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Inclusion Characteristics and Their Link to Tool Wear in Metal Cutting of Clean Steels Suitable for Automotive Applications

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Inclusion Characteristics and Their Link to Tool Wear in Metal Cutting of Clean Steels Suitable for Automotive Applications Inclusion Characteristics and Their Link to Tool Wear in Metal Cutting of Clean Steels Suitable for Automotive Applications Niclas Ånmark Licentiate Thesis Stockholm 2015 Division of Applied Process Metallurgy Department of Materials Science and Engineering School of Industrial Engineering and Management KTH Royal Institute of Technology SE-100 44 Stockholm, Sweden Akademisk avhandling som med tillstånd av Kungliga Tekniska Högskolan i Stockholm, framlägges för offentlig granskning för avläggande av Teknologie Licentiatexamen, tisdagen den 5 maj 2015, kl. 10.00 i Sefströmsalen, M131, Brinellvägen 23, Kungliga Tekniska Högskolan, Stockholm. ISBN 978-91-7595-505-6 Niclas Ånmark: Inclusion Characteristics and Their Link to Tool Wear in Metal Cutting of Clean Steels Suitable for Automotive Applications Division of Applied Process Metallurgy Department of Materials Science and Engineering School of Industrial Engineering and Management KTH Royal Institute of Technology SE-100 44 Stockholm, Sweden ISBN 978-91-7595-505-6 Cover: Reprinted with permission of Sandvik Coromant © Niclas Ånmark Abstract This thesis covers some aspects of hard part turning of carburised steels using a poly-crystalline cubic boron nitride (PCBN) cutting tool during fine machining. The emphasis is on the influence of the steel cleanliness and the characteristics of non-metallic inclusions in the workpiece on the active wear mechanisms of the cutting tool. Four carburising steel grades suitable for automotive applications were included, including one that was Ca-treated. A superior tool life was obtained when turning the Ca-treated steel. The superior machinability is associated with the deposition of lubricating (Mn,Ca)S and (CaO)x-Al2O3-S slag layers, which are formed on the rake face of the cutting tool during machining. Moreover, the transfer of work material to the rake face crater is characteristic in hard part turning of clean steels. It can be because of the lack of sulfides that protect the cutting edge when turning machinability treated steels. This corresponds to the more pronounced crater wear caused by the low-sulfur steels than that of the steels with higher sulfur contents. It was also concluded that the composition of the non-metallic inclusions in the Ca-treated steel is a more important factor than the inclusion number and size, in hard part turning using a PCBN cutting tool. Also, a 3D analysis after electrolytic extraction was found to give a more precise characterisation of non-metallic inclusions than the conventional 2D analysis by SEM-EDS. In turn, better correlations to machinability and mechanical properties can be obtained. Hence, the use of this technique is beneficial for future material development. Finally, the challenge for future metallurgy is to manufacture high-performance steels with improved combined properties of mechanical strength and machinability. Key words: Tool wear, PCBN, Machinability, Steel, Non-metallic inclusions. Preface This thesis summarises the knowledge I gained during my first two years as a Ph.D. student. My main topic has been applied process metallurgy and the work has been conducted partly at the Department of Materials Science and Engineering at KTH Royal Institute of Technology and partly at the Department of Materials and Manufacturing at Swerea KIMAB. I thank the Hugo Carlsson and the Gerhard Von Hofsten foundations for financial support. The main work in this study was conducted within the Member Research Consortium Materials in Machining at Swerea KIMAB. The other part was conducted within a VINNOVA funded project within the national Strategic Innovation Program - National Action for Metallic Materials. Participating companies were AB Sandvik Materials Technology, AB Sandvik Coromant, Ovako Hofors AB, Outokumpu Stainless AB, Uddeholms AB and Seco Tools AB. Millions of thanks are directed to the project committee. I wish to express my sincerest gratitude to my supervisors Docent Andrey Karasev, Professor Pär Jönsson and Doctor Thomas Björk for great support and help during this time. I would also like to acknowledge the great collaboration I had with Anna Ganea, Marianne Collin, Patrik Ölund and Professor Emeritus Sture Hogmark throughout the research linked to hard part turning. Many thanks are also directed to my colleagues at the Department of Materials and Manufacturing, Swerea KIMAB. The same goes also to my friends at and outside the institute. Last but not least, the support from family and foremost my dearest Matilda has been decisive the past two years. Thanks for everything. Niclas Ånmark Stockholm, May 2015 List of appended