Hardenability of steel is an important aspect of steel design because it affects the ability of the steel to develop optimum strength and toughness. Hardenability refers to the ability of steel to form martensite on quenching. It is a measure of the capacity of a steel to be hardened in depth when quenched from its austenitizing temperature, meaning that the steel forms martensite not only at the surface of the steel, but throughout the interior. This is usually a prerequisite for the subsequent tempering treatment for an optimal combination of strength and toughness. Insufficient hardenability can make the tempering treatment ineffective and lead to low uniformity of mechanical properties in a steel component. The two most important factors that influence hardenability of steel are grain size and composition, and in this example, we will investigate composition.
This example shows how the Steel Model Library in Thermo-Calc can be used to find the optimal compositions for an Fe-Mn-C steel to achieve high hardenability for the purpose of strength.
Calculated TTT diagram of Fe–2Mn–1C (wt.%), which shows time-temperature curves for ferrite start (2%), pearlite start (2%), bainite start (2%), 50% and 98% austenite transformation, and the Ms and M50 temperatures for athermal martensite. TTT diagrams are a good place to start in our investigation, as the arrows in the diagram indicate directions towards high hardenability.