Toz metalurjı̇sı̇ ı̇le üretı̇len bir takım çelı̇ğinin kutu alüminyumlama yöntemiyle kaplanması
Pack aluminizing of a powder metallurgy tool steel
- Tez No: 980536
- Danışmanlar: PROF. DR. MURAT BAYDOĞAN
- Tez Türü: Yüksek Lisans
- Konular: Metalurji Mühendisliği, Metallurgical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2025
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Lisansüstü Eğitim Enstitüsü
- Ana Bilim Dalı: Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: Belirtilmemiş.
Özet
Takım çelikleri, kalıp ve kesici takımlar gibi ürünlerde kullanılan çelik türüdür. Bu çelikler yüksek mukavemet, sertlik ve aşınma direnci gibi mekanik özelliklere sahiptir. Takım çelikleri, talaşlı imalat, otomotiv, havacılık ve savunma sanayi gibi birçok sektörde kullanım alanına sahiptir. Çok geniş bir çelik ailesini kapsadığı için farklı kullanım alanlarına ve üretim yöntemlerine göre alt dallara ayrılmıştır. Bu çalışmada toz metalurjisi ile üretilen bir takım çeliği tercih edilmiştir. Toz metalurjisi üretim yöntemi, metal tozlarından istenilen şekilde parça üretilmesini mümkün kılmaktadır. Bu yöntemle üretilen takım çelikleri daha homojen bir yapıya sahiptir. Homojen yapının avantajı ile daha yüksek sertlik, aşınma direnci, tokluk ve mukavemet özellikleri elde edilebilmektedir. Ayrıca bu yöntemle daha karmaşık şekillere sahip parçalar, daha kontrollü kimyasal bileşime sahip malzemeler ve daha iyi boyut kararlılığına sahip ürünler üretmek mümkündür. Takım çeliklerine yüzey özelliklerini iyileştirmek için yüzey işlemleri de uygulanmaktadır. Bu çalışmada uygulanan yüzey işlemi kutu alüminyumlama işlemidir. Kutu alüminyumlama işlemi, malzeme yüzeyinde alüminat tabakası oluşturmak için kullanılan difüzyon tabanlı bir termokimyasal yüzey işlemidir. Malzemenin korozyon ve aşınmaya karşı direncini artırmak için uygulanmaktadır. Endüstride kullanılan takım çeliklerinde maksimum sertlik ve minimum aşınma istenmektedir. ASP 2012, toz metalurjisi ile üretilen bir takım çeliğidir. Su verme işlemi sonrasında sertliği 60 HRC'ye kadar ulaşabilmektedir. Yapısındaki karbür yapıcı elementler ve üretim yönteminin getirdiği avantajla mikroyapısında homojen dağılmış ve ince taneli karbür yapıları bulunmaktadır. ASP 2012'nin ince ve homojen karbür dağılımı; yüksek aşınma kararlılığına ve mekanik yükler altında öngörülebilir performansına katkıda bulunur. Daha ileri uygulamalarda, yüzey sertliği çeşitli yüzey kaplama teknikleriyle artırılabilir. Bu çalışmada, kutu alüminyumlama ile ASP 2012 takım çeliği üzerinde bir alüminat tabakası oluşturulmuştur. Kutu alüminyumlama işleminde aktivatör, sıcaklık, yüzey pürüzlülüğü ve ısıl işlemin etkileri farklı parametreler test edilerek incelenmiştir. 560°C, 600°C, 650°C ve 900°C sıcaklıkları test edilmiştir. Aktivatör olarak ise amonyum florür (NH4F) ve alüminyum klorür (AlCl3) kullanılmıştır. Yüzey pürüzlülüğünün etkisini gözlemlemek için, parlatılmış ve kumlanmış yüzeyler arasındaki fark incelenmiştir. Isıl işlemin etkisini değerlendirmek için, su verme işleminden önce ve sonra kutu alüminyumlama işlemine tabi tutulmuştur. Tüm deneyler için süre 240 dakikada sabit tutulmuştur. Toz bileşimi olarak ise tüm deneylerde %70 Al2O3, %25 Al ve %5 aktivatör kullanılmıştır. Deney sonucunda, kutu alüminyumlama işlemi yüzeyde demir alüminat tabakaları oluşturmaktadır. Kutu alüminyumlamadan sonra optik mikroskop, taramalı elektron mikroskobu, XRD ve mikrosertlik cihazı ile mikroyapısal incelemelerle karakterizasyon işlemleri gerçekleştirilmiştir. Yüksek alaşımlı toz metalurji takım çeliğinin kutu alüminyumlama ile başarılı bir şekilde alüminize edildiği görülmüştür. Kutu alüminyumlama yöntemiyle kaplanmış ASP 2012 ve sertleştirme ısıl işlemi gerçekleştirilmiş ASP 2012 aşınma testine tabii tutulmuştur. Aşınma testi sonuçlarının profilometre, dijital mikroskop ve SEM-EDS ile analizi gerçekleştirilmiştir. Kutu alüminyumlama işleminin aşınma davranışında sebep olduğu davranışlar gözlemlenmiştir. Gerçekleştirilen deneyler sonucunda kutu alüminyumlama işlemi uygulanmış ASP 2012 takım çeliğinin sertleştirme ısıl işlemi uygulanmış ASP 2012 takım çeliğine kıyasla daha iyi aşınma direnci gösterdiği gözlemlenmiştir.
