Ideal critical diameter, the size of round bar that would reach 50 percent martensite at its center if its surface cooled instantly (a quench severity H of infinity). It is calculated from chemistry and grain size using a multiplying factor for each alloying element, so metallurgists can compare steels and heat lots on one number. Real quenches are slower, so the actual critical diameter in oil or water is always smaller than the DI.
This heat calcs out at a DI of 3.2, that's the low end for 4140, watch the cores on the big shafts.
Also heard as
- ideal critical diameter
- ideal diameter
Related terms
Hardenability
A steel's ability to form martensite to a given depth, set by its chemistry and grain size, not its hardness.
Quench severity
How fast a quenchant pulls heat out of steel, usually rated by the Grossmann H value.
Jominy test
End-quench test that measures a steel's hardenability by spraying water on one end of a hot bar.