1 F436 Flat Washer Yellow Zinc Plated . F436 flat washers are manufactured from carbon steel to meet or exceed ASTM F436. Zinc plated material will meet or exceed ASTM F1941. Hot-Dip Galvanized material will meet or exceed ASTM A153 and/or ASTM F2329. Hardness for plain and zinc plated washers is HRC 38-45 and HRC 26-45 for hot-dip galvanized washers per ASTM F436.