Papers by Keyword: Dilatometry

Abstract: The first step in a heat treating cycle is the austenitizing of the as-received material. Despite its importance, this step has received relatively little attention. In this work, the kinetics of austenite formation during continuous heating tests of steel samples with low and high silicon content was determined as a function of heating rate. The microstructural evolution was characterized through dilatometric analysis of cylindrical samples (7 mm × 20 mm), continuously heated in a protective atmosphere at constant heating rates ranging from 2 to 40 °C/min. The critical temperatures and the transformation kinetics were determined from the derivative of the relative length change as a function of temperature. As the heating rate increases the critical temperatures and the transformation temperature range increase; the addition of silicon produces a more marked effect. The transformation kinetics data were correlated using an Avrami-type equation. The kinetic parameter n is nearly independent of heating rate while the parameter k is a strong function of the heating rate; in both cases, slightly larger values were obtained for the high-silicon steel.

Abstract: Variations of a true stress vs. true strain illustrate behaviour of materials during plastic deformation. Stress-strain relationship is generally evaluated by a torsion, compression and tensile tests. Results of these tests provide crucial information pertaining to the stress values which are necessary to run deformation process at specified temperature and cooling rate. Uniaxial compression tests at temperatures below the γ solvus were conducted on nickel based superalloy CMSX-4, to study the effect of temperature and strain rate on its flow stress. On the basis of received flow stress values activation energy of a high-temperature deformation process was estimated. Mathematical dependences (σ_pl -T i σ_pl - ἐ) and compression data were used to determine material constants. These constants allow to derive a formula that describes the relationship between strain rate, deformation temperature and true stress.

Abstract: This paper presents the results of dilatometry and DTA analyses performed on a Ni base super alloy sample on which a ZrO₂/20%Y₂O₃ coating was deposited. The tests were done at similar temperatures as those present in a gas turbine engine at the inlet of the turbine. The purpose is to assess how a turbine blade with a ZrO₂/20%Y₂O₃ coating would behave to changes in volume due to temperature and structural changes. The differential thermal analyses and thermo gravimetric analyses were performed on the DTA PT 1600 Linseis installation. The thermal dilatometry was performed on the L75HX XXXX installation.

Abstract: The influences of the heating rate and annealing duration on martensite formation and its reversion to austenite have been investigated in a 60% cold-rolled metastable high-manganese austenitic Type 204Cu stainless steel. A Gleeble 3800 thermomechanical simulator was used for dilatometric measurements. Cold-rolled steel pieces were either heated up to 1000 °C at various heating rates between 5 °C/s and 150 °C/s followed by quenching to room temperature, or heated and held at temperatures in the range of 450 620 °C for different durations between 0.1 600 s. In heating experiments, dilatation curves revealed an expansion of a specimen starting around 550 °C followed by contraction, both processes depending on the heating rate. These dimensional changes could be correlated to the formation and reversion of a ferromagnetic phase, α-martensite. Some martensite was also formed during isothermal holding in connection with tempering of the pre-existing α-martensite before the following reversion, as established by magnetic measurements. Tempering of martensite was revealed by microhardness behaviour, X-ray diffraction analysis and transmission electron microscopy.

Abstract: Using dilatometric technique, it is the purpose of present research to quantify the kinetics of austenite decomposition during cooling after isothermal hot compression. To reach this aim, based on lattice parameters and thermal expansion coefficients of different phases, a new model was proposed. In this model, the contributions of both isotropic and non-isotropic dilatations were considered. The model was applied to 22MnB5 steel to predict formation of bainite and martensite. Furthermore, the model was developed in order to determine the final fraction of deformation induced ferrite (DIF).

Abstract: Four metal tert-butoxides, namely: aluminum (III) tert-butoxide, Al (Ot-C₄H₉)₃, tin (II) tert-butoxide, Sn (Ot-C₄H₉)₂, titanium (IV) tert-butoxide, Ti (Ot-C₄H₉)₄, and lithium tert-butoxide, Li (Ot-C₄H₉), were used as initiators in the bulk ring-opening polymerization (ROP) of ε-caprolactone (CL). The polymerizations were carried out at 120 °C for 72 hrs. It was found that the Al (Ot-C₄H₉)₃ only partially dissolved in the CL monomer and gave a low % conversion. The Li (Ot-C₄H₉) initiator did not dissolve and gave no polymerization. In contrast, the Sn (Ot-C₄H₉)₂ initiator dissolved completely, albeit very slowly, and gave a high % conversion. The Ti (Ot-C₄H₉)₄ initiator was by far the easiest and quickest to dissolve and also gave a high % conversion. Consequently, Ti (Ot-C₄H₉)₄-initiated polymerization was studied further by dilatometry at 120°C. From the kinetic results, a first-order rate constant, k_p, of 0.120 l mol^-1 min^-1 was obtained.

