Unable to convert pandas object to string

[Bhaskar Dhariyal]

<class 'pandas.core.frame.DataFrame'>
Int64Index: 171594 entries, 0 to 63464
Data columns (total 7 columns):
project_id          171594 non-null object
desc                171594 non-null object
goal                171594 non-null float64
keywords            171594 non-null object
diff_creat_laun     171594 non-null int64
diff_laun_status    171594 non-null int64
diff_status_dead    171594 non-null int64
dtypes: float64(1), int64(3), object(3)

not able to convert desc and keywords to string for preprocessing.
Tried astype(str). Please help