DETERMINATION OF TEMPERATURE DISTRIBUTION IN ACTIVE COVERING DURING PRODUCING OF COATINGS BY CASTING METHOD.

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1 29/38 Solidification of Metals and Alloys, No. 38, 1998 Krzepnięcie Metali i Stopów, nr 38, 1998 PAN Katowice PL ISSN DETERMINATION OF TEMPERATURE DISTRIBUTION IN ACTIVE COVERING DURING PRODUCING OF COATINGS BY CASTING METHOD. KUBICKI Jerzy Instytut Inżynierii Materiałowej, Politechnika Szczecińska Szczecin, al.piastów 17. POLAND STRESZCZENIE W pracy opisano metodykę i podano wyniki badania rzeczywistego rozkładu maksymalnych temperatur nagrzania warstwy aktywnej (składającej się z proszków Al i Cu) po zalaniu formy staliwem L20H17N36S. Stwierdzono, że na wielkość badanych temperatur decydujący wpływ wywierają takie czynniki jak: grubość warstwy aktywnej i szybkość podnoszenia metalu w formie (obniżają) oraz ciśnienie metalostatyczne (podwyższa). Jednocześnie stwierdzono, że do tworzenia się powłok, kontakt ciekłego staliwa ze stopionym materiałem warstwy nie jest nieodzowny. 1. INTRODUCTION In order to reduce the effects of the carburization damaging the cast elements of furnaces (made of creep-resisting cast steel G-X20NiCrSi3617), the author has worked out a method of producing Al-Cu protective coatings directly in a casting mould [1]. This method consists in applying on the inner surfaces of a mould the specially prepared powder mixture of metals (Al+Cu), which after pouring the liquid alloy into a mould and after solidification of a casting forms on its surface a suitable coating. W.Sakwa and others have proved [2 6] that a degree of heating up of the active mixture ingredients is a crucial factor effecting the mechanism and consequently the structure and protective properties of a coating. The author in his paper [1] also pointed out that the temperature has influence on mass transfer from the active covering to a casting. However, in this case the value of the maximum temperature of the heating up of the active covering centre (T max ) has been calculated from the equation elaborated with the use of an analytical model of the heat flow between: casting active covering casting mould. This paper deals with determination of the real temperatures distribution in the active covering.

2 EXPERIMENTAL AND RESULTS The base for the presented here tests forms the technological parameters of the casting experiment, but first of all the results of the temperature measurements taken during the tests. In the course of 43 casting experiments the following parameters have been changed: X 1 Al:Cu ratio in the mixture in the range of 1,5 2,5, X 2 density of the covering of a mould with mixture changing from 0,01 to 0,3[g/cm 2 ], what corresponds with the changes of the active covering thickness from 0,1 to 2,5 [mm], X 3 diameter of a casting, from 20 to 50 [mm], X 4 temperature of cast steel, in the range of [ o C], X 5 metal raising rate in a mould, from 2,8 to 8,6 [cm/s], X 6 average metallostatic pressure in the points of temperature measurements, in the range of 13,6 17,4 [cm]. The construction of the experimental casting moulds, the distribution of the thermoelements in the moulds, the measuring apparatus registering the temperature changes and other details have been presented in the monograph [1], whereas Fig.1 shows the location of the chosen thermoelements and a procedure applied to determine the real temperature of the active covering centre. Fig.1.The location of the thermoelements ( t 1 t 4 ) in a mould and the procedure applied to determine the real temperature of the active covering centre (experiment 20.2 from Table 1). Rozmieszczenie termoelementów ( t 1 t 4 ) w formie i przedstawienie sposobu wyznaczania rzeczywistej temperatury środka warstwy aktywnej (eksperyment 20.2 z Tablicy 1). Line 1 shows the temperature distribution in the active covering for time τ 1, in which the temperature at the boundary: casting-covering ( t 2 ) has reached the maximum (T 2m/1 ), and at the same time at the boundary: covering-mould ( t 3 ) the temperature T 3/1 has been obtained.

3 183 Line 2 shows the temperature distribution for time τ 2 in which the maximum value has been registered on the thermoelement t 3 (T 3m/2 ) and at the same time the temperature T 2/2 on t 2. The points of intersection of the lines 1 and 2 with the axis of the covering centre determine the values of the temperatures T śr/1 and T śr/2.the average value calculated from the above values gives the real value of the temperature of the active covering centre T śr/1-2. Table 1. The compilation of the real temperatures and the periods in which they have been achieved for the chosen experiments from the paper [1]. Zestawienie rzeczywistych temperatur i czasów ich osiągnięcia dla wybranych eksperymentów z pracy [1]. No Exper. No acc.to [1] Location and number of a thermoelement castingcovering centre of covering covering-mould t 2 t 3 T 2m/1 τ 1 T śr/1 T śr/2 T śr/1-2 T 3m/2 τ 2 [ o C] [s] [ o C] [ o C] [ o C] [ o C] [s] No in Table 2 Y 1 Y 6 Y 4 Y 5 Y 3 Y 2 Y , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ,75

