ÖZET c? Günümüzde araçlarda, sabit yapılarda ve personel korumalarında zırh sistemleri çok etkili bir şekilde kullanılmaktadır. Günümüzde kullanılan zırh çelikleri su verilmiş ve temperlenmiş çelikler grubuna girmektedir. Bu tip çeliklerin girdiği ana grup az alaşımlı yüksek mukavemetli yapı çelikleridir. Bu çelikler yaklaşık 600 °C'de ön ısıtma yapıldıktan sonra su, yağ veya havada soğutularak sertleşmesi sağlanmaktadır. Daha sonra yapılan temperleme işlemi ile çeliğin içindeki martenzitik yapının oranının azaltılması sağlanmaktadır. Yapılan ısıl işlemler ve içindeki alaşım elementleri sonucunda malzeme çok yüksek bir soğuma kabiliyetine sahip olmaktadır. Yüksek soğuma kabiliyetine sahip olan bir çeliğin kaynak esnasındaki davranışı ters orantılı olarak değişim gösterecektir. Kaynak esnasında meydana gelen aşın sıcaklık nedeniyle malzeme hızlı soğuyacak ve mekanik özellikleri ters yönde etkileyecek bir iç yapı meydana gelecektir. Bu açıdan zırh çeliğinin kaynak kabiliyetinin etüdü özellikle askeri kullanım söz konusu olduğunda önem kazanmaktadır. 8414 yapı çeliği DİN 17240'a karşılık gelmektedir. Bu çelik M52 T K/M OBÜS'te zırh levhası olarak 6.35 ve 12.7 mm kalınlıklarda kullanılmaktadır. Bu levhaların kaynağında sekiz farklı tipte elektrod kullanılmıştır. Ancak görünürde iyi kaynak verebilen bu elektrodlann hangisinin daha iyi mekanik özellik sağladığı saptanamamıştır. Bu çalışmada bazik karakterli ostenitik elektrodlar, ferrit içerikli demir tozu katkılı rutil elektrodlar ve bazik karakterli gazaltı elektrodlan kullanılarak hazırlanan numunelerle çekme ve çentik-darbe deneyleri yapılmıştır. Ayrıca kaynak bölgelerinde sertlik ölçümleri ve metalograflk incelemeler yapılmış ve 8414 çeliğine en uygun elektrod saptanmaya çalışılmıştır. Yapılan çalışmalar sonucunda Tenacito 70-A demir tozu katkılı bazik karakterli elektrodu ve AS P-307 ve amerikan yapımı MEL E 307-L 15 ostenitik paslanmaz çelik bazik karakterli elekrodlan tatmin edici sonuçlar vermişlerdir. Ancak demir tozu katkılı elektrodlann normal bazik elektrodlara göre kolay tutuştuğu, cürufunun kolay kalktığı ve kaynakçı için rahat kaynak imkanı sağladığı göz önüne alınırsa, tercihin daha demir tozu katkılı bazik karakterli Tenacito 70-A elektrodunun kullanımına doğru kayacağı tahmin edilmektedir.
WELDABILITY OF 8414 (DIN 17240) STRUCTURAL STEEL Keywords: Quenched and tempered steels, weldability of RHA, austenitic and ferritic electrodes Recently, armor systems are used at vehicles, buildings and personal protection very effectively. Hard materials were prefered as armor materials for long years. At usage, high strength metals, especially steels found place. When protection level wanted was increased, it was possible to obtain strength required by alloying different elements. Production of armor steels that is placed low alloyed high strength structural steels is quite hard. Because of very special utilization area, these must be produced at highest quality. During the electric arc welding process of armor plates on tanks, the point which requires the greatest attention is that the toughness of the armor material and the pysical properties which stand the effects of the projectiles during service should not change durring welding. Therefore the factors which influence the weldability of the armor steel should be controlled during welding process. As it is known, there is a quick heating of the metal up to the temperature above its solidus degree, and then a rapid cooling follows it. During the welding of armor steel which can be considered hardenable steel, the cooling rate is faster than its critical cooling rate. Because of this, in the heat affected zone next to the deposited weld metal, there is a formation of martensite which is hard and brittle. In order to avoid this formation, the cooling rate should be reduced. The hulls are huge masses therefore it is not possible to use a preheat in application. Using of austenitic electrodes can solve this problem as theorical. This can prevent the formation of martensite in HAZ. But the results wanted can not be obtained in every time. Weldability of the armor can be increased by using not only austenitic electrodes but rutile electrodes as well. 8414 structural steel which was used at experimental studies, is the quenched and tempered steel. The perlite + ferrite steels that is controlly rolled, fine-grained and hardened have 450 MN/mm2 maximum tensile strength. If the armor steels are classified, there are two armor types according to production methods. Rolled steels and direct casting steels. XIRolled armor steels: - Class I: These are the rolled armor steels that are heat treated toobtain maximum strength against projectile penetration. - Class II: These are the rolled armor steels that are heat treated to obtain maximum strength against hand-bombs and AP projectiles. - Class III: These are generally the armor steels that have increased hardness and notch-impact strength. This type is not used at combat vehicles. It is only used at experimental studies. Structure of armor steels are homogeneous. To determine the ballistic strength of the armor steels, notch-impact strength and hardness values must be known. At the same time, ballistic experiences may be required to determine ballistic strength. The armor plates are fixed at vertical position for testing. A 5052 aluminium plate which has 0.35 mm thickness is placed at back of the armor plate with 150 mm distance. Testing is made according to military specification. The results are discussed by considering crack length according to projectile type and impact rate, particle size that is left from plate back. Ballistic testing is not applied at the rolled armor Class II. Traditional rolled homogeneous armor (RHA) steel plates (MEL 12560) require a quench and temper process to achieve the highest hardness for ballistic protection while still maintaining adequate material toughness. When RHA steel (0.20-0.30 wt % C) is welded into structural components, preheating to minimum of 177 °C is often required to produce sound weldments and avoid weld cracking. Accerelated Cooling/ Direct Quench (AC/DQ) processing of low carbon steels (0.15-0.18 wt %C) utilizes thermomechanical processing to achieve high strength and toughness. Due to reduced carbon content, armor quality AC/DQ steel may be weldable with little or no preheating being required. Weldability of a steel is correlated with so many factors. These can be pre- and post welding heat treatment, choice of the best electrode, determining of heat input during welding. To have a decision about weldability of a steel, it has to be considered these all factors. In this study, first of these factors was made. By using eight different electrodes, the best electrode was determined. Electrodes that is used for preparing the weldment specimens have different chemical compositions, different covering types and different welding processes. Seven of all are the electric arc welding electrode. These have basic and rutile covering. One of all is shielded gas welding electrode. The specimens which is produced according to DIN were tested mechanically. Tensile, notch-impact destructive testing and hardness measurements were made. At the end of these experiments, it was shown that a basic-iron powder added-electrode and two basic electrodes had enough strength. If a preference is made amongst these three electrodes, iron powder added electrode can be prefered. At utilization of IPA- basic electrodes, arc can be burned easily and the slag can be left easily after welding. Also IPA basic electrodes are comfortable for welders. This study is the first step for XIIdetermining the weldabiiity. It is prefered to use only one electrode(IPA-basic) for studying the other steps. 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