Hardenability Design of Steel

About Hardenability of Steel

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 ferrite, pearlite and bainite transformation and the Ms and M50 temperatures for athermal martensite.

ttt-diagram-2022b-with-arrows_copy-01-01 — 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. 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, Bainite, and Ferrite 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, Bainite, and Ferrite 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. 900K is used to check for ferrite start.

Composition Selection

The final step in this example is to overlay the total martensite percentage and start times of pearlite, bainite, and ferrite formation to give an allowable region of compositions to achieve high hardenability.

crossplot_start_times_copy-01 — Total martensite percentage, start time of pearlite formation (2% pearlite, unit: second), and start time of bainite formation (2% bainite, unit: second), and start time of ferrite formation (2% ferrite, paraequilibrium mode, 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.

How to Run the Calculation

To run this example, open Thermo-Calc and navigate to the Help Menu → Example Files…→ Property Models → Steel. This example includes one calculation file:

PM_Fe_07_Hardenability_Design_of_Steel: can be run with the free FEDEMO and MFEDEMO databases, but requires a full license for Thermo-Calc 2021b or newer* and the Steel Model Library.

*The ferrite model was introduced in Thermo-Calc 2022b. The ferrite calculations are therefore not included prior to 2022b.

This example also includes a PDF with in-depth explanations of all the calculations and interpretation of the results.

Read the Full Example

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