EXAMENSARBETEINOM MATERIALDESIGN, AVANCERAD NIVÅ, 30 HP STOCKHOLM, SVERIGE 2016 Evaluation of the impact of different heat treatments on the toughness and tempering resistance for a Cr-Mo-V hot-work tool steel NATHALIE SCHMIDT KTH SKOLAN FÖR INDUSTRIELL TEKNIK OCH MANAGEMENT Abstract High Pressure Die Casting (HPDC) has very high demands on toughness and tempering resistance of the tool. A typical hardening cycle for this purpose consists on austenitizing and quenching in a vacuum furnace followed by three temperings at 600°C. In this work the possibility of optimizing the toughness and tempering resistance in Uddeholm Dievar by adding an extra tempering at the beginning and/or the end of the tempering process is investigated. Extra temperings were performed at 300-400° and hardness levels, impact toughness and tempering resistance were evaluated. Microstructural investigations as well as thermodynamical calculations were also carried out. Results showed no feasible differences between the results of the here conducted tests and those from the common tempering procedures. Keywords: Tool steel, heat treatment, secondary carbides, impact toughness, tempering resistance. i CONTENTS 1 INTRODUCTION .............................................................................................................. 1 1.1 Background .................................................................................................................. 1 1.2 Purpose ........................................................................................................................ 1 2 THEORY ............................................................................................................................ 2 2.1 Hot-Work Tool Steel ................................................................................................... 2 2.2 Toughness and Charpy V-notch testing ....................................................................... 2 2.3 Thermodynamics and kinetics ..................................................................................... 3 2.3.1 Thermo-Calc ......................................................................................................... 5 2.4 Martensite Hardening .................................................................................................. 5 2.4.1 Austenitization ..................................................................................................... 5 2.4.2 Formation of Martensite ....................................................................................... 6 2.4.3 Retained Austenite ............................................................................................... 8 2.5 Tempering .................................................................................................................... 8 2.5.1 The ε-carbide ........................................................................................................ 8 2.5.2 Decomposition of Retained Austenite .................................................................. 9 2.5.3 Cementite ........................................................................................................... 10 2.5.4 Alloy Carbides .................................................................................................... 11 2.5.5 Tempering Resistance ........................................................................................ 12 2.6 Scanning Electron Microscopy .................................................................................. 13 3 METHOD ......................................................................................................................... 15 3.1 Material ...................................................................................................................... 15 3.2 Thermo-Calc .............................................................................................................. 16 3.3 Tempering .................................................................................................................. 17 3.4 Hardness Measurements on Charpy V-specimens .................................................... 18 3.5 Charpy V-Impact Test ............................................................................................... 18 3.6 Tempering Resistance ................................................................................................ 18 3.7 Metallographic Investigation ..................................................................................... 19 3.7.1 Metallographic Sample Preparation ................................................................... 19 3.7.2 Scanning Electron Microscopy .......................................................................... 20 4 RESULTS ......................................................................................................................... 21 4.1 Thermo-Calc .............................................................................................................. 21 ii 4.2 Hardness Measurements on Charpy V-specimens .................................................... 24 4.3 Charpy V-Impact Test ............................................................................................... 25 4.4 Tempering Resistance ................................................................................................ 27 4.5 Metallographic Investigation ..................................................................................... 29 4.5.1 Undissolved primary Carbides ........................................................................... 29 4.5.2 Microstructure .................................................................................................... 30 5 DISCUSSION .................................................................................................................. 31 5.1 Thermo-Calc .............................................................................................................. 31 5.2 Hardness .................................................................................................................... 31 5.3 Impact toughness ....................................................................................................... 31 5.4 Tempering resistance ................................................................................................. 32 5.5 Microstructure ........................................................................................................... 32 6 CONCLUSIONS .............................................................................................................. 33 7 FUTURE WORK ............................................................................................................. 33 8 ACKNOWLEDGEMENTS ............................................................................................. 33 9 REFERENCES ................................................................................................................. 34 Appendix 1 ............................................................................................................................... 37 Appendix 2 ............................................................................................................................... 38 iii 1 INTRODUCTION Steel is one of the most common construction materials in the world. It is found either as the finished product or as the tool used to shape that final product [1]. With its production site and research department located in Hagfors (Sweden), Uddeholms AB is the world leading manufacturer of tool steels, which are used to shape other materials. The tool steels produced by Uddeholms AB have a large variety of applications, including hot-work tool steels for high pressure die casting (HPDC), forging and extrusion of metals. Depending on the application and materials, the hot-work tool steel manufactured at Uddeholms AB can be used when the operating temperature is up to 600°C [2]. 1.1 Background Tools for hot-work applications are used to shape other materials at elevated temperatures. Hot-work tool steels used in such tools needs therefore to have high toughness and good tempering resistance. To obtain these properties, the steels are alloyed with different elements in order to achieve the typical hot-work tool steel microstructure with tempered martensite and alloy carbide precipitates [3] [4]. One of the steels produced by Uddeholms AB is Uddeholm Dievar, a chromium- molybdenum-vanadium alloyed hot-work tool steel with a high resistance to heat checking, gross cracking, hot wear and plastic deformation. It is characterized by good toughness, tempering resistance, hardenability and dimensional stability. These properties make the steel a good choice for the high demanding die casting, forging and extrusion industry [5]. At the research department at Uddeholms AB, it has been suggested that an extra low- temperature tempering at the beginning and/or at the end of Dievar’s regular tempering process may increase the tempering resistance and/or the impact toughness of the steel. Since these properties are of great importance for hot-work tool steel, Uddeholms AB is highly interested in such an improvement. 1.2 Purpose The purpose of this work is to explore the possibility of optimizing impact toughness and tempering resistance on Uddeholm Dievar by adding an extra low-temperature tempering at the beginning and/or in the end of the tempering process. In order to investigate how extra tempering cycles