The Role of the Refractoriness-Under-Load Tester in Evaluating Refractories at High Temperature

Refractories provide structural support and thermal insulation in high-temperature industrial kilns and furnaces. They remain under high temperature and load for long periods, and their dimensional stability directly affects kiln safety and service life. A refractoriness-under-load (RUL) tester evaluates how a refractory deforms under high temperature and load.

Refractories are comparatively strong at room temperature, but at high temperature their internal structure may change and their strength gradually falls. Marked deformation under load can crack or collapse a kiln lining. The RUL test simulates the high temperature and pressure a material bears in service and observes the temperature range from the onset of deformation to failure. The result helps users judge whether a material suits a particular part of a kiln, and gives refractory manufacturers a basis for product development.

An RUL tester usually consists of a heating furnace, a loading system and a deformation-measuring device. The specimen is placed in the furnace under a set load while the temperature rises on a set program. The measuring device records the change in specimen height with temperature and plots a deformation curve. Several characteristic temperatures can be read from the curve, such as the onset-of-softening temperature and the temperature at which deformation reaches a set amount. These temperatures reflect the material's structural stability at high temperature. Heating rate, load and furnace atmosphere are test conditions that must be controlled, because they affect whether results are comparable.

RUL testers are used mainly by refractory manufacturers and by the plants that use refractories. Refractoriness under load is a common index in the quality inspection of refractory bricks, castables and refractory fibers. In high-temperature industries such as steel, cement, glass and ceramics, refractory selection draws on this value. Research institutes use RUL testing to study the high-temperature performance of new materials and ways to improve their resistance to deformation. Different kiln zones place different demands on a material, and the RUL temperature is one of the main references in selecting it.

Specimen preparation and mounting affect the result. Specimen size and shape should meet the requirements of the standard, and the faces should be flat so that poor contact does not load the specimen unevenly. The ram and spacers of the loading device must be kept clean and undamaged. Set the heating program to the standard, and check furnace temperature uniformity before the test. During the test, watch the furnace atmosphere: some materials behave differently under oxidizing and reducing conditions.

After long service at high temperature, the furnace heating elements may age and need regular inspection. The insulation lining the furnace chamber may spall and disturb the uniformity of the temperature field. Keep the mechanical parts of the loading system lubricated, and verify the sensors and displacement gauges regularly. For a heavily used instrument, a record of tests on reference specimens helps monitor its condition. Place the instrument in a well-ventilated area, and protect operators against high temperatures.

An RUL tester provides a means of evaluating performance at high temperature, and its data show how a material behaves under load at high temperature. In the refractories industry this testing is one link in a product quality-control system. Results are comparable and useful as references only when test conditions are standardized and data are recorded consistently. Evaluating high-temperature performance depends on an accurate understanding of service conditions and on data accumulated over time.