Çeliklerde dövülebilirliğin burma, basma ve çekme deneyleri ile incelenmesi

Abstract

Bir dövme işlemini hasarsız gerçekleştirebilmek için şekil verilmek istenen malzemenin şekillendirilebilme kabiliyetinin bilinmesi istenir. Bu nedenle, dövme işlemlerinde malzemeyi hasara uğratmadan istenen şekli verebilmek için " Dövülebilirlik " terimi tanımlanmıştır. Dövülebilirliğin ölçülmesi amacıyla çeşitli deneyler ve sünek kırılma kriterleri geliştirilmiştir. Hiçbir deney yöntemi ve kırılma kriteri tek basma dövülebilirliği ifade etmek için yeterli görülmemektedir. Bu çalışmada, dövmenin tanımı yapılarak dövme yöntemleri hakkında kısa bilgiler verilmiş ve dövülebilirliğin tanımı yapılarak dövülebilirliği etkileyen değişkenler ve dövülebilirliği saptamada kullanılan deneyler açıklanmıştır. Deneysel çalışmalarla, Vickers sertlik deneyleri, basit çekme deneyleri, burma deneyleri, dört farklı sürtünme koşulunda dolu silindir basma deneyleri, iki farklı sürtünme koşulunda halkalı dolu silindir basma deneyleri ve üç farklı çentik yarıçapına sahip çentikli çekme deneyleri AISI 1040, 1050, 4140 ve 5140 çeliklerinin sıcak haddelenmiş, normalize edilmiş ve küreleştirilmiş durumlarına uygulanarak dövülebilirliğin saptanması için uygun deney yöntemleri araştırılmıştır. Basit çekme deneylerinden akma sının, çekme dayanımı, %uzama ve %kesit daralması değerleri elde edilerek dövülebilirlik incelenmiştir. Burma deneylerinden kopmaya yol açan devir sayısı ve moment ölçülerek elde edilen değerlerden efektif birim şekil değiştirme ve kayma gerilmesi değerleri hesaplanmıştır. Kopmaya yol açan devir sayılarının kıyaslanmasıyla dövülebilirlik sıralaması yapılmıştır. Dolu silindir basma ve halkalı dolu silindir basma deneylerinde elde edilen değerlerden, yükseklik azalması ile gerilme elemanları efektif gerilme oranlarının değişimi, çevresel ve eksenel birim şekil değiştirme değişimi ve şekillendirme sınır diyagramları kullanılarak dövülebilirlik araştırılmıştır. Çentikli çekme deneylerinden, deney süresince kesit daralması ölçülerek kesit daralması değişimi ile gerilme elemanları efektif gerilme oranlan değişimi diyagramları elde edilmiş ve bunlarla dövülebilirlik araştırılmıştır. Sonuç olarak, burmada kopmaya yol açan devir sayısı, basma ve halkalı basmada yükseklik azalması ve dövülebilirlik endisi ve basma, halkalı basma, çentikli çekme kopma sonuçlarını birlikte değerlendiren dövülebilirlik eğrileri ile dövülebilirlik kriterleri karşılaştırılmıştır. Çentikli çekme ve basma deneylerinin dövülebilirlik eğrileri ile incelenmesi dövülebilirliğin saptanması için ideal uygulama kabul edilmiştir.

