Borlama yöntemiyle (AISI M50, AISI M2) yüksek hız çeliklerinin ve AISI W1 çeliğinin yüzey performanslarının geliştirilmesi

Abstract

ÖZET Anahtar Kelimeler: Borlama, Termokimyasal İşlem, EKabor, Yüksek Hız Çeliği (HSS), Demir Borür, Sertlik, Kırılma Tokluğu Borlama işlemi, termokimyasal işlemler içerisinde çok yüksek yüzey sertliği ve kızıl sertlik ile düşük sürtünme katsayısı, yüksek korozyon ve oksidasyon direnci gibi üstün özelliklere sahip olması ve çok geniş bir yelpazedeki metalik malzemelere uygulanabilirliği sebebiyle ayrı bir öneme sahiptir. Dünya bor rezervlerinin %63'üne ülkemizin sahip olması, bu konuyu daha da cazip hale getirmektedir. Çelik numunelerin seçiminde, özellikle yüksek hız çeliklerindeki alaşım elementlerinin etkilerini belirleyebilmek için kimyasal bileşim faktörü göz önünde tutulmuştur. Bu amaçla, aynı karbon oranına sahip ve ortak alaşım elementleri miktarları da birbirine çok yakın olan çelikler seçilmiştir. Bu çalışmada, katı ortamda, ticari EKabor toz karışımları kullanılarak su verme yoluyla sertleştirilen AISI Wl çeliği ile molibden esaslı yüksek hız çeliklerinden AISI M50 ve AISI M2 çelikleri 850, 950 ve 1050°C'de 2, 4, 6 ve 8 saat sürelerle borlanmışlardır. Borlama işlemi sonucunda elde edilen borür tabakası, optik ve taramalı elektron mikroskobisi (SEM), enerji dağılımlı x-ışınları spektroskobisi (EDS) ve x-ışınlan difraksiyon (XRD) analizleriyle karakterize edilmiştir. Borür tabakasının mekanik özellikleri de tabaka yüzeyinden matrise doğru, doğrusal bir hat üzerinde sertlik dağılımı ve kırılma tokluğu ölçümleri ile belirlenmiştir. Yapılan çalışmalarda; borür tabakası-matris arayüzeyinin AISI Wl çeliğinde kolonsal bir yapı, AISI M50 ve AISI M2'de ise düzlemsel bir yapı sergilediği; borlama süre ve sıcaklığı arttıkça borür tabakası kalınlıklarının asimptotik bir artış kaydettiği; kaplamanın, borür tabakası, geçiş bölgesi ve matristen oluştuğu; borür tabakasının, (Fe,M)B ve (Fe2M)2B (M: Cr, Mo) fazlarından meydana geldiği; çelik bileşimindeki alaşım elementleri miktarının artmasıyla tabaka sertliğinin de artarak AISI M2'de en yüksek değere ulaştığı; borür tabakası içerisinde karbür partiküllerinin yer almasıyla tabakanın kırılma tokluğunun arttığı, hatta partikül boyutu küçülüp hacim oranının artmasıyla kırılma tokluğunun daha yüksek değerlere ulaştığı belirlenmiştir. AISI Wl çeliğinde, düşük sıcaklık ve sürelerde Fe2B fazı elde edilirken, sıcaklık ve sürenin artmasıyla çift fazlı (FeB+Fe2B) borür tabakası elde edilmiştir. Tabaka kalınlığı 250um'den daha kaim olması durumunda borür tabakalarında bölgesel olarak boylamasına çatlaklar oluşmuştur. XVII