Özet (Çeviri)
Tool steels are a type of steel used in products such as molds and cutting tools. These steels have mechanical properties such as high strength, hardness and wear resistance. Tool steels are used in many sectors such as machining, automotive, aerospace and defence industries. Since it covers a very wide steel family, it is divided into sub-branches according to different usage areas and production methods. In this study, a tool steel produced by powder metallurgy was preferred. The powder metallurgy production method makes it possible to produce parts in the desired shape from metal powders. Tool steels produced with this method have a more homogeneous structure. With the advantage of the homogeneous structure, higher hardness, wear resistance, toughness and strength properties can be achieved. In addition, with this method, it is possible to produce parts with more complex shapes, materials with more controlled chemical composition and products with better dimensional stability. Surface treatments are also applied to tool steels to enhance their surface properties. In this study, a pack aluminizing process was applied as a surface modification method. Pack aluminizing is a diffusion-based thermochemical surface treatment used to create an aluminide layer on the material surface. It is applied to increase the resistance of the material against corrosion and wear. The growing demand for tool steels with enhanced surface durability has made surface treatment methods increasingly important in recent years. In industrial applications, maximum hardness and minimum wear are desired in tool steels. ASP 2012, a tool steel produced by powder metallurgy, was selected as the substrate material. After quenching, ASP 2012 can reach a hardness of up to 60 HRC. Owing to the presence of carbide-forming elements and the advantages of the production method, its microstructure contains finely distributed carbides with a homogeneous distribution. The fine and uniform carbide distribution of ASP 2012 contributes to high wear stability, dimensional accuracy, and predictable tool performance under mechanical loads. For more advanced applications, surface hardness can be increased through various surface coating techniques. In this study, an aluminide layer was formed on ASP 2012 tool steel by the pack aluminizing process. The effects of activator type, temperature, surface roughness, and heat treatment were investigated under different process parameters. The characteristics of the formed layers were analyzed, and the optimum process parameters were determined. Wear tests were then carried out on both the pack-aluminized and conventionally heat-treated ASP 2012 samples under the determined optimum conditions. Pack aluminizing is a diffusion-based surface coating method used to enhance wear and corrosion resistance. This process, also known as pack cementation, allows the desired coating characteristics to be achieved by varying parameters such as activator type, powder composition, temperature, and duration. Due to its easy applicability, low cost, and ability to provide uniform coatings even on complex geometries, it has attracted significant research interest. Moreover, the process enables modification of surface chemistry without significantly altering the bulk properties of the substrate, which is particularly advantageous for high-performance tool steels that require both surface hardness and core toughness. In this thesis, different parameters were varied to obtain the optimum coating. The process duration was kept constant at 240 minutes for all experiments. The powder mixture consisted of 70% Al₂O₃, 25% Al, and 5% activator. The process temperatures were 560°C, 900°C, 650°C, and 600°C. These temperatures were chosen to approach either the austenitizing temperature above 1000°C or the tempering temperature of 560°C for ASP 2012 tool steel. Two different halide activators, NH₄F and AlCl₃, were used in the experiments. To investigate the effect of surface roughness on the coating layer, the substrate material was prepared in both polished and sandblasted conditions. Another variable parameter was the application of a quenching heat treatment prior to the pack aluminizing process. These variations were selected to provide a comprehensive understanding of how thermochemical and microstructural factors influence diffusion, layer adhesion, and the final mechanical performance of the coating. After the aluminizing process, optical microscopy, scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), and microhardness analyses were conducted to evaluate the effects of the parameters and determine the optimum coating conditions. The integration of multiple characterization techniques provided both morphological and compositional insights, allowing the diffusion mechanism and intermetallic phase evolution to be clearly identified. No iron aluminide layer formation was observed at 560°C. Therefore, the pack aluminizing temperature was increased