4 , , , , , , , , , , , , ,75 Table 1 shows the compilation of the temperatures determined as above and the periods in which they have been achieved for 31 experiments described in paper [1]. It should be mention that the results of the temperature recordings and the periods for the other 12 experiments have been rejected as unreliable. To check if there are reliable dependencies between the determined values and the technological parameters of producing, the computer approximation program has been used. The values of parameters X 1 X 6 have been used as independent variables, whereas as dependent variables the data presented in Table 1 have been put in. The obtained equations have been compiled in Table 2. Table 2. The compilation of the equations presenting the influence of the technological parameters on the distribution of the real temperatures and the periods in which they have been achieved. Zestawienie obliczonych zależności ujmujących wpływ technologicznych parametrów wytwarzania na rozkład rzeczywistych temperatur i czasów ich osiągania. Equations Statistical values R S F Y 1 =251,5+812,2 X 3 X 5-215,8 X ,4(X 5 X 6 ) -1 0,86 59,22 25,6 Y 2 =1226,8-272,9 X 6-341,2X ,4 X 2 X 6 0,923 42,21 51,96 Y 3 =1092,4+369X 6-286,7 X 2 X 5-69,6 (X 2 X 5 ) ,8(X 2 X 6 ) -1 0,915 43,33 33,59 Y 4 =1088,5+368,6 X 6-283,3X 2 X (X 2 X 5 ) -1 0,919 42,33 35,32 Y 5 =1491,4-51,2 X ,7 X 2 X X ,1 (X 2 X 5 ) -1 0,908 45,25 30,45 Y 6 =exp(4,61+0,28 X 2 X 5 +0,74ln(X 3 2 )) 0,912 0,182 69,05 Y 7 =-121,5+56,5 X ,8 X 3 X 6 +64,7(X 2 X 3 ) -1 0,951 19,01 85,62 X 1 X 6 have been standardized according to the following equation: X stand = (X i - X min )/(X max - X min ) + 0,5 (1) The graphs of the obtained equations ( Y 2 Y 7 ) for the real variables X 1 X 6 are shown in Figures 2 5, except for Y 1 because of a low value of R. a. b.

5 185 Fig.2. The influence of the covering thickness (X 2 ), metallostatic pressure (X 6 ) and metal raising rate (X 5 ) on the temperature of the active covering centre: a. test 1 (T śr/1 ) acc. to equation Y 4, b. test 2 (T śr/2 ) acc. to equation Y 5. Wpływ grubości warstwy (X 2 ), ciśnienia metalostatycznego (X 6 ) i szybkości podnoszenia metalu w formie (X 5 ) na temperaturę środka warstwy aktywnej: a. dla pomiaru 1(T śr/1 ) wg zależności Y 4, b. dla pomiaru 2 (T śr/2 ) wg zależności Y 5. a. b. Fig. 3. The influence of the covering thickness (X 2 ),metallostatic pressure (X 6 ) and metal raising rate (X 5 ) on the average temperature of the active covering centre (T śr/1-2 ) acc.to equation Y 3. Wpływ grubości warstwy (X 2 ), ciśnienia metalostatycznego (X 6 ) i szybkości podnoszenia metalu (X 5 ) na średnia wartość temperatury środka warstwy (T śr/1-2) wg zależności Y 3. Fig.4. The influence of the covering thickness (X 2 ) and metallostatic pressure (X 6 ) on the maximum temperature at the boundary: covering - mould (T 3m/2 ) acc. to Y 2. Wpływ grubości warstwy (X 2 ), i ciśnienia metalostatycznego (X 6 ) na maksymalną temperaturę występującą na granicy warstwa-forma (T 3m/2 ) wg zależności Y 2.

6 186 Fig. 5.The influence of the covering thickness (X 2 ), the diameter of a mould (X 3 ) and: a. the metal raising rate (X 5 ) on the period needed for achievement of the maximum temperature at the boundary: casting - covering (τ 1 ) acc.to Y 6, b. the metallostatic pressure (X 6 ) on the period needed for achievement of the maximum temperature at the boundary: covering - mould (τ 2 ) acc.to Y 7. Wpływ grubości warstwy (X 2 ), średnicy odlewu (X 3 ): a. szybkość podnoszenia metalu (X 5 ) na czas potrzebny do osiągnięcia maksymalnej temperatury na granicy odlew-warstwa (τ 1 ) wg zależności Y 6, b. ciśnienia metalostatycznego (X 6 ) na czas potrzebny do osiągnięcia maksymalnej temperatury na granicy warstwa-forma (τ 2 ) wg zależności Y DISCUSSION Even after a rough analysis of the data compiled in Table 1 it can be stated that the maximum temperature at the boundary: casting - active covering only in a few cases exceeds the value of 1260 o C i.e. the temperature of the end of solidification of the cast steel G- X20NiCrSi3617, and sometimes it is even lower than the melting point of Cu. Nevertheless, in all cases the coatings on the castings have been found [1]. From the above the conclusion can be drawn that the coatings can be formed without direct contact of the type: liquid -liquid. The author on the basis of the other premises has proved that at the first stage of formation of Al - Cu coatings the infiltration process of the liquid cast steel into the porous active covering takes place, and at the later stage the melting of Al and Cu accompanied by a mass diffusion towards the surface of a casting occur [7]. Analysing the equations shown in Table 2 a conclusion can be drawn that neither a change of the Al:Cu ratio in the mixture - x 1 (despite of different thermophysical properties of these metals) nor a pouring temperature - x 4 (having been changed in the assumed temperature limits) do not influence on the investigated temperatures distribution. As it can be seen from figures 2 4, the great influence on the maximum temperature values in the investigated area have:

