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Effect of Trace Elements on the Mechanical Properties and Microstructure of 18 Wt.% Nickel Maraging Steel

Author: MuBo
Tutor: XuYuSong
School: Jiangsu University of Science and Technology
Course: Materials Science
Keywords: 18 wt.% Nickel maraging steel SCC Strain rate precipitates precipitation strengthening
CLC: TG142.24
Type: Master's thesis
Year: 2011
Downloads: 98
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Abstract


With the development of technology, the requirement to develop performance and machinery of equipment growing quickly, the higher strength and better toughness materials appeared. Therefore, the research of the ultra-high strength steel is one of the major development of materials science. When the maraging steel obtain a high intensity, a certain toughness has loosed. In order to improve the mechanical properties of maraging steel, develop high strength and toughness of maraging steel is particularly important.In this experiment, the compositions of maraging steel was based on 18Ni （250） steel, Nb element was added to improve the microstructure and prosperities. VIM and ESR was used to reduce the impurity elements in the material. Investigative the effect of Nb on the mechanical properties and microstructure of material. Observation of strain rate on SCC behavior. Use the TEM to analysis the microstructure and EDS to determine the precipitate.The results show that: traditional maraging steel in the suction casting process will have the cast dendrites segregation organization. But the segregation of materials adding Nb is better than the traditional materials, the cast can be basically eliminated in high-temperature homogenization treatment（1050℃×2h）. The materials containing Nb after solution treatment（820℃×1h） and aging treatment（480℃×5h） reached the maximum hardness: HRC52.3. Adding Nb makes the material microstructure refinement, uniform, and improve the mechanical properties.When the 18 wt.% Nickel maraging steel in the strain rate of 0.63×10-6s-1, Iδ<35%, showing a clear tendency to stress corrosion, hydrogen embitterment sensitive areas as, but Strain rate of 0.61×10-5s-1 extension of the rate sensitivity index was 34.95%, 29.16%, as a potential danger zone. while the specimen strain rate higher than 1×10-5 s-1, showed a tendency to stress corrosion is not obvious.TEM observation showed that: the main precipitates of 18 wt.% Nickel maraging steel along the {111} direction of the matrix lying on the （Ni,Fe,Co）（3Ti,Mo）type of rod-like precipitates, and its hexagonal crystal structure Ni3Ti System and Ni3Mo the orthorhombic. There are few Fe2Mo spherical precipitates precipitation type, and its structure is hexagonal. 18 wt.%Nickel maraging steel, a high degree of dislocation through the matrix of nano-scale precipitates of precipitation Ni3 （Ti,Mo）, etc.

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CLC: > Industrial Technology > Metallurgy and Metal Craft > Metallurgy and Heat Treatment > Metallic materials > Steel > A variety of steel products : Organization distinguish > Martensitic steel ( Martin steel)
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