%17 kromlu ferritik paslanmaz çeliklerin nokta kaynağında kaynak parametrelerinin tanelerarası korozyon ve çekme makaslama dayanımı üzerindeki etkisinin incelenmesi

Abstract

ÖZET Kromlu ferritik paslanmaz çelikler, uygulama alanında krom nikelli ostenitik çeliklerden sonra, %30'luk bir pay ile ikinci sırayı almaktadır. Yapılan çalışmalar, ferritik çeliklerin bazı korozyon koşullarında, ostenitik çelikler kadar dayanıklı, hatta klorlu ortamlarda gerilmeli çatlak korozyonuna karşı daha dayanıklı olduğunu ortaya koymuştur. Ostenitik paslanmaz çeliklerdeki tehlikeli selektif korozyon şekillerinden biri olan ve ani kırılma olaylarına yol açan taneler arası korozyon, ferritik paslanmaz çeliklerin kaynağında da söz konusu olmaktadır. Ferritik çeliklerin kaynağı sırasında yüksek sıcaklığın etkisinde çökelen krom karbürleri ve nitrürlerinin yanı sıra oluşan tane sınırı martenziti, tane irileşmesi, sigma fazı ve 475° C (temper) gevrekliğinin de çeliğin tanelerarası ko rozyon dayanımını olumsuz yönde etkilediği ileri sürülmektedir. Yapılan araştırmalar sonucunda, ostenitik paslanmaz çeliklerin nokta kaynağında krom karbürü çökelmesinin ve taneler arası korozyon olayının oluşmadığı belirlenmiş tir. Buna karşılık ferritik çeliklerin nokta kaynağında iç yapının özelliğinden ileri gelen çökelme olayının hızlı kinetiği nedeniyle taneler arası korozyon olasılığı artmak tadır. Çalışmada, % 17 kromlu ferritik çelikler için kaynak noktasına verilen ısı girdisi ile doğrudan doğruya bağlı bulunan temel kaynak parametreleri, akım şiddeti ve kaynak süresi arttırılarak ortaya çıkan krom karbürü çökelmesi, taneler arası korozyonun, noktanın çekme makaslama kuvvet davranışı üzerindeki etkisi vasıtasıyla araştırılmıştır. Ayrıca, kromca fakirleşen bölgelerde krom miktarını, korozyon dayanımı için gerekli kritik değerin üzerine yükseltecek sekonder krom difüzyonuna olanak veren, kaynak sonrası ısıl işlemin etkilerinin belirlenmesi amacıyla da bir seri deney yapılmıştır. Bu arada, mekanik deneylerde elde edilen bulgular, metalografik çalışmalarla desteklenmiş ve ısıl işlemin, noktanın sertliği ve çekirdek boyutlarının, çekme makaslama kuvveti davranışı üzerindeki etkileri de incelenmiştir. Çalışmanın sonucunda elde edilen bulgular, kaynak noktasının birim maliyetini arttırmasına rağmen ısıl işlemin, tanelerarası korozyon hassasiyetini gidermek için uygun bir çözüm yolu olmasının yanı sıra, noktanın temperlenmesiyle sertlik dağılımını olumlu yönde etkileyerek, mekanik özelliklerini iyileştirmesi nedeniyle, uygulanma sının faydalı bir işlem olduğu ortaya çıkmaktadır. - vii -

SUMMARY INVESTIGATION OF THE EFFECTS OF THE WELDING PARAMETERS UPON THE INTERGRANULAR CORROSION AND TENSILE SHEAR STRENGTHS IN THE SPOT WELDING OF 17 % CHROMIUM FERRITIC STAINLESS STEELS The ferritic stainless steels occupy, with a 30 % part in the stainless steels consumption, a second place after the austenitic steels, [21. The studies carried out until today have shown that the corrosion resistance of the ferritic steels is at least as great as that of the austenitics and even the resistance to inter and trans - granular stress cracking corrosion in a chloride containing mediums is as much great, [10]. The intergranular corrosion which is a kind of dangerous selective corrosion for the austenitic steels and gives way to a brittle fracture comes also, during the welding of the ferritic steels, to the fore. It has been asserted that beside the chromium carbide and nitride precipitation, the grain boundary martensite, the grain y growth, the sigma phase and the temper (475°C) brittleness, which occur under the influence of the heat of the welding, contribute to diminish the resistance to intergranular corrosion of the ferritic steels, [ 13, 19]. The datas of the carried out studies shown that the chromium carbide precipitation and the intergranular corrosion are not the case in the spot welding of the austenitic steels. The cause of the non - existence of the chromium carbide is the lower welding temperature (between the solidus and liquidus temperatures of the material) and the shorter welding time during the spot welding, in comparison with the fusion welding methos. Thus, the