On the Machinability of High Performance Tool Steels

To my husband Odd Sandberg and to the memory of my brother Alexander List of Papers This thesis is based on the following papers, which are referred to in the text by their Roman numerals. I L-G. Nordh, N. Broqvist, S. Gunnarsson, P. Miller. Machinabil- ity of High Performance Die Steels. Proceedings of the 22nd Int. Die Casting Congress and Exposition, Indianapolis, USA, 2003. II N. Broqvist, S. Hogmark, S. Gunnarsson, E. Coronel. Ma- chinability of modern hot work tool steels. Proceedings of the 7th Int. Tooling Conference, Torino, Italy, 2006. III N. Broqvist, S. Hogmark, S. Gunnarsson, T. Björk, F. Riddar. On the relation between composition, oxidation and machinability of tool steels. The 13th Nordic Symposium on Tribology, Tampere, Finland, 2008. IV N. Broqvist, S. Hogmark, S. Gunnarsson. Properties affecting machinability of hot work tool steels. Proceedings of the 8th Int. Tooling Conference, Aachen, Germany, 2009. V N. Broqvist, S. Hogmark, A. Medvedeva, S. Gunnarsson. On wear resistance of tool steel. Accepted for publication in Jour- nal of Manufacturing Processes, 2012. Reprints were made with permission from the respective publishers. The author’s contribution Paper I Part of planning, writing and evaluation. Paper II - V Major part of planning and writing, part of evaluation and experimental work. Contents 1 Background..................................................................................................9 2 Introduction................................................................................................11 2.1 Tool steels ..........................................................................................11 2.1.1 Manufacturing.............................................................................13 2.1.2 Machinability..............................................................................16 2.2 Cutting tool materials .........................................................................18 2.3 Aim of this thesis................................................................................19 3 Material and characterisation.....................................................................20 3.1 Tested tool steels ................................................................................20 3.2 Cutting tests........................................................................................22 3.3 Characterisation methods ...................................................................23 3.3.1 Mechanical properties.................................................................23 3.3.2 Thermal properties......................................................................25 3.3.3 Oxidation properties ...................................................................26 3.3.4 ThermoCalc simulations.............................................................26 3.3.5 Finite Element Modelling (FEM) simulations............................27 3.3.6 Microscopy techniques ...............................................................28 4 Hypothesis .................................................................................................31 4.1 Toughness – Hypothesis 1 (Papers II and IV)....................................31 4.2 Oxidation – Hypothesis 2 (Paper III) .................................................31 4.3 Austenitizing temperature – Hypothesis 3 (Papers IV and V) ...........32 5 Summary of results ....................................................................................33 5.1 Machinability of steels with varying silicon content (Paper I)...........33 5.2 Influence of toughness on machinability - Hypothesis 1 (Papers II and IV) ............................................................................................................34 5.3 Relation between oxidation properties and machinability – Hypothesis 2 (Paper III) ..............................................................................................37 5.4 Influence of austenitizing temperature – Hypothesis 3 (Papers IV and V)..............................................................................................................41 5.5 Thermal properties of H13 and H13M and their influence on the cutting temperature...................................................................................43 6 Discussions ................................................................................................45 6.1 Silicon content....................................................................................45 6.2 Hypothesis 1 – Toughness..................................................................45 6.3 Hypothesis 2 – Oxidation...................................................................46 6.4 Hypothesis 3 – Austenite formation...................................................47 6.5 Thermal properties .............................................................................48 6.6 Comparison between the hypotheses .................................................49 7 Conclusions................................................................................................50 8 Sammanfattning .........................................................................................52 9 References..................................................................................................54 Preface The research associated with this PhD thesis has been carried out at Re- search and Development Department, Machining Section, at Uddeholms AB in Hagfors, and at the Tribomaterials Group, Ångström Laboratory, at Upp- sala University. The financial and material support from the participating companies in the research project Böhler Uddeholm AG, Sandvik Tooling AB and Oerlikon Balzers Sandvik Coating AB is gratefully acknowledged. I am grateful to many people, who have inspired, encouraged, supervised me and have been helpful during the research and writing this thesis. First of all, I would like to express my gratitude to my supervisor Prof. Sture Hogmark at the Tribomaterials Group, Uppsala University, for all guidance and support. Secondly I would like to extend my gratitude to my industrial supervisor Staffan Gunnarsson, R&D, Machining Section, Uddeholms AB, for all support, for organising the research project and also for valuable discussions and practical guidance through the years. I would like to thank my colleagues at Machining Section and participating personnel at the above mentioned companies for contributing to the success of this work. Finally, I would like to thank my family for their great support. Natalia Sandberg April, 2012 Uddeholms AB, Hagfors 1 Background Tool steels constitute a very special group of alloys having different properties than other groups of steels. Tool steels are carbon, alloy or high-speed steels capable to be hardened and tempered and produced under strict proc- essing. Tool steels are primarily used in dies and moulds for shaping, form- ing, blanking and cutting of other materials at ambient or elevated temperatures. Normally, tool steels are produced in electric furnaces. Special con- trolled heat treatment gives the steels their required properties. The first tool steels were made in the second half of 19th century [1]. The industrial demands on tool steel performance have increased because of the need to reduce costs [2, 3, 4]. Production of large components with thin wall thicknesses, complicated shapes and closer tolerances leads to more damage of the die surfaces, like heat checking and hot wear or even catas- trophic gross cracking of the whole die. This has led to the development of hot work tool steels with higher toughness, higher strength and higher resistance to softening when operating at elevated temperature. There are many ways to improve the mechanical properties of such materials. One way is to decrease the number of non-metallic inclusions and pre- cipitated primary carbides in the steel, which could be achieved by a modifi- cation of the basic chemical composition of the steel. Carbide precipitation is decreased by reducing the silicon content in the steel. The combination of a low silicon content and a raised molybdenum content results in an increased toughness and higher strength at elevated temperatures. However, these die steels with modified chemical composition have been considered difficult to machine in both the soft annealed and the hardened and tempered state, al- though the effect is greatest in hardened material, when comparing with conventional AISI H11/H13 type of grades. Good steel machinability is defined as a combination of low power con- sumption, low tool wear and good surface finish. Tool life is one important factor in metal cutting from an economical point of view. Cutting tool flank wear is influenced by the interaction between the cutting tool and work piece. New hot work tool steels with improved mechanical properties gener- ate relatively high cutting forces and edge temperatures, which, in turn, pro- mote sticking of tool steel to the cutting edge and an associated built up edge formation. This results in shorter cutting tool life. The high machining cost is in most cases negligible in relation to the increased performance in die life, but should still be kept as low as possible. 11 12 2 Introduction 2.1 Tool steels Many grades and compositions of tool steels are known. Most tool steels can be classified according to the American Iron and Steel Institute (AISI). Ac- cording to the classification the tool steels are divided in the following groups:

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