Microstructure and Mechanical Properties of a Tempered High Cr Martensitic Steel
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Microstructure and Mechanical Properties of a Tempered High Cr Martensitic Steel L. Guerra-Fuentes, M.A.L. Hernandez-Rodriguez, P. Zambrano-Robledo, A. Salinas-Rodriguez, and E. Garcia-Sanchez (Submitted October 20, 2016; in revised form April 26, 2017; published online May 24, 2017) Microstructural and mechanical studies have been performed in a high Cr martensitic steel Firth-Vickers (FV535) to analyze the tempering of martensite. Nanoindentation technique was used to determine the hardness and elastic modulus through systematic measurements on martensite and tempered martensite. On the other hand, microscopic studies were carried out to analyze the material in the same condition as received and subsequently observe the microstructural modifications after heat treatment. The precipitation presented in the last stage of tempering was observed by transmission electron microscopy. The results showed the effect of the martensite decomposition on the mechanical and nanomechanical properties of FV535. Keywords
martensitic steel, nanoindentation, tempering, TEM
1. Introduction The 9 and 12% Cr transformable steels with low carbon content (0.1% max) and additions of Mo, W, V, Nb, N, etc., possess high creep-rupture strength, and present good oxidation resistance at relatively elevated temperatures. The principal uses of high chromium martensitic steels are currently components for gas turbines, boilers, and turbines in steam power plants (Ref 1). The material analyzed in this research is the Firth-Vickers FV535, a 12Cr-Mo-VNbWCo steel with application in aeronautical components with excellent mechanical properties at relatively high temperatures (Ref 2). These types of steels present a martensitic structure, and when it is necessary to modify its mechanical properties, a tempering process is performed. The stages of tempering are: (1) segregation of carbon atoms, (2) precipitation processes that occur below 80 °C and consist of the formation of clusters of carbon atoms in the iron matrix, (3) between 80 and 180 °C occurs the precipitation of e-carbide, (4) taking place in the temperature range of 200-350 °C and associated with the transformation of retained austenite into ferrite and cementite; (5) between 250 and 500 °C involves the conversion of the transition carbide into cementite; (6) and the sixth stage, assigned to the dissolution of cementite and the formation of more stable precipitates, only if strong carbide formers as chromium, molybdenum, tungsten, and vanadium are present; and takes place at temperatures between 450 and 700 °C (Ref 3, 4). All these stages promote microstructural modifications in
L. Guerra-Fuentes, M.A.L. Hernandez-Rodriguez, P. ZambranoRobledo, and E. Garcia-Sanchez, Universidad Auto´noma de Nuevo Leo´n, Facultad de Ingenierı´a Meca´nica y Ele´ctrica, San Nicola´s de los Garza, NL, Mexico; and A. Salinas-Rodriguez, CINVESTAV Unidad Saltillo, Ramos Arizpe, Coah, Mexico. Contact e-mail: [email protected].
3500—Volume 26(7) July 2017
FV535 steel, whic
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