The Tempcore process used in recent years is applied for increasing the strength values of the concrete-reinforcing steel bars by quenching. In this book; applicability of Tempcore method to steels was examined by finite elements method using Fortran77 computer program and compared with the results of the experiments. After tests and computer applications approving that test result; it was seen that yield and tensile strengths of steel bars, having chemical compositions mentioned above, had increased and strain values had decreased. The cause of changes at these mechanical values is the martensitic layer on surface occurred by quenching. By inner structure changes; heat capacity and conduction coefficient change, and these effects change the heat transfer characteristics of the material. In bars of different diameters exposed to Tempcore method, it has been seen that yield and tensile strengths had increased and the self-tempering temperature had increased in proportion to increasing time of quenching. In practice, the change in microstructure can not be determined sensitively. In finite elements method, phase ratios for the each node can be calculated so it gives better results.