Question

does friction act to make the velocity of a purely rolling body constant by balancing the other net external torque?

Answer 1

What friction are you talking about?
Air friction can act this way, or spinning friction about the axis of rotation.

Answer 2

spinning friction tangential to contact area

Answer 3

In case of pure rolling body on a fixed base, this force is a static friction force and its power is zero.

Such a force will never balance any external torque.

This is why all systems with rolling bodies can be seen as conservative and solved using mechanical energy conservation.

Answer 4

no i meant to say after an external torque is applied to a purely rolling body friction adjusts itself to prevent slipping of the point of contact..then after sometimes does the velocity becomes constant and friction ceases to act?

Answer 5

"no i meant to say WHILE an external torque is applied to a purely rolling body friction adjusts itself to prevent slipping of the point of contact"
– Correct

"..then after sometimes does the velocity becomes constant and friction ceases to act?"
– If net external torque ceases to exist, velocity will be constant and, although friction is there, the friction force will be equal to zero.