Reasons and preventive measures for the combination screws that are not tightened

Through the repeated inspection and test of the product at the construction site, the situation that the product cannot be tightened due to the existence of grooves, pits, and oil stains on the surface of the combination screws, welding nuts, and washers is ruled out. Through the analysis and comparison with similar products with surface phosphating treatment, it is found that the problem that this batch of products cannot be tightened and cannot reach the value of the pre-tightening force lies in the method and process of surface treatment. Because the oxide layer is too thick, the combination screw cannot be tightened, so that the phenomenon that the pre-tightening force value cannot be met occurs.

During the alkaline oxidation treatment of the combined screw, the higher the concentration of the oxidant in the solution, the faster the oxide film is formed, and the film layer is dense and strong. If the concentration of alkali in the treatment solution is added, the thickness of the obtained oxide film increases. On the contrary, if the content of alkali is too low, the oxide film will be weak and weak. Properly increasing the temperature of the treatment solution can improve the density of the oxide film.

The oxidation treatment time is mainly adjusted according to the carbon content of the workpiece and the oxidation requirements of the workpiece. The higher the carbon content of the workpiece, the easier it is to oxidize and the shorter the oxidation time. The length of the adjustment time and the concentration of the oxidizing solution directly affect the mechanical properties of the workpiece.

Chemical oxidation of high-strength fasteners with residual stress above 10.9 in solution can easily cause "alkali embrittlement", resulting in the failure of the mechanical properties of fastener products to function normally. Therefore, it is necessary to pay attention to the problem of "alkali brittleness" during the alkaline oxidation treatment of the product.