Utilisation de la spectroscopie mécanique dans l'optimisation des traitements thermiques d'aciers au carbone

This work is motivated by the industrial need to improve the thermal treatments of carbon steels used for the production of files. It aims at clearly defining the role that carbon plays in hardness and wear resistance of files. Three steel grades with different carbon content have been studied. The samples underwent various austenizing treatments, including treatments under carburizing atmosphere, followed by a quenching process leading to an essentially martensitic microstructure. This microstructure has been studied by mechanical spectroscopy. Several complementary techniques have been used : microscopy, thermopower, calorimetry, measurement of the microhardness and of the file performance. The damping spectra show three relaxation peaks, attributed to the interaction of interstitial atoms with dislocations in martensite. Two of these peaks, M2 and M4, are related with the presence of interstitial carbon. Peak M2 is related with the migration of geometrical kinks along screw dislocations, and peak M4 with the kink-pair formation and migration on screw dislocations. Both peaks are controlled by carbon dragging. This interpretation is supported by the measured activation enthalpies. The amplitude of peak M4 depends on the interstitial-carbon content, which apparently contradicts its physical interpretation. This dependence is attributed to the increase of the quench-induced dislocation density with interstitial-carbon content. The peak amplitude is in fact proportional to the dislocation density. The mechanical spectroscopy measurements are also sensitive to tempering effects. In particular, the precipitation of transition carbides at 380 K is associated with a decrease of the damping background, attributed to the depletion of martensite from interstitial carbon. The decomposition of retained austenite, occurring at 520 K or 670 K depending on the grade, is associated with a modulus anomaly, attributed to a sudden decrease of the dislocation density in martensite. A damping maximum coinciding with this transformation can also be observed at low frequency. This study lead to the use of new thermal treatments under carburizing atmosphere. The used gas mix gives rise to an efficient carbon enrichment of the samples. However, the increase of interstitial-carbon content may also lead to a higher content of retained austenite, thus reducing hardness. Calorimetry measurements allow the quantification of such content, which varies according to the steel grade and the thermal treatment. Martensite hardness and wear resistance of the files clearly increase with the interstitial-carbon concentration, that can be quantified by thermopower measurements, after correction of the effect of retained austenite. Long carburizing treatments can cause the formation of carbides on the sample surface. The presence of carbides in a high-carbon martensite does not seem to improve the mechanical performances, and can even be detrimental. The present study leads to the assessment of an optimal treatment duration of 30 min, which produces a hard martensite with maximum interstitial-carbon content and limits the amount of retained austenite and carbides. This carburizing time seems to produce the saturation of both the content of interstitial carbon at the steel surface and of the amplitude of damping peak M4. The amplitude of this peak seems to be an effective criterion for the evaluation of the ideal carburizing time. Indeed, the peak amplitude increases with the content of interstitial carbon and decreases with the content of retained austenite and carbides. The estimated optimal carburizing time is compatible with production demands. A cryogenic treatment proves to lower efficiently the amount of retained austenite.

Mechanical properties and microstructure of a high carbon steel

Iva Tkalcec Vâju

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.

EPFL2004

Tempering effects on three martensitic carbon steels studied by mechanical spectroscopy

Daniele Mari, Robert Schaller

Tempering effects have been studied in three martensitic carbon steels by mechanical spectroscopy. The mechanical-loss spectra present a relaxation peak similar to the Snoek-Koster peak in ferrite. The peak amplitude decreases upon tempering, indicating a decrease of the dislocation density. Transition carbides start to precipitate at 380K in all the three grades. This tends to decrease the mechanical loss and to increase the modulus. Retained austenite decomposes around 520K in two of the grades. In the third grade, the presence of Si delays this decomposition to 670 K. The decomposition of retained austenite leads to a sudden decrease of amplitude of the relaxation peak and a modulus anomaly. Both these effects can be attributed to a decrease of the dislocation density in martensite, probably associated with the depletion of carbon atoms in the dislocation core. At low frequency, a mechanical-loss peak associated with the decomposition of retained austenite is visible.

2008

Mechanical properties and microstructure of a high carbon steel

Iva Tkalcec Vâju

EPFL2004