papers This thesis is based on the following supplements: Paper A Ånmark N., Karasev A., Jönsson P.G. The effect of different non-metallic inclusions on the machinability of steels, Materials 8-2 (2015) 751-783. DOI: 10.3390/ma8020751. Paper B Ånmark N., Björk T., Ganea A., Ölund P., Hogmark S., Karasev A., Jönsson P.G. The effect of inclusion composition on tool wear in hard part turning using PCBN cutting tools, Wear (Submitted). Paper C Ånmark N., Björk T., Karasev A., Jönsson P.G. Effect of different inclusions on mechanical properties and machinability of 20NiCrMo carburizing steels, Conference Proceeding of the 6th International Congress on the Science and Technology of Steelmaking. Beijing: The Chinese Society for Metals, May 12-14th (2015) 805-808. The contributions by the author to the papers of the thesis: Paper A: A literature review as well as the major part of the writing. Paper B: A literature review as well as the major part of the experimental work, evaluation of data and writing. Paper C: A literature review as well as the major part of the experimental work, evaluation of data and writing. Contents 1. INTRODUCTION ............................................................................ 1 1.1. AIMS AND SCOPE OF THE STUDY ......................................................................... 1 1.2. CLEAN STEEL ................................................................................................... 3 1.3. NON-METALLIC INCLUSIONS .............................................................................. 4 1.4. BILLET MANUFACTURING.................................................................................... 7 1.5. HEAT TREATMENT ............................................................................................. 9 1.5. METAL CUTTING AND MACHINABILITY ................................................................ 12 1.6. CUTTING TOOLS .............................................................................................. 15 1.7. TOOL WEAR .................................................................................................... 16 2. EXPERIMENTAL .......................................................................... 18 2.1. WORKPIECE MATERIALS AND CUTTING TOOL ..................................................... 18 2.2. MACHINABILITY TESTS ..................................................................................... 19 2.3. ANALYSIS OF SAMPLES .................................................................................... 20 3. RESULTS ..................................................................................... 22 3.1. HARDNESS PROFILES (PAPER B) ...................................................................... 22 3.2. INCLUSION CHARACTERISTICS IN DIFFERENT STEEL SAMPLES ............................. 22 3.3. PCBN TOOL LIFE AND TOOL WEAR (PAPER B) ................................................... 26 3.4. WORKPIECE MATERIAL ADHERED TO THE CUTTING EDGE (PAPER B).................... 29 4. DISCUSSION ............................................................................... 31 4.1. THE GENERAL INFLUENCE OF NON-METALLIC INCLUSIONS ................................... 31 4.2. THE INFLUENCE OF PROTECTIVE SLAG DEPOSITS ............................................... 32 4.3. COMPARISON BETWEEN HIGH-SULFUR STEELS AND THE CLEAN STEELS ............... 33 4.4. COMPARISON BETWEEN CLEAN STEEL AND THE ULTRA-CLEAN STEEL .................. 34 4.5. SUGGESTIONS TO IMPROVE THE MACHINABILITY OF CLEAN STEEL ....................... 35 4.6. ROLE OF NON-METALLIC INCLUSIONS ON THE MACHINABILITY OF CARBURISED STEEL ............................................................................................................................ 35 4.7 COMPARISON BETWEEN 2D AND 3D INCLUSION CHARACTERISATION .................... 36 5. CONCLUSIONS ........................................................................... 37 6. FUTURE WORK ........................................................................... 39 BIBLIOGRAPHY ............................................................................... 40 1 1. Introduction Developments in steel production since the 1970’s have resulted in steel grades with very low level of impurities. More recently, so called “high cleanliness” steels have been introduced by the steel makers around the world. Thereby, responding to the current and future market demands of steel having excellent fatigue strength and impact toughness and/or an improved corrosion resistance. Clean steels are associated with low contents of impurities such as oxygen (< 10 ppm O) and sulfur (< 10 ppm S). The driving force behind these developments has been to provide new steels that can perform well in extremely challenging applications e.g., gear box components for heavy trucks, bars for demanding construction and tubes for highly corrosive atmospheres. Although the new
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