Sıcak iş takım çeliklerinin yüksek sıcaklık darbeli kayma aşınma davranışının incelenmesi
Investigation of high temperature impact-sliding wear behaviour of hot work tool steels
Yazar: BİRSEN BAŞ
Danışman: PROF. DR. HÜSEYİN ÇİMENOĞLU
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı / Malzeme Bilimi ve Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2019
Konu: Metalurji Mühendisliği = Metallurgical Engineering

Özet:
Takım çelikleri, yüksek aşınma ve korozyon direncine sahip olmaları ve yüksek sıcaklıktaki uygulamalarda dayanımlarının iyi olması sebebiyle bir çok endüstriyel uygulamada kullanılmaktadır. Takım çelikleri iyi derecede mekanik özelliklere sahip olmasına rağmen, sürekli kullanılmaları sonucunda tekrarlı yüklere mağruz kaldıkları için çeliklerde bir süre sonra aşınmalar meydana gelmektedir ve bu durum takım çeliklerinin kullanım ömürlerini azaltmaktadır. Uygulanan ısıl işlemler sonrasında elde edilen üstün özelliklerin yanı sıra, aşınma dirençleri ve kullanım süreleri de takım çelikleri için ekonomik açıdan önemli bir kriter olmaktadır.  Kullanım ömürlerini geliştirmek amacıyla çeliklere çeşitli yüzey işlemleri uygulanmaktadır. Takım çeliklerinde en çok kullanılan yöntem nitrasyondur. Nitrasyon işlemi, çeliğin yüzeyine azot difüze ettirilerek çeliğin sertliğinin ve aşınma direncinin artmasını sağlayan termokimyasal bir yöntemdir. Nitrasyon işlemi sonucunda belirli bir kalınlığa sahip difüzyon tabakası oluşmaktadır. Difüzyon tabakasınn üstünde, en dış yüzeyde "beyaz tabaka" adı verilen ve demir nitrür fazlarının (ɣ′-Fe4N, ε-Fe2–3N) bulunduğu bir tabaka oluşmaktadır. Beyaz tabaka, içerdiği demir nitrür fazları sebebiyle kırılgan ve sert bir yapıya sahiptir ve bu nedenle bazı durumlarda istenmemektedir. Beyaz tabakanın istenmediği durumlarda, iki aşamalı nitrasyon işlemi ile nitrasyon potansiyeli (Kn) değiştirilerek beyaz tabakanın oluşması engellenebilir. Nitrasyon potansiyeli, nitrasyon ortamındaki amonyak gazının kısmi basıncının hidrojenin kısmi basıncına oranıdır. Yapılan çalışmada, 1.2344, 1.2367, ve QR090 sıcak iş takım çeliklerinin yüksek sıcaklıkta darbeli-kayma aşınma davranışı incelenmiştir. Su verme ve temperleme işlemleri uygulanarak sertleştirilen bu takım çeliklerine, Kn 2,2 ve Kn 14,0/4,0 nitrasyon potansiyellerinde nitrasyon uygulanmıştır. Ancak numune yüzeylerinin belirli bir pürüzsüzlüğe getirilmesi amacıyla numune yüzeylerine zımparalama ve parlatma işlemleri uygulanmıştır ve uygulandıktan sonra deneylere tabi tutulmuştur. Parlatma işleminden sonra Kn 14,0/4,0 nitrasyon potansiyeline sahip çeliklerde beyaz tabaka gözlenmezken, Kn 2,2 nitrasyon potansiyeline sahip çeliklerin beyaz tabaka kalınlıkları ortalama 1-2 µm aralıklarında ölçülmüştür. Yapılan çalışmalar sonucunda genel eğilim olarak, nitrasyon yüzeyinde 1-2 µm kalınlığında beyaz tabaka bulunan takım çelikleri en yüksek aşınma direnci göstermiştir.

