An Investigation of the effect of variable parameters on the material quality in tempcore process

V ABSTRACT The Tempcore process used in recent years is applied for increasing the strength values of the concrete-reinforcing steel bars by quenching. This heat treatment process comprises three stages: a fast cooling step which is applied to the bar as near as possible to exit of the last finishing stand and leads to a quenched skin; a step during which the core reheats the quenched surface layer by conduction; an air cooling step, leading to quasi isothermal transformation of the remaining austenite of the core. In this study; applicability of Tempcore method to materials having chemical composition of %0. 16-0.21 C, %0. 17-0.22 Si, %0.74-0.83 Mn has been examined by finite elements method and compared. with the results of the experiments. Heat transfer equations and amount of phase tranformation have been calculated by Fortran77 computer programme. After tests and computer applications approving that test result; it has been 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 occured 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 duration of quenching. In practice, the change in internal structure can not be determined sensitively. In finite elements method, phase ratios for the each node can be calculated so it gives better results.