Thermal diffusivity measurements of metastable austenite during continuous cooling

Abstract

The thermal diffusivity of the metastable undercooled austenite is relevant for the quantitative analysis of the carbon and low-alloy steel quench. The standard laser-flash method requires prior thermal equilibrium between the sample and the furnace, which may not be possible to achieve without allowing the metastable phase to transform. Nevertheless, depending upon the steel's hardenability, the thermal transient due to a laser pulse may be much shorter than a cooling transient sufficiently steep to prevent the transformation of the austenite. In one such case, flash measurements were performed during continuous sample cooling and the thermal diffusivity of the metastable austenite was determined by using an extension of the standard analytical model. The adopted analytical model and data reduction procedure are described and the limitations and uncertainties of this method are discussed, also with the aid of a non-linear numerical simulation. The measured thermal diffusivity of the undercooled low-alloy austenite decreases linearly from 5.4 · 10 −6 m 2 s −1 at 1133 K to 4.3 · 10 −6 m 2 s −1 at 755 K; this trend is in broad agreement with one previous set of measurements upon a low-alloy undercooled austenite and with a large number of previous standard measurements upon stable (high-alloy) austenitic stainless steels.

Thermal Diffusivity of Traditional and Innovative Sheet Steels

“…Its thermal diffusivity values are the lowest ones from room temperature up to the Curie temperature of the ferritic steels, while after this point they result to be closer to the values measured for the other steels. The increase of the thermal diffusivity values with temperature observed in the TWIP steel was previously observed also in some other high-alloy austenitic steels [17][18][19] and in low-alloy metastable austenite [6,20].…”

“…The mechanical properties of the welded joints depend on the microstructures obtained in the melted zone and in the heat-affected zones, which in turn depend upon the thermo-physical properties and the phasetransformation kinetics of the steels that are joined (steels which are often dissimilar) [5]. The thermal diffusivity of the steels is one of the most important parameters not only in the heat treatments [6][7][8][9] and in the welding [10] but also during the use of the components, since it influences the wear resistance [11][12]. Nevertheless scarce information are found in literature on the thermal diffusivity of high strength steels.…”

Modeling and Measuring Thermodynamic and Transport Thermophysical Properties: A Review

“…In this method, the front surface of a disk-shaped samp eral millimeters thick) is subjected to an energy pulse, as shown in Figure 7; as a the rear surface temperature increases. This temperature can be measured via an infrared (IR) detector [24] or a thermocouple welded to the sample [25], or an open thermocouple whose terminals are placed in contact with the sample surface. Then, parameter estimation procedures, such as the Gauss method [26], Levenberg-Marquardt method [27], and similar, are used to estimate the thermal diffusivity.…”

Study of the Thermal Diffusivity Variation in Thin Duplex Steel Plates Welded by GTAW Process

“…Abas R. H. and Taieh, N. K. [5] measured the thermal diffusivity and thermal capacity in a temperature range (from 1473 K until ambient temperature) for different duplex steels in order to complement thermophysical data of these alloys. Betini et al [6] found low value for thermal diffusivity for pure argon as shielding gas using conventional GTA welding process in heat-affected region. Matteis P. et al [7] reported the variation of the thermal diffusivity occurring in austenitic steels during the cooling process.…”