It took a long time for x-ray vision to generate heat, too. In "The Courtship of Adelbert Dribble" from the 1944 issue of Action Comics #74, Superman must escape from a rubber-padded room by using heat. Instead of generating heat from his x-ray vision, which he's been using to see through walls for a few years by that time, he uses friction from his hands to generate enough heat to cause the rubber to become brittle, allowing him to use his strength to burst through it. I believe the concept of x-ray vision causing heat was first developed in the late 1940s or early 1950s. Then it was finally named by the 1960s as the separate power of heat vision.