  For the first time, cyclic re-austenitization is carried out for additively
manufactured high-strength low-alloy (HSLA) steels in order to refine the
microstructure by reducing the prior austenite grain (PAG) size. In this work,
HSLA-100 steels processed using laser powder bed fusion (LPBF) technique are
subjected to several cycles of re-austenitization using quenching dilatometry.
Microstructure characterization for every cycle revealed the presence of
bainite, martensite and martensite/austenite (M/A) islands. From the analysis
of the dilatometry curves and extensive microstructure characterization, it was
found that till the 2nd cycle of re-austenitization, both PAG size and
martensite start (Ms) temperature get reduced, while the amount of bainite
transformed decreased and the retained austenite content increased.
Concomitantly, the highest microhardness along with peak nanohardness of the
constituent phases was achieved at the 2nd cycle. Conversely, from the 3rd
cycle, the microhardness, as well as the nanohardness of the constituent
phases, are found to decrease due to an increase in the PAG size. This behavior
is in contrast to the general tendency where a saturation limit is reached
after the peak refinement is achieved. It is found that retained austenite can
act as a pinning particle to obstruct the PAG boundary movement and its
fraction is found to decrease from the 3rd cycle. Hence, the increase in PAG
size after the 3rd cycle can be attributed to the destabilization of effective
pinning particles to hinder the PAG boundary movement during the
re-austenitization.
