An austenitic segment for a steam turbine nozzle assembly, along with related assemblies. Various embodiments include a steam turbine austenitic ring segment having: a body portion sized to substantially fill a pocket in a steam turbine outer diaphragm ring, the body portion having a greater circumferential length than an axial depth or a radial width thereof; and a hook-shaped portion extending radially inward from the body portion, the hook-shaped portion sized to engage a hook-shaped slot in the steam turbine outer diaphragm ring, wherein the body portion and the hook-shaped portion form a unitary structure.