How do chromium nickel and molybdenum affect steel during heat treatment?

Chromium, nickel, and molybdenum significantly influence the heat treatment of steel by altering transformation kinetics and mechanical properties.

Effects on Heat Treatment

These elements generally improve hardenability, which is the ability of steel to form the hard microstructure known as martensite at slower cooling rates than plain carbon steel. By slowing the diffusion of atoms through iron lattices, they delay allotropic transformations, allowing for a deeper hardened zone in the workpiece and reducing internal stresses during quenching.1

Specific Roles

• Chromium: Added to low-alloy steels to enhance hardenability, wear resistance, and high-temperature strength. While it is essential for corrosion resistance, high-chromium ferritic stainless steels cannot be hardened by heat treatment.2
• Nickel: Acts as a strengthening agent through solution hardening. It is often added to high-chromium steels to produce austenitic stainless steels, which maintain strength at high temperatures.12
• Molybdenum: Highly effective at increasing hardenability and contributing to the toughness of quenched and tempered steels. It is also used to stabilize carbides and improve strength and corrosion resistance in various steel grades.32

Collectively, these alloying agents permit tempering at higher temperatures, which generates better ductility while maintaining desired hardness and strength.1