Thermal diffusivity measurements of metastable austenite during continuous cooling

• Published in: International journal of thermal sciences Vol. 47; no. 6; pp. 695 - 708
• Main Authors: Matteis, P., Campagnoli, E., Firrao, D., Ruscica, G.
• Format: Journal Article
• Language: English
• Published: Paris Elsevier Masson SAS 01.06.2008; Elsevier
• Subjects: Analytical model; Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder; Continuous cooling; Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; Experimental; Flash method; Low-alloy austenite; Materials science; Metastable phase; Phase diagrams and microstructures developed by solidification and solid-solid phase transformations; Physics; Steel quenching; Theoretical; Thermal diffusivity; Analytical model; Flash method; Experimental; Thermal diffusivity; Steel quenching; Continuous cooling; Theoretical; Metastable phase; Low-alloy austenite; Metastable phases; Digital simulation; Low alloy steels; Austenite; Analytical method
• ISSN: 1290-0729; 1778-4166
• DOI: 10.1016/j.ijthermalsci.2007.06.014

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.