THE INFLUENCE OF MN STEELS AGEING ON HEAT TRANSFER PHENOMENA. Eva Mazancová, Pavel Koštial, Ivan Ružiak, Milada Gajtanská, Ľuboš Krišťák *

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1 THE INFLUENCE OF MN STEELS AGEING ON HEAT TRANSFER PHENOMENA Eva Mazancová, Pavel Koštial, Ivan Ružiak, Milada Gajtanská, Ľuboš Krišťák * Technical University of Ostrava, Faculty of Metallurgy and Material Engineering, 7.listopadu 5/272, Ostrava Poruba, Czech republic * Technical University of Zvolen, Faculty of Wood Sciences and Technology, T. G. Masaryka 24, Zvolen, Slovak republic ivan.ruziak@vsb.cz Abstract Thermophysical properties under investigation thermal diffusivity, specific heat capacity, thermal conductivity play an important role for designing material under thermal stress. The original austenitic manganese steel, containing approximately.2 wt% C and 2 wt% Mn, was invented by Sir Robert Hadfield in 882. Hadfield`s steel was unique in that it combined high toughness and ductility with high work-hardening capacity and, usually, good resistance to wear. Thermophysical properties have been measured by lumped capacitance method. We have measured three different materials with markings 023, 026, 028. Maximal content of Mn and C was 27 and 0.5 weight percent. Material 026 has the highest content of C 0.5 wt%, Ni wt% and has the highest specific heat capacity and highest thermal diffusivity in non-aged state. From results of thermophysical properties after ageing, one can see the increase of thermal diffusivity up to 20 percent, thermal conductivity up to 5 percent and decrease of specific heat capacity up to 6 percent. All measured values of thermophysical properties are in good agreement with literature data. Keywords: thermophysical properties, ageing, austenitic manganese steels INTRODUCTION Thermophysical properties under investigation thermal diffusivity, specific heat capacity, thermal conductivity play an important role for designing material under thermal stress. From these values heat transfer through the material can be modelled []. Austenitic Manganese steels belong to materials used in automotive industry. This type of material show high deformation strengthening. Tensile strength and ductility of material is often higher than 000 MPa, 50% [2]. The original austenitic manganese steel, containing approximately.2 wt% C and 2 wt% Mn, was invented by Sir Robert Hadfield in 882. Hadfield`s steel was unique in that it combined high toughness and ductility with high work-hardening capacity and, usually, good resistance to wear [3]. Thermal ageing of Mn steels is special heat treatment which use annealing at the temperature close to 600 C to rise diffusion ability of C in austenitic matrix, thus to creation of very fine K-carbides [4]. Thermophysical properties have been measured by lumped capacitance method. The method is based on Newton s cooling law with negligible conduction heat transfer and combined heat transfer of convection and radiation. The validity of model is based on Biot number which must be lower than 0, [5]. Literature values of thermophysical properties of steels have been obtained from [6].

2 2 THEORETICAL BACKGROUND OF LUMPED CAPACITANCE METHOD Lumped capacitance method is special case of Newton s cooling law. In this method heat transfer by conduction is negligible and total heat given to sample can be computed from equation () C - heat capacity of a sample in J.K - T - temperature increase during a heat transfer in K Q C. T, () For a heat flow from a sample to an environment is valid Newton s cooling law in the form [4] Q the heat given to a sample in J dq h S. T T dt h the total heat transfer coefficient of a sample in W.m -2.K -4 S the total heat flow area in m 2 T - the temperature of an environment in K T - the temperature of sample in K Then for lumped capacitance method is valid differential equation [4]., (2) dt m c p h. S. T T dt c p the specific heat capacity of a material in J.kg -.K -., (3) Temperature of sample can be find in the form [4] T dt.exp t / T max, (4) dt max maximal temperature difference between sample and surroundings in K thermal time constant(relaxation time) in s Thermal time constant can be described by equation [4] ρ density of sample in kg.m -3 L thickness of a sample in m The validity of the model is described by Biot number Bi in the form [4]. c p. L, (5) 2. h

3 Bi h. L 2. k, (6) k sample thermal conductivity in W.m -.K - Lumped capacitance model is valid when the value of Biot number is smaller than 0.. In this case heat transfer by conduction can be neglected. 3 EXPERIMENTAL PROCEDURE 3. Measuring apparatus For determination of thermophysical properties of used materials we used new disagned apparatus, which real look is on Figure. Fig. Electronics (left) and measuring apparatus (right) 3.2 Samples In experimental part we have measured thermophysical properties of three materials entitled 023, 026, 028 before and after ageing. For every material we used one sample of dimension cca 0 mm x 0 mm x 2 mm. Composition of used materials is shown in Table. Basic information about preparing of samples for measurement and statistical functions used can be found in [4]. Table. Materials composition wt.% C Al Mn Ni Si Fe 023 0,79 2,33 26,75 0,0 0,98 69, ,484 2,283 27,3 0,9,05 67, ,374 2,205 27,36 0,03 0,05 69,98 In table 2 are shown parameters of ageing.

