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Corrosion-Fatigue Testing on Steel Grades with Different Heat and Surface Treatments Used in Rock-Drilling Applications

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Corrosion-Fatigue Testing on Steel Grades with Different Heat and Surface Treatments Used in Rock-Drilling Applications DEGREE PROJECT IN MATERIALS SCIENCE AND ENGINEERING, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2016 CORROSION-FATIGUE TESTING ON STEEL GRADES WITH DIFFERENT HEAT AND SURFACE TREATMENTS USED IN ROCK-DRILLING APPLICATIONS LUIS MIGUEL BÉJAR INFANTE KTH ROYAL INSTITUTE OF TECHNOLOGY SCHOOL OF INDUSTRIAL ENGINEERING AND MANAGEMENT Abstract Corrosion fatigue is a common failure mechanism in rock drilling components and many other mechanical parts subjected to cyclic loads in corrosive environments. A crucial part in the design of such components resides in the selection of the right materials for the application, which ideally involves testing and comparison of their performance under working conditions. The present work was performed with the purpose of designing a corrosion-fatigue testing method that would allow the designer to compare the performance of different materials exposed to corrosion fatigue, permitting also the comparison with results from dry fatigue testing. The method was designed for rotating-bending machines. Two different steel grades were used during the work, one through hardened and one case hardened. The effect of these heat treatments and of shot peening over corrosion-fatigue behaviour were studied using the proposed method. It was proven that the testing speed has a strong impact on the fatigue life of steel. It was found that, at a fixed stress level, the case hardened and shot peened steel reached 3X10^6 cycles at 2300 rpm, while it failed at only 5X10^5 cycles with a testing speed of 500 rpm. A large beneficial influence of the shot peening was demonstrated. It was also observed that, at fixed testing speed, the shot peening on the through hardened steel can increase its fatigue strength from 190 MPa to 600 MPa under corrosion fatigue. Many cracks were found at the surface of the shot peened parts, which are arrested near the surface by the compressive stress layer from the shot peening. It was also found that, for the non-shot peened parts, case hardening had a slightly higher corrosion-fatigue strength than the through hardened. This might be a result of the compressive stresses from carburization, or due to the higher core toughness of this steel grade. Keywords: Corrosion fatigue, crack, rotating-bending, S-N curve, staircase method, fatigue strength, shot peening, case hardening, through hardening i Preface This work was performed to cover the thesis project in Mechanical Metallurgy as part of the master’s degree in Engineering Materials Science at the Royal Institute of Technology in Stockholm, Sweden. The project was conducted in cooperation with and financed by Atlas Copco Secoroc AB in Fagersta, Sweden. Very special thanks go to all the people who supported me throughout the whole project, without whom it would not have been possible to complete this work. To Göran Stenberg for the right guidance on every step of the project, and for providing me with scientific criteria to achieve a professional work. To Richard Johanson for the invaluable support in several aspects, including all the laboratory training I received, calibration of the instruments, production processes, metallographic sampling and analyses, and for the interesting discussions that helped to shape the conclusions of this work. To Anders Olsson who shared essential insights to the technical debates that guided the project to success. To Jonas Falkestrom for the support I received within the company throughout the whole timeframe to perform this project without any obstacle. I also want to thank Alexander Beronius for the important technical contributions and interesting discussions which had a vital impact on the outcome of the project, and Gabriella Brorson for the guidance on essential technical information and valuable criteria. Special thanks go to my supervisor in KTH, Stefan Jonsson, who followed my project in all technical aspects and provided me with vital insights that helped me take decisive steps. And last but not least, I want to thank my father, who introduced me to the fascinating world of mechanics and metals, for his innumerable teachings in these subjects and in all others. Luis Miguel Béjar Fagersta, June 2016 ii Abbreviations CH Case hardened TH Through hardened SP Shot peened NSP Non-shot peened HCF High-cycle fatigue LCF Low-cycle fatigue iii List of Tables Table 1. Different material scenarios used in the present work ........................................................... 28 Table 2. Metallographic samples selection ........................................................................................... 33 Table 3. Percent Replication .................................................................................................................. 34 Table 4. Average values for number of cycles to failure and time of test for frequency dependence of corrosion fatigue on CH-SP in fresh water ............................................................................................ 35 Table 5. Average values for number of cycles to failure and time of test for frequency dependence of corrosion fatigue on TH-NSP in fresh water .......................................................................................... 37 Table 6. Parameters for Proposed Corrosion-Fatigue Testing Method ................................................ 39 iv List of Figures Figure 1. Definition of cycles and reversals [3] ....................................................................................... 4 Figure 2. Load cycles for sinusoidal, square and triangular load paths. Middle figure: Same as above but twice the frequency. Lower figure: A schematic spectrum load found in many applications. [2] ... 4 Figure 3. Definition of components in a stress cycle [2] ......................................................................... 5 Figure 4. S-N curve for AISI 4340 alloyed steel [7] .................................................................................. 6 Figure 5. Comparison of steel and aluminium fatigue behaviour [6] ..................................................... 6 Figure 6. Illustration of the steps of the fatigue crack evolution on radial and axial cross sections of a cylindrical part [9] .................................................................................................................................... 7 Figure 7. Schematic illustration of the variation of fatigue-crack growth rate da/dN with alternating stress intensity ΔK in steel, showing regimes of primary crack-growth mechanisms [8] ....................... 8 Figure 8. Development of extrusions and intrusions during fatigue [9] ................................................. 9 Figure 9. Schematic illustration of crack initiation, stable crack growth and fracture [2] ...................... 9 Figure 10. Fatigue crack propagation [6]............................................................................................... 10 Figure 11. Corrosion-fatigue and its general effect on the behaviour of steel [12] .............................. 11 Figure 12. Solubility of oxygen in water at different temperatures ...................................................... 12 Figure 13. Effect of NaCl concentration on the corrosion of Fe [18] .................................................... 13 Figure 14. Effect of NaCl concentration on crack initiation and crack propagation [19] ...................... 13 Figure 15. Effect of MnS inclusions in the film rupture/anodic dissolution process [20] ..................... 17 Figure 16. Hydrogen embrittlement ..................................................................................................... 19 Figure 17. The hydrogen embrittlement process [36] .......................................................................... 19 Figure 18. Shot peening compressive stress profile [21] ...................................................................... 21 Figure 19. Rotating-bending machine diagram ..................................................................................... 22 Figure 20. Example of staircase fatigue data [3]; suspensions are tests in which the specimens survived ................................................................................................................................................. 23 Figure 21. S-N testing with a small sample size [3]; suspensions are tests in which the specimens survived ................................................................................................................................................. 24 Figure 22. Rotating-bending fatigue testing machine - Atlas Copco Secoroc AB materials laboratory 25 Figure 23. Rotating-bending fatigue testing machine, rear side - Atlas Copco Secoroc AB materials laboratory .............................................................................................................................................. 26 Figure 24. Specimen mounted on the machine and water collector underneath ................................ 26 Figure 25. Corrosion chamber ............................................................................................................... 27 Figure 26. Geometry and dimensions of specimen ............................................................................... 27 Figure 27. Laser scan micrometer ......................................................................................................... 28 Figure 28. Laser scan micrometer display ............................................................................................
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