Ideal critical diameter

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The ideal critical diameter is a term from materials technology and describes the behavior of steel during so-called martensitic hardening . The larger the ideal critical diameter of steel, the easier it is to carry out such hardening. The ideal critical diameter is a measure, independent of the coolant, of how well steel can be hardened. It depends on the austenite grain size (the average size of the crystals in the material) and the existing carbon content.

The ideal critical diameter is defined as the diameter of a cylindrical sample which, after martensitic hardening with ideally abrupt cooling, contains 50% martensite along the sample axis .

Ideal critical diameter =

in which:

= Austenite grain size
(Approximation formula standardized according to ASTM)
= Carbon content of the sample in% by mass

This means that the ideal critical diameter increases both with an increasing austenite grain diameter and with an increasing carbon content. This is explained by the fact that the grain boundaries and thus the number of germs decrease as the austenite grain size increases. This also reduces the tendency towards diffusion-controlled transformation into bainite or pearlite .

A higher carbon content creates an increased tension in the iron lattice, so that the hardness increases.

These two mechanisms therefore favor martensite formation. This also creates more martensite, which increases the ideal critical diameter.