What’s autofretting?

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Autofrettage is a metal fabrication process that strengthens metal pressure vessels by subjecting them to high pressure to induce residual compressive stress. The process is used in industries such as aeronautical and petrochemical, and also in the manufacture of gun barrels. The technique allows components to handle stress from pressure, temperature, or repetitive use. The process involves several steps, including placing the metal part into a fixture, filling it with water or a petroleum-based substance, and applying pressure. The practical applications of the technique include heavy pipes, fuel rails, hydraulic cylinders, oilfield components, waterjet cylinders, and check valves.

In metal fabrication, autofrettage refers to the process by which metal pressure vessels are subjected to enormous amounts of pressure to strengthen the vessel for industrial use. This process also increases the durability of the metal that makes up the vessel. The goal of the autofrettage process is to induce residual compressive stress to find the pressure point where the metal begins to bend. After finding the maximum elastic limit, the pressure vessel is re-examined and reconfigured to increase the amount of residual compressive stress it can withstand in future use, while preventing fatigue.

There are several steps to the autofrettage metal fabrication process. First, the metal part is placed into a fixture by a member of the plant’s R&D team or a machine. Next, a safety shield is used to protect the operator from any by-products that may be produced by the autofrettage process. Once the shield is secure, the ship will be filled with water or a petroleum-based substance. Finally, a pressure intensifying machine will apply pressure to the part, hold it for several seconds, and release the applied pressure.

A component has been self-fastened once this step-by-step process has gone through several cycles. Next, R&D scientists record important data, such as the amount of pressure applied, elastic limit, and more. Specialized software is used to gather information about statistical controls during the process. The autofrettage technique allows the team to pre-stress a number of components and condition them to handle any stress they may be subjected to during operation, whether due to pressure, temperature or the stresses of repetitive use.

The self-pressing metal experiences extreme pressure that is high enough to bend the pressure vessel part, but not high enough to actually crack or break the part. In addition to its use in industries where pressurized vessels are critical, such as the aeronautical and petrochemical industries, autofrettage is still used in the manufacture of gun barrels. In fact, the history of autofrettage begins with French militias in the 1800s, who used the technique to optimize the durability and shelf life of their guns and barrels.

The practical applications of this technique are numerous. It is used to condition heavy pipes that experience repetitive and cyclic loading and are therefore subject to wear and tear. Popular components using this type of heavy duty tubing include: fuel rails and fuel lines, hydraulic cylinders, oilfield components, waterjet cylinders, and check valves.

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