SQL & PL/SQL

create table orders
(
order_id varchar2(10) primary key,
prod_id varchar2(10),
order_date date,
QTY integer default 1
);

In orders table i want to disable the default constraint value, but i am not able to drop the constraint as well. Right now I used alter statement. like this ALTER TABLE orders MODIFY qty DEFAULT NULL;
Is any possibility to drop or disable the default constraint in oracle?