HEAT ENGINEERING DEVELOPING FIRING PARAMETERS FOR GATE-VALVE CORUNDUM PLATES V. G. Abbakumov, M. S. Glazman, !. Yu. Khlebnikova, and G. E~ Karas ~
UDC 666.762.11.046.4
The firing cycle of refractories critically affects the technieoeeonomic factors of the kilns and the quality of the finished goods. In fixing the best cycle it is necessary to know the thermomechanical characteristics of the products in firing and to define the conditions which ensure rapid firing within the permitted thermal stresses in the goods and the necessary degree of completion of the physicochemical processes of sintering. It is possible to calculate the permitted rates of heating and cooling of ceramics (ineluding refractories) on the basis of thermoelasticity theory [I~ 2] which is satisfactorily confirmed by experimental data obtained by firing ceramic, kaolin, and high-alumina blocks [5]. The determination of rational firing parameters is logical for corundum refractories, in particular, gate-valve plates used in the stopperless casting of the steel~ which is now being intensively used in metallurgy, In firing, the products sequentially pass through the stages of elastic, viscoelastic, and the elastic state again, which are localized in the main kiln zones (preheat, firing, and cooling). The maximum thermal stresses develop in the preheat and cooling zones when the material is in the elastic state, and in the viscoelastic state region these stresses are to a significant extent relaxed. Calculating the thermal stresses in complicated goods with variable thermomechanical and physical factors occurring in the material and in a nonstationary temperature field is associated with considerable mathematical difficulties, which are usually overcome by turning to evaluations based on solutions to problems for bodies of simple shape (plate, cylinder, etc.) bearing in mind local stress concentrations. In firing gate valves, of some interest are solutions obtained for cases of heating (cooling) plates being moved in a medium with a stated temperature, and when the material is undergoing temperature changes linearly with a regular second-degree cycle [6, pp. 96-102; 7, 8, pp. 85-101]. Equations based on these solutions were obtained to calculate the limiting temperature drop between the gasses and the material introduced to them (tg -- tm)ma x and the rate of change of temperature for the goods (dtm/dr)ma x ( I - - ~ ) t~p
t(~E { dt~ ~
An cos~n x8 -
Pn
exp(--Fo)
(1)
(2)
s(l-~)aap
~---dv-]m~x ,K~E6"(1-- 3 ~-) ' where An=
2 sin ~n
.
~6
a~
~n+sin~cos~n ;~n , root of the equation ~ntg~n= Bi; B~=--X--; Fo=-~-;
$ and ~, thermal
expansion coefficients and the Poisson ratio; E, elasticity modulus; X and a, thermal conductivity and temperature conductivity of the material; X, spatial coordinate; ~, half thickness of the plate; a, coefficient of heat transfer; Op, strength of the articles (for refractories, tensile); K, stress-concentration coefficient; and T, time. As the goods are heated the maximum tensile thermal stresses are developed in the center (X = 0), and in cooling -- at the surface (X = 6). From Eqs. (i) and (2) the mechanical and thermophysical characteristics of the material should be determined. In the investigations we determined the factors for corundum green brick with an apparent density of 2.99-3.01 g/cm s made at the Semiluk Factory by dry pressing from the batch conAll-Union Institute of Refractories. Semiluk Refractories Plant. Ogneupory, No. i, pp. 12-19, January, 1979.
12
0034-3102/79/0102-0012507.50
Translated from
9 1979 Plenum Publishing Corporation
FO Z
30
.
c,
n
6OO 500
2
"-. 000 20 a
I0
soo
Eo 200 0 t
I
I
0 200 #00 800 800 7000
TemperatuIe, ~ Fig. i
lOO
I 200
I 600
I 1000
Temperature, "C Fig. 2
Fig. i. Strength changes in corundum specimens during binder burnout: i) weight loss during calcination; 2) compressive strength (Ucomp); 3) tensile strength (Up). Fig. 2. Compressive strength Ucomp of corundum specimens done at holding temperatures).
