Analysis of lost foam casting process for blast furnace cooling wall

I、 Abstract

The cooling wall is an important part in the cooling system of the blast furnace. It is installed in the body, waist, belly and hearth of the blast furnace. It plays an important role in supporting refractory materials, cooling the shell and maintaining the reasonable shape of the furnace.

The cooling wall not only bears high temperature, but also bears the wear of charge, the erosion of slag and the scour of gas flow, so it must have good thermal strength, heat shock, resistance to rapid cooling and rapid heating and other comprehensive properties.The cooling walls can effectively prevent the furnace shell from heat and red. After the lining bricks were eroded by Liang, the cooling walls themselves are mainly protected by slag skin, and the safe production of the blast furnace is maintained.Therefore, the material and performance of the cooling wall determine its working life and even the life of the blast furnace body.The production situation of domestic and foreign iron and steel enterprises proves that one of the keys to the long life of blast furnace is to realize the long life of cooling wall.

II、Product structure

The cooling wall is a strip plate-shaped part, which is composed of the cooling wall body and the cooling water pipe inlaid and cast inside. Each serpentine pipe is formed by mechanical cold bending of the whole steel pipe.For the steel pipe with serious corrosion and pit, it cannot be used as cast pipe. The serpentine pipe should be bent and formed according to the relevant drawings. The deviation of bending radius is allowed to be ±2mm.The tube wall thinning caused by the bend should be less than 20% of the wall thickness, and the deviation of the inlet and outlet of the water pipe should be ±2mm.

III、 Matters needing attention

 Control the dimension of cooling water pipe during bending
（1） Appropriate amount of pipe shrinkage
In the casting process, the steel cooling water pipe cast into the cooling wall will expand with the heat of high temperature casting process, and with the decrease of temperature, the cast steel cooling water pipe will shrink, so it needs to determine its scale before bending.The size of the scale, that is, the ratio of the difference between the elongation and the contraction and the original length, will directly affect the accuracy of the casting size.
a、The reduced scale in the length direction of the cooling water pipe is generally half of the casting reduced scale; 
b、The transverse spacing shrinkage of the inlet and outlet of the cooling pipe is the same as that of the casting body;

（2）"Uniform distribution technology" is adopted between the protective pipe and water pipe

Two fixed rings are used between the pipe and the water pipe, and buffer materials such as asbestos rope and asbestos felt are filled between the fixed rings to ensure that the pipe and the pipe are concentric and have a certain amount of buffer.

• Technical notes

（1）The steel pipe in the cooling wall has a chilling effect during pouring, and the pouring temperature should not be too low or the pouring speed should not be too slow.

（2）Prevent molten iron from impacting tube wall, avoid deformation of steel tube and surface carburization;

（3）Due to the large size difference between the thickest and thinnest parts of the cooling wall, it is necessary to prevent the thick part from overheating.

IV、 Process design

1、The overall size of the product is 1608mm×909mm×959mm, the casting single weight is 2.5T, the material is HT150, used in 350m3 blast furnace.

• Bending of cooling water pipes

（1）The cooling water pipe is made of φ34*4.5 seamless steel pipe, which is pickled in 30% dilute HCL tank before bending to remove the oxide scale and impurities inside and outside the pipe. After bending, the water pipe is tested with 6 ~ 8kg/cm2 water pressure, and the pressure is maintained for 20 ~ 30 minutes. The pipe shall not leak or sweat.

（2）In order to minimize the deformation of cooling water pipe in the bending process, we adopt the cold bending method, so as to avoid the phenomenon of thin pipe wall caused by oxidation of cooling water pipe at the bending point caused by hot bending.At the same time, the bending is not allowed to have wrinkles, concave, peeling and scars.Bent cooling water pipe to pass the ball and water pressure test, qualified after brushing 0.2 ~ 0.3mm thick anti-carburizing coating, after drying can cast into it.

3、 White film cutting and bonding

The white film structure required by the cooling wall is relatively complex and not suitable for mass production by molding machine, so automatic cutting table and manual bonding are used to complete. Use automatic cutting table to cut the cooling wall in layers, fill in the cooling water pipe and repair by manual bonding.

4、 Gating system design

Bottom injection gating system is adopted. ∑S within: ∑S horizontal: ∑S straight is 1:1.2:1.4 (with S as the flow resistance, the gating system is fast, the metal liquid in the sprue flow rate is high), the sprue adopts the diameter of 50mm round runner, the sprue 40mm×40mm, the inner gate is six 25mm×10mm. The gating system is bonded after the third coat of paint.

V、 Process Worker

1、 White film brush

（1）When brushing the paint for the first time, the paint and water are mixed in a ratio of 1:0.6, and the measured water is poured into the mixing bucket.Turn on the blender and pour the paint slowly (the whole bag needs to be poured completely) under the working condition of the blender and stir for more than 2h.Coating uniform without dead block, measuring coating density (baume 60-65) qualified after the coating can be poured into the brushing tank, standing for 5 minutes, can be used,The first coating should not be too thick, so as not to cause surface cracking when drying, affecting casting quality. Because the casting size is large, brush coating method is adopted.

The yellow mold which is well formed and evenly coated on the model is put into the drying room. The distance between the two models is more than 150mm. Electric heating is adopted in the drying room.The temperature of the drying room is controlled at about 45℃, the temperature should not be too high, it has caused cracks in the corners of the yellow mold, the humidity is less than 15%, the yellow mold should not be directly blown by the drying heater, so as not to cause the yellow mold to be heated too high, causing cracks in the corners of the paint.In this environment drying for about 8 hours, at this time the coating thickness of 0.4-0.6mm.

（2）Second coating, coating density is larger than the first time (Baume 68-70). Brush the model after the first drying, and repeat the first drying process.The coating layer thickness reaches between 1.0-1.5mm after drying.After brushing, transport to the drying room, and brushing yellow mold separately placed.

（3）Third time brushing, repeat the second process, three times brushing, appearance evenly coated, no lack of coating,The drying time must be greater than 24 hours for the presence of flowing or inclusion of bubbles.Can use weighing method to detect whether drying.

2、 Buried box modelling

Use 30-40 mesh baozhu sand, the thickness of the bottom sand is 300mm, first shake the bottom sand, vibration time is 50-60 seconds,Then place the model in position, at least 150mm away from the outer wall of the sand box, and bond the straight sprue.For the second vibration, vibration 30-40 seconds, full vibration, time should not be too long, to avoid vibration crack white mold.

After the second shock, sand is added to the top plane of the sprue cup for the third vibration to tighten.Then the top layer of plastic film (3-5 silk), add sand.In order to prevent molten iron from washing sand around the sprue cup into the casting during pouring, alcohol coating should be applied around the sprue to prevent sand washing.

3、pouring

The oven temperature is controlled at 1510℃ ~ 1530℃，Casting temperature is between 1400℃ and 1420℃ (melting point of cooled steel pipe is above 1460℃),Negative pressure -0.05mpa, pouring speed 50 ~ 65kg/s, pouring time 40 ~ 50 seconds,After pouring, keep the pressure at -0.02mpa negative pressure for 15 minutes, keep the temperature for 3-5 hours, then turn the box.

4、Shot blasting cleaning castings, finished products need to go through 5kg/cm2 water pressure test, pressure 20 minutes without leakage, sweating.