“Is the plastic deformation of metals at temperatures above the recrystallization or work-hardening range. Advantages of hot upsetting are low resistance to plastic deformation, a high degree of ductility in the product, refinement of course grains, and improvement of mechanical properties”.
Chipless Machining, Charles Wick, 1960 pg. 21
Essentially, hot upsetting, also known as hot upset forging, is a process by which a round, uniform section of metal (bar, tube, etc.) is compressed by applying heat and pressure, which expands its diameter.