Steel Structure Bolt Standards

Steel structure bolt standards are distinctly different from those of ordinary bolts. What are the specific differences? Can steel structure bolts be hot-dip galvanized like ordinary bolts? We will analyze these questions one by one below.

First, let's look at its standards. Steel structure bolts are manufactured in accordance with the national standard GB/T 1228. This standard specifies that a steel structure bolt assembly consists of one bolt, two flat washers and one nut, so steel structure bolts are usually sold as complete sets. Among them, the matching nuts comply with GB/T 1229, and the flat washers comply with GB/T 1230. The strength grade of steel structure bolts is 10.9, the nuts are Grade 10, and the flat washers are made of reinforced materials. Different from ordinary washers, they are relatively thicker. Bolts must undergo quenching and tempering heat treatment before use. In their original state after heat treatment, they are gray in color, with an average Rockwell hardness (HRC) of around 35. It should be noted that this is an average value, and the hardness of different parts may vary slightly.

In tensile tests, steel structure bolts can achieve a tensile load capacity of at least 600 kN, and the tightening torque shall meet the design requirements (note: torque unit is N·m, not kN). These are the minimum parameters specified in national standards, which means the actual test results must not be lower than these values. During installation, the torque directly affects the firmness of the connection. Excessively high torque may strip the bolt threads, while excessively low torque will lead to loosening. Therefore, steel structure bolts must be installed using standard torque tools such as torque wrenches or twist-off wrenches.

Occasionally, we may find broken steel structure bolts on construction sites. Professional analysis shows that such fractures are not caused by improper heat treatment, but by inherent material defects. When bolts have surface damage, asymmetric force application during tightening, combined with material defects, will easily lead to bolt fracture. However, such fractures are sporadic and will not cause large-scale quality problems. Careful pre-installation inspection of each bolt can effectively prevent such incidents.

Can Steel Structure Bolts Be Hot-Dip Galvanized Like Ordinary Bolts?

This question needs to be explained from several aspects. First, steel structure bolts are high-strength bolts, which are generally not recommended to undergo surface treatments such as hot-dip galvanizing or electrogalvanizing due to their high sensitivity to hydrogen embrittlement. Therefore, in principle, steel structure bolts are not suitable for hot-dip galvanizing. But what if hot-dip galvanizing is explicitly required by the design?

In practical applications, hot-dip galvanized steel structure bolts are indeed used in many scenarios, such as the connection of outdoor utility poles. To ensure the quality of steel structure bolts and prevent fracture, strict process control must be implemented during hot-dip galvanizing. First, bolts must undergo standardized fluxing treatment before galvanizing. This step is crucial-it removes oxide scale and oil stains from the bolt surface and enhances the adhesion between the zinc coating and the base metal (rather than achieving tempering effect). After galvanizing, bolts must be immediately subjected to hydrogen removal annealing: place the galvanized bolts in a constant temperature furnace at 190–230°C for 2–4 hours to allow hydrogen trapped in the metal matrix to escape, thereby avoiding hydrogen embrittlement. At the same time, standardized hot-dip galvanizing process can ensure that the zinc coating on the bolt surface is uniform and dense without peeling. In summary, steel structure bolts can be hot-dip galvanized, but the production process requires extremely strict and careful operation.

The hot-dip galvanizing process for matching steel structure nuts is different from that of ordinary nuts. Due to the fluctuating demand for steel structure nuts, most manufacturers do not keep them in stock in advance. To ensure timely delivery upon receiving orders, the common practice is to galvanize the nut blanks first and then tap the internal threads using a threading machine. As a result, the internal threads of steel structure nuts are not covered by zinc coating. However, there is no need to worry about rusting, because the internal thread holes are coated with special anti-rust oil.

Steel structure bolts are also known as steel structure bolt assemblies, which mainly include two types: hexagonal head type and twist-off type. To ensure consistent and stable performance of the bolts, all components in one set of assembly are usually manufactured from the same batch of raw materials and heat-treated in the same furnace after forming. In terms of performance grade, Grade 10.9 is the most suitable for steel structure bolts. The specification range of steel structure bolts generally starts from M16 and ends at M30, which is another difference from ordinary bolts. The matching high-strength nuts have a boss structure on one side-during installation, the side with the boss should face the connected workpiece. This point needs special attention; other installation procedures are basically the same as those for ordinary bolts.

Steel structure bolts were officially put into use in the 1970s, and most early industrial factory buildings were constructed using these bolts for structural connections. Before being introduced into China, steel structure bolts were a patented product overseas, which could not be directly incorporated into China's national standards. Although many domestic construction fields had a strong demand for steel structure bolts, they were restricted by patent barriers and could only look on helplessly. With the rapid economic development of China, the demand for steel structure bolts surged due to large-scale infrastructure construction and trade imbalances of related products. Meanwhile, many countries formulated their own national standards for steel structure bolts. Based on actual engineering needs, China revised and optimized the relevant standards and finally incorporated steel structure bolts into the national standard system. This enabled the widespread application of steel structure bolts in China, giving full play to their core value in structural connection.

In the production of steel structure bolts, the procurement and testing of raw materials are subject to strict control. Various chemical composition and mechanical property parameters must be verified by authoritative test reports issued by third-party quality inspection institutions before the raw materials can be put into production. Special tools are required for on-site installation of steel structure bolts. Currently, lithium-ion battery-powered electric wrenches are widely used, while the early pneumatic wrenches (which require air compressors) have been basically phased out. The reason is that steel structure bolts are mostly used in outdoor construction sites, where it is inconvenient to connect air compressors to power sources. In contrast, lithium-ion battery-powered electric wrenches are highly portable and efficient.

The production process of steel structure bolts is as follows: after raw materials arrive at the factory, they first undergo pickling to remove surface oxide scale and oil stains, followed by phosphatization to form a dense phosphate film on the material surface. This film prevents material damage during subsequent processing and ensures that the surface finish of bolts meets the requirements after cold heading. Before machining, raw materials also need to undergo spheroidizing annealing and wire drawing. If the material hardness is too high, additional annealing treatment is required to soften it. After cold heading forming, bolts are subjected to quenching and tempering heat treatment to improve their comprehensive mechanical properties and achieve Grade 10.9 strength. Finally, after strict factory inspection and packaging, the finished products can be delivered to customers.

The storage requirements for steel structure bolts are different from those of ordinary bolts-they should be stored in dry and dust-free warehouses. This ensures that during installation, nuts can quickly align vertically with bolt threads and be tightened smoothly. The matching steel structure flat washers are made of the same material as bolts. If outdoor installation work needs to be carried out on rainy days, proper rainproof measures must be taken to prevent bolts, nuts and washers from rusting due to rain exposure. After bolts are installed in place, it is best not to loosen or adjust them easily. Although steel structure bolts are not disposable products, for the sake of connection stability, it is recommended to install them in place in one go and use them for long-term service.

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