What are the benefits, with respect to residual stresses, of a seamless hollow-core forging over rolled and welded plate that is subsequently processed (machining, heat treating, etc.)?
In general, distortion of a component will occur when the stress states of either the individual components or the assembly as a whole shift from one state of equilibrium to a new equilibrium state. The presence of residual stresses in the components act as a source of potential energy similar in nature to a spring fixed in a compressed state. If the fixture holding the spring remains intact, the spring does not expand. However, once the fixture is removed, the spring expands until it reaches a new state of equilibrium—either another fixed point or a point where the potential energy of the spring is expended—and the spring is extended. So, too, will the potential energy in a component due to residual stress remain unchanged until the equilibrium state is altered—either through mechanical means (metal removal or cold/warm straightening, etc.) or thermal means (welding, heat treatment, etc.).
Consider, now, a seamless hollow-core forging that is forged at elevated temperatures with dynamic recrystallization (the immediate formation of stress-free grains upon deformation). The stresses introduced during forging to produce the cylindrical shape are immediately removed through the recrystallization of the crystal structure, resulting in an essentially stress-free forging. Consequently, a rolled and welded assembly possesses significantly higher residual stress than a forged seamless cylinder (hollow-core). Furthermore, no thermal stresses from welding are introduced during formation of the cylinder as a seamless forged hollow-core, unlike the rolled and welded plate method. To summarize, the seamless hollow-core forging is far more stable and possesses a higher degree of structural integrity (no welds!) than a rolled and welded cylinder assembly for an often lower overall cost.