IRWIN Bi-Metal Blue blades combine the best of two metals electro-welded together: The edge of the blade is made from High Speed Steel, which is much harder than standard carbon steel, & has been shown to last up to THREE TIMES LONGER in user trials on plasterboard / drywalling, roofing, insulation, flooring & other heavy-duty materials. The bulk of the blade is made from spring steel, which can resist pressure applied from the side without snapping. The safest blade in use, is ge