Ferritik-ostenitik paslanmaz çelik çiftinin nokta kaynağında kaynak parametrelerinin çekme-makaslama mukavemetine ve tanelerarası korozyona etkisi

Abstract

ÖZET Günümüzde taşıt endüstrisinde, özellikle demiryolu va - gonlarında ve otomobillerde kaporta malzemesi olarak kul lanılan paslanmaz çelik sacların nokta kaynağı ile birleş - tirilmesi büyük önem taşımaktadır. Paslanmaz çeliklerin ferritik ve ostenitik türlerinin karşı karşıya geldiği bu tür konstrüksiyonlarda nokta kaynağı parametrelerinin bağ lantının çekme-makaslama mukavemeti üzerine etkisine yöne lik bir araştırmanın varlığı saptanamamıştır. Literatürdeki bu boşluğu doldurabilmek amacıyla bu çalışmada, ferritik - ostenitik paslanmaz çelik çiftine ait nokta kaynağı bağlan tılarında kaynak parametrelerinin mukavemete olan etkileri saptanmış ve kaynak bölgesinin korozyon davranışı incelen - mistir. Bu çalışmada, deney malzemesi olarak 1 mm. kalınlığın da X 8Cr 17 ferritik ve Xl2CrNi 18 8 ostenitik paslanmaz çelik saclar kullanılmıştır. Nokta kaynağı parametrelerin den elek trod kuvveti 500 daN., ön ve son tutma süreleri de 25 periyod olarak sabit tutulmuşlardır. Isı girdisinin et kisini incelemek amacıyla kaynak akımı 4,5 - 13,5 kA ara - sında arttırılarak değiştirilmiş, kaynak süreleri ise 5,15, 25 periyod olarak seçilmiştir. Karşılaştırma yapabilmek a- macıyla, ferritik ve ostenitik paslanmaz saclardan da aynı koşullarda deney parçaları hazırlanmıştır. Ferritik -osteni tik çifte ait deney parçalarının yarısı kaynaktan çıktığı gibi, diğer yarısı da Strauss korozyon deneyine tabi tutul duktan sonra çekme-makaslama mukavemet değerleri saptanmış tır. Tüm serilerde iç yapı incelenmiş ayrıca çekirdek ke- sitindeki sertlik değişimleri de saptanmıştır. Korozyon deneyi uygulanmamış ferritik-ostenitik çifte ait bağlantıların mukavemet değerlerinin ferritik-ferritik bağlantılardan daha yüksek ve ostenitik-ostenitik bağlantı lardan daha düşük değerlerde olduğu görülmüştür. Kaynak sü resinin 15 periyod olarak seçildiği deney süresinde koroz yon deneyi uygulanmamış parçalarda en yüksek mukavemet de ğeri 8,5 kA akım şiddetinde, korozyon deneyi uygulanmış deney parçalarında ise en yüksek mukavemet değeri 6 kA'de elde edilmiştir. Bu seride korozyon deneyi, mukavemet de ğerlerinde % 50'ye varan azalmalar ortaya çıkartmıştır. Kaynak süresi 25 periyod olan deney serisinde krom karbür çökelmesinin 8,5 kA'den daha yüksek akım şiddetle rinde ortaya çıktığı görülmüştür. Bu seride korozyon de - neyi uygulanmış parçalarla, uygulanmamış parçalar arasın - daki fark 15 periyodluktan daha azdır. Bu çalışmanın sonucunda ferritik-ostenitik sacların nokta kaynağı bağlantılarında korozif ortam koşullarında dahi uygun parametreler seçilerek tatminkâr sonuçlar elde edilebileceği görülmüştür. vi

