MICROSTRUCTURAL CHARACTERIZATION OF LOW CARBON STEELS BY ATOMIC FORCE MICROSCOPY

Abstract

Microstructural  characterization  of  steels  is  normally  performed  using  optical  and  scanning  electron  microscopy  which allows  identifying  the  most  important  phases  and  constituents  in  steels.  Nevertheless,  the  microstructural  complexity  of some  constituents  during  continuous  cooling  hinders  the  characterization  of  them;  especially  bainitic  and  martensitic microstructures provide rich  morphology characteristics tough  to solve properly. In this sense, atomic  force  microscopy was applied in order to provide complementary information to optical and scanning electronic microscopy in continuous cooling  microstructural  constituents  of  a  low  carbon  and  a  Nb  microalloyed  low  carbon  steels.  The  samples  were austenitized, some of them  were quenched directly  from austenitization temperature and others  were cooling to Ar3  and Ar1  temperatures  and  then  quenched.  The  samples  quenched  from  austenitization  temperature  showed  predominantly bainitic and  martensitic  microstructures, some  microstructural characteristics  were only  elucidated  with the combination between scanning electron microscopy and atomic force microscopy. On the other hand, the samples quenched from Ar3 temperature  showed  polygonal  ferrite  and  martensite  morphologies.  Atomic  force  microscopy  has  also  indicated  the occurrence  of  bainitic  ferrite.  Concerning  samples  quenched  from  Ar1  temperature,  the  microstructure  was  essentially ferrite and fine perlite.