A term applied to Open-Hearth steel wire in the .45/75 carbon range either hard drawn or oil tempered. Oil tempered wire of M B and W M B types are the most widely used of all spring wire. Oil tempered wire is more suitable to precision forming and casting operations than hard drawn wire because of close control of tensile strength and superior straightness.
NOTE – M B, H B and extra H B designate Basic Open Hearth steels, while W M B, W H B and extra W H B designate Acid Open Hearth Steels. The chemical composition and the mechanical properties are the same for both basic and acid steel.