Application Example

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.

The Steel Model Library in Thermo-Calc offers several models that make it easy to set up calculations for investigating the hardenability of steel. In this example, the Steel Model Library is used to investigate the possible composition ranges of an Fe-C-Mn alloy to reach a fully martensitic microstructure for a high hardenability of the steel.

Time-Temperature-Transformation (TTT) Diagram

A good place to start an investigation into the hardenability of steel is with a TTT diagram. While a TTT diagram only acts as a rough guide, it will point you in the direction to search for high hardenability. High hardenability steel design means that the steel should have: (1) a high martensite finish temperature (Mf) or, in other words, high martensite fraction at room temperature, and at the same time, (2) a long starting time of formation of other austenite decomposition products such as ferrite, pearlite, and bainite, in order to avoid them, with reasonable cooling rates. This example calculates a TTT diagram for pearlite and bainite transformation and the Ms and M50 temperatures for athermal martensite.