Mechanical properties and microstructure of a high carbon steel
This work is focussed on a high carbon tool steel used for the production of files. The mechanical properties of the files depend on a hardening thermal treatment that produces a martensitic structure. Thus, the motivation for this work is technological: we want to understand the effects of two types of thermal treatment on the file's mechanical properties. The first is a traditional thermal treatment in a cyanide bath (CN), while the other is performed in a conveyor belt furnace (BF). The files resulting from BF treatment have sometimes a poorer performance. In order to improve the quality of BF files, we study the relationship between the mechanical properties and the microstructure resulting from a particular thermal treatment. Transmission electron microscopy does not reveal a change in the microstructure produced by different thermal treatments, yet it does permit visualization of the complex material microstructure, consisting of a martensitic matrix with embedded carbides. On the other hand, thermoelectric power measurements allow a differentiation between CN and BF files in a fast and non-destructive manner: the measurements performed on the whole files show that the thermopower of BF files is lower. This result is attributed to a higher concentration of carbon soluted in the martensitic matrix. The yield stress and compression strength determined by compression testing, as well as the hardness measured by nanoindentation, are higher for the bulk of BF files, emphasizing the importance of soluted carbon in hardening the martensite. The mobility of point defects and dislocations, and their interaction, are studied by mechanical spectroscopy. The martensitic structure is characterized by a rich internal friction spectrum comprising several overlapping relaxation and material transformation peaks. The internal friction measured at room temperature is related to an athermal background due to the interaction between soluted carbon and dislocations. This background is higher for BF files. The internal friction spectrum indicates that, upon heating above 375 K, the material structure irreversibly changes due to tempering processes. Tempering effects on the material are studied combining several experimental techniques. A first stage of tempering, observed by differential scanning calorimetry, starts around 375 K and is related to carbon precipitation in the form of transition carbides. At this temperature, the thermopower and Young's modulus increase, and the internal friction and hardness decrease, implying that all these quantities are sensitive to soluted carbon. The hardness continuously decreases with tempering and no precipitation hardening is observed. A second stage of tempering is observed around 525 K, leading to cementite precipitation. X-ray diffraction allows quantitative determination of the concentration of carbon in solid solution by measuring the tetragonality of the martensite lattice. The evolution of carbon content during tempering is compared to the evolution of internal friction and thermoelectric power revealing a strong correlation between these quantities. Internal friction measured at room temperature is directly proportional to the carbon content. The measurement of the concentration of carbon by X-ray diffraction is used to determine the variation of carbon content in the file profile. Thermopower measurements indicate that BF files have a lower concentration of carbon in the teeth than in the bulk, while the opposite is found for CN files. This is confirmed by X-ray diffraction. Moreover, diffraction performed on the very surface of the teeth reveals a lower carbon concentration in BF than in CN files teeth, which is probably the origin of their lesser performance. It can be concluded that the concentration of carbon in solid solution in martensite is responsible for the steel's hardness and therefore for its resistance to wear.