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INVESTIGATION of LOW OXYGEN HSLA STEEL WELD METAL By

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INVESTIGATION of LOW OXYGEN HSLA STEEL WELD METAL By INVESTIGATION OF LOW OXYGEN HSLA STEEL WELD METAL by Drew White A thesis submitted to the Faculty and the Board of Trustees of the Colorado School of Mines in partial fulfillment of the requirements for the degree of Master of Science (Metallurgical and Materials Engineering). Golden, Colorado Date: _____________ Signed: _______________________ Drew White Signed: _______________________ Dr. Stephen Liu Thesis Advisor Golden, Colorado Date: _____________ Signed: _______________________ Dr. Angus Rockett Professor and Head of George S. Ansell Department of Metallurgical and Materials Engineering ii ABSTRACT Hot-wire gas tungsten arc welding is a process gaining more popularity in industry today due to increased deposition rate, from the resistively heated filler metal introduced by an external wire feed system. A modified version of this system, which incorporates an oscillation mechanism to modify the frequency with which the filler metal enters the molten weld pool was utilized in this work and the effect of varying process parameters on microstructure, weld bead morphology, and inclusion size and distribution are characterized. Hot-wire amperage, wire feed speed, frequency of wire oscillation and heat input were varied on seven wire consumables to determine their effects on acicular ferrite. Optimal microstructural development in high strength low alloy steel weldments is mainly dependent on acicular ferrite, which nucleates on oxide inclusions. However, with the gas tungsten arc process, it is difficult to manipulate weld pool oxygen content to achieve the needed level for optimal acicular ferrite formation. The oxide inclusion population is investigated to determine the morphology, size and spatial distribution and whether they served as nucleation sites for ferrite formation. The microstructure was primarily affected by heat input and composition, with increasing heat input reducing the total acicular ferrite volume percent. These effects directly correlated to inclusion population, with three distinct distributions demonstrated between inclusion radii in the range of 0.1 to 0.75 micron. Composition effects were tied to both an increase in Molybdenum, 0.1 wt. pct. to 0.25 wt. pct., a Titanium peak at 70 ppm and Oxygen content. The oscillation affected weld bead morphology, improved weld wetting but with little effect on final microstructure. Two experimental wire compositions were identified to move forward for additional mechanical testing. iii TABLE OF CONTENTS ABSTRACT ................................................................................................................................... iii LIST OF FIGURES ...................................................................................................................... vii LIST OF TABLES ....................................................................................................................... xvi LIST OF SYMBOLS ................................................................................................................. xviii ACKNOWLEDMENTS .............................................................................................................. xix CHAPTER ONE LITERATURE REVIEW ................................................................................... 1 1.1 Gas Tungsten Arc Welding .................................................................................................. 1 1.2 Hot-Wire Gas Tungsten Arc Welding ................................................................................. 2 1.2 Activated Gas Tungsten Arc Welding ................................................................................. 4 1.3 Microstructure ...................................................................................................................... 7 1.3.1 Classification of Microstructural Constituents ................................................................. 7 1.3.2 Formation of Acicular Ferrite ..................................................................................... 14 1.3.3 Nucleation of Acicular Ferrite .................................................................................... 16 1.3.4 Inclusion Size and Distribution Effect on Acicular Ferrite ........................................ 20 1.3.5 Sympathetic Nucleation of Acicular Ferrite ............................................................... 24 1.3.6 Prior Austenite Grain Size and Acicular Ferrite ......................................................... 26 1.3.7 Carbon Equivalency ................................................................................................... 28 1.3.8 Low Oxygen Weld Metal ........................................................................................... 30 1.4 Zener Pinning and Inclusion Size ...................................................................................... 34 1.5 Alloying Additions in the Weldment ................................................................................. 36 1.5.1 Titanium .................................................................................................................. 36 1.5.2 Molybdenum ........................................................................................................... 39 1.5.3 Zirconium ................................................................................................................ 43 1.5.4 Manganese .............................................................................................................. 44 iv 1.5.5 Aluminum ............................................................................................................... 46 1.5.6 Silicon ..................................................................................................................... 50 1.5.7 Nickel ...................................................................................................................... 50 1.5.8 Chromium ............................................................................................................... 51 1.5.9 Boron....................................................................................................................... 51 1.5.10 Vanadium ................................................................................................................ 53 1.5.11 Niobium .................................................................................................................. 53 1.5.12 Copper ..................................................................................................................... 55 CHAPTER TWO EXPERIMENTAL METHODS ...................................................................... 56 2.1 Material and Weld Preparation ...................................................................................... 56 2.2 Welding Procedure ......................................................................................................... 56 2.3 Light Optical Metallography .......................................................................................... 60 2.4 Hardness Testing ............................................................................................................ 60 2.5 Chemical Analysis.......................................................................................................... 60 2.6 Inclusion Characterization Methodology ....................................................................... 61 2.7 Multipass Weldment Study ............................................................................................ 61 2.8 A-TIG (Adaptive GTAW) .............................................................................................. 62 CHAPTER THREE RESULTS AND DISCUSSION .................................................................. 66 3.1 Heat Input Calculation for Hot-wire Gas Tungsten Arc Welding ................................. 66 3.3 Welding Parameters Study ............................................................................................. 73 3.4 Shielding Gas Effect on the Welding Process ................................................................ 83 3.5 Designed Consumable, Modified E80C-Ni2 ................................................................. 87 3.6 Composition ................................................................................................................... 87 3.7 Inclusion Size Distribution ........................................................................................... 104 3.8 EDS Inclusion Analyses ............................................................................................... 112 v 3.9 Activated Gas Tungsten Arc Welding ......................................................................... 115 3.10 Acicular Ferrite Prediction ....................................................................................... 122 CHAPTER FOUR CONCLUSIONS .......................................................................................... 127 CHAPTER FIVE FUTURE WORK ........................................................................................... 129 REFERENCES CITED ............................................................................................................... 131 APPENDIX A WIRE INCLUSIONS AND MICROSTRUCTURE .......................................... 137 APPENDIX B HARDNESS ....................................................................................................... 149 vi LIST OF FIGURES Figure 1.1 Schematic assembly of a
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