Yüksek azot içeren çökeltme ile sertleştirilebilir ostenitik paslanmaz çelikler
Precipitation hardening austenitic stainless steels
- Tez No: 83134
- Danışmanlar: DOÇ. DR. MEHMET DEMİRKOL
- Tez Türü: Yüksek Lisans
- Konular: Makine Mühendisliği, Mechanical Engineering
- Anahtar Kelimeler: Azot, Ostenitik çelik, Paslanmaz çelik, Sertleştirme, Çelik-metal, Nitrogen, Austenitic steel, Stainless steel, Hardening, Steel-metal
- Yıl: 1999
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Makine Mühendisliği Ana Bilim Dalı
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: Belirtilmemiş.
Özet
ÖZET Günümüzde, daha çok bazı alüminyum alaşımlarında yararlanılan ve bir dayanım artırma yöntemi olan çökeltme sertleşmesi, genelde aşın doymuş katı çözelti oluşturan alaşımların çoğuna uygulanabilir hale gelmiştir. Gelişen imalat teknolojisi ile beraber, azotun, ostenitik paslanmaz çeliklerin akma dayanımı ve korozyon özelliklerinde önemli gelişmeler sağlaması, azot içerikli paslanmaz çeliklere geniş uygulama alam sağlamıştır. Ayrıca bu tür çeliklerin yaşlanabilme özelliği göstermesi de bu malzemelerin talebini arttırmaktadır. Bu çalışmada çökeltme sertleşmesi, yüksek azot içeren yan ostenitik, çökeltme sertleşmesi uygulanabilir bir paslanmaz çelik alaşımına (AM-355) uygulanarak bu işlemin malzemenin mekanik özelliklerine etkisi üzerinde durulmuştur. Azotun ostenitik paslanmaz çelikler üzerindeki yararlı etkileri;. y fazı kararlılığını artırma,. dayanımı artırma (özellikle akma dayanımı),. korozyon direncini iyileştirme, şeklinde özetlenmektedir. Yapılan bu deneysel çalışmada, malzemeye iki farklı sıcaklıkta uygulanan yaşlandırma ısıl işlemleri sonucunda oluşan çökelti yapısının mekanik dayanıma etkisi incelenmiş ve azotun paslanmaz çeliklerde dayanımı artına bir alaşım elementi olarak kullanılabileceği gösterilmeye çalışılmıştır. Bu amaçla farklı yaşlandırma ısıl işlemlerine tabi tutulan malzemeye uygulanan mekanik deneylerle elde edilen veriler ve metalografik inceleme sonuçlan ile AM-355 paslanmaz çeliğinin yaşlandırma işlemi ile yapı ve özelliklerinin nasıl değiştirilebileceği konusuna açıklık getirilmiştir.
Özet (Çeviri)
SUMMARY PRECIPITATION HARDENING AUSTENITIC STAINLESS STEELS As chromium is added to steels, the corrosion resistance increases progressively due to the formation of a thin protective film of Cr2C>3, the so called passive layer. The stainless grades are the most diverse and complex in terms of composition, microstructure and mechanical properties. Chromium is the principal alloying element in a stainless steel, containing %17Cr can be martensitic, ferritic or austenitic, depending on heat treatment and the presence of other elements such as Ni. In discussing the characteristics of stainless steels, it is more convenient to categorize these materials in terms of microstructure rather than composition. Stainless steels are commonly divided into five groups: - Martensitic stainless steel - Ferritic stainless steels - Austenitic stainless steels - Dublex (ferritic-austenitic) stainless steels and - Precipitation - hardening stainless steels. Stainless steels, especially austenitic stainless steels are used in a wide variety of applications because of their good formability and mechanical properties combined with excellent corrosion resistance. The austenitic stainless steels constitute the most widely used type of stainless steel. The influence of the microstructure on the mechanical behaviour of stainless steel is therefore of great interest. Their high alloy content makes these steels expensive and they are used mainly in applications where corrosion resistance or another of their specific properties, oxidation resistance, creep resistance, toughness it low temperature or one of their magnetic or thermal characteristics is of prime importance. It could be found that a change in all physical, mechanical and chemical properties as chromium is added to iron. Among other things, the strength of the iron is increased by alloy additions. However, strengthening by substitutional elements only is not usually very effective. There are three pertinent methods of strengthening stainless steel: a) Work hardening b) Martensitic hardening c) Precipitation hardening XIThe phenomenon of precipitation hardening is the most familiar in aluminum alloys. Precipitation hardening is a principal hardening mechanism in many commercial alloys, including some steels also. Precipitation hardening is based on a fundamental characteristic of solid solutions and the solubility of an alloying element in a solvent metal may decrease as the temperature is lowered. Precipitation hardening stainless steels are martensitic or duplex chromium nickel types containing alloying elements such as carbon or nitrogen which form precipitates during processing. They can be hardened to high strength by levels of successive solution heat treatment and aging, combining very high corrosion resistance and mechanical properties. They are used for gears, fasteners, cutlery and aircraft, steam turbine, jet engine parts. Using carbon as a strengthening interstitial element may lead to corrosion problems because of the tendency to form carbide precipitates with associated local depletion of chromium and resulting sensitization to intergranular corrosion. This disadvantage for carbon in combination with the fact that modern steel making allows the nitrogen concentration to be controlled with high accuracy has made nitrogen very interesting as an alloying element. Apart from its strengthening effect, nitrogen also delays the formations of carbides by reducing the diffusivity of carbon. One primary advantage offered by high nitrogen alloys is high yield and tensile