AISI 4140 çeliğinde farklı yüzey morfolojilerinin gaz nitrasyon işlemine etkisi
Effect of different surface morphologies to AISI 4140 steel on gas nitration process
Yazar: SENA HOŞVER
Danışman: PROF. DR. FATİH ÜSTEL
Sakarya Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2023
Konu: Metalurji Mühendisliği = Metallurgical Engineering

Özet:
Günümüzde yüzey sertleştirme işlemleri (karbürleme, elektrolitik kaplama vb.); aşınma direnci, mukavemet dayanımı, korozyon direnci gibi özelliklerin arttırılmasında en çok kullanılan yöntemlerdir. Nitrasyon ise bu yöntemler içerisinde geliştirilmeye en açık, ekonomik ve boyutsal değişimlere neden olmaması ile en çok tercih edilen yöntemlerden biri olmuştur. Bu çalışmada, AISI 4140 çeliğinin gaz nitrasyon sonrası yüzey morfolojileri, kesit mikro yapıları ve altlık yüzeyin hazırlanma farklılıklarının nitrasyon sonrası yüzeye etkileri incelenmiştir. Yüzeyi 45° ve 90° açı ile alüminyum oksit kumlanmış, aynı şekilde yüzeyi 45° ve 90° açı ile cam küre kumlanmış, yüzeyi kumlanmamış ve parlatılmış numuneler, ultrasonik yıkama yapılarak, 590°C'de 40 dakika gaz atmosferinde nitrasyon işlemine tabii tutulmuştur. Farklı yüzey hazırlama işlemlerinin, AISI 4140 çeliği yüzeyinde geliştirilen nitrür tabakasının homojenliği (beyaz tabaka kalınlığı) ve difüzyon derinliği üzerine olan etkisi ortaya koyulmaya çalışılmıştır. Deneysel sonuçlar; optik mikroskop, yüzey pürüzlülüğü, mikro sertlik ölçümleri yanında SEM ve XRD ile desteklenmiştir. Nitrasyon sonrası numune yüzeylerinde fiziksel değişim (matlaşma) gözlemlenmiştir. SEM ile numuneler kesitten incelendiğinde altlık yüzeye yapılan yüzey hazırlama işleminin, nitrasyon sonrası yüzeyde de etkili olduğu görülmektedir. Yüzey topografyalarında nitrasyon sonrası değişimlerin oluştuğu ve özellikle alümina ile kumlanan yüzeylerde, alümina partiküllerinin yüzeye saplandığı tespit edilmiştir. SEM görüntülerinde alümina ile kumlanmış numunelerin nitrür tabakasının, kompakt bir şekilde oluşmadığı hatta yüzeye saplanan partiküllerin düşmesi sonucu üst yüzeyde krater benzeri yüzey kusurları tespit edilmiştir. Cam küre ile kumlanmış yüzeyler, alümina ile kumlanmış yüzeyler ile karşılaştırıldığında daha düzgün olduğu görülmektedir. Yüzey pürüzlülükleri incelendiğinde; nitrasyon öncesi en düşük yüzey pürüzlülüğünü parlatılmış olan numune gösterirken, en yüksek pürüzlülüğü 90° alümina ile kumlanmış numune göstermiştir. Nitrasyon sonrasında da bu sıralama değişmemiştir. Nitrürlenen numunelere kesitten mikro sertlik taraması yapılarak tespit edilen difüzyon derinliklerinin sonuçlarına göre en düşük difüzyon derinliği 200 µm ile parlatılmış numune de ölçülmüştür. En fazla difüzyon derinliğine sahip numuneler ise 350 µm derinlik ile yüzeyi hazırlanmamış ve 90° alümina ile kumlanmış numunelerdir. XRD difraktogramında nitrürlerin altlık malzemedeki Fe elementi ile birleşerek oluşturduğu Fe2-3N ve Fe4N fazlarının varlığı tespit edilmiştir. Islatma açıları ölçülerek sıvı ve nitrürlü yüzey arasındaki çekim kuvvetleri incelenmiş ve yüzeyine işlem yapılmamış numune ile 45° açı ile Al2O3 kumlanmış olan numunenin nitrasyon sonrası temas açılarının arttığı saptanmıştır.

Abstract:
Surface hardening processes (carburizing, electroplating, etc.); They are the most used methods to increase properties such as wear resistance, strength resistance, corrosion resistance. Nevertless, Nitriding is one of the most preferred method due to economical point of view,  not being a cause of dimensional changes, and it is the most reliable method to developt  among these methods.
The purpose of nitration is to increase the surface hardness. The nitriding process takes advantage of the low solubility of nitrogen in the ferritic crystalline structure to promote precipitation of iron nitrides or alloy nitrides. Atomic nitrogen is introduced to the surface of the material, and as the material cools, iron nitride and alloying element nitrides precipitate on its surface. A nitride layer (binding layer), which is mostly cohesive, forms on the surface. This layer is connected to a diffusion zone where the precipitated nitrides are evenly distributed in the steel matrix, resulting in hardening especially for alloy steel. Because nitrogen lowers the gamma/alpha conversion temperature of iron to 590°C, nitriding temperatures are usually below this temperature. The resulting nitride layer creates high hardness on the material surface, increases corrosion resistance, increases wear and fatigue resistance.
