Common problems of high-strength fasteners in the cleaning process

The cleaning problem of high-strength fasteners is often manifested after heat treatment and tempering. The main problem is not clean rinsing. Due to the improper stacking of fasteners, lye remains on the surface, resulting in surface corrosion and alkali burns or improper selection of quenching oil to rust the surface of the fasteners.

1. Pollution generated during rinsing

After quenching, the fasteners were cleaned with silicate cleaning agent and then rinsed. XX solid matter appeared on the surface. The infrared spectrometer was used to analyze this matter and it was confirmed to be inorganic silicate and iron oxide. This is caused by the incomplete rinsing and residual silicate on the surface of the fastener.

2. Unreasonable stacking of fasteners

Fasteners show signs of discoloration after tempering. Soak them in ether and let the ether evaporate and find oily residues. Such substances are high-content lipids. It shows that the fastener is contaminated by cleaning agent and quenching oil during the rinsing period, and it melts at the heat treatment temperature, leaving chemical burn scars. Such substances confirm that the surface of the fastener is not clean. Analyzed by infrared spectrometer, it is a mixture of base oil and ether in quenching oil. The ether may come from the addition of quenching oil. The analysis results of the quenching oil in the mesh belt furnace confirm that the fasteners are slightly oxidized in the quenching oil due to unreasonable stacking when heating, but it is almost negligible. This phenomenon is related to the cleaning process, not the quenching oil The problem.

3, surface residues

There is a white residue on the high-strength screw. The infrared spectrometer was used to analyze it and it was confirmed to be phosphide. No cleaning with acidic detergent was performed, and inspection of the rinsing tank found that the tank liquid had a higher carbon solubility. The tank liquid should be drained regularly and the concentration level of the lye in the rinsing tank should be checked frequently.

4, alkali burns

High-strength screws are blackened by quenching heat and have a uniform and smooth oil-black outer surface. However, there is an area visible to the naked eye with orange XX in the outer circle. In addition, there are visible slightly light blue or light red areas. The original bar and wire rod are coated with phosphating film to facilitate cold heading and tapping. They are directly heat treated without rinsing, cooled in quenching oil, cleaned with alkaline detergent, blow-dried (not rinsed), tempered at 550°C, hot After removing the anti-rust oil from the tempering furnace, red spots were found on the screw threads.

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 the heat treatment, leaving marks on the surface of the fasteners.

Steel fasteners cannot remove surface alkalis in the quenching oil, which will burn the surface in the high-temperature austenite state, and aggravate the damage during the next step of tempering. The recommendation is to thoroughly clean and rinse the fasteners before heat treatment to completely remove the alkaline residues that cause the fasteners to burn.

5, improper rinsing

For large-size fasteners, polymer aqueous solution is often used for quenching. Before quenching, they are cleaned and rinsed with alkaline cleaning agent. After quenching, the fasteners have rusted on the inside. Analysis with an 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, and it is also recommended to change the rinse water frequently. In addition, adding rust inhibitors to the water is also a good way.

6, excessive rust

High-strength fasteners often see some black streaks. We have also seen in experiments that we have used inorganic and organic cleaning agents to rinse the fasteners before heat treatment. After quenching, there are still black streaks even before heat treatment. Careful cleaning will also leave streaks after heat treatment. An infrared spectrometer was used to analyze the contaminants remaining on the surface, and it was found that there were higher concentrations of sulfur and calcium. Using a small amount of acetic acid and isopropanol, fold a small piece of test filter paper firmly on the dark spot, leaving the dark spot on the filter paper. Analyzing the filter paper with an infrared spectrometer confirmed that calcium, sulfur, iron, manganese and chromium are the main elements.

The presence of calcium and sulfur in the rust spots indicates that this substance is a quenched oil that has 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.