## About the Steel Model Library

The Steel Model Library is a package of property models used to set up calculations in the Property Model Calculator. The library currently includes five models intended for those working in the steel industry: Martensite Temperatures, Martensite Fractions, Critical Transformation Temperatures, Pearlite, and Bainite Models

The Steel Model Library is available for free to all users who have a license for the databases TCFE11/MOBFE6 and/or TCFE10/MOBFE5 and/or TCFE9/MOBFE4, and valid Maintenance & Support Subscription (M&SS).

## Martensite Temperatures Model

The Martensite Temperatures Property Model calculates the martensite start temperature (Ms) based on modeling the transformation barrier. The Model is based on Stormvinter et al. (2012) with subsequent update and extension by Gulapura Hanumantharaju (2018) and Thermo-Calc internal assessment. The partitionless equilibrium temperature T0 is calculated using the thermodynamic database. The model also gives temperatures corresponding to 50%, 90%, and 99% transformations. Martensite fractions are calculated with the same algorithm as in the Martensite Fractions Property Model.

The plot shows all the Ms temperatures of different types of martensite morphologies (lath, plate, and epsilon (hcp)) compared with experimental epsilon Ms values.

## Martensite Fractions Model

The Martensite Fractions Property Model calculates the fraction of athermal martensite based on a model developed in Huyan et al. (2016). It is assumed that the first forming martensite morphology is the only forming one. The first morphology is determined based on the Ms-temperatures of all morphologies.

The image is a plot of the transformation curves showing Fe-Cr-C martensite with intercritical annealing.

## Critical Transformation Temperatures Model

The Critical Transformation Temperatures Property Model calculates critical transformation temperatures for steels. This Property Model can output the transformation temperatures for the following:

**Liquidus:**First austenite or ferrite transformation from the liquid**Solidus:**Liquid fully transformed to solid**A0:**Magnetic transformation (Curie temperature) of cementite. The cementite is paramagnetic above A0 and ferromagnetic below**A1:**Austenite (FCC_A1) transforms to ferrite (BCC_A2) + carbide (cementite or graphite or M23C6)**A2:**Magnetic transformation (Curie temperature) of ferrite (BCC_A2)**A3:**Austenite (FCC_A1) transforms to ferrite (BCC_A2)**A4:**Delta-ferrite (BCC_A2) transforms to austenite (FCC_A1)**Acem:**Austenite (FCC_A1) transforms to cementite**Agraph:**Austenite (FCC_A1) transforms to graphite

The plot shows the distribution of the A1 and A3 phase transition temperatures for a low alloyed steel (Fe-0.3Cr-1.0Mn-0.3Mn-0.18C) when the composition is varied within the specification.

## Pearlite Model

The Pearlite Property Model describes the thermodynamics and kinetics of pearlite formation from austenite during isothermal heat treatment. It is assumed that the overall composition of pearlite is the same as the austenite composition, and growth rate is constant over time. Growth rate and lamellar spacing of pearlite are determined by a criterion where either growth rate or Gibbs energy dissipation rate is maximized. The model considers Gibbs energy dissipation due to formation of ferrite-cementite interface in pearlite, finite austenite-pearlite interfacial mobility, solute drag, and diffusion of elements within austenite and along austenite-pearlite interface. A complete description of the model is available from Yan et al. (2020).

The plot is a TTT (time-temperature-transformation) diagram showing times of start (2% transformation) and finish (98% transformation) as functions of isothermal heat treating temperature in an Fe-0.69C-1.80Mn alloy (mass %).

## Bainite Model

The Bainite Model describes the thermodynamics and isothermal kinetics of bainite transformation from austenite. This Model can calculate bainite start temperature, transformation times, bainite plate lengthening rate, and phase constitution at the final state. In this Model, bainite is modeled as either ferrite or a ferrite-cementite mixture, depending on the driving force of cementite precipitating at the ferrite/austenite interface.

WBs is calculated following Leach et al. (2018) by a driving-force-barrier balance approach. Lengthening rate is calculated by extending the approach from Leach et al. (2019) to account for interfacial mobility and possible cementite formation in bainite. The overall kinetics of bainite transformation is calculated in the extended-volume approach, with considerations for both grain-boundary nucleation and volume nucleation of bainitic plates.

The plot is a TTT diagram which shows the calculated start (2%), half (50%), and finish (98%) curves of bainite as compared to the experimentally measured 1% transformation times. The alloy is a hypereutectoid steel with the composition Fe-0.97C-0.72Mn-0.32Si-1.54Ni-0.8Cr-0.26Mo (mass %).

## Easily Setup TTT Diagrams

The Steel Model Library makes it easy to set up time-temperature-transition calculations using a TTT template that is accessed directly from the homescreen of Thermo-Calc. The template has several preconfigured settings to help you set up and calculate the TTT diagram using the Martensite Temperatures, Bainite, and Pearlite Property Models.

The template also includes a plotting mode called TTT mode, which is used to define the Temperature on the y-axis and Time on the x-axis for all selected quantities. For example, as shown in the image here, pearlite/bainite model results will show transformation times for 2%, 50%, and 98% pearlite/bainite, and time independent results, like the Ms temperature, will be drawn as a horizontal line.

The plot is a time-temperature-transformation (TTT) diagram for an En 18 1% chromium steel (Fe-0.48C-0.86Mn-0.25Si-0.18Ni-0.98Cr-0.04Mo, mass %) with a grain size of 53 micrometer, made using the TTT-template and the TTT plotting mode. The plot shows transformation times for 2%, 50%, and 98% pearlite and bainite and time independent results, like the Ms temperature, which are drawn as a horizontal line.