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Laser Weldability of Type 21Cr-6Ni-9Mn Stainless Steel

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Laser Weldability of Type 21Cr-6Ni-9Mn Stainless Steel LASER WELDABILITY OF TYPE 21CR-6NI-9MN STAINLESS STEEL by Stephen B. Tate A thesis submitted to the Faculty and Board of Trustees of the Colorado School of Mines in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Metallurgical and Materials Engineering). Golden, Colorado Date ____________________ Signed: __________________________ Stephen B. Tate Signed: __________________________ Dr. Stephen Liu Thesis Advisor Golden, Colorado Date ____________________ Signed: __________________________ Dr. Ivar Remanis Professor and Interim Head Department of Metallurgical and Materials Engineering ii ABSTRACT Type 21Cr-6Ni-9Mn (21-6-9) stainless steel is a high-nitrogen, high-manganese austenitic stainless steel that shows attractive mechanical properties and good general corrosion resistance. In this work the laser weldability of type 21-6-9 steel has been studied to understand the influence of material chemical composition and processing parameters on the laser weldability of type 21-6-9 steel. Bead-on- plate fiber laser welds were used to generate weld microstructures for a wide range of processing parameters. Sigmajig testing was used to examine solidification crack susceptibility. Microstructures were characterized using light optical microscopy, scanning electron microscopy, and electron microprobe analysis. For the range of process parameters used, type 21-6-9 steel showed good general laser welding behavior. Bead morphologies equivalent to electron beam welding are possible with fiber laser welding. As travel speed increased, porosity increased, but overall porosity levels were less than 0.05 vol-%. Weld metal solidification mode and ferrite content were influenced by solidification rate, initial chemical composition, nitrogen loss and temperature gradient. An improved method to measure weld pool solidification rate was developed. Using top and side view longitudinal sections, the dendrite growth direction can be better characterized compared to the typical method only using top view sections. A wide range of commercial and experimental heats were used to develop laser weldability diagrams for type 21-6-9 steel at three travel speeds, relating chemical composition and processing parameters to solidification crack susceptibility. For primary austenite solidification, phosphorus showed a larger influence on solidification crack susceptibility relative to sulfur. A relationship of P+0.2S was developed for total impurity content. As travel speed increased from 21 to 85 mm/s, the critical Creq/Nieq for primary ferrite solidification increased from 1.5 to 1.7. Compositions below this critical Creq/Nieq were crack susceptible. iii Both Fe-Cr-Ni-N and Fe-Cr-Mn-Ni-N systems were used to model the solid-liquid interface temperature for ferrite and austenite. The interface temperatures were used to predict solidification mode as a function of solidification conditions. The model calculations and experimental observations both indicate temperature gradient can affect the solidification rate at which the transition between primary ferrite and primary austenite solidification occurs. iv TABLE OF CONTENTS ABSTRACT ................................................................................................................................................. iii LIST OF FIGURES ................................................................................................................................... viii LIST OF TABLES ....................................................................................................................................... xi ACKNOWLEDGEMENTS ........................................................................................................................ xii CHAPTER 1: INTRODUCTION .............................................................................................................. 1 1.1 Project Motivation .............................................................................................................. 1 1.2 Background ......................................................................................................................... 2 1.2.1 High Energy Density Welding ............................................................................................ 2 1.2.2 Austenitic Stainless Steels .................................................................................................. 4 1.2.3 Solidification of Austenitic Stainless Steels ....................................................................... 4 1.2.4 Solidification Crack Susceptibility Testing ........................................................................ 7 1.3 Research Objectives ............................................................................................................ 7 1.4 Organization of Thesis ........................................................................................................ 8 1.5 References ........................................................................................................................... 9 CHAPTER 2: FIBER LASER WELDING OF HIGH-N, HIGH-MN AUSTENITIC STAINLESS STEEL .............................................................................................................................. 12 2.1 Abstract ............................................................................................................................. 12 2.2 Introduction ....................................................................................................................... 13 2.3 Experimental ..................................................................................................................... 13 2.4 Results and Discussion ..................................................................................................... 16 2.4.1 Bead Morphology ............................................................................................................. 16 2.4.2 Porosity ............................................................................................................................. 20 2.4.3 Nitrogen Loss .................................................................................................................... 25 2.4.4 Microhardness ................................................................................................................... 28 2.5 Conclusions ....................................................................................................................... 30 2.6 Acknowledgements ........................................................................................................... 30 2.7 References ......................................................................................................................... 30 2.8 Copyright .......................................................................................................................... 32 CHAPTER 3: SOLIDIFICATION BEHAVIOR OF LASER WELDED TYPE 21CR-6NI-9MN STAINLESS STEEL ........................................................................................................ 33 3.1 Abstract ............................................................................................................................. 33 v 3.2 Introduction ....................................................................................................................... 34 3.3 Experimental ..................................................................................................................... 35 3.4 Results and Discussion ..................................................................................................... 38 3.4.1 Microstructural Characterization: Solidification Mode and Ferrite Content .................... 38 3.4.2 Nitrogen Loss .................................................................................................................... 45 3.4.3 Solidification Rate ............................................................................................................ 49 3.4.4 Constitution Diagram Considerations ............................................................................... 51 3.5 Conclusions ....................................................................................................................... 54 3.6 Acknowledgements ........................................................................................................... 55 3.7 References ......................................................................................................................... 55 CHAPTER 4: MEASUREMENT OF SOLIDIFICATION RATE WITHIN A LASER WELD POOL . 59 4.1 Abstract ............................................................................................................................. 59 4.2 Introduction ....................................................................................................................... 59 4.3 Theoretical Considerations ............................................................................................... 61 4.4 Experimental ..................................................................................................................... 62 4.5 Results and Discussion ..................................................................................................... 63 4.6 Impact on Previous Results ............................................................................................... 66 4.7 Conclusions ......................................................................................................................
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