Cobalt in steel is most closely associated with which property?

• A. Increases corrosion resistance at high temperatures
• B. Known as red hardness, which is the ability to remain hard when red hot
• C. When strength or high hardness at high temps is desired
• D. Improves machinability at room temperature

Answer: (B)

Red hardness—the ability of steel to stay hard when it gets hot enough to glow red—is the property cobalt in steel is most closely associated with. Cobalt helps carbide particles remain stable at high temperatures, so tools made with cobalt-containing steels can maintain hardness during high-speed cutting or hot-working. This is why cobalt is prized in high-speed steels and related tool steels: it keeps the tool from softening as temperatures rise, allowing sustained performance and longer tool life.

The other ideas describe different aspects that aren’t the primary effect of cobalt in steel. Corrosion resistance at high temperatures isn’t the main role cobalt plays here, and while some steels are designed for elevated-temperature strength, the defining cobalt-related trait is the retention of hardness at high temperatures. Machinability at room temperature isn’t improved by cobalt; in practice, adding cobalt can make machining more challenging due to the increased hardness and wear resistance.