[en] Full text: AISI 52100 steel is a hyper-eutectoid ultra high carbon and low alloy steels type (1.0 weight % of C and 1.5 weight % Cr). When conventionally quenched have a coarse microstructure comprising a mix of twinned and lath martensitic matrix, a dispersion of coarse M₃C type carbides and low level of retained austenite. On the other hand special heat treatments, like subzero and cryogenic and microstructure refining routes, are enable to obtain a structure and superior mechanical properties in the material. This work aim to investigate the relation between 5 different heat treatments and the fracture toughness (obtained by chevron methodology - ASTM E1304). Heat treatment consisted of austenitization at 1000 deg C and 840 deg C (conventional treatments) followed by quenching in an oil bath at 100 deg C, and tempering at 200 deg C, with and without subsequent cryogenic treatments. The heat treated samples were characterized by optical microscopy, SEM (fracture surface), Rockwell C hardness, XRD using Rietveld refinement (levels of retained austenite). The high temperature austenitized samples showed a high grain size and high retained austenite level (since after the cryogenic treatment) in martensite free carbide matrix. The conventional heat treatment show a typical microstructure. The cryogenic treatment did not influence the fracture toughness values. The fracture toughness of high temperature austenitization was lower than the conventional heat treatment due the high grain size. The fracture surface shows an intergranular growth and typical quasi-cleavage, respectively. Reference: [1] Silva, W. S., Estudo da influência da austenita retida sobre a tenacidade à fratura do aço Din-100cr-6 (AISI-52100). Rio de Janeiro : s.n., 2000. 55º Congresso Anual da ABM; Ramos, D. R., Investigação de novas rotas de tratamento térmico em aço para rolamento, Dissertação de Mestrado, Escola Politécnica da USP, São Paulo, 2010. (author)