Meeting increasing requirements concerning part quality and process profitability despite difficult-to-machine materials is only possible with a deep process understanding. In this context, knowledge about the process temperature is of decisive im-portance, since it affects material properties such as hardness or forming behavior as well as chemical and physical interac-tions between tool, workpiece and lubricant. A proven thermoelectric method of temperature measurement in machining, forming and blanking is a tool-workpiece-thermocouple. This setup allows an instantaneous measurement of the temperature development during the manufacturing process insitu at the contact area of tool and workpiece. The accuracy of this method depends on the calibration of the thermocouple, for which the Seebeck coefficients of tool and workpiece material have to be determined. Usually, material samples from different batches are used for this purpose although the resulting measurement errors due to slight changes in material properties are hardly known. In this study, the effects of small changes in the chemical composition and hardness on the Seebeck coefficient are investigated for the first time in order to allow a precise quantifica-tion of the measurement error resulting from the calibration process.