Demir çelik sanayinde kullanılan sert dolgu kaplamalarının yüksek ve düşük sıcaklık aşınma özelliklerinin incelenmesi
The study of high and low  temperature tribology behaviour of hardfacing coatings used in iron and steel industry
Yazar: ERTAN YÖNTEM
Danışman: PROF. DR. HÜSEYİN ÇİMENOĞLU
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı / Üretim Metalurjisi ve Teknolojileri Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2014
Konu: Metalurji Mühendisliği = Metallurgical Engineering
Dizin: Demir çelik endüstrisi = Iron steel industry ; Demir çelik sektörü = Iron steel sector ; MAG kaynak arkı = MAG welding ; Sürtünme aşınması = Abrasive wear ; Yüzey aşınması = Surface wear

Özet:
Günümüzde toplu konut inşaatları, yol ve baraj yapımları ve kentleşme artışından dolayı demir çelik mamullerinin kullanımında önemli bir artış olmakla beraber ülkemizdeki demir çelik fabrikaları global taleplere şu anki kapasiteleriyle karşılık verememektedir. Bununla birlikte büyüyen talep artışı sektörde çok önemli ve büyük yatırımları beraberinde getirmektedir. Böylece dünyada ve ülkemizde farklı kullanım alanlarına farklı demir çelik mamulleri üreten birçok yeni demir çelik üretim tesisi kurulmaktadır. Bu yatırımlarla birlikte ülkemiz demir çelik sanayinde çok önemli bir rol oynamaktadır. Her geçen yıl ham üretim kapasitesi artmakta ve dünyada demir çelik üretimi hızlı bir yükseliş göstermektedir.
Demir çelik üretimi hammadde bakımından cevherden üretim ve hurdadan üretim olarak iki alanda incelenmektedir. Bu alanların proseslerinde sıvı çelik aşamasından sonra adımlar her iki alanda da sürekli döküm ve haddeleme prosesleri ile devam ederek nihai ürün üretilmektedir. Üretilen ürünler farklı alanlarda farklı işlevleriyle kullanıldıkları için çeşitlilik göstermektedir. Bu çeşitliliğin içinde inşaat yapımında kullanılan düz ve nervürlü inşaat çeliği, çelik konstrüksiyonların kolon ve kirişlerini oluşturan profiller, farklı alanlarda kullanılan borular, otomobil üretimi ve daha birçok alanda kullanılan saclar önemli yer tutmaktadır.
Demir çelik mamullerinin üretiminde kar marjlarının düşmesi ve artan enerji giderleri demir çelik fabrikalarının kapasitelerinin üstüne çıkmaya zorlamıştır. Bunun yanında üretim artış hedefleri planlı duruşların aksamasına ve yeterli bakım süresi ayırmaksızın üretime devam edilmesine sebebiyet vermektedir. Bu noktada önemli makine parça ve ekipmanlarının daha uzun servis ömürlerinin olması ön plana çıkmaktadır. Aynı zamanda demir çelik üretimi kontinü bir sisteme sahip olduğundan dolayı üretim hattında sadece bir noktada meydana gelen arıza tüm sistemin duruşuna sebebiyet vermektedir. Bu durum mühendislere aşınan parçaların maksimum servis ömrüne sahip hale getirilmesi alanında ar-ge çalışmaları yapılmasını zorunlu kılmıştır.
Aşınan parçaların servis ömürlerinin arttırılmasına yönelik olarak  yüzeylerinin,  farklı kaynak ve farklı kaplama teknikleriyle aşınmaya dirençli sert dolgu malzemeleri ile kaplanması  gerçekleştirilmektedir. Sert dolgu kaplamalarının uygulanmasında elektrik ark kaynağı ve MIG-MAG kaynağı yöntemleri en yaygın kullanılan yöntemlerdendir. Günümüz koşulları göz önüne alındığında, sert dolgu malzemesi (özlü teller) ile yapılan sert dolgu kaynak uygulamalarının uygulama hızı ve yüksek dayanımları uygulama alanlarını her geçen gün daha da arttırmaktadır. Bunun yanında farklı uygulamalar için geliştirilen birçok özlü telin mevcut olması ürün seçiminde çok geniş bir yelpaze sunmaktadır. Bu noktada mevcut uygulama için uygun ürünün seçilmesi önemli rol oynamaktadır. 
