The Pennsylvania State University The Graduate School College of Engineering THE DEVELOPMENT OF ULTRAHIGH STRENGTH LOW ALLOY CAST STEELS WITH INCREASED TOUGHNESS A Dissertation in Industrial Engineering by Paul C. Lynch © 2011 Paul C. Lynch Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy May 2011 The dissertation of Paul C. Lynch was reviewed and approved* by the following: Robert C. Voigt Professor of Industrial Engineering Dissertation Adviser Chair of Committee Richard A. Wysk Professor Emeritus of Industrial Engineering Edward C. DeMeter Professor of Industrial Engineering Chantal Binet Material Process Researcher, Center for Innovative Sintered Products Paul M. Griffin Peter and Angela Dal Pezzo Chair Professor Head of the Department of Industrial Engineering *Signatures are on file in the Graduate School ii Abstract This work describes the initial work on the development of the next generation of ultrahigh strength low alloy (UHSLA) cast steels. These UHSLA cast steels have both ultrahigh strength levels and good impact toughness. The influence of heat treatment, secondary processing using hot isostatic processing (HIP), and chemical composition on the microstructure and properties of UHSLA cast steels have been evaluated. The extent of microsegregation reduction expected during the heat treatment of UHSLA cast steels has also been estimated by diffusion modeling. This new family of UHSLA cast steels is similar in composition and properties to UHSLA wrought steels. However, the heat treatment and secondary processing of the UHSLA cast steels is used to develop microstructures and properties typically developed through thermomechanical processing and heat treatment for wrought UHSLA steels. Two martensitic UHSLA steels, 4340+ (silicon modified 4340) and ES-1 were investigated for this study. For the 4340+ alloy, heat treatment variables evaluated include homogenization temperature and time, tempering temperature, and austempering temperature and time. For the ES-1 alloy, heat treatment variables evaluated include homogenization temperature and time, austenization temperature, cryogenic treatment, and tempering temperature. The effect of high temperature hot isostatic processing (HIP) on the 4340+ and ES- 1 alloys was also investigated. Tensile properties, charpy v-notch impact toughness (CVN), microstructures, and fractographs have all been characterized after heat treatment. The effects of HIP on microporosity reduction in the ES-1 alloy were also investigated. The experiments carried out on the investment cast 4340+ alloy have shown that increasing the homogenization temperature can increase CVN without changing the ultimate tensile strength (UTS) or yield strength (YS) of the cast material. By replacing the homogenization step in the conventional heat treatment process with a high temperature HIP treatment, both the CVN and ductility of the alloy was found to increase while maintaining comparable ultimate tensile strength (UTS) and yield strength (YS) levels as compared to the original homogenization treatment. Austempering the (IC) 4340+ material led to a significant increase in CVN and ductility at the expense of UTS and yield strength as the primarily martensitic microstructure was converted to a mixed martensitic-bainitic structure. iii Excellent impact and tensile properties were possible with vacuum degassed ES-1 cast ingot material when the material was subject to a high temperature HIP homogenization cycle prior to heat treatment. A high temperature HIP homogenization cycle following by a second high temperature homogenization cycle, high temperature austenization cycle, and a low temperature tempering step had the potential to produce cast ES-1 ingot material with 40 ft-lbs. of CVN impact toughness at -40°F while possessing 15% elongation, yield strength > 190 ksi., and an ultimate tensile strength of > 250 ksi. The impact transition behavior of this material showed evidence of the excellent impact toughness exhibited by the material across a wide range of temperatures (-100°F to +212°F). The lower shelf energy for the cast + HIP ES-1 alloy could not be estimated from the impact toughness vs. temperature curves shown, because the lower shelf energy for this alloy occurs at a temperature below -100°F, the lowest impact testing temperature in the study. A high temperature HIP homogenization