Unlike Social Security, which follows you wherever you live, traditional Medicare coverage does not (except in rare emergencies) extend beyond U.S. borders.
Part A is, of course, free since you have paid Medicare taxes throughout your working life. Most expats maintain their Part B coverage, a) in case they decide to seek treatment for an illness or injury in the States, and b) because the penalties for opting out and then rejoining years later can be quite costly.
Some Medicare Advantage plans do provide for limited emergency care in foreign countries. If you are insured by an Advantage plan, check the details of your coverage.
As mentioned in the FAQ about health care abroad, costs are so low overseas that expats have the freedom to join inexpensive national or private plans, or simply pay out of pocket as the need arises.