Yukarıda belirtilen açıklamalar ışığı altında, bu tezde demir çelik fabrikalarında kısa periyotlarda aşınan, değiştirilmesi önemli duruşlara ve üretim kayıplarına neden olan rölelerin daha uzun ömürlü hale getirilebilmesi için en uygun kaplamanın (özlü telin) belirlenmesi amaclanmıştır. Sanayide yapılan uygulamalar için kullanılan ürünlerin teknik özellikleri incelenmiş ve çalışma şartları ön planda tutularak seçim yapılmıştır. Daha sonra seçilen kaynak telleri ile numuneler hazırlanmıştır. Bu numunelerde kaynak metallerinin mikroyapıları incelenmiş, XRD paternleri çıkarılarak fazları tespit edilmiş, sıcak ve soğuk uygulamalar için aşınma testleri gerçekleştirilmiş, aşınma yüzeylerinin mikroyapıları ve yüzey profilleri incelenmiştir. Yapılan test sonuçları değerlendirilerek verimlilik analizleri gerçekleştirilmiş ve sanayi uygulamaları için uygun alaşımın seçimi yapılmıştır.
Çelikhane sürekli döküm röleleri ve haddehane loop rölelerinin çalışma şartları ve servis ömürleri irdelenerek yapılan sert dolgu kaynağı uygulamaları ile rölelerin servis ömürleri arttırılmıştır. Aynı zamanda ömür-maliyet analizi bakımından değerlendirildiğinde yapılan çalışmaların parça maliyetlerinin düşmesinde de önemli bir payı olduğu görülmüştür.

Abstract:
In this day and age, consumption of iron and steel has been increasing due to the raise of collective house constructions, road and barrage constructions and urbanization, nevertheless capacities of Iron and steel plants of Turkey aren't sufficient to meet the global demands. On the other hand, raising of demands cause big and necessary innovations in the iron and steel sector. Therefore many new iron and steel production companies that produce various iron and steel products for different usage purposes are being established in Turkey and in the world. Our country plays a significant role in iron and steel industry owing to these innovations. Every passing year Turkey's crude production capacity has been increasing, therefore iron and steel production of Turkey is rising out fastly comparing with other countries in the world. Turkey's success in the iron and steel industry is evident as it is among the top 10 crude steel producing countries in the world. Crude steel production is expected to continue and reach 47 million tonnes until 2017. Turkey also provides a level playing field in terms of access to raw materials.
Steel is an alloy of iron, with carbon, which may contribute up to 2.1% of its weight. The carbon content of steel is between 0.002% and 2.1% by weight. Common alloying elements include: manganese, nickel, chromium, molybdenum, boron, titanium, vanadium and niobium. Additional elements may be present in steel: manganese, phosphorus, sulfur, silicon and traces of oxygen, nitrogen and aluminium.
Steelmaking is the process for the producing ferrous scrap, steel and iron. In steelmaking, impurities such as nitrogen, silicon, phosphorus and access carbon are removed from the raw iron, and alloying elements such as manganese, nickel, chromium, and vanadium are added to produce different grades of steel. Limiting dissolved gasses such as nitrogen and oxygen, and entrained impurities in the steel is also important to ensure the quality of the products cast from the liquid steel.
There are two production methods of Iron and steel in terms of raw material. These methods are production from ore and production from scrap. In production from ore process, when iron is melted from its ore with commercial processes, it contains more carbon than desirable carbon amount. To produce steel, it must be melted and reprocessed in order to reduce the carbon to the correct amount, at this point other elements can be added. This process is performed in basic oxigen furnaces. In production from scrap process, scrap which has low carbon is melted in electric arc furnaces and after melting process, other elements is added in melting pot furnaces. In the process of these production methods, after liquid steel production step, next steps are same in both method. These next steps are continuous casting and rolling process. 
Continuous casting, also called strand casting, is the process whereby molten metal is solidified into a semifinished billet, bloom, or slab for subsequent rolling in the finishing mills. Molten metal is tapped into the ladle from furnaces. After undergoing any ladle treatments, such as alloying and degassing, and arriving at the correct temperature, the ladle is transported to the top of the casting machine. Usually the ladle sits in a slot on a rotating turret at the casting machine. From the ladle, the hot metal is transferred via a refractory shroud to a holding bath called a tundish. The tundish allows a reservoir of metal to feed the casting machine while ladles are switched, thus acting as a buffer of hot metal, as well as smoothing out flow, regulating metal feed to the molds and cleaning the metal. Rolling is a metal forming process in which metal stock is passed through one or more pairs of rolls to reduce the thickness and to make the thickness uniform. 
