What is the reason for the insufficient hardness of fasteners

Why is the hardness of some fasteners still not enough? This may be due to the lack of fasteners in the process of processing, resulting in insufficient hardness of the fasteners.
The reasons are as follows:
 
1. Underheating: The reason for underheating is that the heating temperature is too low or the holding time is insufficient.
2. Overheating: Overheated austenite dissolves excess carbon and alloying elements.
3. Insufficient cooling rate: The workpiece is caused by insufficient cooling rate or part of austenite and pearlite transformation during the quenching process. The reason is that the cooling medium is improperly selected.
4. Improper operation: for example, the stay in the water is too short during double-liquid quenching.

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