Electric furnaces were gradually developed after the introduction of DC generators in the 1870s. In the early days, there were small resistance furnaces, medium frequency induction furnaces, and steelmaking electric arc furnaces. In the 1960s, controlled atmosphere resistance furnaces, vacuum resistance furnaces, and vacuum arc furnaces were developed. In the 1970s and 1980s, the single capacity of steelmaking electric arc furnace developed to 75t, and it was made into continuous gas carburizing furnace, ion nitriding furnace, vacuum quenching furnace, microwave oven, single crystal furnace, far-infrared furnace, etc. In the 1990s, the 40t DC steelmaking electric arc furnace and the 250tLF type ladle refining furnace were successively developed and put into industrial operation.https://www.vacfurnace.com/technology-frontiers/vacuum-heat-treatment-of-hand-hacksaw-blades/