Papers by Keyword: Dilatometry

Abstract: The austenitization and inter-critical annealing of X4CrNiMo16-5-1 (1.4418) supermartensitic stainless steel were investigated in-situ with synchrotron X-ray diffraction (XRD), dilatometry and differential scanning calorimetry (DSC) under isochronal heating conditions. Austenitization occurred in two stages: the austenitization started at approx. 600 °C, decelerated at approx. 700 °C at 60 to 75 v.% of transformed austenite, and first resumed after heating for approx. 100 °C. This plateau in the transformation curve was more dominant for faster heating rates. Inter-critical annealing at 675 and 700 °C revealed, that austenite can to a certain extent be stabilized to room-temperature. There was good agreement for the transformation curves yielded by dilatometry and XRD. Some deviation occurred due to the different applied heating principles, different temperature monitoring and the impact of surface martensite formation on the XRD measurement. The applicable temperature range for DSC as well as the close proximity of the A_c1- and the Curie-temperature limited the usage of the technique in the present case.

Abstract: Quenching is a critical step during the strengthening age hardening of Aluminium alloys. To obtain optimal technological results, parts should be quenched with the upper critical cooling rate. The precipitation behaviour of Al alloys during cooling from solution annealing and thereby the critical cooling rates are typically investigated by in-situ measurements with differential scanning calorimetry (DSC). Conventional DSCs are limited at cooling rates below 10 Ks^-1. Unfortunately, medium to high strength Al alloys typically have critical cooling rates between 10 and some 100 Ks^-1. Recently it was shown that dilatometry is generally able for in-situ detection of precipitation in Al alloys. Dilatometry allows controlled cooling up to some 100 Ks^-1 and therefore covers the cooling rate range relevant. In this work, we aim to show up and discuss possibilities and limitations of dilatometric detection of quench induced precipitates in 2xxx, and 7xxx Al alloys. The basic method will be presented and results will be compared with DSC work.

Abstract: Cracks were observed when the magnesium silicate hydrate gel cement (prepared by 40% MgO/ 60% silica fume) was dried. This drying cracking is believed to be caused when unbound water evaporates from the binder. The shrinkage upon forced drying to 200 °C of mortars made up from a reactive magnesium oxide, silica fume and sand was measured using dilatometry. The magnitude of the drying shrinkage was found to decrease when more sand or less water was added to the mortars and can be as low as 0.16% for a mortar containing 60 wt% sand and a water to cement ratio of 0.5, which is of a similar order of magnitude as observed in Portland cement based mortars and concretes. A simple geometrical interpretation based on packing of the particles in the mortar can explain the observed drying shrinkages and based on this analysis the drying shrinkage of the hydration products at zero added solid is estimated to be 7.3% after 7 days of curing.

Abstract: In contrast to isothermal aging, few reports document the non-isothermal aging of deformed Al–Mg–Si alloys. The knowledge of non-isothermal aging of pre-deformed Al–Mg–Si alloys is of primary importance to understand the thermal stability as well as to control the microstructure of the final product during industrial processing. Therefore, the present work has been focused to understand the microstructure evolution during the continuous heating of a cold rolled Al–Mg–Si alloy. This has been followed using dilatometry, Differential Scanning Calorimetry, X-Ray Diffraction and microhardness measurement. Based on the results obtained, it is shown that dilatometry is a powerful tool to study phase transformations in deformed Al-Mg-Si alloys, moreover, the microstructural evolution, of the cold rolled sample, can be described as follows: at the earlier stages of the non-isothermal aging, formation and then the reversion of fully coherent GP zones take place. This is followed by the simultaneous occurrence of β” and β’ precipitation and recovery reaction. By continuing aging, the next reactions which will take place are β” and β’ dissolution and recrystallization. Finally, one can observe the formation and then the dissolution of the equilibrium phase β.

