Zırh Çeliklerinde Kaynak Sonrası Isı Tesiri Altında Kalan Bölgenin Özelliklerinin Isıl İşlem İle İyileştirilmesi

Abstract

Zırh çelikleri, değişik karakterli mermilerin çoklu darbesine karşı çatlamaya, parçacıkların kopmasına ve kırılmaya direnç göstermesi amacıyla üretilmektedir. Gerek sivil gerekse askeri amaçlı üretilen zırh çelikleri kara, hava ve deniz araçlarında kullanılmaktadır. Darbe yükünü oluşturan mermilerin geometrik ve mekanik özelliklerine göre çeşitlenen zırh çelikleri, değişik kompozisyon ve mekanik özelliklere sahiptir.Özellikle tank ve benzeri savunma araçlarında kullanılan zırh çelikleri, yüksek dayanım, iyi kaynaklanabilirlik ve düşük üretim maliyetleri sebebiyle, aluminyum alaşımları, seramik, cam ve elyaf takviyeli kompozit malzemelere oranla daha çok tercih edilirler. Yüksek derecede patlayıcı ve parçalayıcı karaktere sahip mermilere karşı zırh çeliklerinin yüksek kalitede homojen bir mikroyapı özelliğine sahip olmaları gerekir. Kompozisyon açısından alaşımlı çelik grubu içerisine girerler.Sertlik ve tokluk özelliklerinin sağlanması amacıyla östenitleme-su verme ve temperleme ısıl işlemlerine tabi tutularak martenzitik bir mikroyapı elde edilir. Zırh çeliklerinin kaynağında bir çok farklı yöntem kullanılmaktadır. Son yıllarda yapılan çalışmalara göre örtülü metal ark kaynağı yönteminin balistik özellikler üzerinde daha iyi sonuç verdiği belirlenmiştir. Zırh çeliklerinin kimyasal bileşimlerine ve boyutlarına göre geliştirilen ısıl işlemlerle birlikte kullanılan kaynak yöntemiyle de kaynak sonrası yapının mekanik özelliklerinin optimizasyonu sağlanabilmektedir. Kaynak sonrası özellikle ısı tesiri altında kalan bölgede mekanik özellikler önemli ölçüde düşmektedir. Kaynak bölgesinde meydana gelen ergime ve kullanılan elektrodun kimyasal bileşimi, bu yapının iyileştirilmesini zorlaştırmaktadır. Isı tesiri altında kalan bölgede ise ergime meydana gelmediği gibi, kaynak bölgesine kıyasla kimyasal bileşimi de ana malzeme ile aynı değerlerdedir. Isıl etki ile meydana gelen mikroyapı değişimleriyle birlikte mekanik özelliklerin değişmesi kaçınılmazdır. Malzemenin bir darbe yükü karşısında dirençli durabilmesi için, ısı tesiri altında kalan bölgenin ısıl işlem ile iyileştirilmesi aynı zamanda balistik sonuçlar içinde olumlu sonuçlar doğuracaktır. Isı tesiri altında kalan bölge literatürde dört ana bölgeye ayrılmıştır. Bu alanlar kaynak bölgesinden uzaklaştıkça sırası ile kaba taneli, ince taneli, kısmen dönüşüm geçirmiş ve temperlenmiş bölgelerdir. Literatürde yapılan araştırmalarda özellikle kısmen dönüşüm geçirmiş ve temperlenmiş bölgelerde mekanik özelliklerin aniden düştüğü belirlenmiştir. Bu çalışmada zırh çeliğinin kaynak sonrası ısı tesiri altında kalan bölgede yer alan kısmen dönüşüm geçirmiş ve temperlenmiş bölgelerin faz özelliklerinin homojenizasyonu ve mekanik özelliklerin optimizasyonu üzerinde yoğunlaşılmıştır. Mikroyapısal çalışmalarda ışık mikroskobu kullanılmıştır. İlk önce orijinal zırh çeliği üzerinde aynı yapı ve sertlik değerlerinin elde edilmesi için ısıl işlem parametreleri geliştirilmiştir. Zırh çeliği üzerinde geliştirilen ısıl işlem parametreleriyle orijinal zırh çeliği ile aynı sertlik ve faz özellikleri elde edilmiştir. Kaynak işlemi Temsa’da yaptırılmış olup kaynak yöntemi bilinmemektedir. Kaynak sonrası yapı literatür baz alınarak belirli bölgelere ayrıldıktan sonra mikroyapısal ve mekanik özellikler incelenmiştir. Daha sonra kaynaklı yapı önceden belirlenmiş ısıl işlem parametrelerine tabi tutulmuştur. Isıl işlem görmüş kaynaklı zırh çeliği mikroyapı ve mekanik özellikleri incelenmiştir. Mekanik özellikler incelenirken mikrosertlik, tokluk ve çekme testi sonuçları kullanılmıştır. Kaynaklı zırh çeliğinde ısıl işlem sonrası kısmen dönüşüm geçirmiş ve temperlenmiş bölgede sertlik değerleri artmıştır. Tokluk ölçümleri ısı tesiri altında kalan bölgelere denk şekilde yapılmıştır. Kaynaklı yapıda ısıl işlem sonrası sertlik değerleri artarken, tokluk değerlerinde de olumlu sonuçlar gözlenmiştir. Çekme testine tabi tutulan ısıl işlem görmüş ve görmemiş kaynaklı zırh çeliği numunelerinin kopma noktaları incelenmiştir. Isıl işlemli numunede kaynak bölgesinin orta kısmında kopma görülürken, ısıl işlemsiz numunede ısı tesiri altında kalan bölgede yer alan kısmen dönüşüm geçirmiş ve temperlenmiş bölgelere çok yakın bir noktada kopma meydana gelmiştir. Kaynak sonrası yapılan işlemler sonucunda özellikle kısmen dönüşüm geçirmiş ve temperlenmiş bölgelerin mikroyapısal ve mekanik özelliklerinin optimizasyonu sağlanmıştır.

