Prediction of hardness in mechanical parts built with hardened and tempered steel

Abstract

The objective of the work is to predict the “U curve” of hardness of a cylindrical part, built in steel quenched and tempered to temperatures understood between 190 and 670ºC. It is assumed that a point of a quenched part has the same hardness that a point that cools down at the same speed, in a Jominy test probe of the same chemical composition, extending the similarity if the compared points are tempered at the same temperature and time. For the comparison of cooling speeds at 705ºC between part and test probe, a finite volumes algorithm is used. The hardness of the points of the Jominy test probes, tempered at variable temperatures, is calculated theoretically with formulas developed starting from the treatment of experimental and theoretical/ideal data, deduced by use of the Just – Kirkaldy formulas, with a modification introduced by the authors, using as model a SAE 4340 steel. The influence of the time is calculated with the formula of Hollomon and Jaffe. All these elements were introduced in a computational tool that can be described in the following way: Entrance data: the diameter of the part, the chemical composition and the size of grain of the used steel, the severity of the quenching means, and the temperature and time of the foreseen tempering. Exit data:

Hardness HRc in points of the part diameter. That result will allow an appropriate selection, so much of the material, as of the variables of the thermal treatment.