cobalt monograph series cobalt monograph series cobalt-base superalloys - 1970 november 1970 cobalt alloy permanent magnets december 1971 cobalt-containing high-strength steels September 1974 Cover Front Replica electron micrograph of HP 9-4-n steel after martensitic quenching, showing self-tempered martensiie with cementite parti- cles. Back Thin-foil electron micrograph of 13 Ni (400) maraging steel aged for 4 hours at 900" F (480° C), showing martensite laths; their granular aspect is due to & very fine precipitate, probably a-FoMo. cobalt monograph series cobalt-containing high-strength steels a critical review of the physical metallurgy of cobalt- containing high-strength steels, and a survey of their processing, properties and uses A. MAGNEE J.M. DRAPIER J. DUMONT D. GOUTSOURADIS L. HABRAKEN Centre de Reoherches MStallurglques, Centre d'Information Centre de Recherches Mitallucgiques, Li&ge, Belgium du Cobalt, Brussels Liege, Belgium CENTRE D'INFORMATION DU COBALT, BRUSSELS 1974 Gentre d'information du cobalt s.a. Centre d'lnformation du Cobalt Rue Royale 66 B-1000 Bruxelles (Belgique). Cobalt Information Center, c/o Battelle Memorial Institute King Avenue 505 Columbus, Ohio 43201 (U.S.A.). Cobalt Information Centre 7 Rolls Buildings, Fetter Lane London EC4A IJA (England). Kobalt-Information Elisabethstrasse 14 D-4 Diisseldorf (Deutsdiland). FOREWORD The first commercial cobalt-containing high-strength steels of the carbide-hardened, maraging and stainless types date back to 1960-1961. They provide a classic example of an industrial break- through based partly on intuitive reasoning, in lhat the final products turned out to exhibit properties which surprised the industrial world, and perhaps even the scientists responsible for their development. Although the introduction of these steels was obviously preceded by several years of laboratory research, the origin of their outstanding properties remained unexplained for quite a time. For example, in his conclusions to the Journees Internationales des Applications du Cobalt, held in Brussels in 1964, the undersigned, reflecting the general consensus of both speakers and audience, had to admit that the role of cobalt in these steels, and more particularly its remarkable strengthening potential when associated with molybdenum, was still obscure. Since then, a considecable amount of information has been generated on cobalt-containing high- strength steels, and further grades have been developed. It progressively became apparent that the difficulty encountered when trying to explain the role of cobalt lay in the fact that its effect, though major, is indirect. In particular, the contribution of cobalt to solid-solution hardening is small, and it is not involved directly in the formation of strengthening precipitates. However, it has a favourable effect on the martensite formation temperature and refines the martensitic structure; it also exerts a decisive influence on the precipitation kinetics, favouring the formation and retention of fine precipitates whose presence results in considerable strengthening; finally, it can participate in an ordering process. Needless to say, much of the new insight into the physical metallurgy aspects of these steels was made possible by the availability of examination techniques of greatly increased sensitivity. The importance of these findings, as well as the growing industrial significance of cobalt-contain- ing high-strength steels, prompted the Cobalt Information Centre to devote the third volume in its " Cobalt Monograph Series " to presenting a critical summary of the knowledge on the three groups of steels under consideration. The task of writing the manuscript was entrusted to the Centre de Recherches Metallurgiques; as was the case for the preceding volumes in the series, the physical metallurgy aspects of these materials were emphasized, but processing, properties and uses were also dealt with, though more concisely. This monograph is based, not only on a comprehensive literature survey, but also on the experience acquired at C.R.M. during its long-standing association with C.I.C., as well as on numerous discussions with specialists in this area of metallurgy. The authors gratefully acknow- ledge the active support received from the following scientists who, in addition to participating in the discussions, also provided initial readings of the manuscript: Dr. J.H. Gross and Dr. S.J. Matas (Chapters III and TV on carbide-strengthened steels); Dr. S. Floreen (Chapters V to VII on Ni-Co-Mo maraging steels); Dr. H. Brandis, Dr. R.L. Caton, Dr. A. Kasak, Dr. A. von den Steinen and Dr. D. Webster (Chapters VIII and IX on stainless maraging steels). They also wish to thank their colleagues both at the Centre de Recherches Metallurgiques and the Centre d'Information du Cobalt for xheir help during the preparation of