There are 4 different types of nitration. In powder nitration, the parts are placed in boxes similar to box cementation. Approximately after 15% nitriding accelerator material is placed at the bottom of the box, the parts to be nitrided are placed. By placing nitriding powders on the pieces, the box is tightly closed and placed in the oven between 520-570°C.
Plasma nitriding is carried out at temperatures between 350- 590 °C. The positively charged ions strike the component acting as the cathode in front of the furnace wall (anode) at high velocities. Initially, this ion bombardment causes cleaning (sputtering) of the component surface and allowing the passive layers to be removed. Consequently It is heated and the component surface is nitrided.
Nitration in a salt bath is as old as nitration in a gaseous environment and still widely in use today. The nitriding applied in the salt bath is carried out between 510-570°C as in gas nitriding. In this process, which is also performed at low temperature, both cyanates and cyanides are included as a bath content.
However, gas nitriding is a thermochemical surface hardening process used to increase wear resistance, surface hardness and fatigue life by dissolving nitrogen and hard nitride precipitates, and a working temperature range of 450 - 590 C° is used.
Since the nitration process is a process that interacts directly with the surface of the material, surface preparation and cleaning before nitration is extremely important. Oils and impurities that may remain on the material surface due to previous processes such as grinding or manufacturing directly affect the nitration process to be performed. The surface preparation processes carried out before the nitriding process also have a great effect on the desired surface properties. Just prior to nitriding, degreasing and abrasive (sand) must be removed. Sandblasting with alumina and glass spheres is a method used to prepare the material surface prior to nitration. Both methods have advantages alumina has higher abrasion resistance and hardness with the provision of more rougher surface preparation. However, glass spheres, are less aggressive and cause less scratches and deformation on the material surface. The method to be preferred is depend on the properties of the material, the nitration process requirements and the intended use.
In this study, the effects of surface morphologies and substrate preparation differences on cross-section microstructures of AISI 4140 steel with a core hardness of 30 HRC after gas nitriding have been investigated. The effects of surface morphologies on diffusion depth and wetting angle have been evaluated. The surfaces of the samples; cut surface, polished surface, sandblasted surfaces with aluminum oxide at an angle of 45°-90° and with a glass ball at an angle of 45°- 90°. After the ultrasonic washing of the samples, the surfaces prepared in 6 different ways, 580 C° Kn controlled nitration  has been carried out in the gas nitriding oven. The effect of the nitride layer formed on the surface of the samples made of AISI 4140 steel, which has different surface morphologies, on the homogeneity (white layer thickness) and diffusion depth has been tried to be revealed. 
Experimental results; Optical microscope was supported by SEM and XRD devices with surface roughness, micro hardness measurements, wetting angle measurement. After nitration, matting was observed as a physical change on the sample surfaces. The white layer thicknesses were measured with an optical microscope and it was determined that the white layer thickness increased in the sandblasted samples depending on the angle. The thickest white layer was seen in the 90° sandblasted with alumina. It was observed that the white layer formation was low in the polished sample. When the surface roughness are examined; The polished sample showed the lowest surface roughness before nitration, while the sandblasted sample with 90° alumina showed the highest roughness. It was revealed that this order of roughness did not change after the nitration process. When the samples are examined from the cross-section by SEM, it is seen that the surface morphology formed on the substrate is also reflected on the surface after nitration. After nitration, there were changes in the surface topographies, and the surface roughnesses were measured lower after nitration. When the surfaces were examined by SEM before nitration, it was determined that alumina particles were stuck on the surface, especially on the surfaces sandblasted with alumina. In the SEM images after nitration, the nitride layer of the samples sandblasted with alumina did not form compactly, and even crater-like surface defects were detected on the upper surface as a result of the falling particles adhering to the surface. Surfaces blasted with glass beads appear smoother than surfaces blasted with alumina. According to the results of diffusion depth detection by microhardness scanning made from the sections of nitrided samples, 200 µm diffusion depth was measured in the polished sample with the lowest diffusion depth. The samples with the highest diffusion depth are the samples with unprepared surface and sandblasted with 90° alumina, with a depth of 350 µm. In the XRD diffractogram, the presence of Fe2-3N and Fe4N phases, which nitrides form by combining with the Fe element in the base material, was determined. The attraction forces between the liquid and the nitrided surface were investigated by measuring the wetting angles, and it was determined that the contact angles of the untreated sample and the sample that was sandblasted with Al2O3 at a 45° angle increased after nitration.
In the literature, there are studies in which processes such as sandblasting before nitration affect the white layer formed after nitration and the depth of diffusion. In the study of J.Baranowska et al. in 2002, it was also found that the white layer thickness in the sample whose surface was etched by electrolytic method was higher than the polished one; It is argued that the diffusion depth is higher in the sample with a rough surface, which is due to this. The results found in this study seem to support Baranowska's work.