Fasteners are usually used to tightly connect two or more components together to ensure that they will not loosen or move relative to each other during use. They help provide necessary mechanical support in the overall structure and ensure the stability of the overall structure. They are widely used in engineering, electronics, machinery, chemical industry, metallurgy and other industries. Especially with the application of new materials, fasteners are also playing an increasingly important role in the field of engineering construction.

Fasteners may account for a small proportion of the cost of engineering structures, but if a problem occurs with the fasteners, it is likely to cause structural damage to a wider range and cause heavy losses to all parties. Even if they are used to secure doors and windows, install pipes and cables, etc., fasteners still have a unique importance. It is precisely because of the particularity of fasteners that their quality is receiving more and more attention in modern industry.

Common quality issues

In application, fasteners may have quality problems due to various reasons, which may easily lead to accidents or damage to machinery or engineering, affecting the overall normal performance. Surface defects are one of the common quality problems of fasteners, which can take many forms such as cracks, dents, wrinkles, cuts, and damage.

There are many types of cracks that appear on fastener surfaces, often caused by different causes. Quenching cracks are caused by excessive thermal stress and strain during the heat treatment process. Forging cracks may occur during cutting or forging processes. Defects such as forging bursts and shear bursts may also occur during the forging process.

Dents are caused by chips or shear burrs or rust on the raw material, which will remain on the surface of the fastener if not eliminated during the forging or upsetting process. Not only are there defects in the raw materials themselves during the processing, or improper behavior in other links such as transportation, it is also easy for the fasteners to be affected by external influences and cause damage such as dents, scratches, and chips.

As connecting elements, fasteners depend directly on their own mechanical properties to perform their function. Failure of mechanical properties or performance to meet technical requirements is a common fastener quality problem. Deficient mechanical properties can cause insufficient load-carrying capacity and other problems, making wear, deformation, and material failure more likely. Fasteners may then come loose or detach, endangering equipment or project safety. Variable loads on the connected components place particularly severe demands on fatigue performance. Combined with environmental effects, inadequate fastener quality can readily result in corrosion, fatigue fracture, and similar failures.

Quality inspection and control

To reduce problems caused by fastener quality defects, strengthen scientific testing and use appropriate methods to ensure quality. Quality inspection and control must reflect actual applications: performance requirements and applicable standards differ across fields, so inspection priorities should differ too. For example, surface defects in high-strength bolts and nuts for steel structures are governed by GB/T 5779.1. For wind-power high-strength bolts and nuts, surface defects must meet the 2 standards mentioned above, and bolts are also subject to surface flaw-detection requirements. Commonly cited standards include JB/T 4730.3—2005 and JB/T 4730.4—2005.

Not only that, other dimensions and geometric tolerances of high-strength bolts, nuts, and washers for steel structures are also different from the technical requirements of high-strength fasteners for wind power. This is also related to the surface treatment methods commonly used for high-strength fasteners for wind power.

Strengthening the quality control of raw materials used for fasteners is also a particularly critical step. Different types and uses of fasteners have different specific material requirements. No matter which material is used, if the material quality is not up to standard, the quality assurance of the fasteners will be impossible. The composition difference of raw materials is crucial to the performance of fasteners. Appropriate raw materials must be selected according to the characteristics and performance requirements of the product to ensure that the mechanical properties of the material meet the needs of the working conditions.

In summary, before fastener products are put into manufacturing, it is also necessary to use scientific methods to conduct quality testing on material alloy elements and composition.

The finished product is the final form delivered and actually used by all parties. Therefore, the quality inspection of finished fasteners is related to the vital interests of many parties. Quality inspection can be carried out from various aspects such as tensile load, fatigue test, hardness test, torque test, finished bolt tensile strength, bolt coating, depth of decarburization layer, etc. For fastener products in specific use cases, the performance of special indicators may need to be assessed, or the indicators must meet requirements that exceed those specified in ordinary standards.

Conclusion

Nowadays, fasteners can be seen everywhere in all aspects of production and life. In addition to controlling the quality of fasteners and choosing fasteners with reliable quality, we should also pay attention to the daily care and maintenance of fasteners, regularly check the fastener connections, replace them in time when faults are found, pay attention to the standardization of the construction process, and reduce the impact of external factors on the quality of fasteners.

(Source: China Steel National Inspection and Quarantine)