ELECTROSLAG WELDING: THE EFFECT OF SLAG COMPOSITION ON MECHANICAL PROPERTIES by JAMES S. MITCHELL B.Sc, Queens University at Kingston, 1973 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE In the Department of METALLURGY We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA August 1977 © James Mitchell, 1977 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of The University of British Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date 7 ABSTRACT Previous studies of the properties of electroslag weld metal have been done using electroslag remelted ingots made under welding conditions. This procedure assumes the electrical and thermal regimes of these pro• cesses to be equivalent. To test this assumption an experimental program was devised in which the remelted metal of an ingot and weld made with each of three slag systems was analysed and the mechanical properties examined. The results show that each process imparted different properties to the remelted metal by alloy and inclusion modification. Consequently the above assumption was proved invalid. Special consideration was given to the effect of inclusion composition and overall distribution toward mechanical properties. ii TABLE OF CONTENTS Page ABSTRACT . ii TABLE OF CONTENTS . iii LIST OF FIGURES vi LIST OF TABLES viii LIST OF SYMBOLS ix ACKNOWLEDGMENTS . x Chapter I INTRODUCTION 1 1.1.1 The ESW Process 1 I.1.1.1 Applications of ESW ..... 1 1.1.2 Properties of ESW . 4 1.1.3 General Properties of Welds ............. 6 1.1.3.1 Weld Metal 6 1.1.3.2 The HAZ . 8 1.2.1 Slag Composition 9 1.2.1.1 Liquid Slag Chemistry 9 1.2.1.2 Slag Reactions 10 1.2.1.3 Slag Electrochemical Reactions 16 1.2.1.4 Comparison of ESR-ESW Slag Requirements . 17 1.2.1.5 Slag Characteristics 19 1.2.1.6 Welding Considerations 21 iii Chapter Page 1.2.2 Inclusions and Mechanical Properties 22 1.2.2.1 Inclusions 26 1.2.2.2 Mechanical Properties 29 1.2.3 Gas Porosity 31 1.2.4 Heat Distribution and Structure 33 1.3.1 Summary 35 1.3.2 Statement of the Problem 37 1.3.3 Experimental 38 II EXPERIMENTAL AND RESULTS . 40 II. 1 Materials . 40 II.1.1 Steel 40 II. 1.2 Slag 40 11.1.2.1 CaF2 43 11.1.2.2 A1203 . 43 11.1.2.3 CaO 43 II. 1.2.4 Si02 43 II. 2 Apparatus 44 II. 3 Procedure 46 11.3.1 Chemical Analysis 46 11.3.2 Metallographic Analysis 50 11.3.3 Mechanical Testing ... 56 III DISCUSSION 71 III. l Slag Effects on Alloy Composition 71 III.2 Slag Effects on Inclusion Population 76 iv Chapter Paee 111.3 Inclusion Distribution 83 111.4 Mechanical Properties 86 IV CONCLUSIONS 92 BIBLIOGRAPHY 95 APPENDIX I 98 APPENDIX II 101 APPENDIX III • 102 v LIST OF FIGURES Figure Page 1 Schematic of Electroslag Welding Process 2 2 Hydraulic Press Housing 3 3 Electroslag Production of a Rotor 3 4 Horizontal Cross-section of Melt Bw 7 5 Effect of Slag Basicity on Resulfurization Ratio . 13 6 Slag Basicity vs Sulfur Content of ESR Ingots for Various Remelting Power Modes 15 7 Electrode Tip Current Density vs Fill Ratio of most Electroslag Processes 18 8 Relationship of Shelf Energy and Volume Fraction of Inclusions 23 9 Effect of Second Phase Volume Fraction on Total Strain to Failure 24 10 Effect of Inclusion Shape on Mechanical Anisotropy of an as Rolled, Low Carbon Steel 25 11 Free Energy of Formation of Some Metal Sulfides . 27 12 Example of Centre-line Cracking 34 13 Macrostructure of an ESR Ingot . 36 14 Experimental ESW Apparatus Used 39 15 Published Specifications of Alloy Welten 80-C ... 41 16 Electrode Guide Assembly, Schematic 45 17 Schematic of ESW Configuration Before Starting, Slag Removed ...... 47 18 Sectioning of ESR Ingot and ESW Metal for Metallographic and Chemical Analysis 49 vi Figure Page 19 Inclusion Distribution of ESR Ingots 52 20 Inclusion Distribution of ESW Metal 53 21 Apparatus Used to Deep Etch Samples for S.E.M. Survey . 55 22 Orientation of Impact Specimens Cut from ESR Ingot 59 23 Orientation of Impact Specimens Cut from ESW ... 60 24 to 29 Impact Energy, Lateral Contraction and Fracture Appearance Data 61 to 66 30 Instrumented Impact Oscillograph and Tracing ... 68 31 Anodic Polarization Curves for Pure Iron 75 32 Relationship of [Al] and [0] in Equilibrium with Alumina of Various Activities 78 33a X-Ray Energy Analysis of an Inclusion in Ci . 89 33b X-Ray Energy Analysis of an Inclusion in Cw . 90 vii LIST OF TABLES Table Page 1 Example of Weld Metal Properties of Heavy Section Joints ..... 