Improvement of notch fatigue properties of ultra-high CM400 maraging steel through shot peening
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Shot-peened CM400 maraging steel was used to study the mechanism of enhanced notch fatigue properties of ultra-high strength materials. After shot peening, the specimen surface became rougher, but the transversal machining traces were reduced. The yield strength was slightly improved while the ultimate tensile strength and hardness maintained constant; as a result, the fatigue limit was promoted by about 1.5 times. The nucleated sites of the fatigue fracture were partly changed from the surface to subsurface/interior of the specimen. To further analyze the influencing factors of fatigue properties, the fatigue damage process may be resolved to two aspects: (a) fatigue damage rate affected by shear deformation and (b) fatigue damage tolerance controlled by the dilatation fracture process. Considering the stress state near the notch tip, the hydrostatic stress and maximum shear stress are considered for better understanding these two aspects. It is observed that the fatigue damage tolerance increased while the fatigue damage rate decreased after shot peening. Therefore, the notch fatigue properties of CM400 maraging steels can effectively be improved.
I. INTRODUCTION
It is well known that enhanced fatigue properties are the safety guarantees for the long-time service of structural materials.1 In the early studies,1,2 researchers indicated that the fatigue limit is proportional to the ultimate tensile strength (UTS); in other words, strengthening is an effective method to improve the fatigue property. On the other hand, the tensile strength of advanced structural materials has been promoted to a very high level with the improvement of different strengthening mechanisms, e.g., the ultra-high strength steels (UHSS) to almost 3 GPa.3 But the fatigue properties are not improved at the same time as expected, and a part of them even exhibit the lowering fatigue properties.4–6 Pang et al.7 have pointed out that fatigue limits showed a parabolic change with the increase of UTS, which has been verified in large number of alloys. Since the loss of fatigue limit is inevitable with strengthening the material, whether the traditional methods are still valid or not for improving the fatigue properties of materials with very high strength? To answer the question above, the UHSS has been selected for this study. The maraging steels, as one kind of UHSS, can possess unique combinations of very high Contributing Editor: Mathias Göken Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2017.358
strength and excellent toughness by adding several kinds of alloying elements.3,8–11 Maraging steels can be effectively strengthened after aging treatment, and its strength belongs to the highest level (;2.7 GPa) in different kinds of steels3,12–14; its toughness is relatively higher than other steels in the same strength level.15 However, when the maraging steels are further strengthened through optimizing their composition and heat treatment techniques, their toughness is unf
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