1. Alkali burn
High-strength screws are blackened by quenching heat and have a uniform and smooth oil-black outer surface. But there is an orange XX visible area in the outer circle. It has been tested that the red area on the screw is caused by alkali burns. The chloride-containing substances and calcium-containing compounds of the alkaline cleaning agent will burn the steel fasteners during heat treatment, leaving marks on the surface of the fasteners.
Iron and steel fasteners cannot remove surface alkalis in quenching oil. It is recommended to thoroughly clean and rinse fasteners before heat treatment to completely remove the alkali residues that cause fasteners to burn.
2. Improper rinsing
For large-size fasteners, polymer aqueous solution quenching is often used, and alkaline cleaning agent is used to clean and rinse before quenching. After quenching, the fasteners have rusted on the inside. Analysis with infrared spectrometer confirmed that in addition to iron oxide, there are sodium, potassium and sulfur, indicating that there is an alkaline cleaning agent on the inside of the fastener, which is likely to be potassium hydroxide, sodium carbonate or similar substances, which promotes its rust. Rinse the fasteners to check whether there is excessive contamination. It is also recommended to change the rinse water frequently. In addition, adding rust inhibitors to the water is also a good way.
3. Excessive corrosion
Some black stripes are often seen in high-strength fasteners. It has also been seen in the test that the fasteners before the heat treatment have been rinsed with inorganic and organic cleaning agents. After quenching, there are still black stripes and even careful cleaning before heat treatment. , Will also leave streaks after heat treatment.
The presence of calcium and sulfur in the rust spots indicates that this substance is a quenching oil that has been dried out, and it is also the evolution of vapor phases during the quenching process. Because the quenching oil is excessively aging, it is recommended to pour out the old oil, add new oil, and implement process supervision and quenching oil maintenance during the entire process cycle.
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