During direct heat treatment (DHT) processes, hot-forged blanks or components are directly cooled by various controlled cooling methods to achieve specified hardness, microstructure, strength, and durability.
Direct heat treatment processes have also made significant contributions to product quality through piece-by-piece handling. They eliminate the need for reheat treatment and reduce thermal energy and CO2 emissions to a much greater degree.
The basic DHT steel has a microstructure consisting of ferrite and pearlite, and the fatigue strength of such steels is not sufficiently high to meet the specifications for most highly stressed automotive parts. The results of early investigations of fatigue strength revealed the weakness of the ferrite microstructure: fatigue cracks initiate from ferrite and propagate until final fracture. The first important factor in increasing the mechanical properties of microalloyed ferrite and pearlite DHT steel is the improvement of ferrite strength. Figure 7 shows the limits of macrocrack initiation.
For more information, click on the link below (subscription required). Then scroll down the Chapter Contents column on the left side of the page, and click on “Microalloyed Steels for Direct Heat Treatment Technology.” Then scroll to Fig. 7.
https://dl.asminternational.org/handbooks/book/9/chapter/113021/Forge-and-Direct-Heat-Treatment-Processes-and-Technologies
