4. 5. 6. 7. 8. 9. i0. ii. 12. 13. 14. 15.

G. E. Solovushkova, Yu. A. Polonskii, R. S. Churakova, et al., Ogneupory, No. 4, 26-29 (1978). G. E. Solovushkova, Yu. A. Polonskii, et al., Ogneupory, No. 7, 5-6 (1980). Yu. E. Pivinskii, V. A. Bevz, and R. Ya. Popil'skii, Ogneupory, No. 4, 50-56 (1981). Yu. E. Pivinskii, Ogneupory, No. 9, 13-17 (1983). R. Ailer, Chemistry of Silica: Solubility, Polymerization, Colloidal and Surface Properties, Biochemistry [Russian translation], Mir, Moscow (1982). Yu. E. Pivinskii and V. A. Bevz, Izv. Akad. Nauk SSSR, Neorg. Mater.,17, No. 9, 1706-1710 (1981). Yu. E. Pivinskii, Ogneupory, No. 8, 15-22 (1983). P. L. Mityakin, in: Refractory Concretes [in Russian], VIO, Leningrad (1984), pp. 43-49. A. G. Dobrovsol'skii, Sllp Casting [in Russian], Metallurgiya, Moscow (1977). S. A. Greenberg, R. Tarnutovski, and T. N. Chang, J. Colloid Sci., 20, No. I, 20-43 (1965). I. F. Efremov, Periodic Colloidal Structures [in Russian], Khimiya, Leningrad (1971), p. 194. F. S. Kaplan, G. L. Kalyada, N. S. Gaenko, et al., Ogneupory, No. 12, 49-54 (1982).

THERMAL STRENGTH, DEFORMATION, AND THERMAL FATIGUE CHARACTERISTICS OF DINAS PRODUCTS BEFORE AND AFTER SERVICE IN THE HIGH-TEMPERATURE ZONES OF THE HOT-BLAST STOVES E. Z. Korol', V. M. Panferov, V. L. Bulakh, and N. V. Pitak

UDC 666.762.2.017:620.178.17

In recent years, dinas refractory bricks are being widely used in the high-temperature zones of the hot-blast stoves of the blast furnaces. Their use makes it possible to increase the blast temperature up to 1300-1350~ and the temperature under the dome (cupola) up to >1500~ At the present time, the temperature under the dome of the majority of the hot-blast stoves amounts to 1300-1400~ The dinas products intended for service in the hot-blast stoves of the blast furnaces have the following properties: refractoriness -- not less than 1690~ the temperature of onset of deformation at a stress of 0.2 MPa -- not less than 1620~ additional growth at 1450~ -not exceeding 0.4%, the minimum ultimate compressive strength -- 3 0 M P a for the checkerwork products and 27.5 MPa for the wall and dome products, the maximum porosity -- 22% for the checkerwork products and 24% for the wall and dome products, density -- not exceeding 2.37 g/ cm 3, thermal conductivity -- 1.8-2.1 W/(m-~ and thermal expansion at 1400~ -- 1.25-1.35%. The Twelfth Five-Year Plan envisages further increase in the blast temperature and for realizing this, dinas refractory bricks are required to serve under the maximum permissible regimes. In view of this, it is necessary to study the thermal strength and deformation characteristics of dinas products and use the obtained data during the operation of the blast furnace stoves. During the service of dinas products in the walls and the hexagonal checkerwork blocks of the blast furnace stoves, the deformation and strength properties of the material undergo changes because of thermomechanical fatigue under the action of thermal cycles and due to chemical and phase transformations. Dinas specimens drawn from the products before and after service were studied at the Institute of Mechanics, MGU and UkrNllO. Their strength, deformation, and thermal fatigue characteristics were determined. Figure i shows the dimensions of the test specimens subjected to single-stage heating and uniaxial loading (tensile and compressive) under isothermal conditions at different temperatures ranging from 20 up to 1600~ Table i gives the designation (marking) and some of the physical properties of the specimens. The DO specimens were Institute of Mechanics, M. V. Lomonosov Moscow State University (MGU). Ukrainian Scientific-Research Institute of Refractories (UkrNllO). Translated from Ogneupory, No. 6, pp. 19-22, June, 1986. 0034-3102/86/0506-0323512.50

