Mechanical properties and microstructure of a high carbon steel

Iva Tkalcec Vâju
EPFL, 2004
Thèse EPFL

Résumé

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.

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https://infoscience.epfl.ch/record/33558?ln=fr

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Iva Tkalcec Vâju
EPFL, 2004
Thèse EPFL

Résumé

Official source

https://infoscience.epfl.ch/record/33558?ln=fr

À propos de ce résultat

Concepts associés (34)

Concepts associés

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Publications associées (74)

Publications associées

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In the quest of materials for the first wall of the future fusion reactor, it has been shown that oxide dispersion strengthened (ODS) ferritic / martensitic (F/M) steels appear to be promising candidates. The inherent good mechanical properties supported by a good thermal conductivity, swelling resistance and low radiation damage accumulation of the base material, such as EUROFER97, are further enhanced by the presence of a fine dispersion of nanometric oxide particles. They would allow in principle for a higher operating temperature of the fusion reactor, above 550°C, which improves its thermal efficiency. In effect, their strength remains higher than the base material at high temperatures. There are however limitations to their application in the reactor, which relate to the intimate link between the microstructure and the mechanical properties. These limitations, such as an increased ductile to brittle transition temperature above room temperature relative to the base material, come directly from the role played by the oxide particles but also from their indirect impact on the overall microstructure, stemming from the procedure of elaboration of these materials. In addition, there is a lack of knowledge on their response to irradiation, which, even though it appears to be promising with a reduced irradiation induced hardening, is not very well understood. In this work we have developed and evaluated ferritic/martensitic steel, EUROFER97, strengthened by a dispersion of yttria particles. (1) The powder metallurgical process used to obtain it was optimized, (2) the basic mechanisms of hardening were identified and (3) the impact of irradiation on its microstructure and mechanical properties were investigated. In order to understand the behaviour of this class of material other oxide dispersion strengthened (ODS) ferritic/martensitic steels based on EUROFER97 and model alloys were also investigated. For the elaboration of the ODS steel the powders of atomized EUROFER97 and yttria are ball milled in a planetary ball mill and the material is compacted by hot isostatic pressing (HIP). The mechanical properties are evaluated by uniaxial tensile test, Charpy test and micro-indentation while the microstructure is investigated by optical microscopy, scanning electron microscopy and transmission electron microscopy. The focused ion beam (FIB) technique is also used. In the study of the ball milling it appears that the optimal time is 20 hours, as the X-ray peak broadening, crystallite size and hardness of the particles remain unchanged at longer times. Yttria nanoparticles clusters are no more detectable by X-ray after 2 hrs of milling, which is attributed here to their incorporation in the matrix, as opposed to dissolution in the matrix as is often suggested in the open literature. HR-TEM performed on the yttria particles before and after incorporation in the steel by ball milling shows that the yttria maintain its bcc equilibrium structure throughout the process. This could suggest that they are not dissolved during ball milling. We have clarified this point by a cross sectional examination of the EUROFER97 ball milled particles with 0.3wt% of yttria in an FIB. By X-ray EDS analysis it shows nanometric regions reach in yttrium and oxygen, which are the yttria particles embedded in the EUROFER97 particles, proving that they are not dissolved by ball milling. The mechanism at the origin of the alloy compaction during HIP was elaborated. The alloy compaction was done through HIPing at 1150°C for 2.5 hrs at a pressure of 190 MPa. This condition is shown to provide the highest bulk density, to a value close to 99.9%. In the optimization process, it is found that temperature plays a larger role than pressure for alloy compaction. The compaction appears to be mainly due to diffusion. Also, it appears that the ball milled particle size plays a major role in the alloy compaction. It is shown that smaller particles size induces better compaction, provided that the densification proceeds through diffusion flow. This further suggests to terminate the ball milling process by 20 hrs as the particles size reaches its minimum. The impact of the ball milling process on the mechanical properties was evaluated. EUROFER97 with and without ball milling, with and without oxide addition presents an average density around 99.8%. In the as received state, casted EUROFER97, atomized EUROFER97, ball milled EUROFER97 and ODS Yttria present microstructure morphology typical of tempered martensite, whereas ODS Ti presents a nano sized grain microstructure with an average grain size of 200 nm. Ball milling and HIPping does have a slight influence up to 400°C, due to the higher dislocation density induced by ball milling relative to casted EUROFER97. It becomes negligible above 400°C, since both casted and ball milled EUROFER97 present similar hardness and microstructure. In ODS Yttria, yttria presents an average particle size of 25 nm. In ODS Ti, Y-Ti-O presents an average particle size of 7 nm. In the following the identification of the hardening mechanisms in ODS F/M steels. A new model was developed to quantify the collective hardening due to different obstacles, assuming that they provide independent hardening mechanisms. The model helped identifying the dominating source of hardening in ODS F/M steels. The base of the model is the dispersion barrier strengthening model, which is given by Δσsparticle or defects = Mαμb(Nd)1/2 for the hardening due small defects or oxide particles and is given by Δσd-d = Mαμb(ρ)1/2 for the hardening due to dislocations. In this approximation, the total hardening is then given by Δσtotal = {(Δσd-d)2 + (Δσparticle or defects)2}1/2 . The model is used to explain the hardening due to oxides, dislocations and irradiation induced defects. It appears that dispersed oxides is dominating hardness up to 800°C, while following a heat treatment at 1000°C it is the dislocations that then dominate the hardness. Deformation mechanisms were investigated using differential strain rate experiments. The activation parameters, i.e. the activation volume and activation energy, were calculated. It appears that there is a decrease in the activation volume from room temperature to 400°C, which is less than 3%. At about 500°C there is a sudden drop in the activation volume that reaches a minimum at 600°C. With further increase in temperature, there is a significant raise in the activation volume. This abrupt decrease between 400°C and 600°C hints at a change in dislocation-rate controlling mechanism, marking two regimes, a low temperature one and a high temperature one. In situ TEM observations show that at low temperature the deformation is mainly due to the formation of the Orowan loops around the particle. At high temperature the activation energy is about 3.4 eV for ODS, whereas it is 1.3 eV for EUROFER97. The calculated activation energy corresponds well to the one of dislocation climb mechanism. This indicates that at high temperature the yttria particles could be overcome by climb, while at low temperature they are overcome by the Orowan mechanism. The peculiar response of the yield stress to irradiation for this class of material was rationalized using the total hardening model we developed. At a dose of 2 dpa, irradiation induced defects are higher in number density than the dispersed yttria particles, which although they are weaker than the yttria particles they can explain the significant hardening observed in the material. Conversely, the shallow impact of irradiation on ODS F/M steels at low doses, relative to base F/M steels, is explained by the fact that the amount of dispersoid is found to be higher than the density of irradiation induced defects.

