Does humming at low volume abuse the vocal cords? I find that if I hum for a long time, I have difficulties while performing.
Humming at low volume shouldn't hurt the vocal folds, in fact it's a good vocal warming up exercise.
But when you say for a long time, how long are you talking about? Humming still puts the larynx and vocal folds to work, if you do it for a really long time they may become strained.
Also you may have a tendendy to hum at lower pitches than your natural voice (I know I have), which will provoke additional strain.
I agree that humming at low volumes should be a great warmup for the voice, as long as you do not force the air out in any way. Don't try to actually sing like that; rather use only barely enough air to make a sound. Doing this, you should be able to life the soft pallet and get a natural vibrato going as well.
I do this quite often and it always makes my voice better. Just don't use too much air!