This graph shows the effect of different cooling rates on the toughness properties (un-notched samples) of AISI 420 and the nitrogen alloyed M333 (AISI 420 Mod). The cooling parameter used is λ8/5, which is the cooling time from 1470 to 930°F in seconds, divided by 100.
Besides the general trend of falling toughness with rising cooling time, it is clearly evident that the nitrogen-alloyed steel grade M333 (420Mod), which has a lower carbon content, shows significantly higher toughness values. These can be further improved by lowering the austenitizing temperature. This behavior correlates well with the reduced tendency to form proeutectoid carbides at grain boundaries. A similar correlation can be found for the steel grades AISI 440B and, for example, the nitrogen-alloyed M340 (440Mod).
Volume 4D, Heat Treating of Irons and Steels ⇒ Tool Steels ⇒ Heat Treating of Mold Steels and Corrosion-Resistant Tool Steels ⇒ Recommended Heat Treating Practices
