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Nanometer‐sized precipitates in tool steels are the most important microstructural constituents determining the mechanical and thermophysical properties. For the improvement of existing steel grades and the development of new tool steels a fundamental knowledge of the precipitation reactions during heat treatments and at application temperature is of vital interest. Thus, a comprehensive characterization of the various precipitates is required. This paper reviews the studies performed on a complex tool steel which hardens by secondary hardening carbides and intermetallic phases using advanced methods for characterizing nm‐sized precipitates, such as transmission electron microscopy, atom probe field ion microscopy and small‐angle neutron scattering. Additionally, computer simulations of the precipitation kinetics were performed using a novel model for nucleation, growth and coarsening of spherical precipitates in multi‐component, multi‐phase systems. It is demonstrated how experiments and simulations fit together and how thoroughly obtained experimental data help to validate and improve the theoretical models.

Steels in hot-work applications are exposed to a complex combination of mechanical, thermal and creep loading conditions. Advanced hot-work tool steels combine precipitation of alloy carbides and intermetallic phases and, therefore, exhibit a higher thermal stability than conventional tool steels.The mechanical and thermophysical properties of a Fe-0.32C-6.5Ni-2.5Al wt% martensitic tool steel are mainly determined by the combined precipitation of secondary hardening carbides and of intermetallic NiAl precipitates of B2-type. This alloy shows a large and rapid hardening reaction upon aging for as short as 60 s at a temperature of 610°C.Atom probe field ion microscopy has been used to investigate both the composition and morphology of the fine precipitates and the matrix present after aging for 0, 30 and 60 s at 610°C. In particular, chemical composition, particle radii and number density of the precipitates have been determined as a function of the aging time. The data obtained from these initial stages of the precipitation processes were analysed by means of statistical calculations.