to 900°C. Due to the formation of voids at this temperature, further experiments were carried out at 650°C and 600°C. In the experiments using NH₄F as the activator, the presence of AlF₃ phase was detected in addition to the iron aluminide layer. Since AlF₃ is an undesired phase, the experiments were continued using AlCl₃ as the activator. The results indicated that AlCl₃ facilitated a more stable aluminizing atmosphere and promoted controlled diffusion of aluminum into the substrate, yielding a dense and continuous intermetallic layer. To examine the effect of substrate surface roughness on the coating, experiments were performed using both polished and sandblasted samples. The results showed that surface roughness had no significant effects on the coating thickness or morphology. To achieve the optimum substrate hardness after aluminizing, samples were also quenched prior to the process. The quenching treatment was found to reduce the coating thickness considerably. This reduction was attributed to the denser martensitic microstructure of the quenched substrate, which restricts the diffusion rate of aluminium atoms. Optical and SEM-EDS analyses revealed that homogeneous and void-free coatings were obtained at 650°C and 600°C. The coating thicknesses obtained under these conditions ranged from 10 µm to 45 µm. Although a coating thickness of approximately 400 µm was obtained at 900°C using AlCl₃ as the activator, the layer exhibited a non-uniform structure. Therefore, it was concluded that the optimal aluminizing temperature for ASP 2012 lies within the intermediate temperature range, ensuring a balance between diffusion activity and microstructural stability. XRD analyses confirmed that in all experiments where coating formation was observed, the main phase of the iron aluminide layer was Fe₂Al₅, accompanied by FeAl and FeAl₃ phases. The AlF₃ phase was detected only in the samples processed with NH₄F activator. Microhardness tests were performed on both the coating and substrate. The hardness of the coating layer ranged between 800 and 1000 HV. For the wear tests, the sample produced at 600°C using AlCl₃ activator, quenched, and sandblasted was selected. The reason for choosing these parameters was to ensure high substrate hardness in addition to surface hardness. Under these conditions, the coating hardness was measured as 874 HV and the substrate hardness as 521 HV. For comparison, a conventionally heat-treated ASP 2012 sample was used. The heat treatment consisted of austenitizing at 1025°C, followed by quenching and triple tempering at 560°C for 1 hour each. Triple tempering was applied to transform retained austenite into martensite and to stabilize the martensitic structure in this high-alloy steel. This approach ensures that the reference sample possesses optimal microstructural integrity, allowing a fair comparison with the aluminized specimens. A reciprocating wear test was used for wear characterization. The test was conducted using a 6 mm diameter alumina ball as the counter material, with a normal load of 1 N, a stroke length of 5 mm, a total sliding distance of 50 m, and a sliding speed of 10 mm/s. All tests were conducted at room temperature. The wear tracks were analyzed using a profilometer, digital microscope, and SEM-EDS. Differences in the wear behavior between the pack-aluminized and heat-treated samples were identified. Profilometric analysis showed that the average wear track cross-sectional area was 407 µm² for the pack-aluminized sample and 508 µm² for the heat-treated sample. Based on these results, the wear rates were calculated as 4.068×10⁻⁵ mm³/Nm and 5.084×10⁻⁵ mm³/Nm, respectively. The pack-aluminized sample exhibited approximately 20% lower wear rate compared to the heat-treated sample. This finding confirms that pack aluminizing not only enhances the surface hardness but also effectively reduces material loss under sliding wear conditions. Panoramic images of the wear tracks were obtained by digital microscopy, followed by SEM-EDS analyses. The wear surface of the pack-aluminized sample exhibited oxide formation and fatigue-induced cracks, likely caused by the brittle nature of the intermetallic layer. The oxide formed on the surface was identified predominantly as Al₂O₃. In addition, localized material loss was observed in the wear region. In the heat-treated sample, surface analysis revealed signs of plastic deformation and oxide formation. The ductile metallic structure contributed to the characteristic wear track morphology, with observable material pile-up along the track edges. The oxide formed on this surface was expected to be Fe₂O₃ due to the room-temperature wear test conditions. Overall, the comparative results demonstrated that the aluminide-coated ASP 2012 offers superior wear resistance and surface stability, validating the effectiveness of pack aluminizing as a promising surface engineering approach for high-performance tool steels.
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