BATH — Contract negotiations are getting under way between Bath Iron Works and the Navy shipbuilder’s largest union.

The Machinists union Local S6 says negotiations begin today.

The current four-year contract expires May 20. A ratification vote on a new contract is scheduled that day at the Augusta Civic Center.

Four years ago, the union negotiated a contract that gave workers annual pay hikes of 3.5 percent to 4 percent.

The Local S6 represents about 3,200 jobs that include mechanics, fitters, welders, painters and electricians.