Geleneksel Toz Metalurjisi Yöntemiyle Üretilen Aısı 304l Ve Aısı 316l Östenitik Paslanmaz Çeliklerin Özelliklerine Sinterleme Koşullarının Etkisi
Geleneksel Toz Metalurjisi Yöntemiyle Üretilen Aısı 304l Ve Aısı 316l Östenitik Paslanmaz Çeliklerin Özelliklerine Sinterleme Koşullarının Etkisi
Dosyalar
Tarih
2014-07-03
Yazarlar
Ak, Hulki Deha
Süreli Yayın başlığı
Süreli Yayın ISSN
Cilt Başlığı
Yayınevi
Fen Bilimleri Enstitüsü
Institute of Science and Technology
Institute of Science and Technology
Özet
Toz metalurjisi, metallerde ergime gerçekleştirilmeden yapılan bir üretim yöntemidir. Bu nedenle özellikle yüksek ergime sıcaklıklarına sahip metallerde kolayca uygulanabilir. Geleneksel olarak yağlayıcı içeren metal tozlarının istenen şekildeki kalıplar içerisinde preslenmesi ve takiben koruyucu bir atmosferde sinterlenmesi işlemlerinden oluşur. Sinterlenmiş parçalarda yoğunluk ve mekanik özellikler sinterleme ve presleme şartlarına bağlıdır ve istenilen özelliklere ulaşmada sinterleme ve presleme koşullarının iyi belirlenmesi gerekir. Değişken yoğunluklardaki sade ve karmaşık şekilli parçaların yüksek üretim kapasitelerinde üretimine elverişlidir. Toz metalurjisi, sağladığı çeşitlilik açısından en kapsamlı şekillendirme ve üretim süreçlerine sahip teknolojidir. Toz metalurjisinin öne çıkan en büyük avantajı yüksek kaliteye sahip karmaşık şekillerdeki parçaların en az kayıpla veya kayıpsız olarak ekonomik bir şekilde üretimine elverişli olmasıdır. Üretim aşamaları şekillendirilip, preslenmiş ve ısıtma yoluyla sinterlenmesi adımlarından oluşan ve günümüz teknolojilerinde ve sanayisinde en önemli kavramlar olan verimlilik, enerji ve hammadde üçlüsünün uyum içinde çalıştığı bir üretim yöntemidir. Paslanmaz çelikler genelde bileşimlerinde yaklaşık %11 krom bulunan çeliklerdir. Bu krom içeriği sayesinde yüzeylere büyük bir kuvvetle tutunmuş, sünek, ince bir oksit tabakası oluşur. Dolayısıyla kimyasal ortamlara karşı pasifize edilmiş olurlar. Bu oksit tabakası, oksijen içeren ortamlarda oluşup dış etkenlerle bozulduğu takdirde kendini yenileyebilen bir tabakadır. Paslanmaz çelikler, farklı ortamlarda gösterdikleri mükemmel korozyon dayançları ile öne çıkan bir malzeme grubudur. Östenitik paslanmaz çelikler, diğer ferritik ve martenzitik paslanmaz çeliklerle kaşılaştırıldığında en yüksek korozyon direnci gösteren paslanmaz çelik türüdür. Ayrıca yüksek sıcaklık kullanımlarında çokça tercih edilir. 700°C üzeri sıcaklıklarda ferritik ve martenzitik çeliklerin oksidasyon dirençlerinde düşüş görülürken, östenitik paslanmaz çelikler 900°C' de dahi oksitlenmeye karşı yüksek direnç gösterirler. Yapılan çalışmada AISI 304L ve AISI 316L paslanmaz çelik tozları tek yönlü presleme ile preslenmiş ve sonrasında bir gün süreyle yağlayıcı giderme fırınında tutulmuş ve ardından sinterlenmiştir. Sinterleme aşamasında iki farklı atmosfer, hidrojen ve vakum atmosferleri, kullanılmıştır. Bir diğer parametre olarak da sıcaklık, 1270°C ve 1330°C, değiştirilerek farklı rejimlerde sinterlenmiştir ve bu farklılıkların etkisi incelenmiştir. Görsel olarak yapılan incelemelerde hidrojen ortamında sinterlenen paslanmaz çelik numunelerin parlak, vakum atmosferinde sinterlenen paslanmaz çelik numunelerin mat bir yüzey görüntüsüne sahip olduğu görülmüştür. Yoğunluk ölçümleri sonucunda, vakum ve hidrojen atmosferlerinde yapılan sinterlemelerde sinterleme