The purposes of producing armour steels are to gain resistance to multiple impaction of bullets which have different characteristic properties and to avoid from cracking and fraction of material. Armour steels are usable in both civil and military purposes like land, air and sea vehicles. These steels vary according to geometrical and mechanical properties of bullets which are producing impact load. For this reason these steels have different chemical compositions and mechanical properties. These class of steels have been used in tanks and defence vehicles, especially. They are more preferred by comparison with aluminium alloys, ceramics, reinforced glass-fiber composit materials because of high strength, good weldability and low cost producible. From the invention of tanks through to the Second World War, tank armour increased in thickness to resist the increasing size and power of anti-tank guns. A tank with sufficient armour could resist the largest anti-tank guns then in use. RHA was commonly used during this period (combined with other plate alloys and cast steel armour), and the power of an anti-tank gun was measured by the thickness of RHA it would penetrate. This standard test has remained in use despite the modern usage of many other types of armour, some of which do not include steel or even any metals. RHA was in common use as primary armour until after World War II, during which a new generation of anti-tank rounds using shaped charges instead of heavy high-velocity projectiles came into use. RHA was ineffective against these and fell out of use. Since World War II, because of a reduction in effectiveness against new weapons (mainly shaped charges and improved kinetic energy penetrators), RHA has been superseded by composite armour, which incorporates air spaces and materials such as ceramics or plastics in addition to steel, and explosive reactive armour. For the testing and calibration of anti-tank guns, the term RHAe (Rolled Homogeneous Armour equivalency) is used when giving an estimate of either the penetrative capability of a projectile or the protective capability of a type of armour which may or may not be steel. Because of variations in armour shape, quality, material, and case-by-case performance, the usefulness of RHA in comparing different armour is only approximate. Currently, most armoured vehicles have their basic structure formed from RHA to lend general strength and toughness. Armoured steel must be hard yet impervious to shock in order to resist high velocity metal projectiles. Steel with these characteristics is produced by processing cast steel billets of appropriate size and then rolling them into plates of required thickness. Hot rolling homogenizes the grain structure of the steel, removing imperfections which would reduce the strength of the steel. Rolling also elongates the grain structure in the steel to form long lines, which enable the stress under which the steel is placed when loaded to flow throughout the metal, and not be concentrated into one area. RHA is called homogeneous armour because its structure and composition is uniform throughout its section. The opposite of homogeneous steel plate is face-hardened steel plate, where the face of the steel is composed differently to the substrate. The face of the steel, which starts as an RHA plate, is hardened by a heat-treatment process. The U.S. armor community is currently engaged in accelerated efforts to deliver lightweight armor technologies that can defeat armor-piercing (AP) projectiles at reduced areal weights that are available across a large industrial base. While many of these programs involve the application of lower-density metals such as aluminum and titanium, the selection of steel alloys is still competitive for many ballistic and structural applications; the ability to fabricate armor components in both commercial and military operational areas with available equipment and personnel is a major advantage of steel solutions. To meet these requirements, the U.S. armor community has increased the availability of quenched and tempered armor steels by updating current steel military specifications and the most important has been the updated MIL-DTL-46100E specification for high-hardness armor (HHA). This improved specification was necessary to supply the large steel demands for combat operations in Iraq and Afghanistan. This HHA specification allows modern continuous processing technologies to be used efficiently and offers a new class of auto-tempered high-hard steels. Armour steels must have high quality and homogenius microstructure and mechanical properties against bullets which have strong explosive and shreddercharacter. Classification is identifed by alloyed and low-medium carbon steels. Heat treatment procedure is austenitization-quenching to contribute the formation of martensite structure. Then by tempering hardness and thoughness optimization is obtained. There are different kinds of welding techniques which are used in armour steels. The resistance against projectile penetration of the various zones in the weldments of armour steels are crucial. The scientific