Talking about several major problems of high and light fastener products
With the development and progress, the demand quality of its industry is also getting higher and higher, so the requirements for fasteners are also constantly improving. High-strength fasteners can meet everyone's needs. At present, high-strength fasteners have good development prospects. However, there are still some problems with domestic high-strength fasteners. According to our understanding, there are mainly the following major problems.
1. Guaranteed load, wedge load is unqualified. Guaranteed load and wedge load are the key items for evaluating the mechanical properties of products under tensile load. , especially important for high-strength fasteners. The guaranteed load is to evaluate the plastic deformation resistance of the product under the specified tensile load. If this index fails to meet the requirements, plastic deformation may occur due to the action of the preload and the working load, thereby reducing the preload and making the connection loose. . The wedge load test is to evaluate the tensile strength and head-to-rod bonding strength of the actual bolt product. This index is unqualified. Under the action of a large working load, especially the impact load or when the supporting surface is subjected to eccentric load, fracture or breakage occurs. U-turn, resulting in connection failure, and even security incidents.
2. Unqualified hardness. Hardness is also an important indicator for evaluating the mechanical properties of fasteners. The materials used for products of each performance level are different. The mechanical property level is an important basis for the selection of fasteners. The working load should be determined according to the performance level during installation and use. For specific materials, the hardness should be controlled at a reasonable level. High hardness may reduce the fatigue resistance of the product. In order to improve the strength of the product, some companies control the hardness at a higher level, causing some products to exceed the standard.
3. The decarburization layer exceeds the standard. The decarburization layer is also a category A item. Due to the decarburization, the surface hardness and strength of the parts are greatly reduced, which seriously affects the surface contact strength and fatigue life of the fasteners, especially the damage to the threaded parts. The main causes of decarburization are decarburization of raw materials, decarburization during material modification and decarburization during product heat treatment.
4. The size is out of tolerance. Fasteners are general-purpose parts and have high requirements for interchangeability. Excess size will directly affect their interchangeability, and even affect the connection strength, anti-loosening performance or life. The main reason for the unqualified size is the improper control of the process. In order to save materials, some sizes are controlled near the lower limit, the equipment is not adjusted in time during the production process, and out-of-tolerance tools and molds are used; the second is poor inspection, process inspection and factory inspection. Not strict, some unqualified batches will pass the customs; third, the measuring tools cannot be regularly checked for measurement, and even the gauges that exceed the wear limit are used.