(tests
sisting of corundum chamotte with a water absorption of 3-6% (fired at 1500-1520~ of the following grain-size composition: 45% fraction 2-0.5 mm, 20% 0.5-0.6 mm, and 35% <0.06 mm. The binder was sulfite yeast residues grade KDZh (MRTU 13-04-66) with a density of at least 1.15 g/cm s. The moisture content of the green brick after drying was below 0.5%. The chemical composition of the fired product was: 94.6% A1203, 2.0% SiO=, 0.6% Fe=Os, 1.3% CaO, 0.6%
MgO, 0.1% K=O, 0.8% Na=O. The green strength was largely due to the organic binder. The drying removed moisture, the articles grew stronger because of the adhesive action of the sulfite residues. The binder burns out in firing and the strength falls. The weight loss was determined after firing in air using the following cycle: heating to the stated temperature at 5 deg/min; holding 3 h, cooling 12-30 h. The weight loss was discontinued at 500-550~ (Fig. I). At this temperature the intense weakening of the green brick due to binder combustion was complete. The tensile strength o_ was determined by the diametric compression of cylinders 36 mm in height and diameter [9]. ~ With this method tests are done at room temperature. To evaluate the permitted thermal stresses in firing it is necessary to know the values Op at high temperatures. Two series of tests were made on this basis. In the first series the specimens were heated in an electric furnace at 5 deg/min, held at the final temperatures of 250, 500, 750, i000, and 1200~ for 3 h, cooled for 10-12 h at 25~ and tested for compressive strength Ucomp , and tensile strength Up. The changes in these factors versus temperature are shown in Fig. i. The lowest values are noted at 400-800~ Rapid weakening of the material occurs at 25-400~ coinciding with the range of rapid loss in weight, i.e., weakening of the green brick is due to bond burnout. The minimum value for Up was 6 kg/cm 2. The rise in strength of the specimens that were tested at room temperature and calcined at 800-1200~ is due to the start of sintering on account of the fusible impurities contained in the raw materials. In the second, serite heating one. The difference was that the ture (Fig. 2). Comparison of the states showed that the difference of the usual spread for strengths
and holding was done with the same schedule as for series compressive-strength test was done at the holding temperastrengths of the fired specimens in the heated and cold at temperatures up to 800-900~ is located within the limits of refractories [i0] and can thus be ignored.
The elasticity modulus was measured by the sonic method [! I] on heat-processes speci= mens. The experimental relationship for the elasticity modulus and temperature is shown in Fig. 3. As the bond burns out the elasticity modulus diminishes. The thermal expansion of corundum green brick was measured by the dilatometric method. The thermal expansion coefficient B remained constant in the 20-900=C range, and was equal to 7.2.10 -~ I/deg C. 13
,
735
O00 9
D,281 0
,ool
I II e' 200 4,05' 800 800 7000
,,
200
+
gO0
+
~'000
Temperature,*C
T ernperat~e, ~C Fig. 3. Elasticity modulus E of corundum specimens.
Fig. 4.
Gate-valve plate.
Fig. 5. Tensile strength Up of fired corundum specimens.
TABLE i. Calculations of the Maximum Temperature Variations between Gas and Material and the Heating Rates of Unfired Corundum Articles (dtm/dr)max, deg C/h of plate
( t g - tin)max, *C of plate O~a
0
20--100
100.--200
b I0 15 20 10 20
2,4
4,10
50
0,495
20 30 40
I i
700 530 350
I
335 250 170
430
205
2,4
3,93
35
0,46
530 265 160
25O 125 75
320
150
2,3
3,61
25
0,43
200 120 I00
95 55 5~
225
105
30
200~300
without with without with aperture aperture aperture aperture
20 300--400
30 40
2,2
3,33
15
0.38
135 80 65
46~0 30
140
65
400--500
20 30 40
2,1
3,06
7,
0,32
70 40 35
33 20 17
70
35
40 50
2,0
2,80.
