Calculating mechanical properties

CENOS is capable of calculating different mechanical properties of parts as a result of induction hardening, such as hardened zone, microstructure and resulting hardness

Phase change calculation

Simulating the hardening process of steel requires understanding the steel phases formed during heat treatment. This is crucial for predicting material properties and performance.

The CENOS platform includes a Phase calculation model that enables accurate computation of steel phases such as austenite, bainite, pearlite, and martensite.

How to use phase calculation?​

Once your geometry is prepared, you can proceed to define the material parameters in the Material customization window. Follow these steps:

• Enable Phase calculation
  Check the Phase Calculation option in the Material Customization window.
• Define Key Parameters
  Specify the following parameters to ensure accurate calculations:
  • TAc1 (if available): The temperature at which the transformation to austenite begins.
  • TAc3: The temperature at which the steel structure becomes fully austenitic.
  • TMs: The temperature at which martensite formation begins.
  • ts: The start time of the phase change, typically defined as when 1% of transformation occurs.
  • te: The end time of the phase change, typically when 99% of the transformation is complete.
• Guidance on TAc1
  If the temperature for TAc1 is not available, you may use the value of TAc3 as a substitute.

An example of how to get data from a TTT diagram.​

To accurately calculate phase changes, you need data derived from a TTT (Time-Temperature-Transformation) diagram. The TTT diagram provides critical information on the timing and temperature of phase transformations. Below is an explanation of how to interpret and adjust the data:

Interpreting the TTT Diagram

• Combining Phases
  • In the Phase Calculation model, bainite, pearlite, and ferrite are treated as a single phase.
  • To account for this, connect the start lines for ferrite and bainite (e.g., using green dots in the diagram).
  • Similarly, connect the end lines for pearlite and bainite (e.g., using red dots in the diagram).
• Defining ts and te Data
  • The start time (ts) and end time (te) for phase changes must be defined within the temperature range from TMs (martensite start temperature) to TAc3 (the temperature at which the structure is fully austenite).
• Extrapolating Missing Data
  • If the end line in the TTT diagram does not extend to the TAc3 line (as in the example), you must extrapolate the line. This step is essential to ensure the Phase Calculation Model functions correctly

An example of how to treat TTT diagram to get the data for ts​ and te​:

Hardness calculation

CENOS offers two hardness calculation models designed to estimate steel hardness after heat treatment. Both models will be described in the chapters below.

How to enable hardness calculation models?​

• Ensure that thermal physics is enabled for the domain where you wish to calculate hardness.
• Once thermal physics is activated, two Hardness Calculation models and a Phase calculation model will become available.
• Select your desired hardness calculation model. Depending on the model, additional input parameters may be required.

Model 1: Chemical composition-based model​

This model calculates hardness using empirical formulas based on:

• The alloy composition (C, Mn, Cr, Ni, Mo, V, Si, Cu)
• The austenitization temperature (TAc3)
• The critical cooling rate (cooling)

Key Inputs and Calculation

• The model requires the end temperature (Tend) to determine the cooling rate:
• vcooling​ = (Ac3​​−Tend)/time
• where time is the duration of cooling from TAc3 to Tend.
• In default configurations, the martensite start temperature (TMs) is used as Tend, as this marks the end of the hardening process. At this point, any remaining austenite transforms to martensite.
• Originally, the model used a default Tend = 100°C, but this parameter can be adjusted to fit specific cases.

Development and Data Sources

This model was developed using data from over 500 CCT diagrams and can be used exclusively for low-alloy steels. For further details, see references [1] and [2].

Limitations

The model applies only to low-alloy steels. Additional constraints exist on the sum of elemental concentrations in the material, as outlined in the provided limitations table. [1]

Model Accuracy

The model’s accuracy has been validated with the following results:

* 48.5 HV mean absolute error
* 38.9 HV standart deviation of the error
* 0.92 R value (see graph below)

A graph showing the correlation between predicted and actual hardness values provides further insights into the model’s performance.

Model 2: Hardness calculation from the table​

Key Inputs and Calculation

This model calculates material hardness based on cooling time. To use this method, you need to provide data in a tabular format with the following parameters:

• Cooling Time (in seconds)
• Achieved Hardness (in any unit, such as HV, HRC, etc.)

Additionally, you must specify:

• Austenitization Temperature (TAc3)
• End Temperature (Tend)

The end temperature (Tend) depends on the data provided in the table. For instance, if the table includes critical cooling times between 800°C and 500°C, set Tend = 500°C.

A reliable source for such material properties is the Ovako Steel Navigator, which provides heat treatment diagrams (see the example below). Similar diagrams to the one needed for this model can be found there.

Another option is to use hardness data commonly found in CCT (Continuous-Cooling Transformation) diagrams. These diagrams display the hardness achieved by the material based on the cooling time elapsed from the austenitization temperature (TAc3).

Take note: In the example diagram below, hardness is presented using different scales, such as HV and HRC. Ensure you standardize the scale to avoid inconsistencies in your calculations. [3].

Important: It is important to recalculate the hardness to one scale and use the same units for all data otherwise, the results will be meaningless!

Example

In this case, the end temperature (Tend) is equal to the martensite start temperature (TMs). Since the TMs temperature varies throughout the process, we recommend using the temperature at the beginning of the process—approximately 380°C in this example.

Below is a preview of the model definition in CENOS:

Limitations

The error of calculated hardness depends on the quality of the given HV table data.

References​

[1] “Empirical formulas for calculating Continuous Cooling Transformation diagrams”, J. Trzaska, 2019

[2] “Empirical Formulas for the Calculations of the Hardness of Steels Cooled From the Austenitizing Temperature”, J. Trzaska, 2016

[3] “Atlas of steels heat treatment”, A. Rose, H. Hougardy, 1972.