The manufacture of many engineering components involves the deformation of metals and alloys. Deformation processes are relatively simple and yield products with good mechanical properties. Traditionally, the feasibility of a forming process was determined to a great extent by trial and error. Even today, many concerns rely on experience and operator skill for the successful manufacture of components. This approach often leads to the production of defective components that have to be scrapped, and time is lost while tools are modified or redesigned. Recent research has shown that forgeability tests can be useful in predicting the behavior of a workpiece under industrial conditions. Thus, the feasibility of a process can be determined in the laboratory at relatively low cost. Many forgeability tests have been used by various investigators whose choice of test appears to be governed by the availability of equipment and the nature of the forming process under consideration. This work involves tests of hardness, tension, torsion, compression, collar and notched tension and examination of these test results. Forgeability Forgeability can be defined as the amount of deformation that a material will withstand without fracture in the forging processes. It is not a unique property of the material, but depends on such process variables as the stress system, strain rate, temperature, and friction conditions can be ductile under hydrostatic pressure. The concept of forgeability can be expressed by the following relationship. Forgeability = ft (material) x f2 (process) where fi is a function of the basic ductility of the material, and f2 is a function of the stress and strain system imposed by the process. Techniques for Testing Forgeability Metal forgeability can be assessed by the use of different techniques such as tension, torsion, bending, and compression tests. The geometry of the test specimen and the test conditions can be varied to provide a wide range of stress and strain states. The investigator can then use the results of such tests to predict the conditions under which fracture will occur. XXV The Tension Test The tension test is widely used to determine the mechanical properties of a material. The uniform elongation of the specimen, its total elongation, and especially the reduction in area at fracture can be used as indices of ductility. However, the amount of deformation possible in a tensile test is limited by necking, which introduces a triaxial stress and leads to failure. The Torsion Test In the torsion test, deformation is caused by pure shear, and large strains are achieved without the limitations imposed by plastic instability. Since the strain rate is proportional to the rotational speed, high strain rates are obtained readily. There is no friction between the deforming specimen and the tools as there is in the compression test. One of the limitations of the torsion test is that the strain rate varies linearly along the radius of the specimen and is at a maximum at the surface. This difficulty can be partly overcome by the use of tubular specimen in which the strain rate is nearly uniform. Although the stress states in the torsion test may represent those of metal- working processes, the deformation is not an accurate simulation of that encountered in industrial processes. This is probably the greatest limitation of the torsion test. The Bend Test The bend test can be used in the assessment of metal forgeability, especially when the geometry of the workpiece is not suitable for compression tests. The stress and strain states on the outer surface of the specimen are similar to those obtained in a compression test and can be varied by alteration of the width-to thickness ratio of the specimens. The Compression Test Compression tests invariably involve the axial compression of cylindrical specimens. The average stress state is similar to that in most bulk deformation processes, but problems due to necking and material reorientation do not arise, and a large degree of deformation can be obtained before cracking occurs. Barreling of the cylindrical surface of a specimen furnishes considerable flexibility for testing of its forgeability by causing failure of the material after an amount of deformation that is characteristic of the forgeability of that material. Friction between the tools and the specimen and Barreling can be controlled by the use of lubricants and, thus, the behavior of the material can be assessed after varying amounts of deformation. Experimental Procedure Materials AISI 1040, 1050, 4140 and 5140 steels in the hot rolled, normalized and spheroidized condition were used in the present investigation. The composition of xxvi the steels tested are given in Table 1. Spheroidized conditions are given in Table 2. Normalization temperatures are given in Table 3. Table 1. Chemical composition of the steels. Table 2. Spheroidized conditions of the steels. Table 3. Normalization temperatures of the steels. Mechanical Tests Hardness measurements were used by Vickers method. Forgeability was determined from of the reduction in area and elongation which were taken from tensile testing. Number of turn until fracture and moment at the fracture were measurement by torsion testing, then forgeability was investigated from these vahies. For the compression tests, cylindrical specimens having a diameter of 10 mm and heights of 15 mm, were turned. Critical reductions leading to fracture were measured by friction conditions. For the determination of the forgeability diagrams, a grid of 1.27 mm square was etched on the free surface of the specimens by permanent pencil. Collar tests were performed by smooth dies and grooved dies. Forgeability diagrams were settled by changing collar thickness and diameter which were 3 mm and 15 mm at the beginning. Notched tension tests for investigation of effect of stress-triaxialhy on the ductile tearing and the cleavage fracture of steels were completed by using specimens having radius of 2 mm, 5 mm, and 10 mm. Experimental Results The results of the tensile tests are given in Table 4. Steels in spheroidized conditions had very high values for the reduction in area and elongation. xxvu The results of the torsion tests are given in Table 5. Steels AISI 4140 and 5 140 had very high and AISI 1040 and 1050 relatively low values for number of turn until fracture. Table 4. Tensile testing results. Table 5. Torsion testing results. Forgeability was studied by the compression tests including the equatorial strains and stresses versus height reduction curves and formability limit diagrams. These curves are illustrated in the figures 1-2-3 XXVUl ee * Smooth -- Grooved -^Teflon - SAE 50 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 (ho-h)/ho Fig. 1 Strain paths for four conditions of lubrication : Spheroidized AISI 4140 steel. O"e/o"o O"m/o"o * Smooth -~ Grooved -«-Teflon.*? SAE 50 oz/ao 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 (ho-h)/ho Fig. 2 Stress paths for four conditions of lubrication : Spheroidized AISI 4140 steel. XXIX 6e 0.8 Homogeneous upsetting - Smooth ?+- Grooved ^Teflon - SAE 50 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Fig. 3 Formability limit diagram : Spheroidized AISI 4140 steel. Forgeability was studied by the collar tests including the collar strains and stresses versus height reduction curves and formability limit diagrams. These curves are illustrated in the figures 4-5. 0.35 ee Fig. 4 Strain paths for two conditions of lubrication : Normalized AISI 5140 steels. XXX 1.2 0.8 0.6 0.4,e 0.2 9 am Oe/ao -? m- t '. Om/Oo 0"z/ Go - Smooth -*- Grooved 0.2 0.4 0.6 (ho-h)/ho 0.8 Fig. 5 Relation between reduction of height and axial and mean stress values : Normalized AISI 5140 steel. In the notched tension tests, reduction of notched area was measured. Changing curves between axial and mean stress - notched area were taken from these values (Fig. 6) 2.5 1.5 0.5 - r=2 -x-r=5 -*-r=10 Fig. 6 Axial and mean stress values calculated by analytical model in the minimum sross section of cylindrical notched specimens : Normalized AISI 1050 steels. XXXI Conclusions Correlation of fracture values among compression, collar and notched tension tests is given in Figure 7. ? Smooth * Teflon ? SAE 50 ? Grooved ? CSmooth x CGrooved ? r=2 ta r=5 A r=10 Fig. 7 Forgeability diagram of fracture values for all materials and every heat treatment condition. In the experimental investigation of this work, tests mentioned before was practiced to different steels and their different internal structures which found out by heat treatment and materials are classified according to their forgeability. For this aim, forgeability curves which give changing between endure and (am/oo)fiactare were used. These curves are very fitting for a lot of industrial forming like forging, upsetting, extrusion, and drawing Classification of forgeability in this work clearly shows that there are very close relation between forgeability and type of stress state. Therefore, we can certainly say that notched tension and compression tests are ideal for estimation of forgeability.