this volume. In particular, they wish to acknowledge the invaluable assistance of Mrs. H. Lefebvre, of the Brussels C.I.C. office, who assumed a largo part of the editorial work, from manuscript finalization to proof checking. The authors have attempted to present a coherent but concise review of the vast amount of information available to date on the three families of alloy steels. They believe that this volume will prove useful to scientists concerned with the strengthening mechanisms of high-strength steels, to metallurgists involved in promoting them, and to engineers in their continuing search for new materials capable of meeting the increasingly stringent service conditions imposed by present-day technology. If this monograph stimulates a reader response either to identify those aspects which require further clarification or to suggest ways of improving properties, then progress will be assured and the authors' task adequately rewarded. Professor L. Habraken contents iNlRHIHfllOV. ' SlKINi.IlitNISi, MttllAMSUS IS HkiH-S IKfcN'li IH SfEf.LS . 2 2i. Solid-Solution Strengthening - 2 I. Phase-Transformation Sirensthening : Bainilic Reactions . 4 2.2.\. Morphology of Bainites 4 2.^.2. Properties o( Bainiies ft 2.2.3. Role of Cobalt 7 2,.;. Phu>e-Transformation Strengthening : Martensiiic Reactions 7 2 .".I. Cieneral Charaaenstics of Martensiiic "(ran>t'ornii:iions 7 I.'.-. Types of Mancnsile 10 :.:O. Morphology of Lath Martensitc 10 I..v4. Morpholoyy of Twinned Manensite II _.?5. Transition from Lath to Twinned Martensite. ... 12 2.}.b. Properties of Martensites 13 2.3.7. Controlled Martensitic Transformation 15 2.-I. Precipitation Strengthening 16 2.4.!. Mechanisms 16 2.4.2. Carbide Precipitation 18 2.4.3. Precipitation of Intermetallic Compounds .... 19 2.5. Strengthening b> Thermomechanicat Treatment .... 20 C \RBii>t-STRt\t;iHfcNi:i) STF.EI.S — PHYSICAL METALLURGY . 22 3.1. Introduction 22 }.2. Continuous Cooling and Isothermal Transformations . • 23 3.2.1. Continuous Cooling Transformation (CCT) Curves 23 3.2.2. Isothermal Transformation Curves 24 3.?. Biiinitic Transformation Structures 24 3.3.1. Bainites Formed on Continuous Cooling .... 24 3.3.2. Mechanical Properties Associated with Continuous-Cooling Bainites 27 3.3.3. Isothermal Bainite in HF 9-4-45 27 3.3.4. Mechanical Properties of Isothermal Bainite in HP 9-4-45 27 3.4. Martensitic Transformation on Quenching 28 3.5. Tempering Reactions 29 3.5.1. HP 9-4-X Steels 29 3.5.2. 5Ni-Cr-Mo Steels 31 3.5.3. lONi-Co-Cr-Mo Steels 32 3.5.4. Retained Austenite 34 3.6. Effect of Alloying Elements on Tempering Response, Strength and Toughness 34 3.6.1. Effect of Carbon 25 3.6.2. Effect of Nickel 35 3.6.3. Effect of Silicon and Manganese 36 3.6.4. Effect of Carbide-Forming Elements 36 3.6.5. Effect of Cobalt 37 3.6.6. Strength/Toughness vs. Structure Relationship . 38 3.7. Concluding Remarks 39 4. CARBIDE-STRENGTHENED STEELS — PROCESSING AND PROPERTIES 40 4.1. Primary Processing 40 4.2. Properties , 42 4.2.1. Strength/Toughness Characteristics 42 4.2.2. High- and Low-Temperature Properties 46 4.2.3. Fatigue Behaviour 47 4.2.4. Stress-Corrosion Characteristics 47 4.3. Secondary Processing 48 4.4. Applications 49 5. Ni-Co-Mo MARAGING STEELS — PHYSICAL METALLURGY . 50 5.1. Background 50 5.1.1. Role of Alloying Elements 52 5.1.2. Compositions 53 5.2. Martensitic Transformation 54 5.2.1. Formation and Morphology of Martensite .... 54 5.2.2. Factors Controlling Lath Martensite Formation. 56 5.3. Ageing of Martensite 57 5.3.1. Precipitation Reactions 57 . 5.3.2. Ordering 60 5.3.3. The Cobalt/Molybdenum Interaction 60 5.3.4. Maraging Kinetics 62 5.4. Austenite Reversion 64 5.5. Strength/Toughness vs. Structure Relationship 67 6. Ni-Co-Mo MARAGWG STEELS — THE CONVENTIONAL GRADES 68 6.1. Primary Processing 68 6.2. Properties 68 6.2.1. Strength/Toughness Characteristics 68 6.2.2. High- and Low-Temperature Properties 72 6.2.3. Fatigue Behaviour .',...' 74 6.2.4. Stress-Corrosion Characteristics 74 6.3. Secondary Processing 76 6.4. Applications 77 7. Ni-Co-Mo MARAGING STEELS — THE ULTRA-HIGH STRENGTH GRADES 77 7.1. Processing 77 7.2. Properties 79 7.2.1. Strength and Toughness 79 7.2.2. High- and Low-Temperature Properties 79 7.2.3. Other Properties 80 7.3. Applications .80 8. STAINLESS MARAGING STEELS — PHYSICAL METALLURGY . 81 8.1. Background 81 8.2. Effect of Alloying Elements on Equilibrium Structures . 83 8.3; Transformation Temperatures and Structures 85 8.3.1. Martensitic Transformation 85 8.3.2. Austenite Reversion 88 8.3.3. Retained Austenite 89 8.4. Grain Size 90 5.5. Ageing Reactions 91 ii.5.1. Iron-Chromium System 91 S.5.2. Fe-Cr-Co and Fe-Cr-Ni Systems 92 8.5.-1. Fe-Cr-Co-Mo and More Complex Ailoys ..... 94 8.5.4. Concluding Remarks 98 8.6. Snengih Toughness vs. Structure Relationship 99 5.6.1. General '99 5.6.2. Effect of Retained Ausienite 99 8.6.3.