Vacuum Hot Pressed Novel 21-4N Valve Steel Strengthened by Y-Ti-O Through High-Energy Ball Milling
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JMEPEG https://doi.org/10.1007/s11665-020-05268-w
Vacuum Hot Pressed Novel 21-4N Valve Steel Strengthened by Y-Ti-O Through High-Energy Ball Milling M. Arun Prasad and E. Pavithra Submitted: 9 July 2020 / Revised: 12 September 2020 / Accepted: 10 October 2020 In recent years, the selection of engine valves materials is based on high temperature performance, longer maintenance life, prominent creep and corrosion and oxidation resistance. Typical engine valve includes materials like 21-2N and titanium alloys. In order to develop a more suitable material to enhance its performance, new nanodispersion strengthened austenitic stainless steels were developed with and without the addition of Y2O3, Ti and MnN from ferro-alloys (ferro-manganese, ferro-nickel and ferro-chrome) to obtain Fe-21Cr-4Ni-9Mn-0.5C-0.4Si as the required composition through powder metallurgy route. These pre-alloyed powders in proper proportion were milled mechanically within a planetary ball mill to obtain the required composition and nanocrystalline structure. The milled powders were subsequently vacuum hot pressed at 1200 °C and 60 MPa by maintaining it at a vacuum level of 1023 mbar for a period of 2 h. Vacuum hot pressed samples were subjected to densification and hardness studies, and microstructural analysis and characterization studies. Density of vacuum hot pressed samples was 97.9% when compared to its theoretical density. With the addition of Y2O3 and Ti as minor alloying elements, very fine austenitic grain structure was formed with the formation of Y-Ti-O complex oxides. The presence of chromium-rich precipitates was revealed during SEM–EDS analysis at the grain boundaries which was attributed due to lower cooling rate of austenitic stainless steel. TEM analysis revealed homogeneous distribution of Y-Ti-O complex oxides within the metal matrix. A marginal increase in hot pressed density was reported with the inclusion of Y2O3 and Ti in the austenitic matrix along with 28% increase in hardness values. Keywords
austenitic stainless steel, ferro-alloys, mechanical alloying, microstructure, vacuum hot pressing, Y-Ti-O
1. Introduction Due to reasonable corrosion and oxidation resistance and excellent creep resistance at elevated temperatures when compared to ferritic steels, austenitic steels are suitable for high-temperature applications (Ref 1), whereas ferritic and martensitic steel have excellent swelling resistance at high temperatures with inferior creep resistance as a result of its BCC structure. Conventional steels limit its usage above 500 C with reduction in creep resistance due to dissolution of precipitates, coarsening of grains and recrystallization. At high temperatures mechanical properties like tensile strength and stress corrosion resistance of austenitic steels are inferior. Grades of different austenitic steels were modified continuously for better high temperature strength through strengthening mechanisms and alloying. For inlet and exhaust valve application, alloys most commonly used are 21-4N material. Availability of
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