sıcaklıklarının artması ile AISI 304L ve AISI 316L paslanmaz çelik numunelerin yoğunluğunun arttığı gözlenmiştir. Işık mikroskobu görüntüleri incelendiğinde, hidrojen ve vakum atmosferlerinin her ikisinde de porozite miktarının sıcaklığın artması ile azaldığı gözlenmiştir. En düzenli gözenekler, AISI 316L paslanmaz çelik numunelerin hidrojen atmosferinde sinterlenmesi sonucunda elde edilmiştir. Sertlik değerleri incelendiğinde, vakum ve hidrojen atmosferlerinin her ikisinde de sinterleme sıcaklığının artması ile AISI 316L paslanmaz çelik numunelerin sertliklerinde azalma görülmüştür. AISI 304L paslanmaz çelik numunelerde, vakum atmosferinde sinterleme sıcaklığının artması ile sertlik artmış, hidrojen atmosferindeki sinterleme sıcaklığının artması ile sertliklerde düşüş gözlenmiştir. Yoğunluk ve sertlik değerlerinin bir arada incelenmesi ile, diğer numunelerin aksine sinterleme sıcaklığının artması ile sertlik değerinde artış görülen AISI 304L paslanmaz çelik numunelerin, yoğunluk değerlerinde , diğer bütün numunelerden farklı ve fazla olarak, %5 'e yakın bir artış görülmüştür. EDS analizleri incelendiğinde, sıcaklığının artması ile bütün numunelerde demir oranında bir düşüş gözlenirken, 1330°C' de vakum atmosferinde sinterlenen AISI 304L paslanmaz çelik numunelerde demir oranında artış görülmüştür. Ayrıca 1330°C 'de vakum atmosferinde sinterlenen AISI 304L paslanmaz çeliğinin XRD analizinde, diğerlerinden farklı olarak, Cr0.2-Fe0.8 piki oluşmuştur. Yani farklı olarak Cr0.2-Fe0.8 (ferrit) fazı oluşmuştur. Sinterleme sonrası hızlı soğumada yaklaşık 1050°C' de tam östenitik katılaşma olurken, yavaş soğumada ilave olarak δ-ferriti meydana geleceğinden yapıda FeCr bileşikleri oluşur. Kromların ferritik demirle bağ yapmaları sonucu Cr0.2-Fe0.8 , nikelin ferritleri bağlama eğilimi etkisiyle ferronikel bileşikler meydana gelmiştir. Bu durum sertliği etkileyen bir unsur olmuştur. SEM görüntülerinde, 1330°C sinterleme sıcaklıklarında hidrojen atmosferinde sinterlenmiş AISI 304L ve AISI 316L paslanmaz çelik numunelerin yüzeyinde SiO2 kabarcıklarına rastlanmıştır. Bu durum düşük soğuma hızlarında, özellikle hidrojenin çiğlenme noktasının yüksek olması sonucu gerçekleşir. Yüksek çiğlenme noktalarında, hidrojen atmosferi yüksek oranda su buharı içerir. Bunun sonucunda da hidrojen gazının reoksidasyon özelliği çok azalır. Yüksek sıcaklıklarda kararlılığı azalan Cr2O3 ayrışır ve kroma nazaran oksijene afinitesi daha yüksek olan silisyum ile SiO2 oluşumu gerçekleşir ve bu da sertliği etkiler. Aşınma deneylerinde ağırlık kayıpları ölçülmüş ve hacimsel aşınma miktarları hesaplanmıştır. Bu sonuçlara göre, AISI 304L paslanmaz çelik numunelerde hacimsel aşınma miktarı, vakum atmosferinde sıcaklığın artması ile azalmış, hidrojen atmosferindeki sıcaklığın artması ile artmıştır. Vakum ve hidrojen atmosferlerinin her ikisinde de sinterleme sıcaklığının artması ile AISI 316L paslanmaz çelik numunelerin hacimsel aşınma miktarında artış görülmüştür. Aşınma dayanımları karşılaştırıldığında, AISI 304L ve AISI 316L paslanmaz çelik numuneleri içerisinde 1270°C' de hidrojen atmosferinde sinterlenmiş numunelerin en yüksek aşınma direncine sahip numuneler olduğu görülmüştür. Sonuçta, 1270°C ve 1330°C' de hidrojen ve vakum atmosferlerinde sinterlenen AISI 304L ve AISI 316L numunelerin özelliklerindeki ve yapılarındaki değişimler incelenmiştir.
