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High Strength, High Toughness Microalloyed Steel Forgings Produced with Relaxed Forging Conditions and No Heat Treatment

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High Strength, High Toughness Microalloyed Steel Forgings Produced with Relaxed Forging Conditions and No Heat Treatment Title Page High Strength, High Toughness Microalloyed Steel Forgings Produced with Relaxed Forging Conditions and No Heat Treatment by Aaron Edwin Stein B.S. in Engineering Science, Saint Vincent College, 2016 Submitted to the Graduate Faculty of the Swanson School of Engineering in partial fulfillment of the requirements for the degree of Master of Science in Materials Science and Engineering University of Pittsburgh 2019 Committee Page UNIVERSITY OF PITTSBURGH SWANSON SCHOOL OF ENGINEERING This thesis was presented by Aaron Edwin Stein It was defended on November 20, 2019 and approved by John Oyler, PhD, Professor, Department of Civil and Environmental Engineering Patrick Smolinski, PhD, Professor, Department of Mechanical Engineering and Materials Science Brian Gleeson, PhD, Professor, Department of Mechanical Engineering and Materials Science Thesis Advisor: Anthony J. DeArdo, PhD, Professor, Department of Mechanical Engineering and Materials Science ii Copyright © by Aaron Edwin Stein 2019 iii Abstract High Strength, High Toughness Microalloyed Steel Forgings Produced with Relaxed Forging Conditions and No Heat Treatment Aaron Edwin Stein, MS University of Pittsburgh, 2019 Three steel compositions were designed and investigated as possible materials for the forging of a wheel hub geometry using the recrystallization controlled forging (RCF) process. Titanium nitride (TiN) technology was utilized to control prior austenite grain sizes (PAGS) and vanadium precipitation to strengthen ferrite. For processing, two cooperating systems were investigated, RCF, for refinement of the PAGS, and controlled cooling, where interrupted direct quenching (IDQ) and indirect accelerated cooling (IAC) provided multiple possible physical property combinations. The response of the steels to thermo-mechanical processing (TMP) was investigated to determine optimal forging conditions for refinement of the PAGS. The maximum reheat temperature to avoid grain coarsening, as well as the minimum forging temperature for complete recrystallization was determined. Ti and V additions were found to elevate both temperatures. After, IDQ and IAC cooling paths were performed on the steels to yield desired microstructure products. Microstructures comprising predominantly polygonal ferrite, bainite and martensite were attainable in the laboratory samples. Finally, the steels were tested at Meadville Forging Company (MFC), a collaborator in this project. Here, each steel was subjected to the MFC standard forging routine and varying cooling paths were used. Mechanical testing samples were machined and tested from the forgings. The strengths increased with cooling rate and alloying, but the fully forged, hot water quenched bainitic samples did not perform well, having lower strengths than the fast air cool to room temperature (ACRT) conditions. This is interpreted to be because of iv the low alloying, specifically the low carbon, limiting the strength of the bainite when present in large bainite phase percentages. The performance of the steels is compared with those of similar steels and discussed from metallurgical standpoints. Methods of improving upon the current design of the steels are discussed. This research has demonstrated that low-carbon compositions, when subjected to proper RCF processing, are capable of being refined to equivalent circular prior austenite grain diameters approaching 10µm. Additionally, ultimate tensile strengths in the non- water quench to room temperature (WQRT) conditions were observed between 550MPa and 720MPa, and approaching 940MPa in the WQRT condition, with good toughness in all conditions. v Table of Contents Acknowledgements .................................................................................................................. xxiii Glossary .................................................................................................................................... xxiv 1.0 Introduction ............................................................................................................................. 1 2.0 Background ............................................................................................................................. 4 2.1 Strengthening Mechanisms ............................................................................................ 4 2.1.1 Strengthening Overview ..................................................................................... 4 2.1.2 Solid Solution Strengthening .............................................................................. 5 2.1.3 Precipitation Strengthening ............................................................................... 5 2.1.4 Grain-Boundary Strengthening ....................................................................... 11 2.1.5 Dislocation Strengthening/Work Hardening/Strain Hardening ................... 12 2.2 Composition and Alloying ........................................................................................... 14 2.2.1 BAMPRI, Meadville Forging Company, TIMKENSTEEL Steel Company Composition ................................................................................................................ 14 2.2.2 Carbon ................................................................................................................ 17 2.2.3 Vanadium ........................................................................................................... 19 2.2.4 Titanium ............................................................................................................. 19 2.2.5 Nitrogen .............................................................................................................. 22 2.2.6 Vanadium and Titanium Precipitates: Solubility and Influences ................ 24 2.2.7 Chromium, Molybdenum and Manganese ..................................................... 34 2.2.8 Silicon, Phosphorous and Aluminum .............................................................. 37 2.3 Austenite Conditioning ................................................................................................ 39 vi 2.3.1 SV Parameter ..................................................................................................... 39 2.3.2 Recrystallization and Temperatures T5 and T95 ............................................ 41 2.3.3 Grain Coarsening and Temperature TGC ....................................................... 43 2.3.4 Deformation and Forging Passes ..................................................................... 46 2.4 Cooling and Transformation ....................................................................................... 49 2.4.1 Cooling Rates ..................................................................................................... 49 2.4.2 RCF Cooling Schedules .................................................................................... 52 2.5 Relation to FIERF RCF Project .................................................................................. 56 2.5.1 Austenite Conditioning ..................................................................................... 56 2.5.2 Cooling and Transformation ............................................................................ 57 2.5.3 Strengthening Employed .................................................................................. 59 3.0 Statement of Objectives ........................................................................................................ 61 4.0 Experimental Procedures ..................................................................................................... 63 4.1 Microstructural Analysis ............................................................................................. 64 4.1.1 Optical Microscopy ........................................................................................... 64 4.1.2 Scanning Electron Microscopy (SEM) ............................................................ 65 4.1.3 Transmission Electron Microscopy (TEM) .................................................... 66 4.1.4 Grain Size Measurements................................................................................. 66 4.2 Mechanical Properties .................................................................................................. 67 4.2.1 Tensile Testing ................................................................................................... 67 4.2.2 Micro Hardness and Nano Hardness .............................................................. 68 4.3 Austenitic Phase Processing ......................................................................................... 68 4.3.1 Initial Convection Furnace Heating Trial ...................................................... 68 vii 4.3.2 Grain Coarsening Trials ................................................................................... 69 4.3.3 Recrystallization Forging Trials ...................................................................... 70 4.4 Cooling Experiments .................................................................................................... 71 4.4.1 Standard Processing Procedure ....................................................................... 71 4.4.2 Air Cooling Experiments .................................................................................. 72 4.4.3 Water Quenching Experiments ....................................................................... 73 4.4.4 Bainitic Cooling Experiments .........................................................................
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