I've heard "grain flow" mentioned on several occasions when discussing forgings. What exactly is "grain flow" and what does it do (or not do) for me.

Forgings are produced using the open-die forging process through the controlled application of compressive stresses while the metal is heated in the plastic regime. The metal, once subjected to the compressive stress, will expand in two other directions unless constrained in either direction. The expanding metal will stretch the existing grains and, if the temperature is within the forging temperature region, will recrystalize and form new strain free grains. The formation of the new grains is not random, however. The new crystal structure is oriented along the direction of the metal flow and can be used to enhance the properties of the forged component by producing a forging that closely follows the outline of the component resulting in even better resistance to fatigue and stress corrosion than a forging that does not contour the component. Other contributors to grain flow are the expansion of microsegregated regions and/or inclusions in the direction of the metal flow. The effect of the elongated microsegregated regions and/or inclusions can be controlled through the use of high quality material and due attention to the forging technique.

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.).

Using this model, it is apparent that the key to minimizing distortion is to select a fabrication process that (1) uses input material with little or no residual stress and (2) will permit a subsequent processing path that introduces as little residual stress as possible. To form a plate into a cylinder will, in most cases, necessarily require stretching the metal beyond its yield point to both hold the cylindrical shape and allow for the subsequent spring-back. If it is assumed that the starting plate is essentially free of residual stress due to processing at elevated temperatures (a large assumption indeed), the equilibrium state of the plate is subsequently changed during rolling through the introduction of tensile and compressive stresses that shift the equilibrium state to that of a cylindrical shape (requiring a weld to hold it in place because of the tendency to spring back, in particular with materials having a high yield strength).

Understanding the proportional relationship between stress and strain (elastic modulus - ironically sometimes referred to as the spring constant) it can be intuitively understood that the stretching will spring back to a new state that now has residual stress present. In addition, the introduction of the longitudinal weldment to complete the cylindrical shape further disrupts the system through the introduction of thermal energy. The severity of the residual stress will increase with increases in either, or all, of the yield strength of the base metal, circumference of the tube, and plate thickness. Once the desired shape is achieved, it will remain in that shape as long as no subsequent processing or service conditions that alters the stress state is performed (machining or welding for example). When the part does distort, additional mechanical work is required to revive the desired shape resulting in an often "circular" manufacturing path. These costs are sometimes (often?) not considered when selecting a rolled and welded cylinder.

Consider, now, a seamless hollow-core Scot Forge 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 Scot Forge 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.