treatment of cast ES-1 ingot material significantly reduces both the average number of pores and the average area fraction or average % porosity. A high temperature homogenization treatment (without pressure) significantly increased the average number and average area fraction (% porosity) of the cast ES-1 material. No retained austenite was found in the as cast, as quenched, or fully heat treated ES-1 ingot samples that were analyzed using XRD analysis. Cryo-treatment led to a small improvement in hardness at the expense of -40°F impact toughness. An initial heat of induction melted, aluminum deoxidized investment cast ES-1 with 0.06 wt % of aluminum showed that the average -40°F and +72°F impact toughness, % elongation, and UTS and YS of the fully heat treated investment cast + HIP ES-1 material lagged significantly behind that of the vacuum degassed cast + HIP ES-1 ingot material. Even though the % elongation and impact toughness of the investment cast ES-1 material changed between heat treatment conditions, the average UTS and YS values remained relatively unchanged throughout the heat treatments for the investment cast study. Etched micrographs of the investment cast ES-1 material showed evidence of significant differences in microsegregation reduction between the samples homogenized at 2125°F for 4 hours and those not homogenized at 2125°F for 4 hours. SEM fracture surface work performed on the investment cast material clearly showed that the induction melted investment and aluminum killed cast material contained iv significant amounts of MnS and Al O inclusions that were not discovered in the vacuum 2 3 degassed cast ingot material. Lastly, the results of a third heat of induction melted, aluminum deoxidized investment cast ES-1 material possessing just 0.01wt% of aluminum showed that the decrease in aluminum content from the first experimental heat did not improve the mechanical properties of the investment cast material. Casting section size (cooling rate) is shown to directly influence the amount of segregation reduction possible during HIP or homogenization treatments of UHSLA steel castings. The segregation reduction possible depends not only on the alloys present and the homogenization time and temperature, but also on the DAS of cast steels. Model estimates show that little, if any diffusion of substitutional alloying elements will occur during the homogenization of steels castings with DAS ≥ 200 μm regardless of the homogenization temperature. Throughout this study, high temperature (1950°F - 2125°F) HIP cycles as well as high temperature homogenization cycle improved the impact toughness of UHSLA cast steels. Diffusion modeling suggests that high temperature HIP cycle can also significantly reduce the microsegregation of substitutional alloying elements and can therefore replace the homogenization step in the heat treatment of UHSLA cast steels. The research suggests that HIP processing improves the toughness by significantly reducing microporosity formed during solidification. v TABLE OF CONTENTS Page LIST OF FIGURES xi LIST OF TABLES xviii ACKNOWLDEGEMENTS xxii CHAPTER ONE – INTRODUCTION 1 CHAPTER TWO – LITERATURE REVIEW 4 2.1 Background of Ultrahigh Strength Steels 4 2.1.1 Roles of Alloys in Strengthening and Toughening of Steels 4 2.1.1.1 Carbon (C) 5 2.1.1.2 Silicon (Si) 6 2.1.1.3 Manganese (Mn) 7 2.1.1.4 Molybdenum (Mo) 8 2.1.1.5 Aluminum (Al) 9 2.1.1.6 Chromium (Cr) 9 2.1.1.7 Nickel (Ni) 9 2.1.1.8 Vanadium (V) 10 2.1.1.9 Tungsten (W) 10 2.1.1.9.1 The Effects of Tungsten in UHSLA Steels 11 2.1.1.10 Titanium (Ti) 13 2.1.1.11 Cobalt (Co) 13 2.1.1.12 Phosphorus (P) 14 2.1.1.13 Sulphur (S) 14 2.2 Ultrahigh Strength Low Alloy (UHSLA) Steel Development 14 2.2.1 Wrought UHSLA Steel Composition 15 2.2.1.1 4340 Steel 16 2.2.1.2 300M (4340+) Steel 17 2.2.1.3 D-6AC Steel 17 vi 2.2.1.4 Hy-Tuf Steel 18 2.2.1.5 Ultrahigh Strength Low Alloy Steels: Transition Carbide Strengthening 18 2.3 High Strength Steels with High Impact Toughness 19 2.3.1 High Alloy Maraging Steels: Precipitate Strengthening 19 2.3.2 Secondary Hardening Steels: Fine Alloy Carbide Strengthening 20 2.3.2.1 AF 1410 Steel 23 2.3.2.2 Aermet 100 23 2.4 High Strength Steel Castings 24 2.5 Eglin Steel 26 2.6 Processing of Cast Steels 29 2.6.1 Steel Melting Practices 30 2.6.2 Solidification and Segregation 32 2.6.3 Steel Heat Treatment 