THE IMPROVEMENT OF SURFACE PERFORMANCE OF (AISI M50, AISI M2) HIGH SPEED STEELS AND AISI Wl STEEL BY BORONIZING TREATMENT SUMMARY Keywords: Boronizing, Termochemical Treatment, EKabor, High Speed Steels (HSS), Iron Boride, Hardness, Fracture Toughness In industrialized societies there is a growing need to reduce or control friction and wear for several reasons, such as to extend the lifetime of machinery and bio-systems, to make engines and devices more efficient, to conserve scarce material resources, to save energy, and to improve safety. These aims have been achieved by design changes, selecting improved bulk materials, utilizing lubrication techniques and surface treatments and coatings. Surface treatments are remarkable choices for a wide range of tribological applications where the control of friction and wear are of primary concern. Bonding is a thermochemical surface hardening process in which boron atoms are difHised into the surface of a work-piece to form borides with the base materials. Industrial bonding can be applied to most ferrous materials such as structural steels, cast steels, Armco iron, gray and ductile cast irons, and sintered iron and steel; nonferrous materials such as nickel-, cobalt-, titanium-, and molybdenum-base alloys and cemented carbides. Depending on the substrate material, the boriding process involves heating of well-cleaned material at 700°C to 1050°C, preferably for 1 to 12h. in contact with boronaceous solid powder (boriding compounds), paste, liquid or gaseous medium. Two important phases are seen in the boride layer: FeB and Fe2B. Single phase (Fe2B) layer is desirable for industrial applications owing to difference being between specific volume and coefficient of thermal expansion of borides and substrates, and less brittleness. The strong covalent bonding of most borides is responsible for their high melting point, Young's modulus and hardness values. In this study, boronizing was carried out in a pack medium, commercially known as EKabor powder. The boriding procedure was carried out 850, 950 and 1050°C at atmospheric pressure for 2-8h. The chemical compositions of base steels used in this study are given in Table 1. Table 1. The chemical compositions of base steels XVIIIAfter boronizing, each group of steels was examined by using classical metallographic techniques. The morphology and type of borides formed on AISI Wl, M50 and M2 steels are closely related to chemical composition of steel. The presence of borides formed in coating layer were confirmed via x-ray diffraction (XRD) technique, optical microscopy and scanning electron microscope backscatter electron image (SEM-BEI) analysis. Both hardness and fracture toughness of boride layers were measured with a Vickers indenter under a load of 0.5 N and 5-10 N load respectively. Optical microscope and SEM cross-sectional examinations of bonded AISI Wl steel showed, the characteristic sawtooth morphology of the boride layer is dominant. But at the AISI M50 and AISI M2, the development of a jagged boride/substrate interface was suppressed, and a smooth interface was formed. Initially, the thickness of boronized layer increases, later on, the growing rate of layer decreases due to changing diffusion of boron atoms. It is well understood that process is diffusional. At higher magnifications, three distinct regions were identified on cross-section of boride steel surfaces, these are; i) surface layer primarily consisting of FeB, Fe2B, CrB, M02B and MoB phases, ii) a transition zone being rich with boron and iii) the steel matrix. The thickness of boride layers depends on the substrate material being processed, boron potential of the boriding compound, process temperature and time. It was found that the thickness of boride layers revealed at 1050°C and 8 hours treatment period on the AISI M2, AISI M50 and AISI Wl steels are ~140um, 215um and 380u,m respectively. It is recognised that mechanical properties of borided alloys depend strongly on the chemical composition and structure of the boride layers. The surface hardness of borided AISI Wl steel ranges between 1314-1995 HV 0.05; on AISI M50 steel, between 1655-2440 HV 0.05 and on AISI M2 steel, between 1812-2497 HV 0.05. The fracture toughness of borides depends on alloying elements in the base steels. It was also noticed that carbide particles in the boride layer play a positive role on the fracture toughness of borides. The values of fracture toughness were changed from 3.11-3.15 MPa.m1/2, 3.89-3.51 MPa.m1/2 and 5.21-4.80 MPa.m1/2 at 1050°C for 6-8 hours borided AISI Wl, AISI M50 and AISI M2 steels, respectively. It also noticed that the longer boronizing time results in low fracture toughness values due to formation of harder FeB phase. By using scanning electron microscopy- energy dispercive x-rays spectroscopy (SEM- EDS) analysis the change in alloying elements distribution was studied. It was found that carbon and silicon are not soluble in the boride layer, and these elements are pushed from the surface by boron atoms. Very little amount of molybdenum is dissolved and formed molybdenum borides. Tungsten and vanadium complex carbides are not dissolved during boronizing threatment. Scanning electron microscopy- backscattered electron image (SEM-BEI) studies showed that, boride layers have some cracks between FeB and F^B phases that is parallel to sample surfaces for longer boronizing time and high temperature. Because FeB and Fe2B formed are under tensile and compressive residual stresses, respectively. However, steels with low alloying elements have not got any FeB phase and no cracks at low temperature and short time. XIX