Influence of the natural aluminium oxide layer on ... - ALU-WEB.DE

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MELTING, RECYCLING & HEAT TREATMENT The new generation <strong>of</strong> <strong>aluminium</strong> heat treatment plants – a vanguard concept Markus Belte and Dan Dragulin, Belte AG The vigorous discussion in <strong>the</strong> <strong>aluminium</strong> heat treatment community about <strong>the</strong> development <strong>of</strong> <strong>the</strong> appropriate technique becomes within <strong>the</strong> framework <strong>of</strong> <strong>the</strong> present work a genuine concrete answer. This paper is completely dedicated to <strong>the</strong> presentation <strong>of</strong> a new generation <strong>of</strong> heat treatment plants; it first introduces <strong>the</strong> fundamentals <strong>of</strong> high speed heat transfer processes and presents practical results and <strong>the</strong>irs <strong>the</strong>rmodynamical analysis and <strong>the</strong>n <strong>of</strong>fers a detailed perspective <strong>of</strong> <strong>the</strong> vanguard concept <strong>of</strong> <strong>the</strong> new generation <strong>of</strong> <strong>aluminium</strong> heat treatment plants. The present work is based on original experiments and investigations obtained by <strong>the</strong> authors and will present techniques never used before in <strong>the</strong> industrial heat treatment <strong>of</strong> <strong>aluminium</strong>. Infrared radiation – <strong>the</strong>oretical aspects The radiation heat transfer process is described by <strong>the</strong> law <strong>of</strong> Stefan-Boltzmann: • ∂Q Q = ⎯⎯ = εσST 4 ∂t (1) • 4 4 Q = εσS [T O - T U ] (2) • Where: Q = heat flow rate; ε = emissivity 1 ; σ = 5.67 x 10 -8 W/m 2 K 4 Stefan-Boltzmannconstant; S = surface <strong>of</strong> <strong>the</strong> emitting body; T = temperature <strong>of</strong> <strong>the</strong> emitting body (Kelvin); T O = surface temperature; T U = environment temperature 2 . The heating rate can be calculated using <strong>the</strong> following formula: mc 100 TWB Ta τ = ⎯ ⎯⎯ [ ξ´(⎯⎯)- ξ´´ (⎯⎯)] α T 3 U TU TU (⎯⎯) 100 (3) 3 Where: m = mass/surface unity <strong>of</strong> <strong>the</strong> product to be heated; c =specific heat; TWB = heat treatment temperature; TU = environment temperature; ξ = f(TWB/TU) A special case <strong>of</strong> <strong>the</strong> heat transfer process is <strong>the</strong> infrared heat transfer process. “Infrared radiation, that portion <strong>of</strong> <strong>the</strong> electromagnetic spectrum that extends from 1 ε Є (0,1] ; for <strong>the</strong> ideal black body: ε = 1 2 The environment could be <strong>the</strong> interior <strong>of</strong> a furnace. 3 According to [3] <strong>the</strong> long wavelength, or red, end <strong>of</strong> <strong>the</strong> visiblelight range to <strong>the</strong> microwave range. Invisible to <strong>the</strong> eye, it can be detected as a sensation <strong>of</strong> warmth on <strong>the</strong> skin. The infrared range is usually divided into three regions: near infrared (nearest <strong>the</strong> visible spectrum), with wavelengths 0.78 to about 2.5 micrometres (a micrometre, or micron, is 10 -6 metre); middle infrared, with wavelengths 2.5 to about 50 micrometres; and far infrared, with wavelengths 50 to 1,000 micrometres. Most <strong>of</strong> <strong>the</strong> radiation emitted by a moderately heated surface is infrared; it forms a continuous spectrum. Molecular excitation also produces copious infrared radiation but in a discrete spectrum <strong>of</strong> lines or bands.” [Encyclopaedia Britannica [1]] “Radiant heating has been used in <strong>the</strong> industry since <strong>the</strong> 1930s, where it was introduced to bake finishes on cars. It took time for <strong>the</strong> new technology to gain acceptance but today <strong>the</strong> technology is used in most industrialised countries especially for drying and curing <strong>of</strong> paints and drying <strong>of</strong> textile, pulp and paper. Infrared energy is unique because it can heat materials or objects without heating <strong>the</strong> air around <strong>the</strong>m. That allows infrared heat to be concentrated exactly where it is wanted without much loss <strong>of</strong> energy.” [2] In <strong>the</strong> case <strong>of</strong> an infrared heating <strong>the</strong> maximum temperature which can be reached depends on <strong>the</strong> environment temperature: T - T U = (T max - T U)(1-e -B . τ ) (4) [3] Where: T = body temperature; τ = duration; T U = environment temperature; T max = maximum temperature; B = coefficient which takes into account <strong>the</strong> process conditions In <strong>the</strong> case <strong>of</strong> a classic heat transfer process performed exclusively through air convection, <strong>the</strong> body temperature can not exceed <strong>the</strong> air temperature. Gas radiation In <strong>the</strong> case <strong>of</strong> flame radiation <strong>the</strong>re are two types radiation: visible and invisible/infrared radiation. The last one is <strong>the</strong> invisible infrared radiation emitted by carbon di<strong>oxide</strong> and steam 4 . The quantity <strong>of</strong> heat transferred through radiation from a <strong>layer</strong> <strong>of</strong> CO 2 respectively H 2O to a grey surface are: 3.2 3.2 Tg Tw qCO2 = ε . 10.35(p . CO2 s) 0.4 [(⎯⎯) - (⎯⎯) 100 100 Tg 0.65 . (⎯) ] Tw 4 After J. H. Brunklaus Fig. 1: Bilateral (green) and unilateral (red) exposure to radiation (not coated, h = 10 cm) (5)[[7]→[6]] 60 ALUMINIUM · EAC CONGRESS 2011 Abbildungen: Belte
q H2O = ε(46.52 - 94.9p H2Os)(p2 . s) 0.6 3 ⎯⎯ 3 ⎯⎯ 2.32 + 1.37 √p 2 s 2.32 + 1.37 √p 2 s Tg Tw [(⎯⎯) -(⎯⎯) 100 100 ] (6)[[7]→[6]] Where: q = quantity <strong>of</strong> heat [W/m²]; p = partial pressure [daN/cm²]; s = <strong>layer</strong> thickness [m]; T g = gas temperature [K]; T w = temperature <strong>of</strong> <strong>the</strong> grey surface [K]; ε = emissivity rapport: A λ = absorbed radiation and A λs = total radiated energy Aλ ε = ⎯ A λs Infrared heating <strong>of</strong> massive <strong>aluminium</strong> castings Electric radiant burners (7)[[7]→[6] Experiment purpose – determination <strong>of</strong>: • heating rate • maximal reached temperature • influence <strong>of</strong> surface quality �� MELTING, RECYCLING & HEAT TREATMENT coated or not as cast 100% graphite coated • distance between <strong>the</strong> infrared source and <strong>the</strong> irradiated surface (h); Experimental conditions: • experimental infrared facility industrial IR installation power : 108 KW/m² nominal power: 37.8 kW (active surface <strong>of</strong> 0.35 m²) air cooling system • test specimen die casting: cylinder head Al-Si-Mg-Cu alloy weight: 26 kg wall thickness: 120 mm • exposure to radiation bilateral and unilateral In <strong>the</strong> case <strong>of</strong> a unilateral exposure to radiation (one part <strong>of</strong> <strong>the</strong> infrared module is switched <strong>of</strong>f) we obtain a heating rate <strong>of</strong> 0.11 °C/s (between 50 and 425°C 5 ). In <strong>the</strong> case <strong>of</strong> a bilateral exposure to radiation we obtain a heating rate <strong>of</strong> 0.43 °C/s. Enhancing <strong>the</strong> installation �� @�� power by 100% (in <strong>the</strong> case <strong>of</strong> a bilateral exposure to radiation) quadruped <strong>the</strong> heating rate by a coeval reduction <strong>of</strong> <strong>the</strong> energy consumption. The unequal temperature maximum is conspicuous (fig.1). Energy consumption: • unilateral exposure to radiation: ~ 55 min 55 • E = ⎯ . 18.31 = 16.78kWh 60 • bilateral exposure to radiation: ~ 15 min 15 • E = ⎯ . 18.31 = 4.58kWh 60 Where: 0.35 m²………………………37.8 kW 0.169 m² 6 ………….……………x kW X = 18.31 kW/side → X = 36.63 kW for a bilateral exposure The coating leads to a considerable enhancement <strong>of</strong> <strong>the</strong> surface heating rate and simultaneously to <strong>the</strong> increase <strong>of</strong> <strong>the</strong> temperature difference between <strong>the</strong> surface and <strong>the</strong> 5 One has to pay maximum attention to <strong>the</strong> solidus temperature (especially for Al alloys containing Cu). 6 casting surface exposed to radiation
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