H13 sıcak iş takım çeliklerinin aşınma dayanımına düşük sıcaklıklarda nitrasyon işleminin etkisi
Effect of low temperature nitriding on H13 hot work tool steel's wear resistance
Yazar: ZAFER CAN TEOMAN
Danışman: PROF. DR. HÜSEYİN ÇİMENOĞLU
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Malzeme Bilimi ve Mühendisliği Ana Bilim Dalı / Malzeme Bilimi ve Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2017
Konu: Metalurji Mühendisliği = Metallurgical Engineering ; Mühendislik Bilimleri = Engineering Sciences

Özet:
Bu çalışmada nitrürleme işlemi ve etkileri takım çelikleri üzerinden anlatılmıştır. Yüzey sertleştirme yöntemlerinden biri olan nitrürleme sonucunda malzeme yüzeyinde ince bir tabaka oluşturularak sertlik artması sağlanır. Oluşan tabaka aşınma, korozyon ve yorulmaya karşı dayanıklıdır. Uygun bir nitrürleme sıcaklığında, azot atomsal olarak parça yüzeyinden içeri yayınarak çeliğin, demir ve alaşım elementleri ile nitrür oluşturur. Karbona göre atom çapı daha küçük olan azot, demir kafesine daha kolay nüfuz eder ve oluşturduğu nitrürler, ince taneli olduğu için çeliğin yüzey sertliğini arttırırlar. Genel olarak nitrürleme işleminde bu 500 -550 oC sıcaklığa kadar çıkılır. Bu noktada işlemin gerçekleştirildiği sıcaklık ve süre söz konusu difüzyona doğrudan etki eder. İşlem sonucunda ise değişik fazlarda bileşikler oluşabilmekte ve bu fazlar yüzeydeki tabakanın gerek kalınlığını gerek morfolojisini özellikle de mekanik özelliklerini ciddi bir şekilde değiştirmektedir. Yapılan işlem sonucunda malzemenin aşınma, korozyon ve yorulmaya karşı dayanım değerlerinde optimum artışı sağlayacak değerler bulunarak uygun difüzyon tabakası oluşturulmalıdır. Nitrürleme işlemiyle birlikte, işlemin uygulandığı malzemeye birçok özellik kazandırılmakla birlikte, temel olarak işlemin arkasında yatan özellikler yüksek yüzey sertliği, yüksek aşınma ve korozyon dayanımı, yorulma ömründe artış ve yüksek sıcaklıklarda yumuşamaya dayanım olarak özetlenebilir.
Takım çelikleri ise sıcak veya soğuk haldeki kesme, dövme gibi yöntemlerle şekillendiren takım ve kalıpların imalatında kullanılan çelik grubuna verilen genel isimdir. Takım çelikleri, takım ve kalıp yapımı gibi temel kullanım alanlarının dışında spesifik özellikler talep edilen makina parçalarının imalatı gibi alanlarda da kullanılmaktadır. Genellikle çok ağır çalışma koşullarında kullanılan takım çeliklerinden, farklı sıcaklıklardaki kullanımları sırasında yüksek hızlarda ve yüksek gerilmeler altında deforme olmaksızın ve aşınmaksızın çalışmaları beklenir.
H13 sıcak iş çeliklerinde yaklaşık olarak 400 oC sıcaklıktan sonra sertlik ve dayanım değerleri düşmeye başlar, 600 oC sıcaklıktan sonra ise bu değerlerde keskin bir düşüş meydana gelir. Bu durumun önüne geçmek amacıyla uygulanan yöntemlerden biride nitrürleme olup, H13 takım çelikleri nitrürlenmiş şekilde yaygın şekilde kullanılan bir malzemedir. İş parçalarında istenen yüzeyinin sert, aşınmaya ve yüzey basıncına dayanıklı, iç kısmının ise tok olması gibi özellikleri sağlamak amacıyla düşük sıcaklıklarda nitrürleme işlemi yapmak ve nitrürleme parametrelerini optimize ederek en uygun şartlara getirerek, en iyi mekanik değerlere ulaşmak amaçlanmıştır.
Yapılan çalışmada, farklı sıcaklıklarda ve sürelerde nitrürlenen numunelere aşınma ve sertlik deneyleri uygulanmış, nitrürleme işleminin aşınma dayanımına etkileri incelenmiştir. Tüm numuneler mikroyapı, mikrosertlik, XRD difraksiyon analizi ve aşınma deneyleri sonucunda oluşan aşınma izi profilleri bakımından incelenmiştir.

Abstract:
This study focuses on characteristics of H13 hot working tool steels after nitriding at lower temperatures, 375 and 400 °C for 4, 8 and 16 h to prevent tempering effect and change in the microstructure via producing non-desirable equilibrium phases and to have the nitrogen mainly dispersed in to the matrix to enhance the surface hardness and wear resistance. Low temperature nitriding process has been proposed for austenitic stainless steels in the improve both the surface hardness and the corrosion resistance. However, low temperature nitriding process is a newly developed and still researched topic for tool steels.
Tool steels is a specialized group of highly alloyed steels and they are used for applications requiring long life at high operating temperatures and heavy working conditions such as heavy cuts, high-speed machining and mould production. Hot work tool steels can retain their high hardness at elevated temperature, which allows them to have a wide range of usage. Therefore, they are preferred in cutting tool industry, with their long lifetime. High working hardness, high wear resistance, excellent toughness and shock endurance are some of the required properties from the tool steels at both room temperature and elevated temperatures. To overcome these expectations, surface modifications and coatings such as nitriding are applied on tool steels.
