Ferritik ve ostenitik çeliklerin karbonlu çelik ile nokta kaynağında kaynak parametrelerin bağlantının çekme-makaslama ve tanelerarası korozyona etkisi

Abstract

Elektrik direnç nokta kaynağı, sağladığı hafiflik, kaynak noktası başına düşük işçilik maliyeti, yeterli düzeyde kaynak dayanımı, uygulama kolaylığı ve yüksek kaynak hızı nedenleriyle otomotiv endüstrisinde, uçak inşaatında, büro ve mutfak eşyalarının imalatında yaygın olarak kullanılmaktadır. Özellikle son yıllarda, uzun ömürlü demiryolu taşıtlarının imaline yönelik projeler, farklı çeliklerin elektrik direnç nokta kaynağı ile birleştirilmesinin önemini bir defa daha gündeme getirmiştir. Konu ile ilgili literatür incelendiğinde, paslanmaz çelik-karbonlu çelik çiftinin elektrik direnç nokta kaynağı üzerine bir çalışma yapılmadığı saptanmıştır. Literatürdeki bu boşluğu doldurmak amacı ile, hazırlanan çelik çiftlerine ait nokta kaynağı bağlantılarında kaynak parametrelerinin bağlantının çekme-makaslama dayanımına etkileri saptanmış ve kaynak bölgesinin tanelerararsı korozyon davranışı incelenmiştir. Bu çalışmada, deney malzemesi olarak 1 mm kalınlığın da ferritik kromlu ve ostenitik krom-nikelli paslanmaz çelik saclarla, karbonlu çelik saclar kullanılmıştır. Ferritik kromlu paslanmaz çelik-karbonlu çelik; ostenitik krom-nikelli paslanmaz çelik-karbonlu çelik çiftleri, elektrot formu ve elektrot kuvveti sabit kalmak koşulu ve kaynak akım şiddeti ile periyodu değiştirilerek beş seri halinde elektrik direnç nokta kaynağı ile birleştirilmişlerdir. Bağlantının dayanım değerlerini saptamak amacı ile bütün seriler çekme-makaslama deneyine tabi tutulmuştur, ayrıca sertlik ve mikroyapı özellikleri ile çelik çiftlerinin temas dirençleri de araştırılmıştır. Çalışmanın sonucunda bu tür bağlantılar için uygun kaynak parametreleri seçilerek tatminkar sonuçlar elde edilebileceği görülmüştür