Rolling is classified according to the temperature of the metal rolled. If the temperature of the metal is above its recrystallization temperature, then the process is termed as hot rolling. If the temperature is below its recrystallization temperature, then the process is termed as cold rolling. Roll forming, roll bending or plate rolling is a continuous bending operation in which a long strip of metal is passed through consecutive sets of rolls, or stands, each performing only an incremental part of the bend, until the desired cross-section profile is obtained. Roll forming is ideal for producing parts with long lengths or in large quantities. After the rolling process, end product are produced. These end products are various since they are used for several industries for several usage purposes. For instance, these products are ribbed structural steel for usage of construction of buildings, the column and beam profiles of steel constructions, pipes for several usage purposes, automobile production and the sheets that are used in many other fields.
Decline of profit margins for iron and steel production and increasing of energy expenses force to the iron and steel plants to work with over capacity. Nevertheless, the cost of production is among the lowest in all of Europe. Besides, more production goals cause to variation of planned downtimes and to continue process spending less time for maintenance. In this case, it comes to the forefront that important machine parts and equipments must have long service life. At the same time, a breakdown at a point in the process causes to breakdown of all system since iron and steel production has a continuous system. This situation demonstrates that it is very important that engineers have to work about research and development to provide maximum service life for weared parts in steel industry.
The cost of wear in iron and steel industry has a great importance. The cost of wear in an integrated iron and steel plants constitute approximately 1% of annual turnover. From this point of view, the cost of wear in iron and steel plants which have large production capacity is to find millions of dollars. Besides, in iron and steel plants, adhesive wear (friction of metal to metal) causes the highest cost of wear in wear mechanisms. Therefore, preventitive actions against adhesive wear are significant and these actions provide advantages against wear mechanisms.
Hardfacing coatings of weared parts are applied with welding methods and several kinds of coating methods. Electric arc welding and MIG-MAG welding methods play a significant role in welding applications. Hardfacing welding applications that were applied with flux-cored wires increase every passing day becuase of application speed and high strength according to today's conditions. In addition, many products which are developed for various applications are available in a wide range of product selection. In this case, selection of suitable product plays an important role for existing application.
Taking into consideration today's conditions, the objective of this thesis is to make sure that the rolls that wear in short periods of time and that cause significant breakdowns and production losses in the iron and steel plants as they get replaced have more service life and are once available for production through repair and maintenance. 
Technical specifications of the products that are used for applications in industry were viewed  and  products that have suitable technical specifications  were selected based on work conditions. The samples were prepared welding with flux cored wires that was chosen right after. Then, samples were milled on planer and grinded on surface grinding machine. Lastly, samples were sanded with various mesh size of sandpaper and polished.
In these samples, micro structure of welding metals was observed, Rockwell hardness was measured, phases were determined with the detection of X-Ray difraction paterns, wearing tests were carried out for high and low temperature applications with ball on disc wearing test and reciprocating wearing test, microstructure and surface profiles of weared surfaces were analyzed. Three welding alloys were evaluated for steel plant continuous casting rolls and two of welding alloys were evaluated for rolling mill loop rolls comparing the analysis result. Productivity was analyzed evaluating of test result that was carried out and suitable welding alloys were selected for industry applications.
Service life of rolls has been increased with applications of hard facing welding investigating the work conditions and service life of steel plant continuous casting rolls and rolling mill loop rolls. Results have been compared with the current applications in iron and steel plants. At the same time, when the applications in this thesis were evaluated in terms of service life-cost analysis, it demonstrates that they provided to decline of part costs. When the industry and thesis studies are taken into consideration for continuous casting rolls, three of welding alloys which are analyzed provide longer service life than AlSl 4140 alloy steel , but Fe-13.2Co-13.1Cr-2.53Mo alloy has the most cost effectiveness and longest service life. When the industry and thesis studies are taken into consideration for rolling mill loop rolls, two of welding alloys which are analyzed provide longer service life than AlSl 1050 carbon steel, but Co-30Cr-8W-1.8C alloy has the most cost effectiveness and longest service life.