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33020240-MIT.Pdf C3WS.OF TECU 1j 0~0 JUN 7 1961 LJB RAPS' DESULFURIZATION OF VACUUM INDUCTION MELTED HIGH STRENGTH STEEL: RELATION TO MECHANICAL PROPERTIES by John B. Dabney A.B. Bowdoin College 1956 S.M. Massachusetts Institute of Technology 1958 Submitted in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF SCIENCE from the Massachusetts Institute of Technology November, 1960 Signature Redacted Signature of Author Department of Me4llurgy Signature Redacted Certified by Signature Redacted Signature Redacted Accepted by If I,,' z WNt Chairman,Departmental Committee / Graduate Students DESULFURIZATION OF VACUUM INDUCTION MELTED HIGH STRENGTH STEEL: RELATION TO MECHANICAL PROPERTIES Submitted to the Department of Metallurgy on November 22, 1960 in partial fulfillment of the requirements for the Degree of Doctor of Science ABSTRACT A method of desulfurizing molten steel during the refining cycle of the vacuum induction melting process has been developed. This method involves the use of a sintered layer of a CaO - 10% CaF2 mixture on the bottom of a magnesia crucible. Final sulfur concentrations of 0.004 weight per cent were consistently attained using an AISI 4300 series steel in which the carbon contentvaried from 0.40 to 0.60 weight per cent. Two grades of charge material which had initial sulfur concentrations of 0.013 and 0.020 weight per cent were used. The final sulfur contents were found to be independent of carbon concentrations and initial sulfur concentrations. In heats melted in magnesia crucibles without the CaO - 10% CaF2 lining, limited desulfurization was obtained at low refining pressures as a result of volatilization. Tensile properties were determined of the sound cast metal, heat treated to a high strength level. Data were analyzed statistically, and the analysis showed the ultimate tensile strength and per cent reduction in area could be satisfactorily estimated as linear functions of the carbon and -1 ii. sulfur concentrations in the steel. The relationships are: UTS = 273,132 (%C) + 1,075,736 (%s) + 143,889 %RA = 136.82(%C) - 1355.5(%S) + 99.0 The reduction in area of desulfurized 4340 steel varied from 37.0 to 40.4 per cent at tensile strengths of 250,000 to 260,000 pounds per square inch. Tensile strengthsas high as 317,000 pounds per square inch were obtained at higher carbon contents. Thesis Supervisors: Merton C. Flemings, Assistant Professor of Metallurgy Howard F. Taylor, Professor of Metallurgy -q iii. TABLE OF CONTENTS Section Page ABSTRACT ----------------------------------------- i LIST OF TABLES ----------------------------------- v LIST OF FIGURES ---------------------------------- vi ACKNOWLEDGEMENTS --------------------------------- vii I INTRODUCTION ------------------------------------- I II LITERATURE SURVEY -------------------------------- 3 A Behavior of Alloying and Impurity Elements in Liquid Iron under Vacuum ------- 3 B Segregation -------------------------------- 7 C Formation of Sulfides ---------------------- 11 D Effect of Sulfides on Mechanical Properties --------------------------------- 14 E Desulfurization Theory --------------------- 17 III EXPERIMENTAL PROCEDURE --------------------------- 21 A General Procedure -------------------------- 21 8 Equipment ---------------------------------- 21 C Melting, Refining, and Casting ------------- 23 D Analyses and Testing ----------------------- 25 IV RESULTS AND DISCUSSION --------------------------- 26 A Desulfurization ---------------------------- 26 B Control of Alloy Analysis ------------------ 28 C Structure ---------------------------------- 29 D Mechanical Properties ---------------------- 30 iv. TABLE OF CONTENTS (Cont'd.) Section Page V CONCLUSIONS ------------------------------------ 33 VI SUGGESTIONS FOR FURTHER STUDY ------------------ 35 VII BIBLIOGRAPHY ----------------------------------- 36 APPENDIX --------------------------------------- 59 BIOGRAPHICAL NOTE ------------------------------ 63 4 ----------40000000000000- V. LIST OF TABLES Table Number Page I Liquid-Vapor Distribution Coefficients ------ 4 II Solid-Liquid Equilibrium Segregation Coefficients in Liquid Iron ----------------- 9 III Mold Material ------------------------------- 40 IV Heat Treatment ------------------------------ 41 V Chemical Analysis --------------------------- 42 VI Gas Analyses -------------------------------- 43 VII Treatment and Variables of Each Heat -------- 44 VIII Results of Tensile Tests ------------------- 45 vi. LIST OF FIGURES Figure Number Pa ge Segregation of sulfur and oxygen in solidifying liquid iron determined for initial concentrations of 0.01 and 0.001 per cent according to Case B -------- 47 2 Experimental Casting ---------------------------------- 48 3 Details of tensile test specimen ---------------------- 