Powder metallurgy is a process, that produce metals without melting. Due to this, powder metallurgy is used for the metals with high melting point. In traditional processes, metal powders which contains lubricants, are compacted in a requested shaped patterns. Then they are sintered with protecting atmosphere. Sintered products have high densities and good mechanical properties according to conditions of compacting and sintering conditions and it is going to be choosen well to get required qualifications. Simple and coplex shapes are produced with high capacities. Compared with other methods, powder metallurgy have the most comprehensive shaping and producing proceses. The most important advantage of powder metallurgy is complex shaped materials are produced with minimum loss or without loss. The steps are, shaping, compacting and sintering. And in powder metallurgy, energy, productivity and raw material are worked in accord as today's neccesity. Stainless steels have generally 11% Cr content and chromium adsorbs to the surface strongly. Strongly adsorbed chromium is occured ductile and thin oxide layer on the surfaces so this formation protects the surface from chemical affects. This layers are formed at atmospheres which contains oxygen. If they are destroyed, the layer will be renewed themselves. Stainless steel also are known with thier high corrosion resistance at different atmospheres. Austenitic stainless steels have the most highest corrosion resistance level compared with ferritic and martensitic stainless steels. Additionally, austenitic stainless steels are prefered at high temperatures. Above 700°C, although ferritic and martensitic stainless steels have low corrosion resistance, austenitic stainless steels have higher corrosion resistance even the temperature is above 900°C. Additionally, the effect of various austenitising and ferritising elements on austenite and ferrite can be expressed in terms of Ni equivalent and Cr equivalent respectively. At 18%Cr, is a minimum required nickel content to promote a fully austenitic structure which is stable at room temperatures. Vacuum sintering seems to have superior characteristics for the optimal sintering, vacuum sintering produces the lowest levels of interstitials, on the contrary to maximum magnetic and corrosion-resistance properties. Through addition of carbon to a water-atomized stainless steel powder, the oxygen content of a sintered part can be decreased to levels approaching those of wrought stainless steels. Additionally, hydrogen become the most used atmosphere for sintering stainless steels. For good corrosion resistance and mechanical properties, the primary goal is to obtain lower the oxygen content of the green part, to prevent reoxidation in the cooling zone of the furnace, and to maintain a low carbon content of approximately 0.03% in austenitic stainless steels. In this study, the effects of sintering conditions on properties are observed. AISI 304L and AISI 316L stainless steel powders were choosen. They were compacted by unidirectional press. After compacting, they were waited in lubricating furnace for one day to vaporise the lubricants. Then they were sintered at two different atmosphere with different temperatures. Hydrogen and vacuum atmosphere were performed at sinterring at 1270°C and 1330°C. Through this difference of temperatures, it was enable to observe the difference on properties. By visiul examinations, it was observed that although the materials which were sintered at hydrogen atmosphere had bright surfaces, the materials which were sintered at vacuum have matt surfaces. This was happened because of the decreasing of oxide reductions. When oxides are reduced totaly, the surfaces will be bright. When optical microscope images were examined, it was observed that porosity value was reduced with the increasing of temperature for both atmospheres. Also when used temperature increase, the pores had more spherical shapes. The minimum porosity level was obtained by hydrogen atmosphere sintering at 1330°C. By density measurements, it was observed that porosity value was reduced with the increasing of temperature for both atmospheres. When hardness rates were examined, it was observed that all material 's hardness decreased with the raise of temperature, except AISI 304L which was sintered at vacuum. Hardness of AISI 304L increased with the raise of temperature at vacuum atmosphere. We analised and compared the hardness and density ratios. Again the density level of AISI 304L which were sintered at vacuum, had the most biggest increasing level of 5% against others. When EDS analysis were examined it was observed that iron level was reduced with the raise of temperature