the grease in the lazy susan type swivel has probably become hardened and thus making it swivel hard. Try taking them outside and flipping them upside down and squirting a little WD-40 or even some sewing machine oil in the roller groves where the ball bearings are. Turn seat back and forth a little and see if that loosens them up. Too much lubricant will make it drip oil everywhere.

Clean the joints with a strong descaller (this will often remove rust too) make sure that rinse them thoroughly as you don't want this to react with any other cleaning chemicals and make sure you choose a descaller that is safe to use on the metal in question as some descallers will discolour stainless steel and chrome.

Now spray a solvent based disperser like WD40 into the joint and finally use some grease to lubricate the joint.

WD-40 can fix that hard to swivel stool :)