Tribological behaviour of high thermal conductivity tool steels for hot stamping

In the last years, the use of High Strength Steels (HSS) as structural parts in car manufacturing, has rapidly increased thanks mainly to their favourable strength to weight ratios and stiffness, which allow a reduction of the fuel consumption to accommodate the new restricted regulations for CO2 emissions control, but still preserving or even enhancing the passengers’ safety. However, the formability at room temperature of HSS is poor, and for this reason, complex-shaped HSS components are produced applying the plastic deformation of the sheet metal at high temperature. The use of hot stamping technology, which was developed during the 70’s in Sweden, has become increasingly used for the production of HSS for the car body-in-white. By using this technology, several improvements have been made, if compared with the forming at room temperature, such as the reduction of spring back and the forming forces, the production of more complex shapes, a more accurate microstructure control of the final piece and the achievement of components with high mechanical properties. The hot stamping process of HSS parts consists mainly in heating a metal sheet up to austenitization temperature and then a simultaneous forming and hardening phase in closed dies, water-cooled, to obtain a fully martensitic microstructure on the final components; in this way, ultimate tensile strength passes from 600 MPa up to 1500-1600 MPa. Anyway, several tribological issues arise when the die and metal sheet interact during the forming process at elevated temperatures; the absence of any types of lubricant due to elevate process temperature and in order to preserve the quality of the part for the later stages of the process chain, leads to high friction forces at interface; moreover, and the severe wear mechanisms together with surface damage of forming dies, can alter the quality of the component and can also have an high impact on the process economy due to frequent windows-maintenance or reground of tools. Furthermore, considering that the thermal conductivity of the die material influences the cooling performance, obtained during the quenching phase, and being the quenching time the predominant part of the cycle time, the productivity of the process is influenced too. On this base tool steels play a capital role in this process, as they strongly influence the properties of the obtained final product and have a strong impact to investment and maintenance costs. The survey of the technical and scientific literature shows a large interest in the development of different coatings for the blanks from the traditional Al-Si up to new Zn-based coating and on the analysis of hard PVD, CVD coatings and plasma nitriding, applied on dies. By contrast, fewer investigations have been focused on the development and test of new tools steels grades capable to improve the wear resistance and the thermal properties that are required for the in-die quenching during forming. The research works reported are focused on conventional testing configurations, which are able to achieve fundamental knowledge on friction behaviour, wear mechanisms and heat transfer evaluation, with both a high accuracy for the process parameters and less information about situations that replicate the thermal-mechanical conditions to which the forming dies are subject during the industrial process. Alternatively, the tribological performance have been studied through costly and time-consuming industrial trials but with a lower control on process parameters. Starting from this point of view, the main goal of this PhD thesis is to analyse the tribological performance in terms of wear, friction and heat transfer of two new steel grades for dies, developed for high-temperature applications, characterized by a High Thermal Conductivity with the purpose to decrease the quenching time during the hot stamping process chain and overcome the limits in terms of process speed. Their performances are compared with a common die steel grade for hot stamping applications. To this aim, a novel simulative testing apparatus, based on a pin on disk test, specifically designed to replicate the thermo-mechanical cycles of the hot stamping dies, was used to evaluate the influence of different process parameters on the friction coefficient, wear mechanisms and heat transfer at interface die-metal sheet. Unlike other research works reported in the literature, which individually analyse the friction, the wear mechanisms and thermal aspects, by means of the methodology used in this thesis, the tribological characterization as a whole is obtained by means of a single approach, in order to analyse the simultaneous global evolution of the tribological system