Testing of thermal shock resistance of refractory materials

YB/T376.1-1995 Test Method for Thermal Shock Resistance of Refractory Products (Water Quenching Method) and YB/T376.2-1995 Test Method for Thermal Shock Resistance of Refractory Products (Air Three parallel standards: quenching method) and YB4018-91 thermal shock resistance test method for refractory products.

1.YB/T376.1-1995 (water quenching method)

① Sample size: length × width × height = (200~230)mm × (100~150)mm × (50~100)mm straight brick. In practice, standard bricks with dimensions of 230mm × 114mm × 65mm are mostly used.

2 Heating method: Insert a section of the sample into an electric furnace at 1100°C. The heating surface of the sample is 50mm from the inside of the furnace door and not less than 30mm from the surface of the heating element. After the sample is put into the furnace, the furnace temperature shall not drop by more than 50℃, and shall return to 1100℃ within 5 minutes, and then be kept at 1100℃ for 15 minutes.

③ Cooling method: Immerse the hot end of the sample into flowing water at 5~35°C with a depth of (50±5)mm, cool with water for 3 minutes, and dry in the air for 5 minutes. If the sample is not in damage condition, place it in the furnace and continue testing.

④Evaluation criteria: The number of thermal shocks required for the area of the hot end surface of the sample to be damaged to reach more than half. ,

Thermal shock resistant high alumina bricks

2.YB/T376.2-1995 (air quenching method)

① Sample size: length × width × height = 114mm × 64mm × 64mm rectangular parallelepiped.

②Heating method: Place the sample into an electric furnace at 950°C. After the sample is put into the furnace, the furnace temperature shall not drop by more than 50°C. It shall return to 950°C within 5 minutes. It shall be kept at 950°C for 30 minutes and shall be placed on a long surface and shall not be stacked. The distance between samples and samples and the furnace wall shall not be less than 10mm.

③Cooling method

Blow with compressed air for 5 minutes. The compressed air nozzle should face the diagonal line of the sample injection surface for 5 minutes. Compressed air is at room temperature and does not contain water droplets. The pressure in front of the nozzle is 0.1MPa, and the distance between the nozzle and the center of the sample injection surface is 100mm.

④Evaluation criteria: After cooling, bend the sample with a maximum stress of 0.3MPa. If the sample is damaged under the action of 0.3MPa bending stress, it is considered to have failed the thermal shock. If the sample withstands the effect of 0.3MPa stress, it is considered to have passed the thermal shock. The test is repeated until the sample is damaged or the thermal shock reaches a predetermined number of times.

Thermal shock-resistant and wear-resistant phosphate bricks

3. YB4018-91 Thermal shock resistance test method of refractory products

① Sample size: length × width × height = 230mm × 114mm × 31mm or 230mm × 65mm × 31mm cuboid.

②Heating method: Heat one side of 230mm×31mm, heat the sample to 800℃ at a rate of 10℃/min, then heat to 1000℃ at a rate of 5℃/min, and keep it warm for 30min.

③Cooling method: Take out the sample, place it on an unventilated sample rack, and cool naturally.

④Evaluation criteria: Measure the flexural strength of the sample before and after a thermal shock, and determine the thermal shock resistance based on the strength attenuation rate.

It can be seen from the above that the thermal shock conditions of the water cooling method are the most severe, the thermal shock conditions of the air quenching method are in the middle, and the thermal shock conditions of the natural cooling method are the mildest. Therefore, they are suitable for testing refractory materials with good thermal shock resistance, average thermal shock resistance and poor thermal shock resistance respectively.

The water cooling method has the advantages of being simple, fast and cheap, and is suitable for daily production inspection of refractory materials used in cement kilns. However, the test results of this method are highly discrete and require many samples for testing. Due to differences in testing conditions, the test results of each laboratory may vary greatly. YB/T376.1-1995 cannot be used to test easily hydrated materials (such as dolomite bricks) and some products with special sizes and shapes.

The thermal conditions of the air quenching method are relatively mild. At present, the thermal shock resistance of refractory materials is getting higher and higher. It is difficult to destroy the refractory materials using the air quenching method, and the testing workload is increasing. At present, the product manuals of many Western companies state that the thermal shock resistance of ordinary lock-inscribed bricks, directly bonded magnesia-inscribed bricks, magnesia-aluminum spinels and various new inscription-free alkaline bricks are >100 times. In this way, it is impossible to distinguish the thermal shock resistance of ordinary magnesia-inscribed bricks, direct-bonded magnesia-inscribed bricks, magnesia-aluminum top-quality bricks and various new non-magnesia-inscribed bricks. Because the thermal shock resistance of these materials is >100 times, domestic manufacturers rarely use the air quenching method to control the production of refractory materials, except for dolomite and other materials. The test conditions stipulated in the YB4018-91 thermal shock resistance test method for refractory products are more mild and are rarely used by people.

