Tikso Dövme Proses Parametrelerinin Çeliklerin Mekanik ve Mikroyapisal Özelliklerine Etkisinin İncelenmesi

Abstract

Bu doktora tez çalışmasının amacı yarı katı faz dönüşüm sıcaklıklarında çeliklerin tikso dövme ile şekillendirilmesi için gerekli parametrelerin incelenmesi ve en uygun parametrelerde endüstriyel bir prototip ürün üretilmesini sağlamaktır. Çeliklerde tikso dövme ile ilgili günümüze kadar laboratuvar ölçekli çok sayıda çalışma yapılmış olması sebebiyle, öncelikle tikso dövme için gerekli koşullar belirlenmiş ve başarılı olan tüm çalışmalarda belirtilen parametreler üzerinde iyileştirmeler yapılmıştır. Yapılan çalışmaların en önemli iki özelliği, tikso sıcaklık aralığı geniş metallerin kullanılması ve ısıtma ekipmanı olarak indüksiyon sisteminin tercih edilmesidir. İndüksiyon sistemi, özellikle metal ve alaşımlarında daha hızlı ve stabil bir ısıtma sağladığı için bu sistem tercih edilmektedir. Bu çalışmanın özgün yanı, hali hazırda ticari olarak kullanılan bir çelik kalitesinde (C70S6) tikso dövme çalışmasının yapılmış olması ve bu çelik kullanılarak üretilen biyel kolunun endüstriyel tikso dövme proses parametrelerin belirlenmiş olmasıdır. Yapılan bu çalışmada otomotiv sektöründe en çok kullanılan C70S6 ve 100Cr6 mikro alaşımlı çelikler tercih edilmiştir. Bu çeliklerin diğer bir özelliği ise, yarı katı faz aralığı geniş olması sebebiyle tikso dövmeye uygun çelikler olmasıdır. Özellikle 100Cr6 yarı katı şekillendirme çalışmalarında en çok tercih edilen mikro alaşımlı çelik türüdür. C70S6 ise 100Cr6'ya göre yarı katı faz aralığı dar olmasına rağmen, otomotiv sektöründe en çok kullanılan mikro alaşımlı çeliktir. Yapılan bu çalışmada öncelikle, mevcut bilimsel çalışmalar değerlendirlmiş ve yarı katı şekillendime çalışmalarının bilimsel ve teknik anlamda ortak noktaları belirlenmiştir. Bu sayede kullanılacak yöntemler ve teçhizatlar konusunda araştırmalar yapılmıştır. Deneysel çalışmaların başarılı olabilmesi için öncelikle proses gereksinimleri tanımlanmıştır. Bu sebeple, ısıtma düzeneğinin en önemli unsur olduğu farkedilmiştir. Yapılan mevcut çalışmalarda farklı indüksiyon sistemleri kullanıldığından, mevcut çelik türlerinin yarı kat faz aralığına doğru şekilde ısıtılması için farklı tip indüksiyon cihazlarında denemeler yapılmıştır. Bu amaçla öncelikle levha tip indüksiyon cihazı kullanılmış ve proses sıcaklığı belirlenerek elde edilen mikroyapılar incelenmiştir. Daha sonra ısıtma rejiminin avantajı kullanılarak yatay indüksiyon fırını kullanılmasına karar verilmiştir. Ancak yatay indüksiyon cihazında, yüksek sıcaklıklarda numunelerin transferinde yaşanan zorluklar ve sıcaklık ve ısıtma süresinin kontrollü olmaması sebebiyle sistem geliştirilerek dikey tip indüksiyon cihazı kullanılmasına karar verilmiştir. Dikey indüksiyon sisteminde her iki çelik için yarı katı faz aralığı belirlenerek, bu faz aralığında seçilen sıcaklıklara kadar numuneler ısıtılmış ve su verilerek elde edilen mikroyapı farklılıkları incelenmiştir. Her iki çelik için farklı tikso dövme sıcaklıkları belirlendikten sonra, otomotiv sektöründe biyel kollarında kullanılan C70S6 mikro alaşımlı çeliğinin belirlenen parametrelerle tikso dövme ile şekillendirilmesi sağlanmıştır. Farklı sıcaklıklarda yapılan denemeler sonrasında, kapalı kalıp sisteminde tikso dövme işlemi ile biyel kolu prototipi üretilmiştir. Elde edilen sonuçlar incelenerek, proses parametreleri geliştirilmiş ve iyileştirme çalışmaları üzerinde durulmuştur. Optimum parametreler ile elde edilen biyel kolu prototipinde çeşitli mekanik deneyler yapılmış ve sonuçlar geleneksel sıcak dövme ile üretilen biyel kolu numuneleri ile karşılaştırılmıştır. Yapılan çalışmalar sonucunda tikso dövme ile üretilen biyel kolunun mekanik özelliklerinin, geleneksel dövme ile üretilen biyel kolunun mekanik özelliklerini yeterli oranda karşıladığı görülmüştür. Tüm bu çalışmalar TÜBİTAK TEYDEB (Proje No: 3130030) projesi kapsamında gerçekleştirilmiş ve başarılı şekilde sonuçlandırılmıştır.

