|
Son yıllarda, ülkemizde paslanmaz çeliklere olan gereksinim her geçen gün artmaktadır. Paslanmaz çelikler sağladıkları korozyon direnciyle, endüstrinin neredeyse her alanında kendilerine yer bulmaktadırlar. Ancak üretimde ve birçok fabrikasyon uygulamalarında bu çelikler deformasyona maruz kalırlar. Isıl işlemle sertleştirilemeyen östenitik ve ferritik paslanmaz çelikler soğuk şekillendirme ile dayanım kazanırlar. Paslanmaz çelikler arasında en yaygın kullanıma sahip tür östenitik paslanmaz çeliklerdir ve en bilinen tipi AISI 304'tür. Fakat günümüzde östenitik paslanmaz çeliklerin yerine ferritik paslanmaz çeliklerin kullanıldığı görülmektedir. Çünkü ferritik paslanmaz çelikler nikel içermedikleri için, östenitik paslanmaz çeliklerden daha ucuzlardır. Ferritik paslanmaz çeliklerin en bilinen tipi AISI 430'dur. Üretimleri esnasında çeşitli soğuk şekillendirme proseslerinden geçen östenitik ve ferritik paslanmaz çeliklerin mikroyapılarının, mekanik özelliklerinin ve korozyon davranışlarının bu proseslerin miktarlarına bağlı olarak değişkenlik göstermeleri mümkündür.Bu çalışmada kullanılan AISI 304 kalite östenitik ve AISI 430 kalite ferritik paslanmaz çeliklerin orijinal hallerinin ve soğuk deformasyona uğratılmış hallerinin mikroyapı, sertlik incelemeleri ve korozyon deneyleri yapılmıştır. Soğuk deformasyonlar basma ve çekme işlemleri ile yapılmıştır. Paslanmaz çelikler satın alındıkları durumda ve tavlama ısıl işlemi yapılmış durumda kullanılmıştır. Böylece soğuk deformasyon oranının malzemelerdeki etkileri incelenmiştir. Tavlama ısıl işleminin ve soğuk deformasyon oranlarının özelliklere etkileri her iki kalite paslanmaz çelik için kıyaslamalı olarak incelenmiştir. Son olarak korozyon testi yapılmıştır. Korozon dayanımı testi, hızlı korozyon testi olarak, 1 M HCl asit içerisinde farklı sürelerde bekletilerek yapılmıştır.Yapılan çalışmada aşağıdaki sonuçlar elde edilmiştir;?Her iki kalite paslanmaz çeliklerde deformasyon oranı arttıkça sertlik değerleri artmaktadır.?Artan deformasyon oranıyla AISI 304 kalite östenitik paslanmaz çeliğin sertlik değerleri daha fazla artmaktadır. Bu sonuç AISI 304 kalite östenitik paslanmaz çeliğin mukavemet değerlerinin deformasyonla daha etkili şekilde geliştirilebileceğini göstermektedir.?Paslanmaz çeliklerin çekme dayanımı / akma gerilmesi oranı, soğuk deformasyon oranı arttıkça azalmıştır.?Paslanmaz çeliklerin sertlik değerlerindeki artış oranı, deformasyon oranı arttıkça düşmektedir.?Soğuk deformasyona uğratılmış paslanmaz çeliklerin yüzey ve yüzeye yakın bölgelerdeki sertlik değerleri merkezlerine göre daha fazladır.?Paslanmaz çeliklerin soğuk deformasyon oranlarının artmasıyla mikroyapıdaki taneler daha fazla şekil değiştirmektedir. Yüzey ve yüzeye yakın bölgelerde taneler deformasyon yönünde daha fazla uzamaktadır.?Soğuk deformasyon ile AISI 304 kalite östenitik paslanmaz çelikte?' martenzit fazı oluşmaktadır.?Tavlama ısıl işleminden sonra paslanmaz çeliklerin sertlikleri azalmıştır.?AISI 304 kalite östenitik paslanmaz çeliğin hızlı korozyon dayanımı AISI 430 kalite ferritik paslanmaz çeliğe göre daha yüksektir.?Paslanmaz çeliklerin soğuk deformasyon oranları arttıkça korozyon dayanımları düşmektedir. Yüzey ve yüzeye yakın bölgelerde korozyon hasarı daha fazladır.
