Cobalt in steel is known as red hardness, which describes which property?

• A. Improves machinability
• B. Known as red hardness, which is the ability to remain hard when red hot
• C. Increases corrosion resistance
• D. Increases ductility

Answer: (B)

Red hardness is the ability of a steel alloy to stay hard as it heats up, specifically when it reaches red-hot temperatures during use. Cobalt strengthens the alloy at high temperatures, so cobalt-containing steels resist softening and keep their hardness and edge even as heat builds in the cutting process. This property is crucial for tools that endure intense heat, like cutting tools, where maintaining hardness at high temps prolongs tool life. The other properties listed—machinability, corrosion resistance, and ductility—describe different aspects of a material and are not what red hardness measures.