This paper presents an experimental study on resistance spot welding of DP and TWIP steel sheets used in automative industry. A timer and current controlled resistance spot welding machine having 120 kVA capacity and a pneumatic application mechanism with a single lever was used to prepare the specimens. Welding periods were chosen as 5, 10, 15, 20, 25 and 30 cycles (1 cycle = 0.02 s) and also welding currents were increased from 6.33 kA up to 15.7 kA in steps of 0.9 kA. The electrode force was kept constant at 4 kN. The prepared welding specimens were exposed to tensile-peel test and micro and macro-structures of specimens were investigated by means of an optical microscope and scanning electron microscope (SEM) in order to see the joinability of DP and TWIP steel sheets by resistance spot welding. Finally appropriate welding current and time were advised to the users.
Steel pipe piles are used to reinforce the grounds. Due to high hardness of the rocky materials, in some cases, the tip surface should be developed with new designs in terms of geometry, material and heat treatment. In this study, a hardfacing welding, which reinforces the application point of the tip surface, was applied on the steel pipe pile shoe tip which was manufactured from S355J2 steel. Wear tests were applied and hardness measurements were made to explain wear behavior. According to the results, the hardened surface of the 3rd layer which was welded with FCH-360 flux cored wire showed higher hardness than other layers. Similarly, the highest wear resistance was obtained in this layer. Martensitic and bainitic structures with ferrite islets were observed from the first layer to the second layer. The bainite and ferrite isles were gradually transformed to martensite and maintained itself from first to third layer. The martensitic structure mainly controlled the hardness and wear resistance. The sizes of the martensite highly affected the hardness and wear resistance of the layer itself.
ÖZET Bu çalışmada, XAR (eXtra Aşınma Dayanımı) çeliklerinin robotik MAG kaynağında kaynak akımının kaynaklı birleştirmelerin mikrosertliği üzerine etkisi incelenmiştir. Bu çalışmada, ağır hizmet makinelerinde kullanılan 4 mm kalınlıktaki XAR 500 çeliğinin MAG kaynak yöntemiyle birleştirilmesinde kaynak akımı yoğunluğunun mikrosertliğe etkisi incelenmiştir. Kaynaklanmış numunelerin hazırlanmasında hız, gerilim ve akım kontrollü MAG kynak robotu kullanılmıştır. 140A, 160A ve 180A olmak üzere üç farklı kaynak akım yoğunluğu seçilmiştir. Kaynak hızı 350mm/dak olarak sabit tutulmuştur. Kaynak hızının etkisini test etmek için 300mm/dak, 350mm/dak, 400mm/dak ve 450 mm/dak. olarak seçilmiş kaynak akım şiddeti 160A' de sabit tutulmuştur. 1 mm kalınlığında MG-2 kaynak teli kullanılmıştır. %86 Ar, %12 CO2 ve %2 O2 içeren karışım gaz kullanılmıştır. Sonuç olarak, bu malzeme gruplarının kaynaklı bağlantılarında ısı tesiri altındaki bölge (ITAB) sertliğinin azaldığı gözlenmiştir. Bu nedenle ITAB sertliğinin azalması durumunda çatlak riskinin azaldığı gözlenmiştir. Elde edilen kaynaklı bağlantılarda optimum sertlik oranını sağlayan kaynak akımı ve kaynak hızı kullanıcılara sunulmuştur.
ABSTRACTIn this study is to investigate the effect of welding current on microhardness in combining XAR (eXtra Wear Resistance) steels with robotic MAG welding method. In this study, the effect of welding current intensity on microhardness in combining XAR 500 steel of 4 mm thickness used in heavy duty machine with MAG welding method was investigated. In the preparation of welded samples, MAG welding robot with speed, voltage and current control is used. Three different welding current densities were selected as welding current intensity 140A, 160A and 180A. Welding speed was kept constant at 350mm/min. In order to test the effect of the welding speed, the welding current was kept constant at 160A and at speeds of 300mm/min, 400mm/min and 450mm/min. MG-2 welding wire with 1mm thickness was used. Mixed gas with 86% Ar, 12% CO2 and 2% O2 was used. Microhardness obtained from welded joints were analyzed. As a results, it has been observed decrease of HAZ hardness on welded joints of this material groups. Therefore, was observed decrease of crack risk in HAZ of weldment when decreased HAZ hardness. The welding current which providing the optimum hardness ratio in the obtained welded joints was presented to the users.
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