With the development and progress, the demand quality of its industry is also getting higher and higher, so the requirements for fasteners are also increasing. High strength fasteners can meet everyone's needs. At present, the development prospect of high strength fasteners is good, but there are still some problems with domestic high strength fasteners. According to our understanding, there are mainly the following problems.
1. Guarantee load, wedge load is unqualified. Guarantee load and wedge load are the key items to assess the mechanical performance of products under tensile load., It is especially important for high strength fasteners. The guarantee load is to assess the product's ability to resist plastic deformation under the specified tensile load. If this index fails to meet the requirements, plastic deformation may occur due to the preload and working load, so as to reduce the preload and loosen the connection. Wedge load test is to assess the tensile strength and head rod bonding strength of bolt products. This index is unqualified. Under the action of large working load, especially impact load, or when the support surface is under eccentric load, it breaks or turns around, resulting in connection failure and even safety accidents.
2. The hardness is unqualified. Hardness is also an important index to assess the mechanical performance of fasteners. The materials used for products with different performance levels are different. The mechanical performance level is an important basis for the selection of fasteners. During installation and use, the working load and installation torque shall be determined according to the performance level. For specific materials, the hardness shall be controlled at a reasonable level. High hardness may reduce the fatigue resistance of products, In order to improve the strength of products, some enterprises control the hardness at a high level, causing some products to exceed the standard.
3. The decarburization layer exceeds the standard. Decarburization layer is also a class a project. Due to decarburization, the surface hardness and strength of parts are greatly reduced, which seriously affects the surface contact strength and fatigue life of fasteners, especially the damage to thread parts. The main reasons for decarburization are raw material decarburization, decarburization during material restructuring and decarburization during product heat treatment.
4. The dimension is out of tolerance. Fasteners are general-purpose parts with high interchangeability requirements. The size out of tolerance will directly affect their interchangeability, and even affect the connection strength, anti loosening performance or service life. The main reasons for the unqualified dimensions are: firstly, the process control is improper. In order to save materials, some dimensions are controlled near the lower limit, the equipment is not adjusted in time during the production process, and the tools and dies with out of tolerance are used; secondly, the inspection is poor, and the process inspection and factory inspection are not strict, which makes some unqualified batches pass the standard; Third, the measuring tools in use cannot be calibrated regularly, and even the gauges exceeding the wear limit are used.