at all materials except AISI 304L which were sintered at vacuum. At AISI 304L which were sintered at vacuum, it was observed that iron level was increased with the raise of temperature. Additionally it was clearly seen that and extra peak was occured at AISI 304L materials which was sintere at 1330°C. This peak is Cr0.2-Fe0.8 (ferritic phase). It was realised that when cooling speed is fast at 1050°C, full austenitic transformation is occured, but if cooling speed is slow, δ-ferrite will be formed near austenite. Because of this, Fe-Cr compounds are formed. With this, nickel wants to eleminate the δ-ferrite and occur ferronickel compound in compositions. So it was observed that this effected the hardness levels. When SEM images of AISI 304L ve AISI 316L, which were sintered at hydrogen atmosphere, was checked, it was seen that SiO2 formations were occured at the surfaces. This will be happened when the slow cooling is performed at high dew point hydrogen is used. Due to high dew point temperature, hydrogen atmosphere consist water vapour with high level. Additionally at high temperature stability of chromium compounds are reduced. Also silicon have higher affinity than chromium. So SiO2 formations are occured. After pin-on-drum wear tests, weight loses and volumetric wear amounts were calculated. It was observed that volumetric wear amount had a ratio with hardness. The volumetric wear amount was increased with the raise of hardness at AISI 304L materials which was sintered at vacuum atmosphere. Also AISI 304L materials which was sintered at hydrogen atmosphere, it was decrased at other material with the raise of temperature. For AISI 316L, volumetric wear amount was increased with the raise of temperature at both vacuum and hydrogen atmospheres. Additionally, the wear resistance of these materials were calculated. The AISI 304L and AISI 316L materials which were sintered at hydrogen atmosphere at 1270°C, had the highest wear resistance levels compared with same of them. As a conclusion, the change of mechanical, structural properties and different formations were observed and analised in our study.
Powder metallurgy is a process, that produce metals without melting. Due to this, powder metallurgy is used for the metals with high melting point. In traditional processes, metal powders which contains lubricants, are compacted in a requested shaped patterns. Then they are sintered with protecting atmosphere. Sintered products have high densities and good mechanical properties according to conditions of compacting and sintering conditions and it is going to be choosen well to get required qualifications. Simple and coplex shapes are produced with high capacities. Compared with other methods, powder metallurgy have the most comprehensive shaping and producing proceses. The most important advantage of powder metallurgy is complex shaped materials are produced with minimum loss or without loss. The steps are, shaping, compacting and sintering. And in powder metallurgy, energy, productivity and raw material are worked in accord as today's neccesity. Stainless steels have generally 11% Cr content and chromium adsorbs to the surface strongly. Strongly adsorbed chromium is occured ductile and thin oxide layer on the surfaces so this formation protects the surface from chemical affects. This layers are formed at atmospheres which contains oxygen. If they are destroyed, the layer will be renewed themselves. Stainless steel also are known with thier high corrosion resistance at different atmospheres. Austenitic stainless steels have the most highest corrosion resistance level compared with ferritic and martensitic stainless steels. Additionally, austenitic stainless steels are prefered at high temperatures. Above 700°C, although ferritic and martensitic stainless steels have low corrosion resistance, austenitic stainless steels have higher corrosion resistance even the temperature is above 900°C. Additionally, the effect of various austenitising and ferritising elements on austenite and ferrite can be expressed in terms of Ni equivalent and Cr equivalent respectively. At 18%Cr, is a minimum required nickel content to promote a fully austenitic structure which is stable at room temperatures. Vacuum sintering seems to have superior characteristics for the optimal sintering, vacuum sintering produces the lowest levels of interstitials, on the contrary to maximum magnetic and corrosion-resistance properties. Through addition of carbon to a water-atomized stainless steel powder, the oxygen content of a sintered part can be decreased to levels approaching those of wrought stainless steels. Additionally, hydrogen become the most used atmosphere for sintering