Despite their high cost, the main reason for the usage of stainless steels, which do not designate much more differences from other steels on the basis of mechanical properties, is that they have a great resistance to corrosion. Although this property is obtained with the presence of chromium as the main alloying element, it is enhanced with the addition of molibdenum and nickel (1). When the stainless steel contain chromium more than 12 %, their strength to the high temperature increases with the increase of chromium content. Because of this property, they have used as steels which have resistance to creep at high temperatures (1,2). In modern industries, their high strength to corrosion and oxidation for the working conditions in high temperatures the high mechanical and physical properties, the hot and cold formability and weldability and being unaffected from the corrosion and creep strength or mechanical properties of this property and their low cost are most important benefits of stainless steels (3). The electric resistance spot welding of ferritic and austenitic stainless and carbon steels, which are used, beginning from the 1900' s, especially for the carbody materials in automotive industry, for decorative aims in construction industry, as well as in aircraft, ship, chemistry, food and space industries have a great importance. In these industries, determination of the suitable welding parameters and the effect of these parameters on the tensile-shear strength of welded joints to obtain qualified welded joints are still important subjects for the constructions made with the same or different types of steel. Electric resistance spot welding offers many advantages to the manufacturers. Especially during last decade, the projects for the production of long-life railway wagons have emphasized the importance of joining of different types of steel with electric resistance spot welding once more (80). The usage of these steel pairs in automotive industry in Europe, USA and USSR have been the main factor for the ongoing scientific researches on spot welding of construc tions made with stainless steels and carbon steels. In this study, done for investigating the effects of welding parameters on the strength and corrosion properti es of the spot welded joints were investigated to determine the welding parameters. The ferritic chromium and austenitic chromium-nickel stainless steel and carbon steel sheets were used as the experiment materials. The ferritic chromium stainless steel-carbon steel and austenitic chromium-nickel stainless steel-carbon steel pairs were joined with the electric resistance spot welding keeping i^he electrode shape and the force constant while changing* the welding current and period. For each selected welding current and period, five samples were welded. For determination of the tensile- shear strength of the joints, the tensile-shear tests were applied to all samples. In addition, the hardness and the microstructure properties as well the contact resistance of joints were investigated. The ferritic chromium and austenitic chromium-nickel stainless steels used for this investigation are AISI 430 (X6Crl7) and AISI 304 (X5CrNil88) respectively, and the carbon steel are low carbon and unalloyed ones. After the experiment materials were cut in dimension of 1X30X100 mm, they were cleaned in ethanol of 96 %, and finished with a clean and cotton cloth for purifying the materials' surfaces from dirts, oils, etc. An electric resistance spot welding machine with single arm, equipped with pneumatic pressure and electronic time and current units was used for the experiments. The electrode force is measured with a manometer located on the bottom arm of machine, and the welding current was continuously controlled with a magnetic field measuring ampermeter. In the experiments, the electrodes which have a truncated cone shape with 6 mm of tip diameter were used. The electrode force was kept constant at 5 kN. The welding times were selected as 5, 15 and 25 periods, and the squeezing and holding times were kept constant at 25 periods for all the samples. The welding current was increased from 4.5 kA to 13.5 kA with the intervals of 1 kA. According to the planned experiment program, five pairs of joints were obtained for each condition. The tensile-shear tests and the micro Vickers tests were applied to the first and second groups respectively. The nugget diameters of the third group of the joints were measured with a measuring microscopy, and the microstructure photog raphs of the fourth group were taken. By examining the grain boundaries of the fifth group with the scanning electron microscopy, the chromium carbide precipitation were investigated. The best joints always exhibited regular, clean, columnar structures and showed only slightly indentation and no cavitation. To determine the hardness distribution of joints, the hardness measurings are done at the base metal, Heat Affected Zone (HAZ) and weld metal. To obtain the effect of welding currents on the nugget dimensions and tensile-shear strength of the welded samples, the nugget dimensions and their ratios were measured. The contact resistance of experiment pieces at different surface conditions were measured in line with the principles given at IIW/IIS Doc. 111-905-88, IIIG-126-88. The results obtained may be listed as follows: 1. The tensile-shear strength of the joint increases, as the welding time and the welding current increase. The maximum tensile-shear strengths were obtained at 25 periods of welding time and of 10.5 kA welding current. 2. Maximum tensile-shear strength values were obtained with the welding parameters for which xi the nugget formation has been completed and this was verified by the nugget dimensions and the dimension rates. 3. Since nugget formation is not completed at short welding times and at low welding currents, the tensile-shear strengths of the joints are not sufficient, and a rupture of separation types were observed. 4. Beginning from certain welding conditions (e.g. after 8.5-9.5 kA of welding currents), it was determined that the intercrystalline corrosion was formed at the electric spot welded ferritic chromium stainless steel-carbon steel sheets and that this formation effects the tensile-shear strength of the joints in opposite way. 5. Above 9.5 kA of current value, an excessive splash at the interface a decrease at the tensile- shear strength of joint was observed for both ferritic chromium stainless steel-carbon steel and austenitic chromium-nickel stainless steel-carbon steel pairs. 6. At very high current values, the formation of excessive indentations on the surfaces and some significant decreases at the quality and the tensile-shear strength of the joints due to the splash at outer surfaces of the sheets were observed. The beginning of splash at the outer surfaces for each group of steel pairs, was determined after 12.5 kA of current value. 7. Before tearing starts and the expulsion of metal occurs, so as to get a satisfactory welded spots, the proper welding current must be selected. 8. For the prevention of chromium carbide formation resulting in the intercrystalline corrosion, especially at the spot welding of the ferritic chromium stainless steel-carbon steel pairs, the stabilized types of ferritic chromium stainless steels are suggested. 9. The results of chemical analysis of the welding nugget, shows that the nugget is subjected to a chromium loss, and therefore it losses its xii stainless property. 10. For both steel pairs, it was observed that the contact resistance of steel sheets surfaces of which were chemically prepared and then brushed, is considerably high. This provides a significant advantage for welding operation.