The trolley-type forging heating furnace is used for heating steel ingots before forging. Its furnace temperature changes with time, and the maximum furnace temperature can reach 1350℃. The working conditions of the trolley-type forging heating furnace are very harsh due to the need for working methods. The workpiece is loaded outside the furnace, and a driving mechanism sends the trolley and the workpiece into the furnace to start heating according to the process curve. When the workpiece is heated to the specified temperature and the insulation is completed, the forging operation can be started, that is, the workpiece is taken out of the furnace and sent to the press to complete the forging operation. At this time, the furnace temperature drops by about 400~500℃ from the working temperature. After the workpiece forging is completed, it is re-entered into the furnace for heating. At this time, there is generally no requirement for the heating rate. This repetition requires high thermal stability of the lining material of the trolley heating furnace.
At present, the refractory layer of heating furnace lining materials mostly adopts refractory bricks, castables and prefabricated blocks, and the insulation layer adopts lightweight clay bricks or diatomaceous earth bricks. There are also zirconium-containing fibers used as lining materials in China, but the service life is not ideal.
Commonly used refractory materials for heating furnaces
1.Traditional refractory materials
Refractory bricks are very mature refractory materials and are widely used in various heat treatment and heating furnaces. Heating furnaces mostly use high-alumina bricks as refractory layers, and usually require a mass fraction of AL2O3 ≥ 65%. High-alumina bricks have good refractoriness and are suitable for occasions such as forging heating furnaces where temperature changes frequently. The construction method of using refractory bricks to build furnace walls is simple and relevant masonry standards have been formed.
According to different use temperatures, different furnace types, and different parts, clear indicators such as brick seam width and masonry error are proposed. When the heating furnace roof adopts a brick vault structure, the maximum width of the furnace is generally not more than 5m, and it should be far away from equipment with strong vibrations such as presses. The thickness of the furnace lining using refractory bricks generally adopts the following dimensions: 345mm thick high-alumina bricks + 230mm thick lightweight clay bricks or diatomaceous earth bricks + 20mm aluminum silicate fiberboard. When the above thickness of the furnace lining is used, the temperature of the outer wall of the furnace does not exceed 70℃.
Since the refractory bricks have been sintered, the furnace baking process is mainly to bake the free water brought in during the masonry process and the sintering of the refractory mud. When using refractory bricks for masonry, the furnace baking time is much shorter than that of the castables. The heating furnace baking time below 50㎡ does not exceed 5d.
2.Refractory Castable
Since the brick vault structure has requirements for the width of the furnace, large heating furnaces, especially those with larger widths, rarely use refractory bricks to build the furnace roof. The furnace roof of a high-temperature and large-span heating furnace is constructed with castables, and a flat-top hanging structure is adopted. At present, castables are widely used in large heating furnaces. The construction of refractory castables is much more complicated than that of refractory bricks. First, the anchors should be made and installed. Anchor bricks are used on the heating furnace to support and fix the furnace. The anchor bricks are connected to the furnace steel structure through anchor hooks. The length of the metal anchor hook must exceed 1/3 of the thickness of the furnace lining, and the spacing between them should not exceed 600mm in the horizontal direction and 500mm in the vertical direction. Waterproof treatment should be done on the contact surface between the insulation layer and the castable to prevent moisture from damaging the insulation material during the construction of the castable. Then make the template according to the drawing. The template size must be accurate and the support must be firm to prevent the template from shifting during pouring. The template should be painted with engine oil before construction, and the convenience of demolding should be considered during installation.
The amount of water added should be strictly controlled during the mixing of the castable. When vibrating with a vibrating rod, the vibration depth should not be too long, and the bubble precipitation in the castable should be the standard, about 2~3s. For heating furnaces, the thickness of the furnace lining is generally 300mm low cement reinforced castable + 230mm lightweight clay brick + 20mm aluminum silicate fiberboard when using castables. The furnace lining of castables usually takes a long time to bake and the heating speed is strictly required, which is generally 10~15d for heating furnaces.
The size of the castable prefabricated block can be changed according to different furnaces and pre-baked. The use of castable prefabricated blocks eliminates the complex construction of castables on site, reduces the workload on site, and also shortens the baking time. When using castable prefabricated blocks, the furnace wall structure is exactly the same as the refractory brick lining structure, and the furnace roof can also be designed as a prefabricated block according to the brick vault structure. It can be said that castable prefabricated blocks and refractory bricks have roughly the same use conditions.
