Desarrollo de aceros rapidos para aplicaciones tribologicas diseño de aleaciones y estudio de tratamientos termicos y criogenicos

Zusammenfassung

Development of high speed steels for tribological applications: alloy design and study of heat and cryogenic treatments, thesis ABSTRACT The present work deals with the study of two main aspects of the processing of high speed steels (HSS): sintering and heat treatment. while the former accounts only for powder metallurgical HSSs, the latter has a great relevance, no matter the fabricatión route this type of materials are made by. First, it has been assessed the sinterability of two HSS powders (AR-K2 and M42HVIG), previously designed at CEIT to be highly sinterable in nitrogen rich atmospheres. The obtamed results have been compared with the theoretical predictions in order to check the thermodynamic calculations. This comparison has allowed to conclude that in order to predict the sintering behaviour of a new designed composition it must be considered every carbón containing hypothetical phase (because the more the free carbón in the matrix, the better the sinterability). This permits to improve the computer aided alloy design and, consequently, has been taken into account to develop an M42HVIG based new alloy of optimum sinterability. 0n the other hand, it has been carried out a deep study about two common heat treatments: tempering and isothermal annealing. Firstly, it has been analysed how and why the different processing parameters affect the hardness values of T42 and M42 HSSs. The existing relationship between those mentioned parameters and the hardness states the fundamentals to design thermal cycles in order to obtain HSSs with tailored hardness (and therefore, with the desired mechanical properties). HSSs are usually employed as cutting tools but the strong demand in the market of automotive components for materials with better wear resistance at higher temperatures has introduced these materials in the field of structural components. That is the reason why in the present work a great effort has been made to attain a material with a hardness value of 50 HRC, which is an adequate value for structural applications. The T42 HSS (with added carbón) has been chosen in order to carry out an exhaustive characterisation by means of Transmisión Electron Microscopy of the corresponding microstructure after tempering or annealing to a hardness value of 50 HRC. it has been found out that a sudden cooling from the austenitising temperature promotes the intragranular precipitation of tungsten rich nanometer sized M6C carbides. These carbides act as a barrier for the laminar growth of the pearlite because they limit the available ferrite/austenite interface for the cementite nucleation. So, pearlite must adopt a globular configuration, although according to the high driving forcé involved in the process, the transformation should lead to a laminar morphology. Moreover, taking into account that the specific use of the developed material will be the fabricatión of valve seat inserts, they have been assessed both the mechanical properties (toughness and fracture strength) and the wear resistance. In this sense, it has been al so paid special attention to the wear of the valve material. The T42 HSS wear behaviour is significantly better than that of the most widely used commercial option, without provoking an increase in the valve wear. Finally, it has been studied the cryogenic treatment of high speed steels. The characteristic features of this process differ substantially from those of the heat treatments. As a result of that, nowadays there is still a lack of understanding of its fundamental metallurgical basis. The present work tries to approach to this problem through the study of the microstructure and mechanical behaviour of the cryogenically treated M35, ASP30 and T42 HSSs. it has been found out that the cryotreatment results in a continuation of the austenite-martensite transformation. In the absence of retained austenite it has not been detected any microstructural change