Type SL Lock Nuts are made of heat-treated carbon steel and zinc plating (ZI). Type SL Lock Nuts are made of heat-treated carbon steel and zinc plating (ZI). For use in sheet hardness of 80 or less on the Rockwell "B" scale.Available in Carbon Steel, Aluminium, Stainless and Stainless Steel, M2-M8, #2-56-1/4-20. Self-clinching lock nut