Reasons and Preventive Measures for Untightening of Combination Screw

In the construction site, through repeated inspection and test of the products, the situation that the products are not tightened due to the existence of grooves, pits and oil contamination on the surface of combination screw, welding nut and washer is eliminated. Through the analysis and comparison with the similar products of surface phosphating treatment, it is found that the problem that the products can not tighten up to the pre-tightening force value lies in the way and technology of surface treatment. Because the oxide layer is too thick, the combination screw is not tight, so that the phenomenon that can not meet the pre-tightening force numerical requirements occurs.

The higher the concentration of oxidant in solution, the faster the formation of oxide film, and the denser and stronger the film. If the concentration of alkali in the treated solution is added, the thickness of the obtained oxide film will increase. On the contrary, if the alkali content is too low, the oxide film is thin and weak. The density of the oxide film can be increased by increasing the temperature of the treated solution appropriately.

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 will be oxidized and the shorter the oxidation time. The length of the adjustment time and the concentration of the oxidizing liquid directly affect the mechanical properties of the workpiece.

Chemical oxidation of high strength fasteners with residual stress above 10.9 grade in solution can easily lead to "alkali brittleness", resulting in the failure of mechanical properties of fasteners. Therefore, attention should be paid to the problem of "alkali brittleness" in alkaline oxidation treatment of products.