Single vs Double Quench
Long term exposure at the carburising temperature leads to austenite grain growth
- On direct quenching this leads to a coarse microstructure in the core with inferior mechanical properties
To counteract this (if necessary)
- cool to room temperature slowly then reheat until the component has just transformed to fine austenite and quench again to restore core properties (Single quench - normally used)
- In some steels quench from carburising temperature, reheat to ~800oC and quench again (double quenching) to get a good hardness profile for deep cases