33 2.6.4 Homogenization 33 2.6.4.1 The Effect of Homogenization Temperature on Toughness 33 2.6.4.2 Diffusion During Homogenization 34 2.6.4.3 Diffusion Models 35 2.6.5 Austenization 38 2.6.5.1 The Effect of Austenization Temperature on Toughness 41 2.6.5.2 Retained Austenite in Steels 43 2.6.5.2.1 The Effect of Retained Austenite on the Mechanical Properties of Steel 45 2.6.5.2.2 Alloying Elements and their Impact on Retained Austenite 47 2.6.6 Tempering 48 2.6.6.1 Temper Embrittlement 50 2.6.6.2 Tempering and Carbide Formation in UHSLA and Intermediate Alloy Steels 53 2.6.7 Austempering 55 2.7 Secondary Processing of Cast Steels 60 2.7.1 HIP of Cast Steels 61 2.8 Summary of Chapter 2 67 vii CHAPTER THREE – EXPERIMENTAL PROCEDURES 69 3.1 Approach 70 3.2 Materials 71 3.2.1 Investment Cast 4340+ Steel 71 3.2.2 Cast Ingot ES-1 Material 72 3.2.3 Investment Cast ES-1 Material 73 3.3 Investment Cast (IC) 4340+ Heat Treatment Studies 76 3.3.1 Experimental Heat Treatment Procedures for (IC) 4340+ (300M) Steel 76 3.4 Cast ES-1 Ingot Heat Treatment Experimentation 80 3.4.1 Initial Heat Treatment Procedure for Cast ES-1 Ingot 80 3.4.2 Microporosity Study Heat Treatment Procedure for Cast ES-1 Ingot 82 3.4.3 Cryo Quenching and Retained Austenite Study for Cast ES-1 Ingot Material 82 3.4.4 Transition Carbide Characterization Study for ES-1 Material 83 3.5 Investment Cast ES-1 Screening Experiments 84 3.6 Mechanical Testing Procedures 86 3.7 Microstructure Evaluation 86 3.7.1 Pore Size and Pore Reduction Quantification 87 3.8 Fractograph Characterization 87 3.9 Crystallographic Characterization 88 3.10 Impact Transition Curves 88 3.11 Modeling Studies 88 CHAPTER FOUR – MODELING MICROSEGREGATION REDUCTION IN UHSLA STEEL CASTINGS 89 4.1 Diffusion in Steel Castings 89 4.2 Microsegregation Reduction Modeling 90 4.3 Diffusiviety Coefficient Selection 94 4.4 Model Implementation 96 4.5 Model Visualization 98 4.6 Model Estimation Results 99 4.6.1 Microsegregation Reduction During Homogenization / HIP 99 viii 4.6.2 Diffusion Model Verification 104 4.6.3 Microsegregation Reduction During Full Heat Treatment 106 CHAPTER FIVE – RESULTS 107 5.1 Results for Investment Cast (IC) 4340+ Cast Steel Screening Experiments 107 5.1.1 Study #1: Effect of Homogenization Temperatures 108 5.1.2 Study #2: HIP of Investment Cast 4340+ (300M) Steel 112 5.1.3 Study #3: Austempering of Investment Cast 4340+ (300M) 115 5.1.4 Study #4: Austempering + HIP of Investment Cast 4340+ 118 5.1.5 Summary of the Initial Experimentation of 4340+ Cast Steel 122 5.2 Results for Cast ES-1 Ingot Experiments 123 5.2.1 Impact Transition Curves and Mechanical Properties 124 5.2.2 Micrographs and Fractographs 125 5.2.3 Estimation of Microsegregation Reduction 129 5.3 Results for Cast ES-1 Ingot Porosity Reduction Study 131 5.4 Results for Cryo Quenching Study for Cast ES-1 Ingot Material 134 5.5 XRD Retained Austenite (RA) Study Results for Cast ES-1 Ingot Material 136 5.6 Results for Transition Carbide Characterization Study for ES-1 Material 139 5.7 Results for Investment Cast ES-1 Screening Experiments (Heats 1&2) 140 5.7.1 Mechanical Property Results 142 5.7.2 Micrographs and Fractographs 142 5.7.3 Microsegregation Reduction Estimation for Heat Treatments 1-6 149 5.8 Results for Investment Cast ES-1 Screening Experiments (Heat 3) 151 5.9 Summary of the Initial Experimentation with ES-1 Cast Steel 152 CHAPTER SIX – DISCUSSION 154 6.1 Steel Melting and Pouring Practices 154 6.2 Heat Treatment 156 6.2.1 Homogenization 157 6.2.2 HIP Homogenization 159 6.2.2.1 Microporosity Reduction During HIP and Homogenization 162 ix 6.2.2.2 Microsegregation Reduction During Homogenization and HIP Homogenization 166 6.2.3 Austenization 173 6.2.3.1 Retained Austenite in UHSLA Cast Steels 175 6.2.3.2 Cryo Quenching of cast ES-1 Steel 177 6.2.4 Austempering 178 6.2.5 Tempering and Temper Embrittlement 179 6.3 Low Temperature Tempering of Lower Carbon Content UHSLA Steels 182 6.4 Eglin Steel (ES-1) Alloy Cost Analysis 183 6.5 Cast + HIP vs. Forged ES-1 Routing Summary 184 CHAPTER SEVEN – CONCLUSIONS 187 RECOMMENDATIONS FOR FUTURE STUDY 190 APPENDIX 192 APPENDIX A.1 Cast ES-1 Ingot EPMA Study 192 APPENDIX A.2 Cast + HIP ES-1 Ingot Study Results 198 APPENDIX A.3 Cast + HIP ES-1 Ingot Microporosity Study Results 203 APPENDIX A.4 Investment Cast ES-1 Study Results 206 BIBLIOGRAPHY 209 x