6
0,29
37 31
18 15 13
6O
3084
60 !~
2,0
60
30
500--600
600--900
30
27 12
4
The thermal conductivity X and the temperature conductivity a are shown in Table I. At temperatures about 900~ the glass phase softens, the material begins to change from the elastic state to the viscoelastic state, the tensile and compressive strengths diminish, but in the viscoelastic and plastic regions the thermal stresses relax, a n d t h e danger of cracking is reduced. We calculated the heating parameters for corundum tiles in a gate valve for a steel ladle (Fig. 4) and an unbounded plate, of the same thickness. As the plate was being fired in the setting, it was mainly heated through the surfaces perpendicular to the axis of the
14
aperture;so, to locate the temperature stresses or limiting heating parameters it is possible to use the relationships (I) and (2). The values of the criteria Bi and Fo were determined by using the results of the investigation into the heat-exchange in tunnel kilns [12, 13]. Calculations took into account the various setting schemes using three values for the heat-transfer coefficients in each range of temperature. When calculations were made with K = 2.1, computed on the basis of the geometrical characteristics of the corundum plate [14], the stress concentrations near its aperature were also considered. The results for heating the green articles are given in Table i. In the range studied the average heating rate for the plate according to the calculations should be about 45 deg C/h, and not exceed 30 deg/h in the danger zone (500-900~ The thermomechanical characteristics of the fired specimen required to calculate the cooling zone were studied. It was found that they vary slightly in the elastic region and can be taken to be Op = 200 kgf/cm 2 (Fig. 5), E = 1.7.106 kgf/cm 2, thermal expansion coefficient ~ = 7.2.10 -6 i/deg C, Poisson ratio ~ = 0.2 The results (Table 2) show that the cooling rate of corundum plates should not exceed 50-100 deg C/h, and furthermore, when green articles are being heated, with a material temperature rise, the permitted rate of heat exchange is reduced. The difference in the thermomechanical properties of the green and fired articles, in particular Op, means that in the danger zone the maximum rate of heat exchange in the preheat zone is almost one half of that in the working zone, which should be considered in designing furnaces. In the danger sections the maximum temperature drop should be slight (see Tables 1 and 2). The operation of a tunnel kiln approximates to the optimum in proportion to the reduction in the ratio of the interval between car pushings to the firing cycle for the goods, i.e., the approximation to the scheme of contraflow stabilization. Since the firing conditions are improved in this case, we should endeavor to organize the kiln operation so that the cars are pushed frequently over a short distance, and ideally with continuous movement. Further development of the firing cycle for corundum plates was done in a kerosene-fired periodic kiln with a volume of 0.3 m 3. The gate-valve plates, grade ZS-60, were made at the Semiluk Factory using the factory technology. One of the unfired plates was cut into pieces and used to measure the open porosity and apparent density (Fig. 6). A significant irregularity in the distribution of open porosity over the volume of the plate is noted. The plates were placed in three types of setting (Fig. 7) and fired with different cycles, viz. the heating rate was varied (in the range 30-200 deg C/h); the cooling rate (40ii0 deg C/h), the firing temperature (1450-1500~ and the holding time at the firing temperature (4-9 h). The results are shown in Table 3. Trial No. 1 imitated the heating cycle for green goods close to the plant schedule, in which the heating rate in the flue-gas extraction system reached 170 deg C/h. Experiments confirmed the inadmissibility of such a sudden heating of the green articles which leads to extensive cracking. Trial No. 2 produced high loss from deformation, indicating too high a firing temperature. The distribution of open porosity over the volume of the plate is shown in Fig. 8. This shows that the original heterogeneity of density (see Fig. 6) produces heterogeneity of properties in the fired goods. Trial No. 3 produced acceptable goods by using setting I, and the firing in setting II produced fine cracks while the lower plate in setting III was deformed. In trial No. 4 firing was done with a calculated cycle (Fig. 9), the articles (3 plates) being established in the setting of type I. Firing yielded quality articles, without cracks, hairline cracks, or unacceptable deformation. Thus, investigations confirmed the validity of the calculations, facilitated the determination of a rational type of setting, and of the most intensive firing cycle. The parameters worked out are optimal for periodic kilns in which they can be realized in practice. For a tunnel kiln they are maximal, showing the boundaries of the permitted intensification of the heat processing. This is because in the larger part of the tunnel kiln length the firing cycle is determined by the conditions of heat exchange in the kiln space, which, without excessive complications to the design of the furnace, cannot meet the
15
TABLE 2. Calculations of the Temperature Differences between the Material and Air, and Cooling Rates of Fired Corundum Articles
'~
~, ke..a1/ m.h.*C
h.C
Temp. difference i Cooling rate, Jbetween rnaterialand I Co, deg C / h , a. zo~. at~ ArM-A, Cofptat } of plate m~/h
without ape~tuxe
with ][without with apem~ze/apertuxe aperture
20--100
10 15 2O
3,70
5,43
186 130 93
90 62 44
237
113
100--200
10 20 30
3,67
5,43
186 130 93
9O 62 44
237
113
200--300
20 30 40
2,80
3,96
186 130 108
90 62 52
173
82
300--400
20 30 40
2,56
3,46
186 131 101
90 60 45
151
72
400--500
40 50 60
2,42
3,13
100 93 70
45 45 35
136
65
50 60
2,39
2,95
I00 93 72
46 44 35
129
61
60 80 100
2,34
2,73
72 62 54
30
119
56
40
500--600
600--900
P" %
34 26
//7,o1~1 IZf31f#,~
Zg
fl, g / c m ~ - - ~ - - ~ ] ~ I~01 ~,05 I S0ZIZ~ Iz~5[z~qsI~,oz ~L3,oz ! ~ t~[ .j~,OlS,O~ s,os ] s,o I~,~1
,\\\\\\\~\\
Fig. 6
\\\, \\\
Fig. 7
Fig. 6. DistriBution of open porosity, P, and apparent density p~ in green corundum plates. Fig. 7.
