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Bolt
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Adequacy Assessment & Reliability Assessment
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materials
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Heat treatment is a standard process for all bolts, which involves exposing the bolt to extreme temperatures in order to harden the steel. Threading is usually applied before heat treatment, either by rolling or cutting when the steel is softer. Rolling works much like cold forging, and involves running the bolt through a die to shape and mold the steel into threads. Cutting involves forming threads by cutting and removing steel.
Since heat treatment will change the properties of the steel to make it harder, it is easier and more cost-effective to apply threading beforehand. However, threading after heat treatment will mean better fatigue performance.
Heat Treating – Once threaded, the bolts are heat treated, which arranges the mechanical properties of the steel to the required strength specified by ASTM. They are put into large ovens in a controlled environment and introduced to regulated temperatures for an amount of time deemed necessary to get the bolt strength into the parameters required by the ASTM A325 specification. This two part process is referred to as quenching and tempering.
Production Process
Heat Treating – Once threaded, the bolts need to be heat treated. This is the same two part process (quench and temper) required for cold forged structural bolts. They are put into large ovens in a controlled environment and introduced to regulated temperatures for an amount of time deemed necessary to get the bolt strength into the parameters required by the ASTM A325 specification.
Cold forging starts with large steel wire rods, which are uncoiled and cut to length. The grade of steel is standardized across the industry, according to the requirements of ISO 898‑1. Using special tooling, the wire is then cold forged into the right shape. This is basically where the steel is molded, while at room temperature, by forcing it through a series of dies at high pressure. The tooling itself can be quite complex, containing up to 200 different parts with tolerances of hundredths of a millimeter. Once perfected, cold forging ensures bolts can be produced quickly, in large volumes, and with high uniformity.
A summary of the production process:
Wire - Uncoiled, straightened and cut to length.
Cold forging - Molding the steel into the right shape at room temperature.
Bolt head - Progressively formed by forcing the steel into various dies at high pressure.
Threading - Threads are formed by rolling or cutting.
Heat treatment - The bolt is exposed to extreme heat to harden steel.
Surface treatment - It depends on the application. Zinc-plating is common to increase corrosion resistance.
Packing/stocking - After quality control to ensure uniformity and consistency, the bolts are packaged.