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Abstract

Wear is one of the key problems with many types of engineering components as it can limit their life expectancy. The same is true for tools and tool steels, which are the focus of this doctoral thesis. Tool-wear response and behaviour are closely related to a material’s final microstructure and properties. Most of the existing literature is focused on the commercially available steels where the heat-treatment regimes are selected in a manner to achieve the highest hardness so as to improve the wear resistance. However, the consideration of a specific microstructure at different hardness levels and understanding the influence of the microstructure constituents on the tribological properties are not yet properly clarified. Furthermore, it is of great interest to tribologically evaluate conventional tool steels to their full potential, as more specialised materials are much more expensive. The aim of this doctoral dissertation was to investigate and describe the influence of the different microstructures of conventional tool steels (different carbide morphology, orientation, fraction, size, type) and the corresponding matrix hardness on a tool steel’s wear properties and its behaviour during wear.
The variation in chemical composition results in a different volume fraction, type and the size of the hard particles in the matrix microstructure, i.e., carbides. In addition, the microstructure is influenced by altering the heat treatment. The results show that the best wear properties are provided by a fine-grain matrix with a grain size ASTM 5 or finer, achieved by the proper selection of the austenitization temperature. On the other hand, improper austenitization can result in an increased matrix grain size or the presence of residual carbides, which consequently deteriorate the mechanical properties and the wear resistance. The presence of different carbides in terms of fraction, type and size (morphology) in the martensitic matrix has different effects on the wear, being matrix-hardness dependent. At high hardness levels a high fraction of large eutectic carbides (M7C3 type) provides an improved abrasive wear resistance. However, when the bulk hardness drops below a certain value (<54 HRC) a larger amount and a homogeneous distribution of smaller secondary carbides (M23C6 and MC type) combined with a very small amount of larger ones provides the best results. Matrix hardness is important for preventing carbide removal from the matrix, especially in the case of an abrasive wear mechanism. In this case the wear is intensified due to the plastic deformation of the tempered martensite matrix, followed by carbide cracking and their removal from the matrix. In the case of adhesive wear, the wear resistance is in general improved by an increased fraction of carbides, especially large ones. However, as the contact conditions become milder those carbides can start to act as cutting edges, traps for wear debris and areas of transferred material build-up, thus leading to high overall wear rates.
Thermal loading of the tool steel results in a significant decrease in its tensile and yield strength. However, the wear response at elevated temperatures is not solely governed by a change in the mechanical properties and a drop in the hardness caused by the decomposition of the tempered martensitic matrix, but also by the formation of an oxide glazed layer. A softer matrix promotes its formation, so leading to reduced wear rates.