crossing time through the critical temperature interval is also shorter, because of the water cooling of the copper electrodes, which increases the cooling rate of the spot, [ 7, 8, 14, 18]. An intergranular corrosion during the spot welding of the ferritic stainless steels may be awaited because of - viii -a quick precipitation cinetics as a result of its structure. Although the welding time of the spot welding is shorter than that of the fusion welding, this time is enough for the precipitation of the chromium carbides, because the carbon atom dissolves less and diffuses quicker at the room temperature in a ferritic structure. On the other hand, the diffusion rate of the chromium atom is lower compared with that of the carbon atom and the diffusion time of the chromium is not sufficient to keep the 12 % Cr level necessary for the resistance to intergranular corrosion. Therefore the latter may appear at any time, during the spot welding of the ferritic stainless steels, [18, 19, 30, 37]. In this work, the existance of the chromium carbide during the spot welding of the 17 % chromium ferritic stainless steels will be studied by evaluating the influence of the intergranular corrosion upon the tensile shear strength of the spot under different welding parameters (welding current, welding time), it is to keep in mind that the amount of heat generated within the spot depends on these Q= 0.24 I2R`t (W/sec) H w t w./ where 0.24 (W/cal) is the conversion factor, I (A) the welding current, R (il) the total resistance of the weld circuit and t (sec) the welding time, [ 46, 47]. The heat balance in the spot can be shown as Q-i + Qo = Qo + Qa equivalance hence Q., is the heat amount, which is generated within the spot and Q2 is the heat amount, which is imported from the copper electrodes to the spot, [46, 48]. The heat amount, necessary to build up a nugget is termed Q~ and that which is dispersed within the material by conduction and through the environment by radition Q^ where Q, = 0.24 CLGT + C9G (cal) and Q4 = A TtA (cal) C, (J/gr°K) is the specific heat of the material, G(gr) the mass of the nugget, T (°K) the melting point, Cr,(cal/gr) the latent heat, A ( j/sec m K) the heat conductivity of the material, t (sec) the welding time and 1 (m) the total length of spot welded test piece, [46, 49]. p - ix -The total resistance R is the sum of the contact resistance (R ) and material's resistance (R ). *- in On the other hand, with the aim, a serie of tests has been carried out to determine the influence of the heat treatment which follows the welding and renders possible a secundary chromium diffusion through the impoverished zone to that the chromium content will be raised to that' critical value necessary for the resistance to corrosion, [ 10, 19, 24, 30, 32, 33, 35 ]. In the meantime the data of the mechanical tests are supported by metallographical studies. In the extend of this studies, the hardness and more specifically the hardness distribution has been investigated according to the Vickers method along a line paraleli to the horizontal axis of the spot and in the nugget and also in the HAZ. Together with the determination of the influences of the heat treatment upon the mechanical characteristics. Apart that, the influence of the current on the width h and diameter d and the ratio h /d of the nugget and the relation between the ratio h /dn and tensile shear strength of the spot have been studied. n Accordingly, the following test programme has been carried out: - After welding, the test pieces were attached to a tensile testing machine without any treatment in order to determine the optimum welding parameters for the ferritic stainless steels and the tensile shear strength with these parameters. - The spot welded pieces have been treated in a corrosive medium and broken off afterwards. - A secondary diffusion temper, boiling in the corrosive medium and the tensile shear strength test has been successively applied to the spot welded specimens. During the essays, the test pieces have been prepared from a X 6 Cr 17 steel sheet, with the dimensions of 30x100x1.5 mm. and spot welded with a 90 kVA electro - pneumatic