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.
About the Example
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.
Calculated TTT diagram of Fe-2Mn-1C (wt.%), which shows time-temperature curves for 2%, 50%, and 98% pearlite transformation and bainite transformation and the Ms and M50 temperatures for athermal martensite. Arrows in the diagram indicate directions towards high hardenability.
Martensite Fractions Model
The next step in the process is to investigate how Mn and C contents influence the amount of martensite at room temperature. The Steel Model Library includes a Martensite Fractions Model which makes it easy to calculate the total martensite percentages as a function of Mn and C, revealing how the Mn and C contents influence the amount of martensite at room temperature.
Pearlite and Bainite Formation
We then calculate the pearlite start time and bainite start time when varying both Mn and C contents. The Steel Model Library includes Pearlite and Bainite Models, allowing you to easily set up these calculations. The nose of the start time of pearlite from the TTT diagram is used, which was about 800 K, and the nose of the start time of bainite, about 700 K, to give us a temperature for the calculation.
The final step in this example is to overlay the total martensite percentage and start times of pearlite and bainite formation to give an allowable region of compositions to achieve high hardenability.
Total martensite percentage and start times of pearlite and bainite formation (2% pearlite or bainite, unit: second) as a function of Mn and C contents for Fe-Mn-C. Calculated using the Steel Model Library in the Property Model Calculator in Thermo-Calc.
Download the Example
This example includes a PDF with in-depth explanations of all the calculations and interpretation of the results. It also includes a calculation file that can be downloaded and run in Thermo-Calc if you have a license for the relevant software and databases.