Six common problems of high-strength 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

The fastener shows signs of discoloration after tempering. Soak it with ether and let the ether volatilize and find an oily residue, which is a high content of 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. For analysis 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 confirmed 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 (Guide: explain to reduce the chance of fastener locking)

There is a white residue on the high-strength screw. The infrared spectrometer was used to analyze it and it was confirmed that it was a phosphide. No acidic detergent cleaning was carried out, and the inspection of the rinsing tank found that the tank liquid had a higher carbon solubility. The tank liquid should be poured out 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 slightly light blue or light red areas visible. 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 burn. Alkaline cleaners containing chloride substances and calcium compounds will burn the steel fasteners during heat treatment, leaving scars on the surface of the fasteners. Iron and steel fasteners cannot remove surface alkalis in the quenching oil, so that the surface burns in the high-temperature austenite state, and aggravates the damage during step 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, it is cleaned and rinsed with alkaline cleaning agent. 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.

6, excessive rust

High-strength fasteners often see some black streaks, and we have also seen in experiments that the fasteners before heat treatment have been rinsed with inorganic and organic cleaning agents. After quenching, black streaks are still found even before heat treatment. Careful cleaning will also leave streaks after heat treatment. 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. With a small amount of acetic acid and isopropanol, fold a small piece of test filter paper on the dark spot and wipe it firmly, leaving the dark spot on the filter paper. Analyzing the filter paper with 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 quenching 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.

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