Steel TTT & CCT estimator

Enter a carbon or low-alloy steel composition (up to 1 % C) to estimate its isothermal (TTT) and continuous-cooling (CCT) transformation diagrams, critical temperatures and as-quenched hardness.

Critical temperatures (°C) & hardenability

Continuous cooling results

Method & accuracy

Models used

  • Ferrite, pearlite and bainite kinetics: Li, Niebuhr, Meekisho & Atteridge, Metall. Mater. Trans. B 29 (1998) 661, a refinement of the Kirkaldy–Venugopalan model.
  • Continuous cooling: Scheil additivity rule applied at constant cooling rates.
  • Martensite: Ms and the Koistinen–Marburger rate constant from van Bohemen (2012). Ms is lowered by carbon rejected into austenite during ferrite formation.
  • Critical temperatures: Ae1 and Ae3 (Andrews), Bs (Li et al.). Eutectoid carbon is taken where Ae3 meets Ae1, so it shifts with alloy content.
  • High-carbon steels (up to 1 % C):
    • The carbon solubility in austenite (Acm) comes from a fit to the Fe–C diagram, ln C = 3.141 − 3402/T − 0.37 Cr, with the Cr term calibrated to 52100.
    • Austenitizing below Acm leaves carbide undissolved. Kinetics, Ms and hardness then use the carbon and Cr actually dissolved, with Cr assumed to concentrate 5× in the carbide.
    • Proeutectoid cementite uses the pearlite rate law below Acm, scaled to start alongside pearlite. This is an assumption, not a fitted model.
    • Martensite hardness is capped at the value for 0.65 % C, the level where it stops rising. Large amounts of retained austenite count as soft (300 HV), and cementite as 900 HV.
  • Boron: the boron hardenability factor 1 + 1.5(0.9 − C) slows ferrite, pearlite and bainite. It is scaled by the free boron left after N combines with Ti (TiN) and then with B (BN), and reaches full effect at 10 ppm free B.
  • Critical cooling rate: the slowest constant rate at which ferrite, pearlite and bainite each stay below 1%.
  • Hardness: Maynier et al., by phase, weighted by phase fraction. Retained austenite is counted as martensite.

Validation

  • Critical cooling rates match Li et al.'s published values for 4140 (25.6 vs 24.8 °C/s) and 4340 (6.0 vs 6.4 °C/s).
  • Measured CCT hardness: RMS error of 27 HV for 23MnNiCrMo5-3 (Schindler et al., 2020) and 28 HV for S960MC (Moravec et al., 2022).
  • Measured start temperatures (S960MC, a boron steel): RMS error of 48 °C, compared with 93 °C without the boron term.
  • 22MnB5 (boron steel), whose measured critical cooling rate is about 27 °C/s: the model gives 82% martensite at 27 °C/s and a critical rate of about 125 °C/s. Without the boron term it gave 0% martensite at that rate, so hardenability of lean boron steels is still underestimated.
  • Ms: within about 10 °C of typical handbook values for 4140, 4340, 5140, 8630 and 1080.
  • 52100 undissolved carbide: 6.7 / 5.9 / 5.2 % after 815 / 843 / 871 °C, against reported values of about 8 / 6 / 5 %.
  • As-quenched hardness: 52100 oil-quenched from 845 °C gives 63 HRC (typical 64–66). 1095 water-quenched gives 65 HRC (typical 65–66).

Expected accuracy

  • Transformation temperatures: typically ±20–50 °C. Ms: about ±15 °C.
  • Transformation times (nose positions): typically within a factor of 2–3 of measured diagrams.
  • Phase fractions: about ±10–20 %. Hardness: about ±30 HV (as-quenched and after continuous cooling).
  • Grain size has a large effect. Coarse grains (low ASTM number, for example after austenitizing at 1000 °C or higher) slow ferrite markedly.

Limitations

  • Valid for carbon and low-alloy steels from about 0.05 to 1.0 % C (Mn ≤ 2, Ni ≤ 3.5, Cr ≤ 2, Mo ≤ 1). Not valid for tool, stainless or high-speed steels.
  • Above about 0.6 % C the kinetic equations are extrapolated, so transformation times there are rougher, within a factor of about 3–5. Hardenability of 52100 is probably underestimated: the model's critical rate is about 100 °C/s after 845 °C.
  • Retained austenite is underestimated in high-carbon steels: typically 2–5 % predicted, against 8–15 % often measured in 52100.
  • Nb in solution, and boron above about 30 ppm, are not modelled.
  • Only cementite (Fe,Cr)₃C solubility is modelled. Alloy carbides (Mo, V, Nb, Ti) are assumed fully dissolved. Prior austenite grain size has a strong effect, so enter a measured value if you have one.
  • Treat results as estimates for screening and comparing grades. For critical parts, confirm with dilatometry and calibrate using the kinetic factor.