1. Alkali burn The high-strength screws turn black afte […]
1. Alkali burn
The high-strength screws turn black after quenching, and the outer surface is even and flat, showing oil black. It was found to be caused by alkali burn. Therefore, it is impossible to remove the surface alkalinity in the quenching oil of steel fasteners, otherwise it will burn the surface at high temperature and aggravate the damage during tempering. It is recommended to thoroughly clean and rinse the fasteners before heat treatment to completely remove the alkaline residues that may cause fastener burns.
2. Improper flushing
For large fasteners, aqueous polymer solutions are usually used for quenching. Before quenching, the alkaline cleaner is used for cleaning and rinsing. After quenching, the inside of the fastener rusted. Therefore, it is necessary to change the flushing water frequently to ensure that the fasteners will not rust during the flushing process.
3. Excessive rust
Some black stripes often appear in high-strength fasteners. Black streaks are contaminants and dry quenching oil left on the surface. They are the evolution of the gas phase during quenching. Due to the over-aging of quenching oil, it is recommended to add new oil.