Rsearch the mechanism of refractory castable oven crack
1.Preface
 Modern metallurgy furnace building materials, refractory castable are used more and more, it is made up of refractory aggregate and binder mixing dry, mainly adopts hydraulic binder, by adding water or other liquid mixing after use, to casting, vibrating and tamping method, without heating sclerosis of solidification. It compared with the traditional brick clay refractory materials, construction is convenient, rigor, integral sex is good, high strength, long service life and so on a series of advantages. Refractory castable in about 6% of the water must be added into the construction process, in this way, in the masonry had a lot of free water and water of crystallization. How to make the moisture in refractory castable security quickly ruled out, has always been a problem in metallurgical furnace masonry baking.refractory cement 
Traditional oven curve is drawn up based on the following ideas: oven heating speed depends mainly on lining masonry thermal expansion stress generated by the size, the general temperature at 10 ~ 20 ℃ / h rate; Free water precipitation around 150 ℃, 150 ℃ or so precipitation crystallization water, in the two temperature points to holding 40 ~ 50 h; Considering the thickness direction of heat transfer resistance, again in 600 ℃ heat preservation;Considering the heat transfer and mass transfer resistance, make deep along the thickness direction in the masonry water completely ruled out, and silica complete crystal shape transformation, at about 870 ℃ heat preservation for 10 h. The traditional oven method mainly exist the following problems:
(1)Baking time is long, generally need 7 ~ 10 days, severely restricted the equipment overhaul period;
(2)Baking once early warming speed too fast, there will be a furnace explosion, explosion accident, furnace body peeling thickness can beUp to 150 ~ 200 mm, rework, equipment damage and other serious consequences;
(3)Bake in the early, local temperature suddenly increases, can cause a local scale, the castable masonry flaking thickness up to 20 ~50 mm, adverse effects on the furnace life, especially for heavy oil burning stove, bake oven with wood, tarps, early similar accident easily. Castable through tests, and the mechanism of masonry crack when baking, can make a more scientific and reasonable firing curve.refractory cement 
2Test method for
In general, at the bottom of the furnace is of the longest time in baking furnace, main from the upper part of the single side ruled out because the ministry of water, so we choose the bottom of the furnace as experimental simulation objects. Placed at the bottom of the sample simulation of the actual heating furnace hearth structure, to ensure that the water out mainly through the surface on the single side. Sample size: 500 mm (long)(width) x 500 mm x 350 mm (thick), it can ensure that the size of the sample similar to the real geometry and boundary conditions are similar, which are of heat transfer in the thickness direction is given priority to. 125 mm in the middle of the material surface and away from the center, 50 mm per casting is high, and put a 50.5 mm respectively NiCr - NiSi galvanic, buried thermocouple before use cement board hammer and dull ramming to pan pulp surface, pouring and 350 mm high, step by step to 0 #, 8 # galvanic on 20 mm from charge level surfaceHigh places. When pouring over 64 h after demoulding, sample charging with fibre blanket around the plug. In order to flexibly and accurately control temperature, test zone of furnace temperature control. Sample material selection heating furnace Al2O3 content is 65% of the widespread use of low cement castable.  
3, Test the required equipment and instrument
     (1)It can realize the automatic control of furnace 1, 20 kw rated power, rated voltage, 0 ~ 380 v ,mains frequency 50 hz, phase number for three phase, the heating element is carbon, silicon series.
     (2) A 12 point temperature recorder.
     (3) Diameter of 0.5 mm, 1.5 m long NiCr - NiSi thermocouple ,  16.
     (4) A multimeter. 
     (5) A   temperature gauge.
4. The test process
Test the fast heating and slow heating  two test methods.