If a new refractory material is developed, it is recommended to use a thermal shock fatigue test to characterize the thermal shock resistance of the material based on the flexural strength loss trend of the sample during the test.

4. Thermal shock fatigue test

In cement kilns, the damage of refractory materials is a gradual process, and the thermal shock experienced by the kiln lining is far less severe than the water cooling test specified in YB/T 63376.1-1995. Therefore, it is more in line with the actual situation to use the forced air cooling test method to conduct thermal shock and measure the strength loss during the test to evaluate the thermal shock resistance of refractory materials. The recommended technical conditions for thermal shock fatigue testing are as follows:

① Use the standard size of high temperature flexural strength specimen 25mm×25mm×150mm to make shaped or unshaped refractory material specimens using existing molds.

②The RJX-14 silicon carbon rod electric furnace is used for heating. The furnace size (length × width × height) is 500mm × 250mm × 200mm. The electric furnace power is 14kW. There are 12 SiC heating elements. The number of adjustable voltage levels is 12. The reference operating voltage is 220V. Current intensity 60A.

③Cooling is carried out in a special air box. The air box is equipped with a fan at one end, with a power of 370kW, a rotation speed of 2800r/min, an air volume of 8.5m3/min, and a wind pressure of 780Pa. The length of the nozzle of the air box is 300mm, and the size of the main part is 1000mm×400mm×200mm. The bellows is equipped with a sample rack that can be flexibly rotated, and 20 samples can be placed on the rack at a time. The use of air boxes can prevent the cooling air from being lost midway, and the rotation of the sample rack can ensure uniform cooling of each specimen.

④ In the test procedure, the number of test strips of the seed material should not be less than 7. Generally, at least 2 test strips are needed for the 0th and last thermal shock. For example, arrange 2 test strips each for 0 times and 10 times, arrange 1 test strip each for 3 times and 7 times, and have 1 test strip as a spare.

During the test, first put 2 pad bricks (230mm×64mm×32mm clay bricks) into the furnace to preheat, raise the furnace to the predetermined temperature, and keep it warm for more than 30 minutes. Then, open the furnace door and use iron pliers to pull out the pad bricks, place the pad bricks on the shovel, place the shovel on the refractory fiber felt, quickly lay the test strips, and then send them into the furnace. After the sample is put into the furnace, the furnace temperature is required to return to the predetermined temperature within 5 minutes. After the sample is kept at the predetermined temperature for more than 15 minutes, the furnace door can be reopened to cool the test strip with air blast.

When cooling, first turn on the cooling fan, take out the pad bricks from the furnace, use iron tongs to pick up the test strip and place it evenly on the sample rack. While cooling by blowing air, rotate the sample rack. After cooling for 7 minutes, the test strip can be placed. Take out the strips, place them on the pad bricks, and reinsert the pad bricks containing the samples into the furnace. A thermal shock takes about 30 minutes. In order to reduce heat dissipation, the test should be performed quickly, the furnace door should be closed at all times, and the removed pad bricks should be covered with refractory fiber.

⑤Selection of heating and cooling conditions: 950℃ air cooling, 1100℃ air cooling, 950℃ water cooling or 1100℃ water cooling. Materials with poor thermal shock resistance, such as dolomite bricks and magnesia bricks, can be air-cooled at 950°C; materials with good thermal shock resistance, such as spinel bricks and high-alumina bricks that resist spalling, can be air-cooled at 1100°C. Materials with excellent thermal shock resistance, such as silicon carbide, should be water-cooled at 1100°C. In short, the test is to divide the refractory material samples into three, six, and nine grades in order to select the best from the bad, and then select the best from the best.

⑥Selection of the number of thermal shocks. The failure mechanisms of refractory materials in thermal shocks are different, and the strength loss trends are also very different.

Some composite refractory materials are made from different raw materials. These raw materials have significantly different coefficients of thermal expansion. After firing, residual stresses may exist in the material. During thermal shock, thermal stress reduces strength, but annealing increases strength. Therefore, some materials may have abnormal strength curves after thermal fatigue testing.