Art experiment devoted to the heat flux estimation in a workpiece during a machining process by turning is presented. The method is based on temperature measurement from thermocouples embedded in the workpiece, close to the heated surface. A model that expresses the heat flux according to the temperature at the sensors is developed. The stationary and linearity assumptions are used in order to decompose the three-dimensional original problem into two bi-dimensional problems. This decomposition can be realized given the difference between the cutting speed and feed velocity in two orthogonal directions. The temperature on the machined surface is calculated from the estimated heat flux and the heat transfer model in the workpiece. The application concerns hard steel machining, using a CBN insert tool. Three parameters are placed into evidence from this application: the temperature magnitude on the machined surface, the thermal gradient in the workpiece, and the 'thermal persistence' that represent the heating time of the machined surface. This study leads to a better understanding of the influence of temperature during a hard steel turning process.