Estudo de correlações tipo taxa de resfriamento x microestrutura x dureza da liga resistente ao calor 28Cr48Ni5W3Co
Abstract
The 28Cr48Ni5W3Co alloy is widely used in furnace systems and various
petrochemical plant equipment. In this work, the directional solidification of this alloy
in a transient heat extraction regime was carried out, obtaining cooling rates according
to the dendritic length-scales and enabling a correlation between both. An analysis
was performed by sizing the secondary dendritic spacings (λ2) finding lower values (~
16 m) near the metal/mold interface and higher values along the solidification
direction (~ 120 m). The directionally solidified alloy casting showed an equiaxed
morphology, with dendritic growth along its entire length. The thermodynamic analysis
by CALPHAD method showed, from the Scheil model, that at the time of the first
measurements with the thermocouples inside the casting there was no eutectic liquid
present, resulting in rates that do not describe the beginning of solidification, but
instead represent a global processing cooling rate. Within the range of the obtained
cooling rates (from 0.03 ºC/s to 22 ºC/s), the analyzes revealed that increasing the
cooling rate, in addition to reducing the secondary spacing, reduces the carbide size.
The microstructure formed is composed of an austenitic matrix rich in nickel and
chromium with carbides precipitated at the interdendritic portions. The secondary
dendritic spacings were related to the cooling rate, and with this, an adjusted equation
was generated that described the behavior of the microstructure along the casting.
Finally, the Brinell hardness results indicated that there was a high correlation with the
cooling rate.
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