Analysis Of Tensile Properties Of High-Strength Bolts

A small bolt plays an indispensable role. In particular, the installation and application of high-strength bolts have strict requirements for their tensile strength. When calculating the tensile load-bearing capacity of a bolt, we need to multiply its stress cross-sectional area by the nominal tensile strength value specified in its design, and then determine whether the result falls within the allowable range. To fix large-scale equipment, high-strength bolts must be grouted integrally with the cement foundation to prevent severe vibrations generated by heavy machinery during operation. During installation, it is necessary to inspect whether the specified thread dimensions of high-strength bolts meet the requirements, and whether the chamfer at the thread tail and the undercut at the joint between the thread and the bolt head are qualified. Especially for traditional high-strength water stop bolts, unlike ordinary bolts, they cannot be disassembled after installation. High-strength water stop bolts adopt a one-time connection method and become permanently non-removable once installed.

In the fastener industry, the vast majority of threads are right-hand threads, which are tightened by rotating clockwise (from left to right) and are also the standard threads we commonly use. For left-hand threads, the tightening direction is reversed. Since most people are right-handed and left-handed individuals are in the minority, the thread design of high-strength bolts generally adopts the right-hand thread scheme. Some high-strength double-ended studs have threads at both ends. In some special application environments, one end is designed with right-hand threads and the other with left-hand threads. By simply rotating the middle adjusting part in one direction, the threads at both ends can be tightened simultaneously, which is quite ingenious. The left pedal of the bicycle we ride uses left-hand threads-this is because our pedaling direction is clockwise, and the left-hand thread design ensures the pedal gets tighter as we pedal. Isn't human wisdom remarkable?

Another crucial process in the production of high-strength bolts is heat treatment. Due to the high hardness of the raw materials used for high-strength bolts, the materials must be annealed for softening before cold heading forming. After forming, heat treatment is required to enhance the mechanical properties of high-strength bolts to meet the design specifications. Whether the heat treatment meets the requirements is vital to the final quality of high-strength bolts. Although heat treatment may seem simple in operation, it is essential to clarify the operational responsibilities of workers in each post and improve their professional knowledge of heat treatment. High-strength bolts undergo thermal expansion and contraction during the heat treatment process. Therefore, each heat treatment worker must be equipped with dedicated testing tools to monitor the dimensional and performance changes of bolts at all times, preventing quality defects.

After heat treatment, the surface of high-strength bolts takes on a matte black-gray appearance. During the transportation and counting process, bolts of different specifications are likely to be mixed together. Thus, scientific design of heat treatment furnaces for high-strength bolts is also necessary to facilitate batch processing and reduce mixing risks.

Carbon steel bolts are classified into different strength grades after heat treatment, including Grade 4.8, Grade 8.8, Grade 10.9, and Grade 12.9. Among them, Grade 4.8 bolts are ordinary bolts, Grade 8.8 bolts are medium-carbon steel bolts, and only Grade 10.9 and Grade 12.9 bolts are categorized as high-strength bolts. Of course, there are also Grade 14.9 ultra-high-strength bolts, but they are rarely used in daily applications. Bolts of different strength grades correspond to different mechanical parameters such as hardness.

Taking Grade 10.9 bolts as an example, after heat treatment, the nominal tensile strength of the bolt material reaches 1000 MPa, and the yield strength ratio of the material is 0.9. Therefore, the nominal yield strength of the bolt material is 1000×0.9 = 900 MPa.