|
|
In recent years, the necessity for stainless steels in our country increases every day. Stainless steels are widely used in various applications related to corrosion resistance. But this steels are exposed to deformation in production and many fabrication applications. Austenitic and ferritic stainless steels can not be hardened by heat treatment. On the other hand, cold working can harden such stainless steels. The most common stainless steels are austenitic steels, well-known as AISI 304 type. But today, ferritic stainless steels have been used instead of austenitic stainless steels. Because of ferritic stainless steels don?t contain nickel, they are cheaper than austenitic stainless steels. AISI 430 is the well-known type of ferritic stainless steels. It is possible that the microstructures, mechanical properties and corrosion behaviours of austenitic and ferritic stainless steels subjected to various cold-forming processes during their production can show variability.Austenitic stainless steels are the most popular type of stainless steel because of their excellent formability, corrosion resistance in various aggressive environments and weldability. The presence of chromium (% 16-28 wt.) and nickel (% 3,5-32 wt.) near to the small contents of the carbon (usually below % 0,1 wt.) assures a stable austenitic structure in the whole range of the temperature (from the temperature of solidus to the room temperature). Moreover can contain such elements as molybdenum (% 2-6 wt.), titanium and niobium. After supersaturation in water from 1100°C steel a single-phase austenitic structure with high corrosion resistance and without carbides extractions was obtained. In the supersaturated state austenitic stainless steels are characterized by high plasticity and relatively low strength (Rm about 550 MPa; Rp0.2 about 200-250 MPa). They are widely used in chemical, petrochemical, machinery, automobile, nuclear and shipyard industries.One of the possible mechanisms of the plastic deformation in steels type 18-8 is strain-induced martensitic transformation leading to the transition of paramagnetic austenite into ferromagnetic martensite. Austenite has a FCC crystal structure, while martensite at low carbon concentration crystallizes in BCC structure. Martensite is harder and stronger than austenite. Some alloying components are inhibitors of martensitic transformation in austenitic stainless steels. It is well known that N, Mn, and Cu are such inhibitors, because they broaden the area of the occurrence of the ? phase. However in such elements as: Mo, W, Si, Ti, Nb, V are put into steel in order to increase the strength properties and corrosion resistance contribute to restricting the range of the occurrence ?.The mechanical behaviour and evolution of phases in metastable materials, particularly Fe-Cr-Ni steels, have been the subject of experimental and theoretical studies for a long time. The reason is that these materials are quite often used in modern engineering. However, their mechanical properties are not fully investigated, because the character and the intensity of phase transformations depend on many factors, such as strain rate and strain level, stress state and regime of mechanical loading, and temperature.Ferritic stainless steels present typical Cr contents in the range % 11 to 17, with low Ni and C levels in their chemical composition. Their good corrosion and oxidation resistance is related to the Cr content. They also present low toughness and a ductile-brittle transition temperature close to or somewhat higher than room temperature. Ferritic stainless steels are frequently cold formed to their final application and differ to the austenitic stainless steels by presenting higher YS and a low n (strain hardening coefficient) values. These differences lead to the fact that they are far less adequate than the austenitic ones, for applications that involve stretching operations, despite that they may be used for deep drawing operations. The higher levels in YS and the lower ductility of the ferritic stainless steels conduce to lower levels of conformability in relation to the austenitic types, hence leading to less demanding stampings.Mechanically, the differences in between the austenitic and ferritic types are more evident. Measured by HV, Rp0.2 or Rm, most ferrites equal the austenitic steel types. However, ferritics possess higher yield strength (Rp0.2) and lower tensile strength (Rm). In general, their mechanical properties are comparable to high strength carbon steels. A major difference in between the ferrites and the austenites is the elongation, i.e. the possible deformation until breakage. For the austenitic AISI 304 or 316 groups, the minimum elongation is around % 45 meaning that these steel types may be stretched and deformed very much, before they break. In contrast, the ferritic types posses a minimum elongation of % 18-20 which means that they are much less useful in the case of mechanical deformation, such as pure stretch forming. On the contrary, ferritics are more suitable for deep drawing, such as complex exhaust systems. With regards to cold forming, the ferrites are comparable with carbon steels, and less powerful machines are needed in comparison with austenitics. Notably, alloys like AISI 430 are widely used in i.e. England and Italy for catering purposes. However, do not expect to be able to make a very complicated double kitchen sink from a ferritic stainless steel. In such a case, the traditional AISI 304 is better. Another notable difference is the mechanical properties at extreme temperatures, i.e. notch toughness (AV) and creep strength, respectively, although Nb stabilized ferritics deform less than austenitics in response to long term stresses. Unlike the austenites, the ferrites may become brittle at very low temperatures, and they do not maintain their excellent tensile stress at very high temperatures (typically 700-800 ºC and above). In addition, long-term exposure to temperatures in between 400 and 550 °C may give rise to?475°-brittleness?, an ?illness? which may also attack the duplex stainless steels in the same temperature range.The microstructure, hardness investigations and corrosion tests of original states and cold deformed states of AISI 304 grade austenitic and AISI 430 grade ferritic stainless steels that have been used in this study were carried out. The cold deformations have been performed by the tensile and pressure operations. Stainless steels have been used with purchased and made of annealing heat treatment states. Thus, the effects of cold deformation rate on materials have been investigated. The effects of annealing heat treatment and cold deformation rates to properties for both grades of stainless steels have been investigated by comparing. Finally, corrosion test has been made. Corrosion resistance test, as rapid corrosion test, has been made as waiting in 1 M HCl acid for different periods.The following results have been obtained in this study;?The hardness values increase as deformation rate increases in both grades of stainless steels.?The hardness values of AISI 304 grade austenitic stainless steel increase more with increasing deformation rate. This result indicates the strength values of AISI 304 grade austenitic stainless steel can be improved more effectively with deformation.?Tensile strength / Yield stress ratio of stainless steels decreases as the cold deformation rate increases.?The rate of increase in hardness values of stainless steels decreases as deformation rate increases.?The hardness values in the surface and close to the surface regions of cold deformed stainless steels are greater than the centers.?With increasing cold deformation rates of stainless steels, the grains in microstructure change more shape. In the surface and close to the surface regions, the grains elongate more in the direction of deformation.??? martensite phase forms in AISI 304 grade austenitic stainless steel with cold deformation.?The hardnesses of stainless steels decreased after annealing heat treatment.?The rapid corrosion resistance of AISI 304 grade austenitic stainless steel is higher than AISI 430 grade ferritic stainless steel.?Corrosion resistance decreases as the cold deformation rates of stainless steels increase. Corrosion damage is higher in the surface and close to the surface regions. |