The purpose of this doctoral thesis study is to examine the thixoforging parameters of steels at semi solid phase transformation temperatures and to produce an industrial prototype product with the most suitable parameters. Since there are many laboratory scale thixoforging studies for various steels in the literature, the required parameters for the thixoforging have been determined and improvements have been made on the process parameters specified in all these successful studies. The two most important characteristics of the subjected thixoforging studies are the use of metals which have wide thixo temperature range and the preference of the induction heating system as an equipment. The novelty of this study arises from a commercial grade steel (C70S6) was employed and industrial thixoforging process parameters of a prototype connecting rod made of this steel was determined. In this study, the most commonly used C70S6 and 100Cr6 micro alloyed steels which are also preferred in the automotive industry have been preffered for our trials. Another feature of these steels is that they are suitable for thixoforging applications due to their wide range of semi-solid phase. In particular, 100Cr6 is the most preferred type of micro alloyed steel in semi-solid forging works and C70S6 is the most widely used micro alloy steeled in the automotive industry, although the semisolid phase range is narrow. Based on this reason, limited laboratory scale trials was performed by using this type of steel until now. In this study, firstly, the existing scientific studies were evaluated and the common points of the semi-solid shaping studies in scientific and technical terms were determined. In this way, researches have been made on the methods and equipments to be used. During the study, firstly, semi-solid phase intervals were determined for both steels and the samples were heated up to the selected temperatures in this range and the microstructure differences obtained by water quenched were investigated. Both, numerical and theorical ways were used for identifiying semi solid intervals and phase diagrams for both steels. Especially, ThermoCALC software was used for determining semi solid temperatures and intervals, then trials in different induction devices helped us for confirming all outputs of ThermoCALC software. In order for the experimental studies to be successful, the process requirements are defined first. Therefore, it has been recognized that the heating device is the most important element. Since different induction systems are used in the current studies, different types of induction devices have been tested for the proper heating of the existing steel alloys to determine semi solid phase range. For this purpose, plate induction device was used firstly and microstructures obtained by determining process temperature were examined. This plate induction device is already used in forged hand tools production line for hardening of tool surfaces. Plate induction device helped us for first trials of thixoforging temperatures which we want to reach out. Thanks to this device, the effects of the temperatures mentioned in the literature were confirmed on the microstructure of the steels. It was then decided to use the horizontal induction furnace using the advantage of the heating regime.In addition, homogenous heating can be obtained by horizontal induction furnace by using different type of thermocouples. Due to some issues on heating control inputs and transfering problem between furnace and die tools, vertical induction furnace were designed and further trials were performed in this developed system. By using new vertical induction system, different thixoforging temperatures were confirmed which was already mentioned by ThermoCALC results previously. After experimential studies at elevated temperatures, the prototype of the connecting rod was produced by thixoforging method in the closed die system. For this purpose, 1/7 scales of commercial connecting rod was designed for trials and thixoforging was done within selected temperatures. In order to manufacture the connecting rods by thixoforgingprocess, a fully closed die assembly was designed.In this design, the sample workpiece is first placed on the punch of the lower die. After closing the upper and lower dies, lower punch is moved upward by a mechanical press with a capacity of 400 tons into the closed die cavity to form the billet. The connecting rod was ejected from the die and cooled down to room temperature in air. After determining different thixoforging temperatures for both steels, it was ensured that C70S6 micro alloyed steel used for connecting rods in automotive industry was shaped by thixoforging with selected heating parameters in