Plan of setting for plates.
P,%
7500
r"
I~oi 9,7 I fo,o n,3
~a ! ~o,o
~
I~1 ,al I/O, Slf~_
9,yf4Ol 9,z I 9,s I=Z
IMSg,~lI 13J7I~1
.,~al.t.~l ~,~9 I s I,~ I ~ l ~ Z l ~,Sz I S~8 g ~
11311s s
7200 7 800
O0 10 20 30 ztO 50
80 707~
Time, h F i g . 8. rosity,
Distribution
of
open po-
P and apparent density, p, in fired plate
16
Fig. 9. Calculated firing schedule for plates.
TABLE 3. Test Results for Firing Industrial Corundum Plates Grade ZS-60
Firing parameters temp. rise, "C z .=
~
1!
[.
"
9 ~'l~ degC/h I~
9
9
!0--150~
i, " "
170 100
9
50
ff
40
4
11
40
111 (top) III (bot-
14,0 3,23 1600 Fine c~acks 15,2 3,19 2330 11,1 3,33 11950 12 313 28' 2010 Not determined 1400 Not determined
tom)
3 I 20~147C
4
Inspection results for fired plates
,8~,
I 1 I 2
~
OR
I~ ~ 1 20--800 800--1450
Propertiesof firedplates
,~
50
20.--900 (Ta~e 1) 900---1470
6
6
Deformation to support surface The same Curving Normal Deformation, elliptic apertures
13,0 3,24 1590 Normal 13,7 3,22 1050 Fine cracks 50 II1 (top) Not detedr-. mined 960 Normal I11 (bot- INot determined Deformation, elliptic tom) apertures 50*
1
13,513,231 1500 Normal
* C0oling from 1470 to 900"C at 50 deg C~ h; from 900 to 20"C with the cycle shown in Table 2, demands of the maximal firing cycle. Therefore, the actual cycle for firing goods in a tunnel kiln is always longer than the maximal cycle, i.e., in theory, the periodic kiln may ensure a greater speeding up of the firing process than can the tunnel kiln. At the same time the maximal production cycle is needed for an analysis of the projected or actual firing cycles for goods in tunnel kilns, since it means we can reject unacceptable cycles and select the optimum from the possible versions with respect to heat-exchange conditions. The permitted difference in temperatures between gas and material affords us the possibility of planning a schedule for moving the car stock, taking into account the local disturbances due to its discrete nature. The technological firing schedule thus developed is used for improving the heat treatment of gate valves in a tunnel kiln at the Semiluk Refractories Factory.* As a result of this work, a kiln cycle has been introduced in which the average heating rate of the plates in the preheat zone is 33 deg C/h (in the danger zone -- 28 deg C/h), holding at maximum temperatures 6 h, and cooling rate 52 deg C/h. Local thermal shocks were eliminated as not corresponding to the maximum permitted technological schedule. This achieved a significant intensification of the firing process, reducing the cycles from 104 to 78 h, and improving the quality of the refractories obtained. LITERATURE CITED i. 2.
3. 4. 5. 6.