researches that had been accomplished recently revealed that the ballistic limit has been found to be highest in the case of shielded metal arc welding (SMAW) weld and least in respect offlux cored arc welding (FCAW) welds. The heat-affected zone (HAZ) of SMAW was resistant to penetration and exhibited the highest ballistic limit and the HAZ of FCAW exhibited an intermediate performance. Optimization of mechanical properties of armour steel can be obtainable after welding process by developed heat treatment parameters which can be affected by thickness and chemical composition of material. Quenched and tempered steels of high hardness are used in armour applications where resistance against projectile attack is needed. When these steels are exposed to weld thermal cycles they exhibit heat-affected- zone (HAZ) softening, this softening leading to degradation in ballistic performance. The degree of softening in the HAZ is a function of the weld thermal cycle, which is a characteristic of the welding process. The softening characteristics depend also on the kinetics of the phase transformations of the steel and are a function of the chemistry of the steel. Improvement of weld zone’s mechanical properties by heat treatment is diffucult to achive because of occuring different chemical composition that dissimilar unaffected material which is provoked by melting and electrode composition that is used. But the HAZ is heat treatable as compared with weld zone by the reason of remaing chemical composition same. Mechanical and microstructure properties change is inevitable in consequence of heat distribution. Improvement of HAZ by heat treatment beneficial not only to obtain resistance to impact load but acquirement of positive consequences about balistic properties. HAZ is seperated four main zone based on literature. These zones can be identify as become distance from welding zone; “grain-growth HAZ”, “fine-grained zone HAZ, “partially austenized and tempered HAZ” and “tempered HAZ”. According to recent scientific researches, mechanical properties of “partially austenized and tempered HAZ” and “tempered HAZ” are decreasing instantly after welding process. In this thesis emphasised on improvement and homogenization of mechanical and microstructure properties in “partially austenized and tempered HAZ” and “tempered HAZ” after welding process of armour steel. Light microscope has used in microstructure examinations. Firstly original armour steel has analysed by hardness and microstructures researches. Afterward heat treatment parameters have developed on original armour steel to apply on welded armour steel. Results of experiments have shown that the parameters of heat treatment have sufficent values to obtain same hardness and microstructure properties on heat treated armour steel when compared with original armour steel. Parameters has found that at 950 Cº austenization for 30 minutes. After quenching, tempering has completed at 200 Cº for 30 minutes. Microstructure examination results shown that phases in original and heat treated armour steel are lath type martensite. Welding process has completed in Temsa and welding technique which is used is unknown. Microstructures has seperated critical zones base on literature informations after welding process. Classification of zones has used in order to examine microstructures and mechanical properties detailed Results have shown that microhardness has decreased instantly in “partially austenized and tempered HAZ” and “tempered HAZ”. Microstructures of these zones has found in form of ferritic and perlitic phases. Grain size of ferrit in “partially austenized and tempered HAZ” is observed lesser than “tempered HAZ” which verify by microhardness results. Because hardness is decreasing while transition from “partially austenized and tempered HAZ” to “tempered HAZ”. Heat treatment parametres have used in welded armour steel. Microhardness, charpy v-notch impact and tensile test results have used in examination of mechanical properties. Microhardness values of heat treated armour steel have increased in especially “partially austenized and tempered HAZ” and “tempered HAZ” after welding process. Charpy v-notch impact test has implemented on HAZ. In HAZ of heat treated welded armour steel acquired that not only hardeness values have increased but also toughness values have shown positive results. Heat treated and non-treated welded armour tensile test specimens have examined by where the seperation is occured. Heat treated armour steel’s seperation has occured in the middle of welding zone. Non-heat treated tensile test specimen has seperated in HAZ which is so close to the “partially austenized and tempered HAZ” and “tempered HAZ”. Yield strengths values of tensile test specimen have found undetermined. Tensile strength values of heat treated armour steel has found low in compariosn with non-heat treated welded armour test specimen. Optimization of microstructure and mechanical properties of heat affected zone , especially in “partially austenized and tempered HAZ” and “tempered HAZ”, is achieved by heat treatment after welding process.