Steel Properties

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.

The Martensitic Steel Strength Property Model calculates the strength and/or hardness of martensitic steels. It is based on the Intrinsic Strength and Solid Solution Strength models available in the General Yield Strength model with the addition of a strengthening contribution from the formation of martensite. The Martensite Fractions model is used to evaluate the fraction transformed austenite upon quenching. Furthermore, the effect of tempering as a function of tempering temperature and time is also modeled.

The plot shows the calculated hardness after tempering versus tempering temperature and time compared to data from Grange, R.A. and Baughman, R.W. (1956) Hardness of tempered martensite in carbon and low alloy steels, Transactions of American Society for Metals, Vol. XLVIII, pp.165–197.

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 %).

The Ferrite Model describes the thermodynamics and isothermal kinetics of ferrite transformation from austenite. This Model is specific to the so-called grain-boundary allotriomorph, or polygonal ferrite. (Widmanstätten ferrite is treated in the Bainite Model.) This Model can calculate diffusional growth rate constant, grain-boundary nucleation rate, and isothermal transformation times of ferrite.

The overall kinetics is calculated in the extended-volume approach, with considerations for grain-boundary nucleation and a mixed-control growth of ferrite particles. Nucleation rate of ferrite allotriomorph on austenite grain boundaries is calculated from classical nucleation theory assuming a pillbox shape of the ferrite nucleus, following Lange et al. (1988). Growth is calculated using a mixed-control model considering diffusion in austenite and a finite interfacial mobility, and any solute-drag effects. Composition of the growing ferrite is determined from prescribed conditions, which is simpler and faster than solving diffusion and local-equilibrium equations rigorously (like in DICTRA).

The plot shows the calculated ferrite start (2%) of an Fe-0.11C-3.28Ni (mass %) alloy, in orthoequilibrium (OE) and paraequilibrium (PE) modes, as compared to the experimental ferrite start data.

The TTT template includes a plotting mode called TTT mode, which is used to define Temperature on the y-axis and Time on the x-axis for all selected quantities. For example, as shown in the TTT diagram here, transformation times for 2% ferrite/pearlite/bainite and 50% and 98% austenite transformation are shown as functions of reaction temperature. Additionally,  time independent results, like the Ms temperature, are drawn as horizontal lines. 

The plot is a 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 micrometers, made using the TTT template and the TTT plotting mode. The TTT diagram shows transformation times for ferrite start (2%), pearlite start (2%), bainite start (2%), austenite transformation 50% and 98%, and time independent results, like the Ms temperature, which are drawn as horizontal lines.