Which fabrication method is sometimes used for constructing vessel shells and does not create a longitudinal shell weld?

The correct answer is hot forging, which is sometimes used for constructing vessel shells without creating a longitudinal shell weld. Hot forging involves shaping metal through the application of high pressure and temperature, which changes the properties of the material and allows for the formation of shapes without the need for welding seams. This method is particularly advantageous for producing components that require high strength and integrity, as the process enhances the mechanical properties of the metal while eliminating potential weaknesses introduced by welding.

In contrast, welding typically relies on joining two or more pieces of material through melting and fusing them together, creating a longitudinal weld that can be a site for potential stress concentration and failure. Rolling is another fabrication method that involves forming flat sheets of metal into cylindrical shapes; however, this process usually requires a weld to join the edges together into a completed shell. Extrusion, while useful for creating shapes, also generally involves producing seams that can lead to longitudinal welds if the final profile requires it.

The unique nature of hot forging allows for the creation of robust components from solid or partially processed materials, which can lead to enhanced performance in pressure vessel applications by avoiding weld-related issues.