9 1987 Plenum Publishing Corporation

323

6,.MPa a

3

3 *o

I

2

l

30 2O

t

b

d

2

f3 10

o-55 a

o,,oo~ o.olo o.ozs#, g

b

Fig. 1

o,oo4

o,o12

o,o2oG, %

Fig. 2

Fig. i. Specimen configuration and dimensions for thermomechanical tests under uniaxial tension (a) and compression (b). Fig. 2. Isothermal tensile (a) and compressive (b) deformation curves of the DO ( ) and DS (. . . . --) specimens and after i000 thermal cycles (--,--~--) at the following temperatures: !) 20; 2) i000; 3) 1200; 4) 1400; 5) 1500; 6) 1600~ ~, M~ fOr j

-

p O,O7O

j ,c

o,o12

O,0O8 ~oo~

o

o,oo~

o,olz

o,ozoep%

w

o

7

2

~

Fig. 3 Fig. 3.

(-

8t.h

6

Fig. 4

Isothermal uniaxial compressive deformation curves of the DM ( - - ) and D ) specimens at the temperatures: i) 20; 2) i000; 3) 1200; 4) 1400; 5)

1600~ Fig. 4. Creep deformation e~ of the dinas walling products DO ( - - ) and DS (-- -- -~ under a uniaxial compressive stress oF = 1 MPa at 1300~ (i) and 1400~ (2).

O,07Z

3

o,olz i i O,008

0.00# #

I

I

r

9

I

-.--.t---,.. I

2 Fig. 5

#

I

6

I

8 t,h

Fig. 6

Fig. 5. Creep deformation e~ of the dinas walling products DO (--) and DS (. . . . ) at 1500~ under uniaxial compression at a stress of 0.5 MPa (I) and 1 MPa (2). Fig. 6. Creep deformation ~ of the dinas checkerwork products before service DM ( ) and after service D (. . . . . ) when heating up to 1500~ under a stress of 1 MPa (i), 0.5 MPa (2), 0.7 MPa (3), and 0.35 MPa (4).

324

TABLE I. imens

Desig' nation

Properties of Dinas Spec-

Open po- Apparent Demi~y, rosity, % d~m_~/, gl g/cm" cm ~

, . o

20,34

DO DS: DM D

2,35 2,33 2,36 2,37

1,87 2,04 1,90 1,91

14,0

19,7 18,8

0,016

OOO8 o~oo~ I

I

#

8

|

12

1

16t.h

Fig. 7. Creep deformation EPI of the dinas checkerwork products before service DM ( ) and after service D (. . . . . . ) at a O temperature of 1500~ (i) and 1600~ (2): - - ) oz = 0.5 MPa; -) ~ = o.35 MPa. sawed out from the straight wall products (according to GOST 20901--75). Similarly, specimens were mechanically cut out from the hexagonal checkerwork products (designated DM). The specimens for studying the characteristics of the material after service were made from the dinas products that served for three years in the wall lining (DS) and in the checkerwork of a hotblast stove (D) of the No. 3 blast furnace of the "Azovstal'" steel plant. The shape and size of the specimens were chosen in such a manner as to provide for uniform heating and stress state in the working portion. The specimens were radiation heated in air using molybdenum disilicide heating elements maintaining a heating rate not exceeding 5-6=C/min in the 20-I000~ range and not exceeding 10~ at temperatures above IO00~ This regime ensured uniform bulk heating of the specimens. The specimens were held at the test temperature for a period of 15-20 min prior to loading. Figure 2 shows the deformation curves oa ~ c~ (oa is the stress and ~z is the strain) of the specimens of the wall products under uniaxial tension and compression. An analysis of the elastic limit and the ultimate strength at different temperatures shows that up to 1200-1300~ deformation of dinas is mainly elastic and at 1400~ it is elastoviscoplastic; the material has virtually no load-bearing capacity at 1570-1600~ At the temperatures ranging from i000 up to 1450-1500~ there is a sharp reduction in the values of the modulus of elasticity, the proof stress, the elastic limit, and the tensile strength. In order to establish the nature of variation of the modulus of linear elasticity (Young's modulus) E = ~z/e~ during the heating process, tests were carried out according to the following heating and loading program: a given specimen was heated according to the aforementioned regime up to a temperature Tn = Tmaxn/N (where Tma x is the maximum working temperature, Tma x = 1600~ N is the number of subdivisions of the temperature range O-1600oC, ~ N = 8-16; and n is the number of intermediate values of temperature in this range, n = i, 2, 3, ..., N) and was held for a period of 10-15 min; thereafter, loading and unloading was carried out within the linear range of the ox ~ cx diagram and the initial portion of the diagram was recorded; this was followed by heating up to a temperature Tn+ z and the loading-unloading cycle was carried out; and the procedure was repeated up to T = T m . This program made it n ax possible to plot the temperature dependence of the modulus of elasticity: T(T) = E(T)/E (20~ quite accurately. Figure 2b shows the deformation curves of the dinas specimens drawn from the wall products after service (DS) for a period of three years under the conditions of prolonged action of thermal cycles and cyclically varying (from oxidizing to slightly reducing) aggressive chemical environment. Besides this, Fig. 2b gives the data obtained on the specimen drawn from the original product (DO) that was subjected to cyclic heating according to the charac-

