  Radial migration is an important process in the evolution of the Galactic
disk. The metallicity gradient of open clusters and its outliers provide an
effective way to probe for this process. In this work, we compile metallicity,
age, and kinematic parameters for 225 open clusters and carry out a
quantitative analysis of radial migration via the calculated migration
distances. Based on clusters with age $< 0.5$ Gyr, we obtain the present-day
metallicity gradient of $-0.074 \pm 0.007$ dex/kpc. Along this gradient
distributes three sequences, and clusters in the upper, the middle, and the
lower groups are found to be old outward-migrators, in-situ clusters, and
inward-migrators, respectively. The migration distance increases with age, but
its most effective time is probably less than 3 Gyr. The metallicity gradient
breaks out at $R_g$ (guiding center radius) $\sim11.5$ kpc, which is caused by
the lack of young open clusters in the outer disk and the presence of old
outward-migrators in the upper sequence. It shows that this boundary is related
to the different effects of radial migration between the inner and outer disks.
We also found many special open clusters in and near the outer disk of $R > 11$
kpc and a steeper metallicity gradient from the inner disk of $R_g < 7$ kpc,
which tells a complicated evolution history of the Galactic disk by different
effects of stellar radial migration.
