Vacuum Furnace Outgassing

Vacuum degassing in the furnace pulls gases and moisture out of the workpiece, which would otherwise cause trouble during welding. The part is heated under vacuum so that impurities diffuse out of the surface. If you skip this, you get pores in the weld and an unclean melt. It's important that the parts are free of grease and dry beforehand, otherwise the dirt burns in the furnace and does nothing. The exact furnace values depend on the material, but the principle is: clean in, clean out.