Abstract: The master sintering curve (MSC) of nanocomposite WC-MgO was constructed based on the combined-stage sintering model. Nano-sized WC-4.3wt%MgO powder with average particle size of 35nm was synthesized by high-energy ball milling, and then uniaxially pressed at the pressure of 500MPa to fabricate green compacts. The shrinkage response of the compacts, used to construct the master sintering curve, were studied by dilatometric runs at two constant heating rates of 5°C/min and 10°C/min up to 1900°C. Using the estimated activation energy, the master sintering curves were established and compared to acquire an optimum value (Q=361.8 kJ/mol). The obtained MSC was validated by non-isothermal sintering with the identical green compacts. The results demonstrate that the MSC can be applied successfully to predict and control shrinkage level and final density during heating up regardless of heating rates.

Abstract: Annealing of martensite/austenite microstructures leads to the partitioning of carbon from martensite to austenite until the chemical potential of carbon equilibrates in both phases. This work calculates the volume change associated with this phenomenon using theoretical models for the carbon partitioning from martensite to austenite. Calculations are compared with experimentally determined volume changes. This comparison reveals that in the case of steels with higher contents of austenite-stabilizing elements, reported volume changes are satisfactory predicted assuming a low mobilily martensite/austenite interface. In the case of a steel with lower additions of austenite-stabilizing elements, experimentally measured expansions are considerably larger than predicted ones. The large measured volume expansions probably reflect the decomposition of the austenite.

Abstract: Innovative treatments like quenching and partitioning (Q&P) have been recently proposed to improve the combination of strength and ductility of high strength steels by stabilization of significant fractions of retained austenite in a microstructure of tempered martensite. The decomposition of austenite into bainite and carbides precipitation are the two main competitive processes that reduce the content of retained austenite achievable at room temperature. A medium carbon low-silicon steel (0.46% C and 0.25% Si) has been studied to identify in which limits the austenite can be enriched in C and stabilized by Q&P, although a silicon content well below 1.5%, commonly used to retard cementite precipitation, is adopted; indeed, high Si contents are detrimental to the surface quality of the product due to the formation of adherent scale in high temperature manufacturing cycles. The heat treatments have been carried out with a quenching dilatometer, investigating the carbon partitioning process mainly below Ms, where cementite precipitation is not activated. The dilatometric curves show the progressive enrichment of carbon in the untransformed austenite and the occurrence of austenite phase transformation during the isothermal holding below Ms. A range of temperatures and times has been found where a content of about 10% of retained austenite can be stabilized at room temperature, a percentage much lower than the theoretical maximum achievable with the carbon content of this steel.

Abstract: A new approach for case hardening of powder metallurgical steels is surface densification prior to heat treatment, hence avoiding hardening to the core caused by open porosity. With regard to this process chain a porosity and carbon dependent model of the transformation kinetics is essential. In powder metallurgical materials the transformation behavior is mainly influenced by the chemical composition, homogeneity and porosity. Using a prealloyed powder, e.g. Astaloy 85 Mo, a homogeneous distribution of alloying elements after sintering can be assumed and the transformation behaviour is mainly determined by pores and the carbon profile caused by case hardening. The effect of carbon is widely known but up to now, only a few details about the effect of porosity on the transformation can be found in literature. It is reported that a decreasing relative density causes a reduction of incubation and overall isothermal transformation time. In the present study, the transformation kinetics of a powder metallurgical steel based on Astaloy 85 Mo were investigated for the carbon levels 0.5 and 0.8 wt% as well as the relative densities 6.8, 7.2 and 7.8 g/cm³. The investigations were carried out using a high-speed quenching dilatometer. The isothermal time temperature transformation diagrams for this powder-metallurgical alloy are presented and Avrami-type equations are fitted to the measured data. A good correlation can be found for the transformation model and the experimental results verifying the used modeling approach showing the potential to be applied within case hardening simulations.