Microstructure Refining and Strengthening of Martensitic Steel
The improvement of delayed fracture (DF) and fatigue resistance for steels, especially the former of martensitic steel, through microstructure refining and strengthening are the main concern in this chapter. Firstly, the effects of microstructure refineme
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The improvement of delayed fracture (DF) and fatigue resistance for steels, especially the former of martensitic steel, through microstructure refining and strengthening are the main concern in this chapter. Firstly, the effects of microstructure refinement in toughening and improving DF property of martensitic steels are studied. Secondly, methods for strengthening grain boundaries such as reducing the amount of segregated embrittling elements at grain boundaries and controlling grain boundary carbides are discussed. Thirdly, the effect of controlling hydrogen trap through microalloying element carbides on improving DF resistance is investigated. Fourthly, the effect of cleanliness on the fatigue performance of martensitic steels is discussed. Finally, new high strength martensitic steels ADF series with excellent combination of toughness, DF and fatigue resistance as well as its characteristics and industrial applications were introduced.
Introduction Higher-strength materials are objectives for which material scientists have strived for many decades. Among these efforts, martensitic steel was the earliest success, which is now still widely chosen for the structural component applications (Hsu,1999) . This is because tempered martensitic steels can offer a good balance between high strength and excellent toughness by deliberately adjusting compositions (carbon and other alloying elements) and designing specific thermal processes. Fatigue and delayed fracture are the two important mechanisms for the failure of metallic materials in service. With increasing the strength of martensitic steel, its toughness generally decreases, and its fatigue failure susceptibility increases, particularly its delayed fracture susceptibility increases remarkably. Therefore, the
6 Microstructure Refining and Strengthening of Martensitic Steel 30] application of higher strength martensitic steel requires the improvement of its toughness, fatigue and delayed fracture resistance simultaneously. In late 1990s, Japan, Korea and China have all implement national research programs with a huge financial support, named as "Ultra Steel", "High Performance Structural Steels for 21st Century" and "New Generation Steel", respectively. Research and development of 1500 MPa grade steel for practical applications was one of the important subjects among all of these programs (Sato, 2000; Lee, 2000; Weng, 2000).
6.1
Challenges of High Strength Martensitic Steel
Toughening of high strength martensitic steels has been an important subject of extensive studies over past several decades and remarkable achievement has been obtained. Therefore, the improvement of delayed fracture and fatigue resistance is mainly concerned thereafter. 6.1.1
Delayedfracture
Delayed fracture (DF) is a phenomenon in which components such as bolt or pre-stressed concrete steel bar suddenly fail after a certain period of duration after loading. It is a kind of environmentally enhanced embitterment, resulting from the interaction among materials, environment and stress (Mats
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