Analysis of the reasons for cleaning problems of high-strength fasteners

1. Pollution generated during rinsing

After quenching the fasteners, they were cleaned with silicate cleaning agent and then rinsed. A solid substance of XX appeared on the surface, which was analyzed using an infrared spectrometer and confirmed to be inorganic silicate and iron oxide. This is due to incomplete rinsing leaving silicate residue on the surface of the fasteners.

2. Unreasonable stacking of fasteners

After tempering, there are signs of discoloration on the fasteners. Soaking them in ether and letting the ether evaporate, oily residues are found, which are high content lipids. The fasteners were contaminated with cleaning agents and quenching oils during the rinsing period, melted at the heat treatment temperature, and left chemical burn scars. These substances confirm that the surface of the fasteners is not clean. Using an infrared spectrometer for analysis, it is a mixture of base oil and ether in quenching oil. Ether may come from additives in quenching oil. The analysis results of quenching oil in the mesh belt furnace confirm that the fasteners have slight oxidation in the quenching oil due to improper stacking during heating, but it is almost negligible. This phenomenon is related to the cleaning process, not the quenching oil.

3. Surface residue

There is white residue on the high-strength screw, which was analyzed by an infrared spectrometer and confirmed to be phosphide. The acidic cleaning agent was not used for cleaning, and during the inspection of the rinsing tank, it was found that the tank solution had high carbon solubility. The tank solution should be regularly poured out and the concentration level of the alkaline solution in the rinsing tank should be checked regularly.

4. Alkaline burn

High strength screws have a uniform and smooth oil black outer surface due to quenching waste heat blackening. But there is an orange XX visible area on the outer ring. Additionally, there are visible areas that appear slightly light blue or light red. The original bar and wire rod are coated with phosphating film for cold heading and tapping, and are directly heat treated without rinsing. They are cooled in quenching oil, cleaned with alkaline cleaning agent, blown dry (without rinsing), and tempered at 550 ℃. The hot state is taken out of the tempering furnace and immersed in anti rust oil. As a result, red spots are found on the screw threads.

After testing, the red area on the screw is caused by alkali burns. Alkaline cleaning agents containing chloride substances and calcium compounds can burn steel fasteners during heat treatment, leaving scars on the surface of the fasteners.

Steel fasteners cannot remove surface alkaline substances in quenching oil, resulting in surface burns in high-temperature austenitic state and exacerbating damage during the next tempering step.