Designing a Heat Treatment to Achieve Ductile Advanced High Strength Steels Abdellatif LAARICH Materials Engineering, master's level (120 credits) 2020 Luleå University of Technology Department of Engineering Sciences and Mathematics Abstract Heat treatment is a way to significantly change materials properties. When presented with materials that lack certain mechanical properties, it is possible to change its chemical properties and microstructures by applying heat. This can help achieve better yield strength, ductility and toughness. This project discusses the effects of multiple distinct heat treatment methods for several materials in order to improve ductility and elongation without diminishing strength. The materials in question are High Aluminum Steel and Strenx 700MC steel, the first being under development and the second being a commercially available steel. These steels show promise to be used as high ductility, high strength, and 3rd generation steels. The heat treatments can change the mechanical proprieties of the base materials in order to optimize these steels for applications in vertical access solutions. The heat treatments in this project were Quenching and Partitioning (QP), Quenching and Tempering (QT), Austempering (AUST), Intercritical Heat Treatment (IHT) and other usual heat treatments such as Double normalizing (D-Norm). First, the most beneficial type of the above mentioned heat treatments was selected for each steel and series of heat treatments were performed in order to identify and optimize the best method for each steel. Then, heat treated samples underwent a series of tests to numerically quantify their properties and compare them to the existing steels in Alimak’s applications. The results show that Quenching and Partitioning is the most promising heat treatment for optimizing strength and ductility in High Aluminum Steel, with elongation values up to 19% together with yield strengths of 700 MPa. For Strenx 700MC a combination of temperature and time was found that gave an elongation of above 25% with a yield strength of 450 MPa. The explanation for the good properties was partly grain refinement and phase transformations during heat treatments. i Acknowledgements I am blessed to have great parents who taught me that hard work always pays off so for that, thank you dad Mr. El Arabi LAARICH and mom Mrs. Khaddouj El Omari. For guidance and help along the project I would like to thank: Marta-Lena Antti, my examiner and supervisor Farnoosh Forouzan, my co-supervisor Hadi Torkamani, my co-supervisor Dennis Johansson, my industrial supervisor from Alimak I am also grateful to: Esa Vuorinen, for sharing his knowledge Lars Frisk for helping me with the tensile test experiments My friends from AMASE, (especially Chihab) for sharing good moments with me All my friends from the university. شكرا ! Thanks! Tack! “Do not belittle any good deed, even meeting your (ﷺ) brother with a cheerful face.” Prophet Muhammad Abdellatif LAARICH Luleå, June 2020 ii Table of contents Abstract .......................................................................................................................................................... i Acknowledgements ....................................................................................................................................... ii List of Figures ................................................................................................................................................ v List of Tables ............................................................................................................................................... vii 1. Introduction ............................................................................................................................................ 1 1.1. Steel ............................................................................................................................................... 2 1.1.1. Iron-carbon phase diagram ..................................................................................................... 2 1.1.2. Isothermal transformation diagram (TTT diagram) ................................................................ 3 1.1.3. Advanced high strength steel ................................................................................................. 5 1.2. Stress and strain ............................................................................................................................. 5 1.2.1. Concepts of stress and strain .................................................................................................. 5 1.2.2. Stress-strain curve ................................................................................................................... 6 1.3. Ductility and strength in steels ...................................................................................................... 7 1.3.1. Work hardening ...................................................................................................................... 8 1.3.2. Grain size reduction ................................................................................................................ 8 1.3.3. Solid solution strengthening ................................................................................................... 8 1.4. Heat treatments ............................................................................................................................. 8 1.4.1. Quenching and Partitioning .................................................................................................... 9 1.4.2. Quenching and Tempering .................................................................................................... 10 1.4.3. Austempering ........................................................................................................................ 11 1.4.4. Intercritical Heat Treatment ................................................................................................ 13 2. Materials and Methodology ................................................................................................................. 14 2.1. Alimak steels ................................................................................................................................ 14 2.2. Chosen steels ............................................................................................................................... 14 2.2.1. High Aluminum Steel............................................................................................................. 14 2.2.2. Strenx 700MC ........................................................................................................................ 15 2.3. Heat treatments ........................................................................................................................... 15 2.3.1. Heat treatments performed on High Aluminum Steel ......................................................... 16 Quenching and Partitioning ........................................................................................... 16 Quenching and Tempering ............................................................................................. 17 Austempering ................................................................................................................. 17 2.3.2. Heat treatments performed on Strenx 700MC ..................................................................... 18 Other usual heat treatments ......................................................................................... 18 iii Intercritical Heat Treatment ......................................................................................... 18 2.4. Microstructure characterization .................................................................................................. 19 2.5. Phase identification ..................................................................................................................... 20 2.6. Mechanical testing ....................................................................................................................... 20 2.6.1. Sample preparation............................................................................................................... 20 2.6.2. Micro-hardness measurements ............................................................................................ 20 2.6.3. Tensile testing ....................................................................................................................... 21 2.6.4. Conversion of elongation A25 to A50 ................................................................................... 22 3. Results and Discussions ........................................................................................................................ 23 3.1. Microstructure characterization .................................................................................................. 23 3.1.1. As-received and heat treated High Aluminum Steel ............................................................. 23 3.1.2. As-received and heat treated Strenx 700MC ........................................................................ 26 3.2. Phase identification ..................................................................................................................... 28 3.3. Mechanical testing ......................................................................................................................