Effect of surface treatment on mechanical properties of high strength fasteners
Abstract: It is generally believed that the mechanical properties of fasteners are related to the material of the fasteners, the parameter setting and forming method of the cold forming process, the heat treatment process control and the parameter selection, and the effect of surface treatment on the mechanical properties of the fasteners. Not big, just solve the appearance or corrosion resistance of the fasteners and extend the service life of the fasteners. This is not the case. This is a deviation from the standard understanding. If the surface treatment method is not suitable or the operation does not meet the process parameters, it will directly affect the mechanical properties of the fastener, and is not limited to the surface corrosion resistance or Exterior. The following is an analysis of the effect of the surface treatment of high-strength fasteners on its mechanical properties: First, the effect of oxidation treatment on the performance of high-strength fasteners 1, the problem description The company produced a bolt with ML40Cr, M22×1.5×120-6g and strength grade 10.9. The surface is oxidized and the finished product is sent to the user. During the assembly process, the user applies the value according to the set pre-tightening force requirement. As a result, the tightening process does not reach the value of the pre-tightening force requirement. 2. Causes and preventive measures After repeated inspections and tests on the product at the construction site, the surface of the bolts, nuts and washers is not tightened due to the presence of grooves, pitting, oil stains, etc. After analysis and comparison with similar products with surface phosphating, it is found that the problem that the batch of products does not reach the value of pre-tightening force is in the surface treatment method and process. Since the oxide layer is too thick, the fastener is not tightened, so that the phenomenon that the preload force value is not reached is caused. During the alkaline oxidation treatment, the higher the concentration of the oxidant in the solution, the faster the rate of formation of the oxide film, and the dense and firm film layer. If the concentration of the alkali in the treatment solution is increased, the thickness of the obtained oxide film is increased. On the contrary, if the content of the alkali is too low, the oxide film is weak. The temperature of the treatment solution is appropriately increased to increase 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 a high-strength fastener with a residual stress of 10.9 or higher in the solution is liable to cause "alkali brittleness", resulting in the mechanical properties of the fastener product not being properly exerted. Therefore, it should be noted that the problem of "alkaline brittleness" should be solved when the product is treated with alkaline oxidation. 3. Conclusion The method of surface treatment of fasteners has a direct relationship with the surface treatment process. It should be carried out strictly in accordance with the requirements stipulated in the process documents. For fasteners with a height above 10.9, alkaline oxidation treatment should be used with caution. Second, the impact of electroplating treatment on the performance of high-strength fasteners 1, the problem description In October 2009, the company produced a combination nut with a performance class of 10, a nut body and a washer riveted together, and the surface treatment was galvanized. The gasket breaks when the user assembles. 2. Causes and prevention measures At the site, the gasket material, specifications, dimensions and metallurgical structure of the heat treatment were observed, and no abnormalities were found by careful study and analysis. The reason is because the product does not completely eliminate hydrogen embrittlement during the plating surface treatment. The atomic hydrogen generated during the electroplating of the fastener during the electroplating of the cathode, the acid pickling and the electroplating process is easily adsorbed by the base metal and enters the inside of the metal, thereby forming a hydrogen embrittlement hidden danger. When a hydrogen atom enters a steel matrix, under stress, it will cause a decrease in toughness and bearing capacity, causing the product to break or suddenly brittle failure. This is the delayed brittle failure or hydrogen stress fracture caused by hydrogen embrittlement. Products with this phenomenon cannot be found to have a reduction in toughness by a conventional tensile test. In order to reduce the hydrogen embrittlement sensitivity of the fastener after plating, any steel fastener with tensile strength ≥1050MPa should be prepared before the plating, and should be avoided in the alkaline or acidic solution. The process requires elimination of hydrogen embrittlement. 3. Conclusion High-strength fasteners shall be specified in the technical documents and marked for timely elimination of hydrogen embrittlement when selecting the plating surface treatment. When the elimination of the hydrogen embrittlement treatment is incomplete or overlaps with the heat treatment temper brittleness, the function of the high-strength fastener is easily lost, and the consequences are very serious. Silicon Briquette,Silicon Slag Briquette,Silicon Carbide Briquette,Silicon Alloy Briquettes YINCHUAN BINHE ABRASIVES CO.,LTD. , https://www.nxbhzhang.com