49 4 Estimated ultimate tensile strength as a function of carbon and sulfur as based on experimental data ------- 50 5 Correlation of estimated ultimate tensile strengths with experimental data -------------------------------- 51 6 Estimated per cent reduction in area as a function of sulfur and carbon contents based on experimental data - 52 7 Correlation of estimated per cent reduction in area with experimental data -------------------------------- 53 8 Casting #11 ------------------------------------------- 5+ 9 Casting #114 ------------------------------------------ 54 10 Casting #16 ------------------------------------------- 55 11 Casting #26 ------------------------------------------- 55 12 Casting #11, Air melted, 0.009 per cent sulfur --------- 56 13 Casting #15, Vacuum melted, 0.020 per cent sulfur ------ 56 14 Casting #16, Vacuum melted, 0.013 per cent sulfur ------ 57 15 Casting #17,Vacuum melted, 0.009 per cent sulfur ------ 57 16 Casting #20, Vacuum melted, 0.004 per cent sulfur ------ 58 vii. ACKNOWLEDGEMENTS The author of this thesis wishes to express his sincere appreciation to Professor Merton C. Flemings for his enthusiasm and invaluable advice in all phases of this in- vestigation. He is also indebted to Mr. Richard W. Willard for aid in preparing the statistical analyses. To all others without whose assistance this work could not have been successful, thanks are also extended. -4 I. INTRODUCTION Recent work has indicated that many factors have important roles in determining the tensile and impact ductilities of cast, high-strength, low- alloy steel. Variables such as micro-porosity(l), as-cast grain structure(l), the shape and number of inclusions(2), and the relative amounts of impurities such as sulfur and phosphorous(3,4 ,5) have been observed to limit these pro- perties in air melted steel. Improvement of the ductilities of these steels by vacuum melting and refining is significant and may generally be attributed to (1) a cleaner steel due to a lower oxygen content, (2) less gas-formed micro-porosity due to a lower concentration of residual gases, and (3) a lower concentration of impurities due to a high purity charge and the removal of substantial amounts of impurities having high vapor pressures. Present vacuum melting techniques, though, are unable to remove any great amount of sulfur( 6,7). To achieve optimum quality, therefore, the initial material or charge, must be as free from sulfur as possible(6 ). An effective method of desulfurization has been published which does not involve the use of a slag(8 '9). Sulfur removal by this method in heats melted in air is not particularly good with respect to final sulfur contents except at high carbon levels. However, under a carbon monoxide atmosphere and by use of stronger deoxidizers such as silicon or aluminum consistent final sulfur concentrations of 0.001 weight per cent and better are reported. As excellent deoxidation under vacuum can be achieved by small amounts of carbon alone(lOPl,12), application of this method of desulfurization to the vacuum refining of molten steels should produce results which are equally as good. -2- Since significant increases in ductilities of air-melted, high- strength, low-alloy steels have been reported to accompany desulfurization from 0.023 to 0.008 per cent(3) and from 0.010 to 0.003 per cent(5) it may also be possible to produce similar improvements in vacuum melted steels. Should such effects occur to an appreciable extent the following possibilities present themselves: 1. The ductilities currently attained for vacuum melted steels may be further improved. 2. The sulfur content of the charge may be increased without sacri- ficing the ductility of the product. 3. Tensile and yield strengths of the steel may be increased through modification of alloy content while still maintaining adequate ductility. -3- II. LITERATURE SURVEY A. Behavior of Alloying and Impurity Elements in Liquid Iron Under Vacuum. Most purification during vacuum refining results from the distilla- tion of atoms or molecules from the liquid metal surface. The rate at which an element volatilizes from a liquid metal bath is proportional to the partial pressure of that element. Ideally, the maximum rate of volatili- zation of a pure element is given by: Wmax = 3.5P (M/T)1/ 2 (13) Eq. I where Wmax = gm/cm2 /sec P = Vapor pressure of the element ( dynes/cm2 ) M = Molecular weight of the element (grams) T = Temperature (*K) For iron at 1600*C. this equation gives a value of about 37 grams/cm2 /sec which is obviously several orders of magnitude too great. This can in part be accounted for by the following factors: (1) the supply of heat to the surface from which the evaporation is occurring is not great enough to maintain this rate
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