Nitriding is one of the surface hardening methods which provides increased surface hardness by forming a thin layer on the material surface. It is a method which has been used for a long while for improving surface properties of many ferrous and nonferrous metals. This formed thin layer has elevated wear, corrosion and fatigue resistance. The interest in nitriding process has been growing and it has become more and more recognized as a proven surface engineering process day by day. Generally, nitriding process conducted up to 550°C temperature. Depending on the nitriding temperature and nitrogen chemical potential at the material surface, the process may cause the formation of a continuous iron nitride surface layer. Underneath this layer, a diffusion zone will form containing nitrogen in solution and, in case of strong chemical affinity of nitrogen with the alloying elements, nitride compounds can be formed.
From beginning of the last century, nitriding processes has become an important and proven surface treatment and interest in the subject of nitriding has grown day by day. Particularly, during the past 10 years nitriding has been recognized as a very effective and satisfactory process for industrial demands. 
Based on these industrial demands, many techniques developed, and many studies carried out to determine the optimum process parameters. Depending on these process parameters such as temperature and time, it is possible to obtain different properties and results.
It is well known that, the temperature is crucial on the diffusion mechanism of the nitrogen and therefore, on nitriding process. Also, the process time is another important factor which effects the nitriding mechanism and as an economic factor.
It is known that, with the increasing temperature, hardness and strength values of the steels starts to decrease. For H13 hot working tool steels, approximately after 400 oC, hardness and strength values start to decrease and after 600 oC, a sharp drop is observed. Most expected characteristics from hot working tool steels are their toughness and high temperature endurance. For preventing this drop on the mechanical properties, nitriding is a preferred method which have been used for H13 hot work tool steels. Also, one of the other advantages of the nitriding process is, nitrogen is diffusible in metals especially at low temperature. Thus, nitriding becomes a very simple process without serious problems such as the problem of distortion. 
In this study, H13 tool steels was chosen due to the large application on tools production. Wear of extrusion dies has an important technological and economic significance due to cost to prevent die failure from thermal cracking, erosive wear, soldering and corrosion or a combination of these processes. H13 die steel is characterized by its high hardenability and toughness, but a relatively low wear resistance. The savings that result from increasing the lifetime of the dies by the enhancements of its mechanical properties have motivated researchers to optimize surface treatments suitable to these materials. Therefore, investigating the influence of nitriding process on the friction and wear behavior of H13 steel has become an interesting subject	. 
The samples were first quenched at 1050 oC for 1 h, cooled in air to room temperature and tempered three times for one hour at 650, 550 and 500 oC and cooled in air to room temperature.
After the pre-heat treatment, the decarburized layer that formed at the surface during heat treatment was removed by grinding. Then, the samples were polished with abrasive particles up to 3 μm and cleaned in a phosphoric acid-pure water mixed ultrasonic bath. Gas nitriding were applied to the samples. The temperatures for the different treatments were 400 and 375 oC for different durations under %40N and %60 NH3 atmosphere. The nitriding layers were observed using both SEM (Hitachi TM-1000) and optical microscope (Leica DM 6000M). It was possible to identify the diffusion zone and measure the thickness of the formed layers and detect the forming of the compound layer. Wear tracks were also observed with both SEM and optical microscope. Hardness results were obtained using a Vickers microhardness tester (Wilson Tukon 1102). The result was the average of five microhardness values. Both surface hardness' and nitride layers' hardness' measured. The phases formed by different nitriding treatments were verified by X-ray diffraction. The X-ray diffractometer (GBC MMA 027) used monochromatic Cu Kα radiation. The XRD patterns were obtained using Cu Ka radiation (λ= 0.154 nm) at 35 kV and 28.5 mA. The samples were scanned over 2θ angles of 20-80o at a step of 0.02o and a scanning speed of 2o min-1. Pin on disk equipment was used to evaluate the effects of the nitriding treatments on the wear resistance. The test was performed on a tribometer (CSM High Temperature Tribomemeter) on room temperature and 600 oC. Friction coefficient was monitored through the measurement of the friction force during the wear test. The counter-bodies were polished alumina spheres. The tests were performed on 3 N load, 250 m sliding distance, 10 m/sn sliding speed and 3,52 mm wear track radius. The wear volumes were obtained by the volume loss, quantified through the track profile that was measured with a profilometer (Veeco DEKTAK). 
This study, focused on nitriding process' performed up to 400 oC to prevent tempering effect and change in the microstructure via producing non-desirable equilibrium phases and to have the nitrogen mainly dispersed in to the matrix to enhance the surface hardness and wear resistance. It is finally concluded that, it is observed that, applying the nitriding processes on higher temperatures and for longer durations increases the diffusion layer thickness. XRD patterns show that, as a result of the nitriding γ'-Fe4N and ε-Fe3N starts to form on 400 oC with longer durations while γ'-Fe4N is not observed on the 375 oC. The nitriding treatments improved the wear resistance on 25 oC severely. On the other hand, with the increasing nitriding temperature and durations there is an even decrease on the wear resistance of the samples on 600 oC. The friction coefficient did not suffer big changes with the different nitriding processes. But, with the effect of the oxidation wear there is a big decrease with the increasing testing temperature.
The nitriding treatments promoted a higher hardness surface to all the materials. It can be seen that, hardness values increase with the increasing temperatures and duration along with the diffusion layer thicknesses.