Cleaning Problems of High - strength Fasteners

1. Pollution generated during rinsing
 After quenching, fasteners are cleaned with silicate cleaner and then rinsed. XX solid substance appears on the surface. This substance was analyzed by an infrared spectrometer, which confirmed it is inorganic silicate and iron oxide. This is caused by residual silicate remaining on the fastener surface after incomplete rinsing.

2. Unreasonable stacking of fasteners
 Fasteners show signs of discoloration after tempering. When soaked in ether and after the ether volatilizes, oily residue is left behind. This kind of substance is lipid with high content, which indicates that fasteners are contaminated by cleaning agents and quenching oil during the rinsing stage. It melts under the heat treatment temperature, leaving chemical burn scars, and such substances confirm that the fastener surface is not clean. Analysis by infrared spectrometer shows it is a mixture of base oil and ether from quenching oil. Ether may come from additives in quenching oil. The analysis result of quenching oil in a mesh belt furnace confirms that due to unreasonable stacking when fasteners are heated, slight oxidation of fasteners occurs in quenching oil, but it is almost negligible. This phenomenon is related to the cleaning process, not a problem with the quenching oil.

3. Surface residue
 There is white residue on high-strength screws. Analysis by infrared spectrometer confirms it is phosphide. Acid cleaning agent was not used for cleaning, and inspection of the rinsing tank found that the tank liquid has a high carbon solubility. The tank liquid should be drained regularly, and the concentration level of the lye in the rinsing tank should be tested frequently.

4. Alkali burns
 After quenching, high-strength screws are blackened by residual heat, which gives them a uniform, flat black outer surface. However, there are orange XX areas visible to the naked eye on the outer ring. In addition, there are also visible areas with a slight light blue or light red tint. The original bar stock and wire rod are coated with a phosphating film to facilitate cold heading and threading. They are directly heat-treated without rinsing, cooled in quenching oil, cleaned with alkaline cleaning agent, blow-dried (no rinsing), tempered at 550°C, taken out of the tempering furnace in a hot state and immersed in antirust oil. As a result, red spots are found at the thread of the screws.

 After testing, it is confirmed that the red areas on the screws are caused by alkali burns. Alkaline cleaning agents containing chloride and calcium compounds will all cause burns to steel fasteners during heat treatment, leaving spots on the fastener surface.
 Surface alkaline substances cannot be removed from steel fasteners in quenching oil, which causes surface burns when the fastener is in a high-temperature austenitic state, and the damage is aggravated in the subsequent tempering process.