The reheating furnace is an important thermal equipment used in the rolling mill to heat the billet. When the reheating furnace is newly built, minor repaired, medium repaired, and overhauled, the owner has strict requirements on the construction quality and construction period. Refractory castables have the advantages of short construction period, good insulation effect, and high overall strength. They are widely used in heating furnace roof, furnace wall, water beam, flue and other parts. The construction work requires the water temperature to be above 15°C and the curing temperature to be above 5°C. In order to ensure the quality of construction, heat preservation measures must be taken during winter construction to prevent frost damage.
When the heating furnace is constructed in winter, due to the low ambient temperature, the fluidity is poor and the solidification time is long, which brings certain difficulties to the construction.
Taking calcium aluminate cement combined with refractory castable as an example, the influence of low-temperature pouring construction on refractory castable mainly has the following aspects:
(1) The construction temperature is too low, the mixing quality is not good, and it is extremely difficult to form.
(2) The fluidity becomes worse, the porosity of the material increases, and the physical strength decreases.
(3) The hydration reaction of cement is slow, which affects the expansibility. When the temperature is lower than 0°C, the free water begins to freeze, and when the temperature reaches -15°C, almost all the free water freezes into ice, causing the hydration and hardening of the cement to stop completely. When water is transformed into solid ice, its volume increases by about 9%, causing internal stress, causing the relative displacement of aggregate and cement particles and forming ice aggregates inside, causing local structural damage. The density of water is the highest at 4°C. When the temperature drops below 4°C, the volume of water has actually begun to expand, which will damage the newly formed cement hydrate structure of the refractory castable. If the free water freezes and freezes, the strength after thawing will be very low, and the surface of the castable will be layered off.
(1) After frostbite, it manifests as ice patterns, spiral patterns, whitish color, unevenness and other phenomena on the surface of the frozen material. The material looks wet, the edges and corners are darker in color, and it is easy to break. Just use the naked eye. It can be seen that there are fossil-like ice crystals on the edge and bottom of the aggregate pit, which are fragile and have almost no adhesion to the steel (anchor), and there are many large pores in the material.
(2) In terms of condensation and hardening, the temperature is low and the condensation and hardening speed is slow, which affects the construction progress. When the temperature is negative, it will not only fail to harden normally, but will also have an adverse effect on the construction structure, which will cause a large area of the casting body to peel off during the oven, which will affect the normal production.
Insulation measures to be taken in winter construction
In order to ensure the construction quality and the strength of the pouring body, and avoid the material from being frosted, it is generally required that the construction environment temperature should not be lower than 5℃, and necessary heat preservation measures should be taken. Through years of on-site construction technical guidance and practice, the following insulation measures for winter construction of refractory castables have been summarized, and good results have been achieved.
Measure one
Before construction, use canvas or plastic film to cover the top of the heating furnace and the air leakage of the heat storage box, which can prevent cold air from blowing into the furnace, so as to achieve the effect of heat preservation. It is worth mentioning that this method must ensure the safety of fire prevention and the safety of high-altitude operations during construction. Cross operations such as steel structure welding and other hot operations will occur at the construction site. Pay attention to fire prevention to avoid igniting the canvas or plastic film.
Measure two
Arrange a certain number of iodine tungsten lamps in the furnace, or arrange a certain number of heating fans, and use the principle of light source heat radiation to increase the ambient temperature in a certain area of the furnace. At the same time, it is necessary to prevent personnel from getting an electric shock, reasonably plan the direction of the wires and the layout of the iodine tungsten lamp or the heating fan, so as not to hinder the operation and place it safely.
Measure three
Put a few stoves made of oil drums in the furnace to ignite coke or wood to increase the temperature in the furnace; you can also burn wood at the bottom of the furnace, and the firewood can be supervised by a dedicated person to increase the temperature in the furnace.
Measure four
Pipes are arranged at the bottom of the furnace to pass steam to increase the ambient temperature in the furnace. This method is better, that is, it is safe and can make the temperature in the furnace more uniform and lasting. Another method is to install several heaters in the furnace to blow hot air to the construction parts in the furnace to increase the temperature of the construction environment.
Measure five
Heat the refractory castable with stirring water, and the water temperature should be controlled at 15~35℃. Or direct ignition to heat the water storage bucket, or steam the water storage bucket or direct electric heater heating, and finally heat up the stirring water. The water temperature must be controlled as required, not too hot or too cold.
Conclusion
(1) During the construction process, it is necessary to choose appropriate insulation measures to ensure the construction quality, so that the construction period can be completed on time and production can be put into operation smoothly.
(2) According to the actual situation of the site and the requirements of the local government for environmental protection, one or several methods can be selected and used on the basis of ensuring safety for the five kinds of thermal insulation measures introduced in the article.
