What is the reason for the insufficient hardness of fastener

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.

Haiyan Xinglong Fastener Co., Ltd. was established in 1994. It is a professional manufacturer of fastener products and a self-operated import and export enterprise approved by the Ministry of Foreign Trade. The company is located in the Haiyan Qinshan Economic Development Zone on the north shore of Hangzhou Bay, on the coast of the East China Sea. It is 110 kilometers north of Shanghai and 82 kilometers south of Hangzhou. It is located in the middle section of Haiyan Qinshan Avenue. The geographical position is superior and the transportation is very convenient. The North and South Lakes, the world's wonder "Haining Tide" and many scenic spots are within easy reach. The company covers an area of ​​8,000 square meters, a building area of ​​more than 5,000 square meters, and more than 100 employees. It has invested a lot of money to introduce technology, equipment transformation, workshop expansion and factory environment construction. The company has more than 100 cold rutting machines and dozens of ancillary equipment, with a monthly output of 800-1000 tons and an annual output of about 10,000 tons.

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