What's the difference between music wire and hard drawn?

Both hard drawn wire and music wire gain their strength through cold drawing the wire from a large diameter rod to its final size. There are three significant differences. First is the chemical composition of the wire. Music wire contains more carbon and less manganese than hard drawn wire. Additionally, the allowed levels of contaminants such as phosphorus and sulfur in music wire are more restrictive.

The second key difference is in the wire’s strength. Because of the additional carbon, music wire can be drawn to significantly higher tensile strengths than hard drawn wire. Finally, the processing of music wire is done in a manner to provide a finished surface with smaller allowed defects than hard drawn wire. Since surface defects are one of the most common initiation sites for fatigue cracks in springs, smaller surface defects (and their corresponding reduction in stress concentration) enable music wire to be used in high cycle fatigue applications. Hard drawn wire is best suited to static or very low cycle service conditions.