4 Table 2. Ageing parameters Temperature Time Heat treatment 560 C 5min Annealing 3.3 Experimental results Thermophysical properties of material were measured four times for each sample. Mean values of total heat 4 transfer coefficient 2 hw.m. K, specific heat capacity c p J.kg. K, thermal diffusivity mm 2. s thermal conductivity parameters of fit for un-aged samples., for un-aged samples are shown in Table 3. In table 4 are shown statistical Table 3. Thermophysical properties for non-aged state Value 4 2 hw.m. K c J.kg. K mm 2. s p Bi 023,09 2, ,83 2, 83 5,78 0, 54 52,77 2, 06 0, ,02, ,93 9, 35 8,52 0, 92 58,93, 45 0, ,82, ,30 5, 84 6,57 0, 78 58,26 2, 7 0, 0004 Table 4. Values of statistical parameters for non-aged state Value 2 R RMSE 023 0,997-0,999 0,07-0, ,996-0,999 0,06-0, 028 0,997-0,999 0,07-0,5 From tables 3 and 4 is good to see, that condition of lumped capacitance method, Bi lower as 0., 2 R close to and RMSE close to 0, are fulfilled. From table 3 is also good to see very high repeatability of thermal properties, percentage error of thermophysical properties don t exceed 5 percent. In table 5 are shown literature values of thermophysical properties for carbon steels which were used as reference values. Table 5. Literature values [5] Value c J.kg. K mm 2. s p MIN 450,5 24,3 MAX 208 8,5 65,2

5 From measured values of thermophysical properties of materials 023, 026, 028 in non-aged state is good to see, that these are in good agreement with literature values. Thermophysical properties of material were measured four times for each sample. Mean values of total heat 4 transfer coefficient 2 hw.m. K, specific heat capacity c p J.kg. K, thermal diffusivity mm 2. s thermal conductivity parameters of fit for aged samples., for un-aged samples are shown in Table 6. In table 7 are shown statistical Table 6. Thermophysical properties for aged state Value 4 2 hw.m. K c J.kg. K mm 2. s p Bi 023,5 0, ,85 4, 35 9,05 0, 3 6,04 0, 88 0, ,24 0, ,97 3, 52 8,78 0, 06 6,80 0, 26 0, ,60 0, ,35 0, 85 8,8 0, 65 60,9 0, 70 0, 0004 Table 7. Values of statistical parameters for aged state Value 2 R RMSE 023 0,997-0,999 0,07-0, ,998-0,999 0,07-0, ,997-0,999 0,07-0,0 From tables 6 and 7 is good to see, that condition of lumped capacitance method, Bi lower as 0., 2 R close to and RMSE close to 0, are fulfilled. From table 6 is also good to see very high repeatability of thermal properties, percentage error of thermophysical properties don t exceed 5 percent. In figure 2 are compared values of specific heat capacity for non-aged(blue bars) and aged state(red bars). In figure 3 are compared values of thermal diffusivity for non-aged(blue bars) and aged state(red bars). In figure 4 are compared values of thermal conductivity for non-aged(blue bars) and aged state (red bars).

6 Thermal conductivity (W/m/K) Thermal diffusivity (mm2/s) Specific heat capacity (J/kg/K) , Brno, Czech Republic, EU Ageing influence on specific heat capacity Denomination of samples Non-aged state Aged state Fig 2. Comparison of specific heat capacity for non-aged state(blue bars) and aged state(red bars) Ageing influence on thermal diffusivity Non-aged state Aged state Denomination of samples Fig 3. Comparison of thermal diffusivity for non-aged state(blue bars) and aged state(red bars) Ageing influence on thermal conductivity Denomination of samples Non-aged state Aged state Fig 4. Comparison of thermal conductivity for non-aged state(blue bars) and aged state(red bars)

7 From presented graphs and tables 3, 6 is good to see that ageing decrease the specific heat capacity up to 6% and increase thermal diffusivity, thermal conductivity up to 20%, resp. 6%. 4 CONCLUSIONS From presented results we can conclude: - Ageing influenced thermophysical properties of used Mn steels - Ageing affect specific heat capacity, thermal diffusivity and thermal conductivity - Percentage error of thermophysical properties measurements were lower than 5%, which indicate excellent repeatability - All measured values of thermophysical properties are in good agreement with literature values - Statistical parameters and values of Biot number fulfill the conditions of lumped capacitance method. ACKNOWLEDGEMENTS This paper was created in the project No.Cz..05/2..00/ Regional Materials Science and Technology Centre within the frame of the operation program Research and Development for Innovations financed by the Structural Funds and from the state budget of the Czech Republic. This paper was financed also by the project Zevní fixace, MPO TIP FR-TI3/88. REFERENCES [] LIENHARD, J.H. Heat transfer, University of Oxford, [2] MAZANCOVÁ, E. Nové typy materiálů pro automobilový průmysl fyzikálně inženýrské vlastnosti vysokopevných materiálů legovaných mangánem a slitin hybridů kovů pro uskladnění vodíku. VŠB-TU, GEP ARTS s.r.o, Ostrava, 2007, s [3] [4] FROMAYER, G., BRŰX, U. Steel Research Inter., 77, 2006, 9-0, [5] KOŠTIAL, P., KOPAL, I. Vyšetrovanie termofyzikálnych parametrov I, Exponenciálny model chladnúceho telesa, VŠB TU Ostrava, 200, s. -. [6]