SUMMARY THE EFFECT OF WELDING PARAMETERS TO THE TENSILE-SHEAR STRENGTH AND INTERCRYSTALLINE CORROSION IN THE SPOT WELDING OF FERRITIC-AUSTENITIC STAINLESS STEEL Stainless steels are those alloy steels that have a normal chromium content of at least 12 percent with or without other alloy additions. The stainlessness and corrosion resistance of these alloy steels are attributed to the presence of a passive oxide film on the surface. When exposed to conditions that remove the passive oxide film, stainless steels are subject to corrosive attack. The rate at which a stainless steel develops a passive film in the atmosfere depends on its chromium content [ 1,2, 16 J. Stainless steels are commonly divided into the following general groups: (1) Chromium Martensitic (2) Chromium Fer r i tic (3) Aus teni tic (4) Precipitation-hardening The first three groups are characterized by the predominant metallurgical phase present when the stainless steel is placed in service. The fourth group contains those stainless steels that can be strengthened by an aging heat treatment. At present, fixing the ferritic and austenitic types of stainless steels by means of electric resistance spot welding is of great importance in the field of chemistry, food and automotive industry and aeronautics. Stainless steels are preferable because of its resistance to oxida tion, sufficient mechanical properties and high resistance to corrosion in extremely high working temperatures. In the last years, stainless steels are widely used in the manufacture of kitchenware, in the field of medicine and architectural designs. As it is widely used in various fields, scientific researches have been held to improve the spot welding of the best quality. In USA, after it was understood that viistainless steel could be used in the manufacture of airplanes and wagons, researches of all kinds have been supported [ 4,5]. Furthermore, it is also known that in the American automotive industry; and in the production of wagons and automobiles in Europe, especially in Germany; the use of stainless steel sheets have been increasing 5. All of these developments gave rise to studies on the spot welding of stainless steels. The welding current, welding time, electrod force and electrod form are the basic parameters of the electric resistance spot welding. Many researchers were made especially on the effect of welding parameters on tensile- shear strength and corrosion behaviours of the junctions of austenitic chromium- nickel stainless steel of spot welding [5,31,34,35,36,37,38,39 ]. Limited research was made on the certain types of ferritic chromium stainless steel [27,3l]. Researches have been concentrated on the Long-Life of automobiles so as to find a solution to the problem of corrosion in the automotive industry and to minimize to economical factors of poduction [6,7]. While examining the literature on this subject, it was seen that research is being made on the proto-types of automobiles where stainless steels are used for the first time in European automotive factories. In all the applications until the present time, spot welding of austenitic stainless steels are widely used. However, as f erritic stainless steels are cheaper and nickel is rather a strategical material, and also the bright and decorative aspect of ferritic types made it possible to be used widely. But, due to the economical factors, partly ferritic and partly austenitic contructions are to be considered recently. While examining the literature on this subject, no research has been found on the spot welding of ferritic- austenitic stainless steel. In order to fill this gap, it was thought helpful to make a study on the behaviour of ferritic- austenitic stainless steel under spot welding. Mainly, the welding current though the electric resistance spot welding parameters; and the effect of welding time on the tens ile- shear strength of the junction are studied. And also the effect of parameters on intercrystalline corrosion is studied. In this investigation, as test material stainless steel sheet 1 mm. thick X8 Cr 17 ferritic and X12 CrNi 18 8 austenitic stainless steels were used. A squaring sheare machine is used for shearing test pieces from stainless vixisteel sheets of 1 x 30 x 100 mm. Test pieces were cleaned with metil alcool and then rubbed with a piece of soft cloth. In determining the dimensions of the experiments, factors such as removing the effect on sides; making the fracture occur in the welding region during the tensile- shear test; minimizing the consumption of the materials were considered carefully. Pieces in the experiment were put together 30 mm. and spot-welded just at the middle. Electric current and time controlled and pneumatically pressures resistance spot welding machine which had a power of 120 kVA was used in the experiment. Electrode force was determined by measuring the manome ter which was placed between electrodes. The values of welding current were measured with an ampermeter which was installed at the bottom arm of the machine. This ampermeter was capable of measuring magnetic field and of a wide field. The time of spot welding, squeeze and hold time were determined with an electronic equipment on the machine. Pure copper electrodes which had cut- conic pressing surfaces with a diameter of 6 mm. were used in the experiment. With respect to the development of the experiment, first ferritic-ferritic stainless steel pairs, then austenitic- austenitic stainless steel pairs, were welded. As a result of the preliminary studies, the electrode power was controlled at a constant 500 daN. Welding times were changed as 5, 15 and 25 periods. Squeezing and holding times were constant and given as 25 periods. Welding current was made to start 4,5 kA and increased to every other 1 kA and finished at 13,5 kA. For each conditions, at least six pairs of pieces were welded together. And there of these pairs were also made tensile-shear tests. Strauss Corrosion Test was also applied to the ferritic-austenitic stainless steel pairs and after this test, they were applied the tensile-shear test. Macroscopical, microscopical and hardness controls were made to the nugget. Tensile speed was taken as constant and was 7 mm per minute. Three different types of breaking were observed. These were seperation breaking, breaking as buttoning and breaking in the style of tearing as seen in ferritic-auste nitic stainless steel pair. ixIn the corrosion test (TS 3157), Strauss Corrosion Solution was used. The prepation of the test solutions was used. The prepation of the test solutions was as follows: First 115 gr. CuSo4. 5 H`0 made dissolved in 700 ml. of pure water. Then 115 ml.H2So4 (p= 1,84 gr/ml.) was added. After adding 57,5 gr- of electrolitic copper chips, the solution was increased to 1150 ml. by pure water. The pieces were boiled in this solution for 15 hours. Corrosion test was not applied and the ferritic-austenitic juctions which this test was applied was not only given a tensile-shear test but also examined metalografically. Adler solution was used. Later, Vickers Hardness Test was also applied to the weld nugget. In this work, the strength and corrosion effect of electric resistance spot welding parameters in ferritic- austenitic stainless steels were examinated. And in this research which was made to determine the suitable spot welding parameters, the results can be summarized as follows: 1) In fixing 1 mm. thick X8Crl7 ferritic stainless steel sheets; by using 25 periods of welding time and when the currend is chosen between 8,5-9,5 kA, juctions are supplied with a high strength. 2) In the spot welding of X12CrNi 18 8 austenitic stainless steel sheets; under the same welding conditions, juctions of high strength are also avaliable (welding time 25 periods and the current is 8,5 kA).. 3) In the electric resistance spot welding of juction of ferritic-austenitic stainless steel pair, the welding time should be 8,5 kA if the pieces would not be in an acidic contact. 4) The tensile-shear strength of the juction of ferritic-austenitic stainless steel pair is said to be medium compared with the tensile-shear strength of only the ferritic or only the austenitic stainless steel. 5) Intercrystalline corrosion was determined to occur above certain welding conditions in ferritic-austenitic stainless steels. And also it was determined that this affected the strength of junctions in an undesirable way. 6) The values of strength in the juctions of spot welding of ferritic-austenitic stainless steel in 25 periods are said to be medium with regard to the juctions of 5 and 15 periods. But in an acidic conditions, 25 periods of welding time and a current in a range of 7,5-8,5 kA should be chosen.7) In the spot welded junctions of ferritic-austenitic stainless steel, 9,5 kA should not exceeded. Because in high currents, the strength would decreases by means of excessive expulsion. Even in higher values, deep electrode traces and expulsion occur on the above surface of the sheet. These conditions would destroy the quality of the juction. The above given values of currents must not be exceeded. 8) In the contructions where electric resistance spot welding is used to fix ferritic-austenitic stainless steels; it would be better to use ferritic stainless steel sheets (X8 CrNbl7) which are staibilized with Nb to prevent the existence of chrome carbide which causes intercrystalline corrosion. XI