The main problems in the processing of high-strength fastener products

(1) Guaranteed load, wedge load is unqualified

Guarantee load and wedge load are key items for evaluating the mechanical properties of products under tensile load (Class A), which is especially important for high-strength fasteners. Guaranteed load is to assess the ability of the product to resist plastic deformation under the specified tensile load. If this index does not meet the requirements, plastic deformation may occur due to the pre-tightening force and working load, thereby reducing the pre-tightening force and loosening the connection .

Wedge load test is to assess the tensile strength of the bolt product and the bonding strength of the head and rod. This index is unqualified. Under the action of larger working load, especially impact load or the supporting surface is subjected to eccentric load, There is a break or a U-turn, causing the connection to fail, or even a safety accident.

(2) Unqualified hardness

Hardness is also an important index for evaluating the mechanical performance of fasteners. The materials used in products of various performance levels are different. The mechanical performance level is an important basis for the selection of fasteners. The performance level should be followed during installation and use. Determine the working load and installation torque.

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. (Guide: Introduction to the commonly used tool processing technology for boring machining center)

(3) Excessive decarburization layer

The decarburization layer is also a category A project. Due to decarburization, the surface hardness and strength of the parts are greatly reduced, which seriously affects the surface contact strength and fatigue life of the fastener, especially the harm to the threaded part. The main cause of decarburization is decarburization of raw materials, decarburization in the process of material reforming and decarburization in the process of product heat treatment.

(4) Out-of-poor size

Fasteners are general-purpose parts and require high interchangeability. Oversize will directly affect their interchangeability, and even affect connection strength, anti-loose performance or life. The main reason for the unqualified size is the improper process control. 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 the out-of-tolerance tool and die are used;

The second is the weak inspection, the process inspection and the factory inspection are not strict, which makes some unqualified batches pass the customs; the third is that the measuring tools cannot be regularly measured and verified, and even the gauges that exceed the wear limit are used.

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