3.Refractory Fiber
Commonly used refractory fibers are as follows according to their use temperature:
① Ordinary refractory fibers, use temperature <1000℃
② High-purity refractory fibers, use temperature <1050℃
③ High-aluminum fibers, use temperature <1100℃
④ Zirconium-containing and chromium-containing fibers, use temperature <1300℃
⑤ Polycrystalline mullite fibers, use temperature >1300℃. ①~④ types of fibers are made by melt blowing, so their organizational structure is glass organization. The instability of glass organization will produce crystallization, that is, it will transform into a crystalline phase at a specific temperature. Therefore, the use temperature of polycrystalline mullite fiber is higher than that of other fibers. The use temperature of ordinary refractory fibers, high-purity and high-aluminum fibers is increased because the content of AL2O3 increases and the content of impurity Fe2O3 decreases. At present, zirconium-containing fibers are mostly used in heating furnaces in China. Since the use temperature of zirconium-containing fibers is close to or even the same as the working temperature of the heating furnace, their service life is not ideal. After long-term use, the fiber shrinkage and powdering phenomenon is more serious. The linear shrinkage of zirconium-containing fiber at 1200℃ is 2.2%, and the linear shrinkage of chromium-containing fiber at 1200℃ is 1.7%. Its operating temperature is 30~50℃ higher than that of zirconium-containing fiber, so its performance in heating furnace is better than that of zirconium-containing fiber.
Since heavy refractory materials have large heat storage capacity and long construction and maintenance cycles, the use of refractory fibers as furnace lining materials can be a new direction for the development of heating furnaces. When the full fiber lining structure is used, the weight of the furnace lining per square meter is about 0.1~0.15t, while the weight of the furnace lining per square meter when the castable is used is 1.5t. It is calculated that the investment of using all chromium-containing refractory fibers is about 20%~30% higher than that of using castable linings. The increase in investment is still within the acceptable range.
Common structures of heating furnace lining
At present, the lining structure commonly used in trolley-type forging heating furnaces is mostly castable. Although the trolley-type forging heating furnace is an intermittent furnace, the single use time is very long and the heat storage loss in a single operation process accounts for a low proportion of all heat losses. In addition, the use of castables in heating furnaces has been very mature. Therefore, major domestic heavy machinery factories are not very enthusiastic about using refractory fibers. We briefly introduce several typical lining structures used in heating furnaces.
1.All Brickwork
The furnace wall and furnace roof are built with refractory bricks. This structure is mostly used in heating furnaces with a smaller furnace width. It is simple to construct and has a short construction period.
2.Furnace wall casting material furnace roof prefabricated block
When using this structure, attention should be paid to the construction process of the castables, otherwise it will have a great impact on the service life of the furnace lining. The furnace roof adopts a prefabricated block arch structure, which can be used on a wider heating furnace, but the weight of the furnace roof is greatly increased and the furnace body steel structure is required to have a higher strength.
3.The furnace wall and the furnace roof are all made of castables
When this material is used, the furnace roof is a suspended flat roof structure. This structure is a mature technology and is widely used in continuous heating furnaces in metallurgical enterprises. It is suitable for use on large-width trolley heating furnaces.
4.Full fiber structure
The furnace walls and the furnace roof are all made of refractory fibers. A heating furnace with this structure has been built in China, and the following problems have occurred during use: Due to the low fiber strength, the iron oxide scale produced by the workpiece has a great damage to the furnace lining; Due to the high temperature near the burner bricks, the fibers in contact with the burner bricks shrink greatly, which is easy to form a thermal short circuit and has a great impact on the steel structure of the furnace body; Due to the high temperature of the furnace roof, the fiber shrinkage is greater than that of the furnace wall, so the service life of the furnace roof is also very short.
Summary: In combination with the above, the use of refractory fibers in high temperature heating furnaces (temperature above 1250℃) can achieve good results as long as the method is appropriate. While ensuring the service life, the initial investment is not significantly increased compared to the use of traditional materials for furnace lining. However, the use effect is not very good, sometimes the wear resistance and iron oxide erosion resistance are poor.
