, double-or single- phase boride layers of definite compositions having a characteristic morphology are formed. Boron source in the boronizing process can be either solid, liquid or gaseous. Main component of boronizing bath can be any boron compounds of minerals such as B4C, Na2B40?, B2H6 etc. Boronizing of steels generally can be done at 850-1000° C for 2-6 hours depending on the desired depth of the boride layer. Properti es of boronized layer primarly depends on the chemical compo sition of steel, boronizing temperature, time and applied additional heat treatment after proces.- V For industrial applications of boronizing» the boronizing medium must have high boron diffusion potential which results in single-phase boride layer of Fe2B. In past years» chemical composition of the boronzing baths are generally patented in the industry. Initial part of this work was carried on the optimization of bath composition by using the indigenous boron resources. For this purpose, molten borax baths with additives were choo sen. Specimens were prepared from an AISI SAE 1042 steel rod 20 mm in diameter. They are boronized at 940 °C for 5 hours. Influence of addition of NaCl, Sic, B`03 and Ferrosilicon in to the borax bath on the boride layer was systematically reported. It is observed that addition of ferro silicon up to 30 % into the borax bath gives the single-phase boride layer of Fe2B and thickness of the layer increases as the amount of ferro silicon increased. On the other hand, addition of calcine boric acid in to the borax and ferro silicon system caused an increase in boronized layer thickness about 3-4 times. Addition of calcine boric acid more than 18 % caused the apperance of FeB layer beside the Fe2B was reported. As the required boride phase and optimum working conditions are concerned, addition of not more than 25 % ferro silicon and 18 % calcine boric acid was recommended. Second part of this work was mainly concerned with the search of the properties of boride layer and its under neath. To do this, hardness measurements, optical metallog raphy, x-ray mi cro-ana lysis and <* -autoradiography of cross sections of the borided samples were studied. Under the borided layer, the presence of a transition zone as much as an order of magnitude thicker than the borided layer was- VI - observed. The micro structure and hardness of this zone are found to be sensetive to the cooling rates after boronizing. The other remarkable point in the transition zone was the presence of large grains (ASTM grain size No 2) than those in the core region (ASTM grain size 7) The distribution of silicon over a.cross section of the sample starting 60 um below the surf ace through the core region was also studied. Although the boronizing bath used contains silicon, it was found that the silicon level is essentially the same across the sample and equal to the silicon level of the original material. From these observa tions it is concluded that the diffusion of silicon is negligible during the boronizing proces. Although boronized outer layers are vdsible under the optical microscope» the boron distribution in the subla yers cannot be determined non-destructiveiy by conventional methods (such as microprobe analysis) because of the low atomic number of boron. The tracing of boron and its compo unds in the steel matrix requires detection using methods which have a high sensitivity and a high resolution and are on a microscopic scale. The best technique for the solution of this ©^autoradiography which is based on the well-known nuclear reaction 5 0 i J If an emitted ©^particle loses its energy in a solid nuclear track detector material» it leaves behind a latent track which can easily be revealed by chemical etching.- VII - Although this technique has been employed previously in order to determine the boron distribution in some steels, no work has been found in the literature on the application of a solid state nuclear track detector in the study of boronized steel. In contrast, the physical mechanism of boron diffusion and the effects of boron on the solid state trans formations remain unknown and have not been explained yet. This work aims at providing some data on the distribution, and hence the role, of boron in boronized steel. ' Boronized specimens were mounted in a cold-setting resin with hard filler and were then fine polished. As a low background detector material for the detection for c< partic les released from the interaction of boron ( B) nuclei with thermal neutrons, cellulose acetate butyrate (CAB) was selec ted. A piece of CAB film (a commercial product of Agar Aids, G. Britain) 100 urn thick is applied gently on the polished specimen surface which had been wetted with butanol. Care was taken to avoid trapping any bubbles which impair image qua lity. Replicated samples were dried under room conditions for 2 days. For neutron irradiation, specimens were enclosed in a water-proof aluminium capsule and irradiated in the revol ving cavity of the reflector of the TRIGA Mark II reactor operated by the Institute for Nuclear Energy, Technical University of Istanbul, A thermal fluence of approximately 15... 10 nvt proved to be quite satisfactory in producing rep roducible results with low background levels. After a cool ing period of 2 days, the detector material was peeled off the irradiated specimen and etched in a 29% KOH bath at 333 K with intermittent rinsing and inspection under an optical microscope to avoid overdeveloping which increases the background. Under the given conditions the proper etching stage is reached at approximately 12 min, beyond 25 min,- VIII - overetching occurs. Each specimen was accompanied by another unboronized steel specimen to observe the background effects of other radiation. The o(. tracks in the detector materials were studied by optical metallography, and the track densities were assess ed by a Quant imet 720 system. An autoradiography clearly show the presence of a large amound of boron segregation in addition to a fine distri bution of boron in the transition zone. However, because of the low resolution of the optical microscope, the presence of ultrafine precipitation cannot be detected. From the microscopic examination of solid state nuclear track detector replicas by comparing the track contrast of the well-identified FeB and Fe2B layers with that of large unknown segregations of boron compounds in the tran sition zone, the latter is predicted to be FejB. At present this technique does not permit the proper identification of fine boron precipitates, i.e whether they are boride or any other boron compounds. The last part of this work is concerned about the influence of the bath composition on the kinetics of boro- nising. In these series of experiments two different boroni-' zing bath one containM66% borax, 14% calcine boric acid and 20% ferro silicon` and second contain 66% borax, 14% calcine boric acid and 20% SiCwwas used and systematic information has been obtained on the rate of boronisation. Diffusion parameters were also calculated from the experimental data, for boride layer.- IX - The findings of this investigation can be summarized as. follows I 1) Optimum composition of the boronizing bath was found to be FerroSilicon 25%, calcine boric acid 18%, rest borax for boronizing at 940°C for 5 hours. The activation energy for boron diffusion at the boride layer was found to be 31 Kcal/ Mole °K for AISI 1042 steel with recommented optimal bath composition. 2) Molten borax bath containing ferro silicon causes the formation of the single-phase of Fe2B. 3) Addition of calcine boric acid in the borax-ferro- silicon system increases the thickness of the boride layer about 3-4 times. However addition of more than 18% causes the appearance of FeB beside the F^B. 4) There is a transition zone under the borided layer of boronized AISI-SAE 1042 steel. The microstructure and hardness of this zone are sensitive to the cooling rates after boronizing. 5) Boron can diffuse homogeneously into the regions under neath the borided surface layer, boron is present mainly in solid solution but also as excess boron precipitates (borides and / or borocarbides) at fault sites. This behaviors has been attributed to the presence of a marked boron-vacancy interaction. 6) The formation of the transition zone is typical of the effect of boron in solid solution, and the distributionX of boron compounds in this zone does not affect the transformation kinetics of austenite. 7) Although the boronizing bath contains silicon, the diffus ion of silicon during the boronizing processis negligible.