Magnetic materials – cobalt content in cobalt steel for permanent magnets Permanent magnets are made from high carbon steel alloyed with approximately what percentage of cobalt?

Introduction:Permanent magnet materials must exhibit high coercivity and good retentivity. Historically, cobalt steels and later AlNiCo and rare-earth magnets have been used for strong permanent magnets. This question targets a classic composition range for cobalt steel magnets used in engineering contexts.

Given Data / Assumptions:

• Material class: cobalt steel (prior to or alongside AlNiCo developments).
• High carbon steel is alloyed with cobalt to increase coercivity.
• We choose the commonly quoted cobalt percentage.

Concept / Approach:

Cobalt addition to high carbon steel significantly improves magnetic hardness (coercive force). Traditional references describe cobalt steels for permanent magnets containing roughly one-third cobalt by weight, i.e., about 35%. This composition yields improved ability to retain magnetization compared with plain carbon steel.

Step-by-Step Solution:

Verification / Alternative check:

Classic materials texts list cobalt steels with ~35% cobalt for permanent magnets, preceding the extensive use of AlNiCo and rare-earth magnets. This aligns with the option given.

Why Other Options Are Wrong:

15% / 20%: Lower cobalt content typically does not deliver the same coercivity as the established 35% composition.

45%: Higher than common cobalt steel compositions cited for traditional permanent magnets, impacting cost and properties.

Common Pitfalls:

Confusing cobalt steel compositions with AlNiCo alloys; assuming any cobalt addition is sufficient without considering the established optimum ranges.

Final Answer:

35% cobalt