actueted welding machine, [ 7, 8]. In order to ensure that the preparation of the specimens will correspond to the practical conditions, the pieces have not been subjected to any superficial treatment apart washing and rinsing. The overlapped test pieces have - x -been jointed with a single spot and a current of 5.500 to 15.500 A, and steps of 1.000 A, welding time of 5, 15, 25 and 50 periods and an electrode force of 5.0 kN, I 7, 8, 9). The welding has been executed with unalloyed copper electrodes having a 6 mm. diameter conical tip. The current has been determined with a Messer - Griesheim amperemeter (typ SM 12A) and the electrode force with a in situ, realised pressure gauge. The specimens have been subjected to an heat treatment for one hour at 800°C for the secondary chromium diffusion and cooled in a calm environment, [37]. The boiling in a corrosive medium has been carried out with a modified Strauss - Monypenny solution according to DIN 50914, [ 57, 58]. The hardness distribution in the spot has been determined with a Vickers hardness testing machine, using a diamond tip under loads for 1 kg and 2 kgs. The width h and the diameter d of the nugget has been measured with a microscope having a scaled oculer. In this study, the behaviour of the resistance spot welded 17 % Cr ferritic stainless steels has been investiged and the following conclusions have been reached: 1) Under welded conditions, the tensile shear strength of the spot has increased together with the welding time and current. Ultimate tensile shear strength has been obtanied during a 25.0 periods time, at a interval of 7.500 - 10.500 A. Thus, as with the aforesaid parameters. / the formation of the nugget is completed, its dimensions and their ratio confirm the results. 2) The tensile shear strength of the specimens subjected to intergranular corrosion shows a significant diminution compared with those which are not in a corrosive environment. 3) But, by increasing the welding time and the welding current of the pieces which are in a corrosive medium one gets, because of the high heat amount generated within the spot and therefore the prolonged cooling time, a chromium diffusion from the chromium rich zones towards the impoverished ones (as a secundary chromium diffusion) and a decrease in the intergranular corrosion sensitiveness. - xi -4) The specimens subjected to a heat treatment after the welding, the chromium balance in the chromium impoverished zones is got by a controlled secondary- diffusion and the susceptibility to intergranular corrosion is diminished. 5) In the specimens welded with a short welding time and low current, the nugget being not completely formed, the tensile shear strength for each test conditions remains insuffiscent and the separation occurs as a break off. 6) The welding parameters chosen to joint a 1.5 mm. thick 17 % chromium ferritic stainless material by spot welding are the values which give the highest tensile shear strength: a- The values of the tensile shear strength under the welding conditions, with a welding time of 25.0 periods and a current range of 7.500 - 10.500 A has a meaning only when the piece is not subjected to any corrosive media. b- Thus, these tensile shear strength values determined under the welding conditions, with a welding time of 25.0 periods and a current range of 7.500 - 10.500 A are obtanied for the most favorable nugget dimension ratios. c- For the specimens subjected to corrosive media after the heat treatment, the ultimate tensile shear strength values are obtained with 7.500 A and 8.500 A for 25.0 periods. Although that the heat treatment practised after the welding increases the cost of the spot welding (manufacturing cost), besides being the most safe way to eliminate the sensivity to the intergranular corrosion, it stabilizes the distribution of the hardness by diminushing it and ameliorates the mechanical conditions. As a conclusion, it will be suitable to pratice a heat treatment of a duration and temperature stated after the welding to the pieces of 17 % chromium ferritic stainless steel jointed with resistance spot welding and which will be used in a corrosive environment. - xn-