7 187 1.The thickness of the active covering (x 2 ); an increase of the thickness causes a decrease of the temperatures under investigation (with one exception).this effect remains in accordance with the results obtained in the paper [1], where the maximum temperature of the covering centre (T max ) was calculated from a simplified analytical model. This decrease can be attributed to the heat absorption by the material of the covering and the deeper cast steel infiltrates into the mixture the bigger is heat absorption. 2.The metal raising rate in a mould (x 5 ); an increase of which causes a significant decrease of the investigated temperatures. This effect can be explained as follows; an increase of the metal raising rate favours the infiltration of the cast steel into the active covering [7], what causes that heat absorption is bigger. It is clearly visible for the coverings thicker than 1mm and periods close to τ 2 ( Fig.2b and 3). At low rates (x 5 =2,8 cm/s), the penetration depth of alloy is negligible, that is why a decrease of the temperature of the covering centre is noticeable for the coverings thinner than 1mm and after longer periods. This statement can be elicit by comparison of figures 2a-3-2b(in the quoted order).the explanation of this phenomenon can be found in a slow process of carrying away of heat by conduction. 3.The metallostatic pressure; the higher is this pressure the higher are the temperatures, and the bigger is their increase the further from the boundary:metal - active covering are located the points of their measurements. This effect can be explained as follows; an increase of the pressure causes that the active covering consolidates after having been partly melted (for periods close to τ 2 ) and consequently the heat conduction of the coverings increases. It is clearly visible for the coverings thicker than 1 mm. Analysing the graphs in figure 5 from the viewpoint of periods needed to reach the maximum temperatures of the heating up of the covering, the following can be said: a change of the covering thickness (x 2 ) has a slight influence on both periods (τ 1 and τ 2 ), a period τ 1 ( boundary: casting - covering ) is getting longer with an increase of the diameter of a casting (x 3 ), as simultaneously an increase of the temperature T 2m/1 is observed what results from the equation Y 2 (Table 2), a period τ 2 (boundary: covering - mould) is getting longer with an increase of the metallostatic pressure, as simultaneously an increase of the temperature T 3m/2 is observed (Fig.4). The explanation of the influence of the metal raising rate on period τ 1 and the diameter of a casting on the period τ 2 is rather difficult at the present stage of the investigation. However, further experiments will be carried out and their results will be published. 4. CONCLUSIONS 1.The determined values of the real temperatures at the boundary:casting - covering show that the formation of the Al-Cu coatings on the castings made from G-X20NiCrSi 3617 cast steel in most of the cases takes place without direct contact between the liquid cast steel and the active covering. 2.The biggest influence on the values and distribution of the temperatures in the active covering have: - thickness of the active covering, - metal raising rate in a mould,

8 188 - metallostatic pressure. 3.The complexity of the physicochemical phenomena occurring in the investigated process makes it difficult to interpret the obtained results, so the further investigation of the subject is required. REFERENCES [1].Kubicki J.: Odlewnicze powłoki ochronne Al-Cu na staliwie żarowytrzymałym. Prace Naukowe Politechniki Szczecińskiej Nr 529, Instytut Inżynierii Materiałowej Nr 17, Wyd. Politechniki Szczecińskiej, Szczecin, [2].Sakwa W: Żeliwo, Wyd.Śląsk, [3].Sakwa W.,Piłkowski Z.: Wytwarzanie powłok ochronnych na odlewach żeliwnych na drodze infiltracji ciekłego metalu w formie. Archiwum Hutnictwa, 2, 1967, s [4].Sobanskij N.W. et al.: Formirovanije litogo metallokeramičeskogo sloja v čugunnych stalerazlivočnych izložnicach. Izv.Vysś.Učebn.Zav.Čern.Metall. 1987(4) [5].Furman E.L. et al.: Povierchnostnyje upročnienije otlivok tugoplavkimi legirujuščimi obmazkami. Izv.Vysš.Ucebn.Zav.Čern.Metall., 8,1987,s [6].Gawroński J. et al.: Stopowe warstwy kompozytowe na odlewach staliwnych. Krzepnięcie Metali i Stopów. PAN Oddz. Katowice, 1995, z.24, s [7].Kubicki J.: Hipotetyczny mechanizm powstawania odlewniczych powłok Al-Cu na odlewach staliwnych. Mat.IV Konf. Zjawiska powierzchniowe w odlewnictwie. Kołobrzeg 98, Wyd. Politechniki Poznańskiej (w druku).