stainless steels. For good corrosion resistance and mechanical properties, the primary goal is to obtain lower the oxygen content of the green part, to prevent reoxidation in the cooling zone of the furnace, and to maintain a low carbon content of approximately 0.03% in austenitic stainless steels. In this study, the effects of sintering conditions on properties are observed. AISI 304L and AISI 316L stainless steel powders were choosen. They were compacted by unidirectional press. After compacting, they were waited in lubricating furnace for one day to vaporise the lubricants. Then they were sintered at two different atmosphere with different temperatures. Hydrogen and vacuum atmosphere were performed at sinterring at 1270°C and 1330°C. Through this difference of temperatures, it was enable to observe the difference on properties. By visiul examinations, it was observed that although the materials which were sintered at hydrogen atmosphere had bright surfaces, the materials which were sintered at vacuum have matt surfaces. This was happened because of the decreasing of oxide reductions. When oxides are reduced totaly, the surfaces will be bright. When optical microscope images were examined, it was observed that porosity value was reduced with the increasing of temperature for both atmospheres. Also when used temperature increase, the pores had more spherical shapes. The minimum porosity level was obtained by hydrogen atmosphere sintering at 1330°C. By density measurements, it was observed that porosity value was reduced with the increasing of temperature for both atmospheres. When hardness rates were examined, it was observed that all material 's hardness decreased with the raise of temperature, except AISI 304L which was sintered at vacuum. Hardness of AISI 304L increased with the raise of temperature at vacuum atmosphere. We analised and compared the hardness and density ratios. Again the density level of AISI 304L which were sintered at vacuum, had the most biggest increasing level of 5% against others. When EDS analysis were examined it was observed that iron level was reduced with the raise of temperature at all materials except AISI 304L which were sintered at vacuum. At AISI 304L which were sintered at vacuum, it was observed that iron level was increased with the raise of temperature. Additionally it was clearly seen that and extra peak was occured at AISI 304L materials which was sintere at 1330°C. This peak is Cr0.2-Fe0.8 (ferritic phase). It was realised that when cooling speed is fast at 1050°C, full austenitic transformation is occured, but if cooling speed is slow, δ-ferrite will be formed near austenite. Because of this, Fe-Cr compounds are formed. With this, nickel wants to eleminate the δ-ferrite and occur ferronickel compound in compositions. So it was observed that this effected the hardness levels. When SEM images of AISI 304L ve AISI 316L, which were sintered at hydrogen atmosphere, was checked, it was seen that SiO2 formations were occured at the surfaces. This will be happened when the slow cooling is performed at high dew point hydrogen is used. Due to high dew point temperature, hydrogen atmosphere consist water vapour with high level. Additionally at high temperature stability of chromium compounds are reduced. Also silicon have higher affinity than chromium. So SiO2 formations are occured. After pin-on-drum wear tests, weight loses and volumetric wear amounts were calculated. It was observed that volumetric wear amount had a ratio with hardness. The volumetric wear amount was increased with the raise of hardness at AISI 304L materials which was sintered at vacuum atmosphere. Also AISI 304L materials which was sintered at hydrogen atmosphere, it was decrased at other material with the raise of temperature. For AISI 316L, volumetric wear amount was increased with the raise of temperature at both vacuum and hydrogen atmospheres. Additionally, the wear resistance of these materials were calculated. The AISI 304L and AISI 316L materials which were sintered at hydrogen atmosphere at 1270°C, had the highest wear resistance levels compared with same of them. As a conclusion, the change of mechanical, structural properties and different formations were observed and analised in our study.
Açıklama
Tez (Yüksek Lisans) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 2014
Thesis (M.Sc.) -- İstanbul Technical University, Institute of Science and Technology, 2014
Thesis (M.Sc.) -- İstanbul Technical University, Institute of Science and Technology, 2014
Anahtar kelimeler
Toz Metalurjisi,
Östenitik Paslanmaz Çelikler,
Sinterleme,
AISI 304L,
AISI 316L,
Powder Metallurgy,
Austenitic Stainless Steels,
Sintering; AISI 304L,
AISI 316L