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The energy that a material absorbs during its fracture is a very important parameter in the steels. In most parts of the applications in the Market, it is searched a kind of steel that can be resistant and in the same time can have tenacity so in that way, they can be able to avoid catastrophic and fragile failures. On this way, the heat treatment of hardening and tempering is one of the alternatives to be applied for changing the mechanical properties of a big variety of steels. Like this, it is very important that the influences of different conditions of heat treatment that are applied in the same alloy be studied, evaluating the mechanical and microstructural properties which were obtained after the heat treatment. This article has the aim to evaluate the behavior of tenacity of SAE 8640 steel, for that, fifteen pieces of tests were made, they were submitted to the same hardening process and separated in three categories of tempering process (200°C, 400°C e 600°C). It was performed metallographic analysis to get the images of the microstructure as stereomicroscope and electron microscope scanning to get the images of fractured surface together with a hardness test and rehearsal Charpy respecting the ASTM E-23 standard, after the analysis, it was possible to conclude that the material which was submitted to 600°C heat treatment had its hardness decreased and its tenacity increased together with a more ductile behavior than when it was submitted to the materials to200°C e 400°C which kept similar characteristics. [ABSTRACT FROM AUTHOR]