5 2 Commercial Fluxes used for Electroslag Processes 20 3 Chemical Analysis of Alloy Welten 80-C, as Received 42 4 Remelting Conditions of Ingots and Welds ...... 48 5 Chemical Analysis of Ingot and Weld Metal 51 6 Inclusion Area Fraction, Spacing and Diameters . 54 7 Inclusions Observed on Deep Etched and Ductile Fracture Surfaces 57 8 Observed Morphology of MnS Type Inclusions 58 9 Instrumented Impact Energies of CVN Specimens Tested at 170°C 70 10 Electroactive Surface Areas . 73 11 Summary of Impact Test Data 87 12 Value of. n for Initial Slag. Compositions 101 viii PRINCIPAL SYMBOLS A in metal phase A in slag phase A as a pure liquid A as a pure solid activity of specie A in slag activity of specie A in metallic solution free energy change standard free energy change free energy change at specified temperature equilibrium constant parts per million temperature weight percent ix ACKNOWLEDGMENTS I would like to sincerely thank my research supervisors Dr. A. Mitchell and Dr. E.B. Hawbolt for their assistance and guidance throughout this study. The financial assistance of a National Research Council Scholarship is gratefully acknowledged. x CHAPTER I Introduction I.1.1. The Electroslag Welding Process Electroslag welding (ESW) is the deposition of a filler metal supplied as an electrode through a liquid slag to effect fusion of metallic members, usually plates, see figure 1. Energy for the process is derived from electric resistance heating of the liquid slag. The current path is between the electrode and parent metallic plates. In practice the process is limited to a few orientations and electrode configurations. Weld metal is laid in place by gravity, thus welding can only proceed in the vertical or near vertical direction. The electrode must be of sufficient cross section to accommodate large electric currents and supply weld metal to the joint. This factor and also the alignment in the gap, to prevent short circuiting are critical. These conditions set a minimum to the weld gap and material thicknesses that may be used. However there is no maximum gap or thickness applied to the process. Thus electroslag welding is primarily used in joining material of large cross section with one pass. I.1.1.1. Applications of ESW Electroslag welding has been used for the assembly of castings to produce large machine parts. Hydroelectric turbines, pump housings and press frames have been cast in sections and welded into final products (Figs. 2,3). 1 2 ELECTRODE RUN - OUT BLOCK PARENT PLATE SLAG BATH METAL POOL WELD METAL STARTING SUMP FIG. I •• SCHEMATIC OF ELECTROSLAG WELDING PROCESS. FIG. 2 : HYDRAULIC PRESS HOUSING. REF.43. FIG. 3= ELECTROSLAG PRODUCTION OF A ROTOR . REF. 54. 4 Also the fabrication of rotors has been accomplished by electroslag welding a series of electroslag refined ingots prior to forging, eliminating much of the waste encountered in conventional production practice. As with all technology popular with industry, the reason for the adaptation of electroslag welding is economic. A weld of large cross section may be constructed with a one pass technique resulting in a saving of welding hours. The soft thermal profiles characteristic of the process reduces the need for preheating. These advantages, despite the necessity of a post heat treatment required in some cases, are the reason why the I process is being used. l' 1.1.2. Properties of ESW Mechanical properties required of large section welds are different from those of thinner welds. Tensile strength and elongation specifications vary with weld metal thickness, see table 1, usually the greater the weld section, the lower are these requirements. Bend tests are also performed on welded structures. This is a simple estimate of weld performance and is confined to thinner weld assemblies. The results from this type of test are empirical, the outer fiber elongation and nature of cracking only are reported. Another test often specified for welds is the impact test. This test requires that a minimum absorbed impact energy be obtained from areas within the welded joint at specified temperatures. Again although this test is empirical, some relations to structurejand processing have been correlated. As a result of the solidification sequence it experiences, weld metal has a cast structure. Of this, the grain size and nature of non-metallic TABLE I : EXAMPLE OF WELD METAL PROPERTIES OF HEAVY SECTION JOINTS.