Abstract:
Hot work tool steels, which have high resistance to softening at high temperatures, are mostly used in many metal forming applications as  moulds. Their enhanced properties are provided by alloying elements such as Cr, Mn, Mo, V. Additonally their tribological properties should be excellent in aspect of extending their service life. Especially, to be used succesfully at elevated temperatures they must protect their properties such as hardness, wear resistance, and oxidation resistance  at high temperatures. However, wear scars can form on their surface after a while when exposed to high  constant pressures. Wear scars shorten tool steel's lifetime that is very critical point in aspect of economical situation in industrial applications. In order to improve the surface properties of steels, several surface modification methods are applied and thermochemical heat treatments are generally used. The most used surface modification method is nitriding for tool steels because it enhances the wear resistance, corrosion resistance, hardness, and fatigue life of steels. Also, it improves resistance of the surface against softening at high temperatures. Nitriding is a thermochemical process based on diffusion of nitrogen atoms into the metal surface. Upon nitriding two layers are obtained on the surface which are compound layer (white layer) and diffusion layer. Compound layer is located at the outer surface and consist of ɣ′-Fe4N and ε-Fe2–3N phases that are very hard and brittle. Tribological properties of the nitriding surfaces depend on the phase composition and thicknesses of the white layer. In general the white layer that includes ɣ′ phase is more ductile and wear resistant as compared to that of the ε phase. Occurrence and thickness of the white layer is controlled by nitriding potential (Kn) which is the partial pressure ratio of ammonia to hydrogen in the nitriding atmosphere. Nitriding potential can be adjusted by changing the time and temperature of nitriding process which also influences the thickness of underlying diffusion layer. Since the moulds used in metal forming operations are exposed to high complex stresses, simulating their working conditions in laboratory must be done by a tester providing impact-sliding contacts. In this test, firstly impact load is applied on the sample surface in the vertical plane and then sliding contact is imposed throughout the horizontal plane. This loading states can be repeated for desired cycles under present testing loads.  In this study, the effect of nitriding on the high temperature impact-sliding behaviour of three different hot work tool steels (1,2344, 1,2367 and QR90) was investigated. Using tool steels are classified according to AISI standard as 1.2344, 1.2367, and QR090 which has special patent. Nitriding was employed under two different Kn values (2,2 and 14/4) after application of quenching and tempering to the tool steel samples. 

Nitriding which has Kn 2,2 potential occurs at one step process. The NH3 and CO2 gases were used for this gas nitriding process. Firstly, the furnace was heated up to 300 °C at 100 % NH3 atmosphere. Then, CO2 gas was introduced in the furnace and the temperature increases up to 580 °C. Samples were hold 120 minutes at this 90 % NH3 and 10 % CO2 atmosphere until the nitriding potential reach Kn 2,2. The cooling occurred at 100 % NH3 atmosphere. The gases amounts and nitriding potential were measured by the furnace during all nitriding processes. The other nitriding process has two steps to obtain different nitriding potentials. The furnace was heated 500 °C at 100 % NH3 atmosphere and waited 30 minutes to obtain Kn 14,0 in first step of nitriding. In second step that was aimed decrease the nitriding potential to prevent occuring the white layer that includes the iron nitrides phases (ɣ′-Fe4N, ε-Fe2–3N). After the furnace was heated 530 °C and waited 120 minutes, Kn 2,2 was obtained. Thanks to this process, the white layer effect on wear behaviour can be investigated. Samples were grinded and polished prior to impact-sliding test. Optical microscopic examinations revealed that grinding and polishing processes completely removed the compound layer of the samples nitrided under Kn 14/4 while for the samples nitrided under Kn 2,2 white layer remained on the surface at a thickness of about 1-2 µm. When the hardness was measured throughout the cross section from the surface to the core, it was seen that the hardnesses decreases with the end of the diffusion layer. This situation was proved to be hard diffusion layer consequence of nitriding treatment. In addition to optical microscope examinations, XRD analsysis was applied to the nitrided samples. While iron nitrides peaks were observed for white layer samples, the other samples had just iron peaks. Impact–sliding test were conducted on preheated (600 °C) samples for 1000 cycles under  impact load of 3,5 kg and pressing load of 15 kg. Sample's temperature was measured as 250 0C after test. After the tests, the  depth and wear area of the tracks formed on the surfaces of the samples  were measured with 2-D surface profilometry and examined by Scanning Electron Microscope. Oxides were observed on the worn surfaces of the quenched and tempered samples while flakes and oxide islands were detected on the worn surface of the nitriding samples. Additionally some cracks were seen on the sliding regions of worn nitrided samples. Then, cross-section examination of the wear track revealed evidences of plastic deformation on the worn surface of the quenched and tempered steels some cracks were detected beneath the worn surface of the nitrided samples. According to the results of the 2-D profilometrric measurements the lowest wear loss was obtained from the samples having 1-2 µm thick white layer on its surface. When the grades of the examined steels are considered, 1,2344 exhibited better impact-sliding wear resistance than 1,2367 and QR90.