Autofrettage

The tube is subjected to internal pressure of sufficient magnitude to enlarge the bore and in the process the inner layers of the metal are stretched in tension beyond their elastic limit.

The next step is to subject the compressively strained inner layers to a low-temperature treatment (LTT) which results in the elastic limit being raised to at least the autofrettage pressure employed in the first stage of the process.

[1] Early in the history of artillery, people observed that, after firing a small number of rounds, the bore of a new gun slightly enlarges and hardens.

[7] The problem of strengthening steel gun barrels using the same principle was tackled by French colonial artillery colonel Louis Frédéric Gustave Jacob, who suggested in 1907 to pressurize them hydraulically and coined the term "autofrettage".

[9] However, implementing such a technique on an industrial scale required numerical methods to approximate the solutions of transcedental equations of plastic deformation, which were developed in France during WWI by math professor Maurice d'Ocagne and Schneider engineer Louis Potin.

The amount of initial underbore and oversize of the die are calculated to strain the material around the bore past its elastic limit into plastic deformation.

The method has been patented by the Norwegian oil service company, Meta, which uses it to connect concentric tubular components with sealing and strength properties outlined above.