EPFL2008

Chargement

Al-Cu-Fe quasicrystalline coatings and composites studied by mechanical spectroscopy

Mechanical spectroscopy measurements were performed on two types of materials: decagonal quasicrystalline Al-Cu-Fe-Cr coatings deposited on a mild steel substrate and aluminium or magnesium matrix composites reinforced with icosahedral quasicrystalline Al-Cu-Fe particles. The internal friction spectra of the substrate with three different thicknesses of the coating indicate that the internal friction of such composites is mostly caused by the quasicrystalline coating and that the contributions of the steel substrate and of the interface are small. The shear modulus measured in torsion increases with temperature, while the Young's modulus measured in flexion behaves normally. This shear modulus anomaly is interpreted as due to a solid friction between cracked segments of the quasicrystalline coating. This phenomenon can also explain the broad athermal maximum, which was found to occur in isochronal internal friction measurements. A quantitative model successfully reproducing the observed behaviour was developed. Finally, the reversible high-temperature exponential background was interpreted as due to the onset of the brittle-to-ductile transition, which may be associated with dislocation motion controlled by collective phason flips in the quasicrystalline coating. The measured activation enthalpy is similar to the value that was deduced from compression tests performed on icosahedral Al-Cu-Fe bulk material. Aluminium or magnesium matrix composites reinforced with icosahedral Al-Cu-Fe particles were processed by gas pressure infiltration and characterised by X-ray diffractometry, electron microscopy observations, micro-hardness measurements, and compression tests. The internal friction spectra of these composites also show a high-temperature exponential background, while measurements of the matrix alone or of the matrix with Al2O3 short fibres exhibit a different behaviour. The difference can be explained by a partial phase transformation of the matrix due to the presence of the quasicrystalline particles. The exponential background is probably caused by dislocation motion in the matrix, however, the effect of the quasicrystalline reinforcement can be neither excluded nor confirmed with certainty.

EPFL2003

Chargement

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

Ronan Martin

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.

EPFL2009

Chargement

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

Ronan Martin

EPFL2009

EPFL2008

Al-Cu-Fe quasicrystalline coatings and composites studied by mechanical spectroscopy

EPFL2003