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Martensite and austenite behavior in cryogenic, subzero and stress relief treatments


There are claims in the literature that AISI D2 tool steel properties are improved when cryogenic treated (CT), in summary, it is said that there is a more completed martensitic transformation and the temper carbides are more refined and homogeneously distributed in the matrix. Additionally, before CT, usually is carried out a stress relief treatment (SRT) to avoid catastrophic failures. The phase transformations able to happen in such temperatures must be in the matrix of the steel. So, this work will compare the behavior of AISI D2 with a clean steel, which composition is the composition of the matrix of the AISI D2, in order to analyze the mechanisms that are occurring without the influence of primary and ledeburitic carbides. There are evidences that subzero temperature (~-80ºC) is more efficient to martensitic transformation if compared to cryogenic temperature (~-196ºC), martensite grows athermally, but, it is necessary time and temperature for nucleation of such phase. It is also said that there is a "martensite conditioning" (aging) during CT, producing carbon cluster at martensite dislocations (or twins), such clusters will act as nucleus for temper carbides. It was supposed that the SRT would not cause any phase transformations, but it was seen that such treatment has a big austenite stabilization effect and that there is an increase in the austenite volumetric fraction due to SRT. It is proposed a study considering the phase transformations occurring during SZT, CT and SRT. It will be carried out a set of thermal treatments with combinations of different thermal cycles and will be evaluated the microstructures and phases present by Scanning Electron Microscopy and X-Ray Diffraction. (AU)

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