N. N. Lebedev, Temperature Stresses in Elasticity Theory [in Russian], Leningrad-Moscow (1937). A. D. Kovalenko, Thermoelasticity [in Russian], Kiev (1975). S. A. Blokh, Steklo Keram., No. i0 (1974). Sh. I. Plyatt, Inzh.-Fiz. Zh., No. i0 (1960). Ya. A. Landa, Tr. Inst. VlO, No. 39 (1967). V. P. Isachenko et al., Heat Transfer [in Russian], Moscow (1969).
*The work was done together with specialists at the factory.
17
7. 8. 9. i0. ii. 12. 13. 14.
E. I. Morozov and Ya. Bo Fridman, in: Strength and Deformation in Nonuniform Temperature Fields [in Russian], Moscow (1962). N. Yu. Taits, Technology of Hea~ing Steel [in Russian], Moscow (1962) o R. Spriggs, L. A. Brissette, and T. Vasilos, Mater. Res. Stand., ~, No. 5 (1964)~ U. D. Kingery, Introduction to Ceramics, Wiley (1960). M. N. Bluvshtein, N. K. Senyavin, et al., Ogneupory, No. 4 (1969). R. S. Bernshtein and S. M. Ivanov, Tr. Inst. VlO, No. 30 (1960). V. G. Abbakumov, Ogneupory, Noo i (1967). A. M. Wahl, and R. Beenwkes, Trans. Am. Soc. Mech. Eng., 56 (1934).
THERMAL AND TEMPERATURE CONDUCTIVITY OF QUARTZ CERAMICS IN THE RANGE 500-1900~ E. Ya. Litovskii, I. V. Men~, and A. V. Bendikov
UDC 666.762.2.017: 620.179.132.001.5
Quartz ceramics cover a number of important refractory materials widely used in industry and new techniques [1-3]. Their thermophysical properties, however, have until now been studied at temperatures of 500-1200~ which is much below their operating temperatures. The present authors studied the thermophysical properties of quartz ceramics obtained by slip casting.* The temperature conductivity was experimentally determined in the 5001900~ range; the specific heat was calculated taking into account the actual phase compositions; then the resulting data was used to calculate the thermal conductivity. Using literature recommendations and our own method, we made a theoretical calculation of the thermal conductivity taking into account the contribution of thermal radiation. The temperature conductivity was measured by the monotonous heating method using plane specimens.t Specimens were cut from goods with a porosity of 24%. The pore-size distribution in the specimens, determined by the mercury porometric method, is shown in Fig. I. The impurity concentrations in the quartz ceramics (spectral analysis results) were: 0.029% CaO, 0.014% MgO, 0.23% A1203, 0.072% Fe203, 0.017% Ti02. The original specimens, consisting of amorphous Si02, formed I0 • 3% cristobalite when heated to 1700~ during determinations of temperature conductivity, and 70 i 7% when heated to 1900~ Microstructure-analysis of specimens showed that the cristobalite consists of a dispersed phase in the form of isometric crystals uniformly distributed in the matrix of amorphous silica. Results for temperature conductivity a(T) and specific heat c(T) are shown in Fig. 2. Specific-heat data for the constituent phases of ceramics (fused quartz and cristobalite) are taken from [4]. The dependence of thermal conductivity on temperature, calculated by means of the relationship a(T) and c(T), are shown in Fig. 3 (curve I) and satisfactorily agree with known experimental data (curves 2, 3). As stated, the quartz ceramic is semitransparent to thermal radiation, and in analyzing its thermal conductivity in a wide range, especially above 1000~ it is necessary to consider the radiation constituent of thermal conductivity. The contribution of thermal radiation was assessed by two different methodso Thermal conductivity of heterogeneous materials is currently calculated in most cases with a method described in [5-8]. In a heterogeneous medium a characteristic elementary nucleus separates, consisting of a solid phase and a space filled by gas (port). The thermal conductivity of this nucleus is taken to be equal to the thermal conductivity of the entire medium. The thermal conductivity of the solid phase is assessed through experimental data *Specimens were provided by G. E. Solovushkova. #E. Ya. Litovskii, "Study of thermal and temperature conductivity of refractory materials in the 200-1800~ range," Author's Abstract of Candidates Dissertation, Leningrad (1970)~ All-Union Institute of Refractories. Translated from Ogneupory, No. i, pp. 20-23, January, 1979.
18
0034-3102/79/0102-0018507.50
9 1979 Plenum Publishing Corporation
