Estudo das propriedades mecânicas do aço ao boro 50B35 com diferentes tamanhos de grãos
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Data
2015-07-22Autor
Santos, Ricardo Jonatas Cerutti
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Boron steel has excellent hardenability due to the addition of small quantities of
boron. Boron atoms reduces the nucleation rate of ferrite and bainite [1]. It is
known that austenitizing temperature and time and cooling rate are the main
factors responsible for hardenability loss on boron steel. The austenitizing
temperature relates to the boroncarbide precipitation that happens during the
heat treatment. The boroncarbide precipitation is related to the austenitization
temperature during heat treatment. The austenitization time defines the grain
size condition that during cooling may or not transform into martensite which is
decisive to the final properties. This study was conducted to simulate different
grain sizes for a given boron steel (50B35), keeping the austenitizing
temperature and the cooling rate constant, so that the only variable was the
grain size. Therefore, the objective of this study was to understand the grain
size effect on boron steel by keeping or not the hardenability effect and
characterize the non-martensitic phases like ferrite and bainite and its
consequences on the mechanical properties. It was observed fully martensitic
structure for finer grain size (ASTM N.5), loss of hardenability characterized by
the presence of bainite, for intermediate grain size (ASTM N.4) and
hardenability recovery, by increasing the martensite percentage for bigger
grains size (ASTM N.3). To conclude, there is a limit for the boron atom to lead
the hardenability effect, from a given grain size the effect of hardenability is
driven by the grain size itself, like in a normal carbon steel. There was loss on
the ductility when increasing the austenitic grain size. For intermediate grain
size both yield and tensile strength were reduced and it can be associated to
the presence of bainite. For bigger grain size, both yield and tensile strength
were recovered to levels near those observed at that for smaller grain size but
without recovering ductility.