The process known as Tempcore is used to produce high resistance rods by the formation of a surface layer of quenched and tempered martensite that surrounds a core made of ferrite and pearlite. Such a mixed structure is result of processing hot rolled rods through waters headers that reduce the temperature at the surface below that for the transformation into martensite. This structure is tempered by the heat flowing from the core of the rod, which transforms into ferrite and pearlite while the rod is in the cooling beds. Such processing produces a significant increase in yield and ultimate tensile strength, while maintaining adequate ductility. The economic advantages of this process are huge in comparison with those that require alloying elements or further metal working to improve mechanical properties. A series of experimental trials were carried out in a pilot plant in which parameters such as reheating temperature, water flow and processing time were varied to study their effect on the mechanical properties of carbon steel rods and on the structures formed in the bars. The study is being complemented by the thermal modelling by the finite element method.