325

teristic cycle of the hot-blast stoves: average temperature 1350~ amplitude !00~ notal period 30 min; trapezoidal cycle; dwell at the maximum and minimum temperatures of the cycle i0 min; and the duration required for increasing and decreasing the temperature 5 min. The values Of the modulus of elasticity Eo(T) and Eo (13500C) of the original specimen were determined from the linear portion of the o~ - s~ diagram at 20~ and during the process of heating up to 1350~C; the magnitude of E n (1350~ after n thermal cycles was also determined in the same manner. The complete ~ ~ s: diagram was plotted after subjecting the specimen to i000 thermal cycles (see Fig. 2b, dot-and-dash line). This procedure makes it possible to qualitatively evaluate the effect of thermal cycles on the thermal strength, deformation, and thermal fatigue characteristics of the refractory materials. It can be seen from Fig. 2b that the ultimate strength of the specimens decreased from 16.5 MPa (in their original state) up to 12.5 MPa after i000 cycles and up to 6~5 MPa after three years of service and their fracture strain decreased from 0.012 up to 0.008%. This fact indicates that cyclic heating leads to softening and embrittlement of the material [i]o The same conclusion can be drawn from an analysis of the deformation curves of the specimens drawn from the checkerwork products under uniaxial compression (Fig. 3). Consequently, thermal fatigue is one of the main properties determining the life of dinas bricks. A single experiment in which the specimen is subjected to I000 cycles is inadequate for obtaining a reliable quantitative evaluation; however, this experiment also confirms the conclusion that the thermomechanical characteristics in the unloaded condition vary slowly when the number of cycles is small (up to 2o103). Under tensile loading conditions, thermal ~ycles can significantly affect the long-term strength (thermal fatigue) and even when the load is small, the values of the tensile strength a t and the fracture strain et decrease sig ~ nificantly. Figures 4 and 5 show the creep characteristics (determined under uniaxial compression at temperatures up to 1300-1600~ of the dinas specimens drawn from the walling products before service and after three years service; and Figs. 6 and 7 show the creep characteristics of the specimens drawn from the c h e c k e ~ o r k products before and after service. The steady-state creep rate of the DM specimens at a temperature of 1500~ and a stress of 1.0 MPa amounts to 5.10 -4 i/h and the steady-state creep of the D specimens at a stress of 0.7 MPa amounts to 6-10 -4 i/h, or when referred to a stress of 1.0 MPa by linear approximation, it amounts to 8.6o10 -4 I/h. The results are comparable within the scatter range of the creep characteristics. CONCLUSIONS The deformation and strength characteristics of the dinas specimens (drawn from the industrial walling and checkerwork products before and after service) were studied under uniaxial tension and compression at a temperature up to 1600~ It was established that their ultimate strength decreases with increasing test temperature and that the load-bearing capacity is insignificant at 1570-1600~ when subjected to cyclic heating (1250 ~ 1450~ in the laboratory tests as well as during service in a hotblast stove operating under alternating oxidizing and reducing environmental conditions, dinas bricks exhibit embrittlement and softening (their tensile and compressive strength characteristics differ by almost I0 times). The obtained data showed that the limiting temperature at which the strength characteristics of dinas abruptly decrease under a constant stress and multiple cyclic action of alternating atmospheres amounts to 1600~ t h i s f a c t must be taken into account when designing and operating hot-blast stoves. LITERATURE CITED i.

326

V. M. Panferov, R. M. Mansurov, V. K. Trincher et al., "Optimum stressed roof," Tr. Insto Mekh. Mosk. Gos. Univ. im. M. V. Lomonosova (M. V. Lomonosov Moscow State University), Moscow, No. 34, 64 (1974).