  (1) Rapid warming (destructive) test will give to a maximum of 20 kw electric power, to see sample heating rate and surface quality. Samples in the furnace heating time is 2180 min, sample surface temperature of 1076 ℃.Free water high evaporation temperature is 117 ℃, the corresponding saturated vapor pressure is 0.18 MPa. When the trials to 1340 min, let out a whoop of bombs from the furnace. Experiment, sample center surface heating rate (164 ~ 818 ℃ range) of 67 ℃ / h, no heat preservation process, and in the center of the sample is 270 mm from upper surface free moisture out time of 1220 min, the speed is 4.7 mm/min. Heating up end of the test sample after cooling, its from a furnace observation: sample surface except for some tiny crack and two large cracks, one of the treaty of 350 mm long, almost across the surface of the sample. In vertical direction, the sample on the right side is 90 mm from upper surface is 300 mm long horizontal cracks; On the left side of the 35 mm from the surface has a 250 mm long horizontal cracks.refractory cement 
   (2) The flat heating test
The heating test adopts the method of control sample surface heating rate.
The specimen in a furnace, heating for 3450 min, surface temperature of 1096 ℃, no heat preservation process, the test free moisture out time of 1680 min sample center, eliminate speed is 5.6 mm/min. End of the test sample after cooling temperature rise, the pull out from the furnace observation: sample surface, no obvious crack only some very small cracks. That can ensure the quality of sample baking temperature of the request.
5.The test results analysis
(1)The castable masonry crack formation mechanism in the process of heating temperature curve, sample depth direction from different time bombs during a sample at 1340 min, free water has completely from the masonry at this time, from the center of the sample125 mm and 270 mm on the surface temperature is 156 ℃; Sample center maximum temperature gradient in the direction of depth in between the sample surface and 50 mm away from the surface, up to 6.6 ℃ / mm; 125 mm depth direction from the center of sample is the largest Temperature gradient in the sample surface and is 85 mm from the surface, up to 6.0 ℃ / mm, these positions in conformity with the crack location. Second, rapid heating test of sample internal vaporization temperature up to 117 ℃, the corresponding saturated vapor pressure is 0.18 MPa, crack of the sample; Gentle heating test of sample vaporization temperature up to inside140 ℃, and the corresponding saturated vapor pressure is 0.38 MPa, samples did not crack. The castable in 11038 ℃ when the compression strength of mpa, is greater than that of sample internal steam saturation pressure, masonry crack is not due to the thermal diffusion is too strong, the partial pressure of cause, but due to the baking heat up too fast, make along temperature gradient in the direction of thickness is too big, masonry expansion stress caused more than masonry shear stress.
(2)The castable masonry internal temperature change law in the process of heating, free water sample before entering the state of vaporization.The temperature gradient is minimal. When free water enter into the state of evaporation, it to absorb large amounts of heat of vaporization,Free water evaporation and basic constant temperature, make sample thermal resistance is quite large. As the evaporation When a layer of water vaporized in masonry, this layer of masonry with basic constant speed, the farther the heating speed is slower. In this way, the evaporation surface temperature difference within the above sample is more and more big; The difference in temperature of evaporating surface within the following sample smaller and smaller. Shows that the temperature stress is mainly formed in the process of evaporation.When vaporized sample within the largest temperature difference. The internal temperature of the specimen in the process of evaporation is nonlinear relationship with the depth of the sample, to the surface, from inside the masonry is gradually increasing temperature gradient, temperature stress is gradually increased.refractory cement 
(3) Oven improvement to reduce the internal temperature of the masonry stress curve, heating speed as small as possible; To shorten the oven time, the surface heating speed the sooner the better. For conventional oven system, when the masonry surface temperature reaches150 ℃, 350 ℃, respectively for a long time of heat preservation process. According to the gentle heating test results,When for thermal insulation masonry surface temperature of 150 ℃, the water in the sample doesn't boil; When the temperature of masonry surface For thermal insulation at 350 ℃, the moisture inside the sample vaporization rate will be slow or even stop. And because of these two temperature gradient is less than the time of completion of vaporizing temperature gradient, so in the heat preservation temperature gradients within the sample there will be no obvious improvement; And when the masonry of the free water evaporation after the completion of the surface temperature of 750 ℃ or so when proper insulation, can more effectively reduce the masonry, the temperature difference and reduce the temperature stress. Economic and reasonable oven system should be: at room temperature to 750 ℃ or so have been at 10 ~ 12 ℃ / h speed slowly heat up, when the masonry surface temperature reaches.After 750 ℃ for 10 ~ 15 h insulation, so on the premise of guarantee the quality of masonry, the oven time than traditional oven time shortened 2 days or so.