strengths. Nitrogen also is a strong austenite stabilizing element, thereby reducing the Ni required for this purpose. High nitrogen stainless steels are thermally unstable and susceptible to nitride precipitation. In the absence of strong nitride-forming elements such as Al, Ti, V or Nb, Cr precipitation occurs in high nitrogen stainless steels during elevated temperature exposure with sufficient duration. Precipitation hardening is consisting three distinct steps. The first is the solution treatment, second is the quenching and the third is aging treatment. The principles of precipitation hardening are illustrated in Figure 1 schematically. If a two phase alloy of composition X is heated to temperature Ti, the p phase will dissolve into a the phase. If the alloy is then quenched rapidly to room temperature, the a phase will not have time to transform p phase. The resulting alloy will remain single phase and relatively soft with highest unstability. The purpose of the solution heat treatment is to obtain in solid solution the maximum practical concentration of the hardening solutes. The rate of solution formation increases with temperature, because of increased diffusion rate due to thermal activation. Consequently, the most favorable temperature for effecting maximum solution is very near that which melting occurs. But melting must be avoided, because there may be a decrease in both strength and ductility of alloy. xnLIQUID + a a + p 100 A %B Figure 1. Phase diagram of an alloy system which has the capability of precipitation hardening. Quenching is the cooling of alloy from solution temperature at a rate great enough to retain all the second phase in a supersaturated solid solution. The objective of quenching is to preserve as nearly as possible the solid solution formed at the solution heat treating temperature by rapidly cooling to some lower temperature, usually near room temperature. The solute atoms that diffuse to grain boundaries as well as the vacancies that migrate to disordered regions, are irretrievably lost for practical purpose and fail to contribute to the subsequent strengthening. The precipitation or aging treatment requires holding the material at the proper temperature until the desired increase in hardness and strength occurs. Since the supersaturated solution is unstable, there is a definite tendency for the second phase to precipitate. The slip planes are preferred since they have maximum interplanar spacing. As this process continuous, distortion of the planes increases and in effect, inhibits slip along those planes, thus resulting is a stronger alloy. Precipitated structures are highly significant, because they frequently indicate the metallurgical condition of the alloy, its mechanical characteristics and corrosion behaviour. The precipitates are fine particles of phases that were in solution in alloy at an elevated temperature and precipitated from solid solution at a lower temperature. Precipitates form at grain boundaries, dislocations and vacancies. Their size, shape and location depend on the thermal conditions leading to their formation. xmObviously, precipitation and resulting properties are functions of time and temperature of the heat temperature. T3<T2<T, AGING TIME AT THE TEMPERATURE Figure 2. Effect of aging time and temperature on the hardness of precipitation hardening alloy When the alloy is reheated to temperatures between T2 and T4 for a time (fig. 1) the (3 phase forms as fine precipitates within the a matrix. The fine P precipitates in the grains can strengthen alloy as shown in figure 2. The mechanical properties of the alloy depend on the aging temperature and time. To obtain optimum properties we must control both time and temperature with the proper combination depending on the alloy. Excessive temperature or time at temperature will not develop maximum strength and hardness. The purpose of this work, is to investigate a more practical and applicable heat treatment which optimizes the strength, hardness and ductility of AM-355 precipitation hardened austenitic stainless steel by controlling the precipitation process and precipitate microstructure which determine the properties of alloy. For this purpose aging treatment was used, which involves the solution heat treatment after quenching the material to the room temperature and then aging which are at 460°C for 3 hours and at 550°C for 3 hours. Test specimens which contain high-nitrogen was produced as hot rolled bars. Solution annealing at 1050°C for 1 hours followed by water quenching insured that all nitrogen present in the material was dissolved interstitially. After solution treatment, annealed materials were aged at 460°C and at 550°C for 3 hours. xivStandard sound specimens of 6 mm diameter and 30 mm long were used in tensile testing. Hardness testing was performed on metallographically prepared cylindrical sections using a diamond pyramid indentor with 1000 gf load. Microstructures were characterized by optical microscopy of metallographically prepared specimens. For metallographic analysis, Vilella's reagent (5 ml HC1, 1 g picric acid, 100 ml ethanol (95%) or methanol (95%)) solution was used as etchant and the specimens were immersed for a few seconds to 15 minutes. The following conclusions can be extracted from the studies: There are the formation of chromium nitride and/or iron-carbide precipitates in the structure of the alloy after aging treatment. The precipitates that form at 460°C are finer than that form at 550°C. Because of these fine particles formed at temperature of 460C, the hardness and strength of alloy are higher than that of 550°C. J tJC'*^kİî^yti“”'" xv
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