the range of 1400-1460°C During all experiments, die temperature was controlled in the range of 100 and 150°C. At the temperature of 1400°C, the die could not be completely filled by the material and only the big end of the connecting rod could be formed. Increasing the forging temperature by about 20°C (1420°C), although a larger volume of the die could be filled by the material, it was still insufficient for obtaining the connecting rod in its final form and some materials resulted in flash formation around the big end. Thixoforging temperature of 1440°C exhibited almost complete formation of the big end without any flash formation. At this temperature, however, the small end of the connecting rod could not be properly obtained, and a fold formation in the big end toward the beam section of the connecting rod was also observed. Further increasing the forging temperature to 1460°C to see the effect of much higher temperature, although provided material flow in some extent toward the small end, the obtained shape was far from the desired geometry of the connecting rod and the flash formation around the big end was reappeared. According to the results of thixoforging experiments performed in this study, the samples thixoforged at 1440°C resulted in better forms of the connecting rod among the others. In those samples, liquid volume fraction was estimated around 0.35. On the other hand, those samples still contain some imperfections such as incomplete formation of the small end and fold formation in the big end toward the beam section of the connecting rod, both of which need to be corrected. The induction heating device and the mechanical press were located very close to each other to minimize the temperature drop as much. By analyzing the obtained results, optimum process temperature was determined due to flash ratio and visual inspection results about surface quality. Then, process parameters were developed and improvement studies were emphasized for new trials. Thixoforging experiments were repeated by using the modified die design, and it was shown that these imperfections on the connecting rod could be eliminated. Various mechanical tests and microstuctural examinations were performed on the connecting rod prototype obtained by optimum parameters and the results were compared with the connecting rod samples produced by conventional hot forging. For the microstructural examinations, samples were prepared in the standard metallographic procedure and examined by optical microscope and scanning electron microscope after etching. A scanning electron microscope equipped with EDS was also used for semiquantitative identification of the second-phase particles in the microstructure of as-quenched samples. Prior austenite grain size was determined according to the lineal intercept method, while volume fraction was estimated by the point counting method. Interlamellar spacing of pearlite was calculated as the mean true spacing in accordance with the procedure proposed. Hardness was measured under a load of 10 kg. At least five measurements were taken on the surface of each sample, and the results were averaged. Tensile test samples which were machined from the thixoforged connecting rods were subjected to tensile test at a crosshead speed of 2 mm/min. Samples which were produced by conventional hot forging at 1280°C were also subjected to tensile test for comparison. Following the tensile tests, fracture surface of the samples was examined by a scanning electron microscope. In order to check the thixoforged connecting rods against any defects arising from the forming process and die design, finite element analysis was performed by a finite element analysis software. As a result of this study, the mechanical properties of the thixoforged connecting rod meet the mechanical properties of conventional hot forging. Thixoforging of C70S6 steel has been assessed by using a scaled-down model of a commercial vehicle connecting rod. The current work has demonstrated that C70S6-grade steel can be successfully thixoforged at 1440°C in the form of scaled-down connecting rod without excessive flash formation. Mechanical test results showed that hardness and strength values after thixoforging can be maintained at almost similar levels with those presented by conventional hot forging. However ductility is significantly decreased due to grain boundary ferrite network upon thixoforging. This results tell us for the success of this study and also means thixoforging may used for different type of forged parts instead of conventional hot forging for several industrial application. All of these studies were carried out within the scope of TÜBİTAK TEYDEB project (Project No: 3130030) and were successfully completed.