Abstract: For understanding and describing the materials behaviour during phase transformations, dilatometer measurements are commonly used in the scientific community. Generally, standard CCT diagrams are generated by means of dilatometer experiments. However, in many cases, real weld cycles are more suitable for studying the materials behaviour instead of using CCT diagrams. In this regard we talk about so-called in-situ diagrams.In this work, two different devices have been used to obtain CCT as well as in-situ diagrams of a low alloyed steel 10GN2MFA. The goal was to compare the results obtained on the one hand by using a widely used standard dilatometer (Bähr DIL-805A/D) and on the other hand by the usage of a thermomechanical simulator called Gleeble^®3800. The paper should finally show advantages as well as disadvantages of using different devices and to keep in mind that the optimal measurement method depends on different parameters. Realized experiments also have revealed the range of operational and diagnostic potential of used devices.

Abstract: 04X13H5M5К9Л is a stainless maraging steel with high strength and excellent toughness. This alloy has been selected for making Impeller casting which is subjected to localized stresses reaching as high as 700 MPa at the tip of its vanes. The impeller rotates at 19000 RPM for 253 seconds.This alloy comprises of austenitic – martensitic dual phase at room temperature, in which martensite exhibits Body Centered Cubic (BCC) structure with moderate hardness (HRC 30) and high toughness. The required strength is achieved by precipitation of second phase particles in soft martensite matrix at suitable aging temperature. There is always a chance of reversion of martensite to austenite on heating either for extended period or at higher temperature. Thus optimization of aging temperature and time becomes highly critical keeping in mind the functional stress requirements of the component. The optimization has been done using Differential Scanning Calorimetric (DSC) analysis, dilatometric studies and experimental iterations of heat treatment temperatures and aging time.Different combination of strength and ductility could be achieved by varying heat treatment parameters. Martensite start and finish temperatures and austenitic reversion temperatures were established through DSC and dilatometric studies. Effect of subzero temperature on properties and microstructure of this steel is also presented here. Scanning Electron Microscopy and Energy Dispersive Spectroscopy were also carried out to analyze phases in different heat treatment conditions.

Abstract: The β-phase transforms to α′, α and α" within a range of temperature from the β-transus (T_β) to about 600°C, considering no external stress is applied. Two types of microstructure were obtained: acicular martensite when rapidly cooled and lamellar α/β when slowly cooled from the β phase field. The sequential transformation of β into α′, α-phase, α₂, and α" was revealed as peaks on the coefficient thermal expansion (CTE) curves, however, reversed transformations: α"→β, and α→β, were revealed by the DSC thermograms. The presence of β, α′, α, α₂ and α" was identified by means of XRD analysis and HRTEM.

Abstract: To develop CCT diagram using DSC requires the use of multiple devices in order to measure across the range of cooling rates required to develop the diagram. In the current work one dilatometer is used to characterize the precipitation reactions of the AA7020 alloy. Precipitation and dissolution reactions resulted in changes in the rate of change in the coefficient of thermal expansion. This was used to determine the start and finish temperatures of the MgZn₂ precipitation reaction and produce the CCT diagram. Good agreement was found between the results of this technique and DSC results from the literature.

Abstract: Cobalt and cobalt-manganese spinel ferrites have magnetostrictive properties suitable for application in magneto-electric and magneto-mechanical transducers. In this work, copper-substituted ferrites of these compositions were processed by means of the ceramic method and their sinterabilities were evaluated by dilatometric thermal analyses. The results obtained suggest that copper affects the solid-state reactions for the spinel formation and lowers the required sintering temperature for the ferrites. However, the densification obtained with sintering of the copper-substituted ferrites at 950 ^oC for 6h was only 64%, which indicates that further adjustments on the processing route must be made in order to obtain higher densities.

Abstract: A one step quenching and partitioning process was applied to a 0.2%C-2.0%Mn-0.5%Cr-1.5%Si steel by quenching austenitised samples to several different temperatures below the experimentallydetermined martensite start temperature of 397 °C and isothermally partitioning them beforequenching to room temperature using a quenching deformation dilatometer. These treatmentsyielded predominantly martensitic microstructures containing 5.6 vol.% to 7.5 vol.% retained austenite,as measured by x-ray diffraction. In each treatment, strong dilation was recorded during isothermalpartitioning, with little indication of phase transformation during subsequent cooling to room temperature.This behaviour lends weight to the idea that an isothermal phase transformation occurred duringpartitioning, and that the final microstructure is a mixture of athermally and isothermally formed constituents.These results also suggest that the final microstructure of this steel is mostly formed beforeand during partitioning.