May 21 st – 23 rd 2014, Brno, Czech Republic, EU IMPACT OF OXIDE SCALE ON HEAT TREATMENT OF STEELS Miroslav RAUDENSKÝ, Martin CHABIČOVSKÝ, Jozef HRABOVSKÝ Brno University of Technology, Heat Transfer and Fluid Flow Laboratory, Brno, Czech Republic, EU, raudensky@fme.vutbr.cz Abstract Oxidation is an inherent aspect of steel production and heat treatment. Oxide scale layers commonly impact surface quality and material loss during steel processing. This paper is focused on the study of the influence of the oxide layer on cooling intensity. Spray cooling of a hot steel surface is considered. Typical examples are secondary cooling in continuous casting, interstand and run-out table cooling at hot rolling, and heat treatment and other metallurgical processes where controlled temperature regimes are required. Cooling intensity is primarily affected by spray parameters such as pressure and coolant impingement density. Though not frequently reported, even thin layers of oxides can significantly modify cooling intensity. This effect is prevalent when cooling steel surfaces at high surface temperatures. The influence of oxide scale layers on cooling intensity was studied using experimental measurements and numerical analysis. Experimental measurements compare the cooling of scale-free surfaces and oxidized surfaces. Experimental investigations show a difference in the cooling intensity. Numerical analyses were prepared to simulate sample cooling with different oxide scale layers and thermal conductivity. Even a scale layer of several microns can significantly modify the cooling intensity. Keywords: scale, oxide, Leidenfrost temperature, spray cooling, cooling intensity 1. INTRODUCTION Oxidation is an inherent aspect of steel production and heat treatment. Oxide scale layers commonly impact surface quality and material loss during steel processing. The scale layer also affects the cooling process. As reported in [1] and [2], the oxide layer can significantly influence the cooling intensity. Oxide scales covering a steel surface have a much lower thermal conductivity than steel (15-60 W m -1 K -1 ). The thermo-physical properties of the scale layer depends on the percentage of different types of oxides present ( and ) and the compactness of the scale layer. Thermal conductivities of different type of oxides are shown in Fig. 1. The thermal conductivity of a scale layer is approximately 3 W m -1 K -1 [1] or lower, due to the presence of air voids (0,062 W m -1 K -1 air at 900 K) in the scale layer. Scales on a steel surface act as an insulation barrier, decreasing the heat exchange between the steel and surrounding area. This “insulating” effect of scales can be easy to describe mathematically using the analogy of heat transfer through a wall with insulation. Let Qs be a heat flux from an oxide-free steel surface at temperature Ts to the surrounding temperature . This is described by Newton's Law of Cooling: where HTC is a heat transfer coefficient. The heat flux from the oxidized surface QP at oxide surface temperature TP is: