Removal of Non-metallic Inclusions from Nickel Base Superalloys by Electromagnetic Levitation Melting in a Slag
- PDF / 1,442,011 Bytes
- 8 Pages / 593.972 x 792 pts Page_size
- 98 Downloads / 219 Views
DUCTION
SUPERALLOYS have been used in a variety of applications. These include gas turbines, steam turbines, military appliances, medical devices, offshore piping, and heat exchangers.[1,2] The nature of these applications requires materials that can withstand harsh operating conditions. Non-metallic inclusions are among factors that can limit operation conditions of superalloys. Non-metallic inclusions are particles dispersed throughout the matrix of the alloy. Inclusions are usually compounds of metals (iron, silicon, aluminum, etc.) with non-metallic elements such as oxygen, sulfur, and nitrogen.[3–5] Inclusions can adversely affect mechanical and physical properties of superalloys. Mechanical properties such as fracture toughness, impact resistance, fatigue resistance, and hot working conditions are greatly reduced by the presence of inclusions.[6–9] Incoherency between lattice planes of the matrix and the inclusion results in stress concentration and appearance of cracks that could ultimately lead
MOHSEN HAJIPOUR MANJILI is with the Department of Materials Science and Engineering, University of Wisconsin Milwaukee, Milwaukee 53211. MOHAMMAD HALALI is with the Department of Materials Science and Engineering, Sharif University of Technology, Azadi Street, Tehran 14588, Iran. Contact e-mail: [email protected] Manuscript submitted May 21, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
to failure of the component. Larger inclusions are generally more detrimental to the mechanical properties of superalloys. It has also been reported that hard and brittle inclusions are more harmful than soft inclusions.[3,5,6] It is therefore of great interest to reduce inclusion content and size in superalloys. Three main sources of non-metallic inclusions have been identified in superalloys.[10] These include: 1. Impurities originating from the starting material(s) and carried over to the final product. 2. Impurities and inclusion content from scrap material used in superalloy production. 3. Inclusions formed during the processing of the alloy. Standard methods employed to process and refine cast superalloys include secondary vacuum induction melting (VIM), electroslag refining (ESR), and vacuum arc remelting (VAR). A combination of these techniques is used in the processing of most superalloys and special steels.[11] VIM process serves to remove some of the inclusions by filtering and/or slag–metal reactions, extract dissolved gases such as oxygen and nitrogen, and evaporate harmful impurities such as lead and bismuth. In addition to decreasing gas content and enhancing mechanical properties, the ESR process is particularly useful in removing non-metallic inclusions. VAR helps to remove nitrogen and other dissolved gases. The alloy is also homogenized while some of the inclusions are removed. Electron beam cold hearth refining (EBCHR) has been used to some extent for refining some alloys.[12] It must, however, be mentioned that this technique is costly, and some researchers have
expressed doubt about its usefulness.[13] Electron bea
Data Loading...
