Study on the anti shrinkage process of ingot mould for typical large iron castings

Ingot mold is a kind of necessary ingot equipment used in mold casting production in steel plant. Although its shape and structure are simple, it belongs to large-scale iron castings because of its large outline size, wall thickness of more than 200 mm and weight of more than 15 t. Because of the large wall thickness, shrinkage cavity and porosity are easy to appear on the top surface of ingot casting. However, the working conditions are harsh, the working stress is complex, and the inner surface is easy to crack.

Based on the above characteristics, the metallurgical factor design is particularly important in the Shrinkproof process design of this series of castings. At the same time, due to the large wall thickness of this series of castings, there are differences in the selection of two solidification principles in riser design.

Therefore, taking a certain type of ingot mold as an example, the casting process of this part is discussed based on the purpose of preshrinking

Technical requirements for ingot casting

Ingot mold is a professional mold for steel casting. It has strict requirements on the outline dimension of the casting. The weight of the main dimension piece is 108t.

Technical requirements:

(1) Chemical composition (%): 4.0-4.4c, 0.5-0.9si, 0.6-0.8mn, P < 0.1, s < 0.05;

(2) The casting shall be free of defects such as porosity, shrinkage cavity, porosity, slag hole, etc;

(3) The inner surface shall be flat and smooth without cracks and holes.

Casting process analysis of the casting

(1) The wall thickness of the casting is large, and the hot spot is large. The riser process design and chemical composition are strictly required to prevent shrinkage cavity and porosity on the upper surface of the casting.

(2) . due to the influence of the working environment of the casting, the casting is prone to crack, so the P and s content is strictly required.

(3) , the shape of the casting is relatively separated, the wall thickness is large, the internal and external surface of the sand mold is under great pressure, and the mold wall expansion is easy to occur, so that the internal and external surfaces of the casting are prone to shrinkage and dimensional deformation due to the bulging problem. Therefore, the strength of sand mold is required to be high.

Casting process design

According to the chemical composition, pouring temperature, pouring system, mold conditions and other technological factors of the casting, the following design will be carried out:

1) Chemical composition

The service life of ingot mold is related to its thermal conductivity, which requires high chemical composition, so it requires high carbon content and low silicon composition. According to the chemical composition required by the drawing, its material is hypereutectic cast iron. If it is required by the drawing, there is a high probability of shrinkage defects. Therefore, based on the above considerations and through consultation with experts, the final requirements for carbon and nitrogen content (%) are as follows: 3.6-4.0c, 1.0-1.4si. In actual production, in order to ensure the service life of castings, the upper limit value of carbon content is 3.9%, and the lower limit value of silicon is 1.1%. If the composition is used, the casting material is hypoeutectic cast iron. If the carbon content is increased, the probability of shrinkage cavity and porosity will be reduced. If the Si content is increased properly, the mechanical properties of the casting will also be guaranteed.

For the content of sulfur and phosphorus, although they have a certain ability to promote the improvement of various properties of castings, considering the factors of crack prevention, the final requirements for the content of sulfur and phosphorus (%) are as follows; s ≤ 0.03, P ≤ 0.04.

2) Pouring temperature

The ingot mold belongs to thick wall and easy to shrink and crack casting. If the pouring temperature is too high, the castings are prone to shrinkage defects and hot cracking; if the temperature is too low, the fluidity of molten iron is poor, the castings are also prone to shrinkage defects. Therefore, in view of the above considerations, the pouring temperature of the casting is reduced appropriately, and the final pouring temperature of the casting is determined to be 1270-1300’c. In actual production, the pouring temperature should be taken as low as possible.

3) Selection of gating system

The average wall thickness of the casting is 545mm, and the height is 2800mm. It is a typical large-scale cylinder casting. Therefore, according to the requirements, the top pouring system is suitable for this kind of casting. Because of the advantages of sequential solidification and slag removal, shrinkage defects and slag holes in castings are reduced.

For the selection of riser, there are two design directions, one is to place the riser directly in the middle of the upper surface of the casting according to the principle of sequential solidification, the other is to place the riser at the edge of the casting according to the principle of balanced solidification. Therefore, the following two plans will be analyzed.

If according to the first scheme, the hot spot circle d at the upper end of the casting is 545mm. Therefore, according to the formula D1 = 1.2-1.5d and H = 2.5D, the riser diameter D is at least 600mm and the height h is 1300mm. The single feeding distance is 3600mm, so the number of risers is more than 4. According to this plan, the yield of casting process is low. Moreover, because the riser diameter is too large, the riser root is prone to shrinkage, which affects the surface quality and mechanical properties of the casting upper surface.

The second scheme is to place the flash riser at the edge of the upper surface of the casting. Through calculation, only four risers with a diameter of 140mm can be placed, but for the sake of safety, six risers with a diameter of 140mm are finally determined to be placed evenly.

4) Mold conditions

Because of the large expansion of the ingot model wall, the probability of bulge on the internal and external surface of the ingot model is large. In order to avoid the above problems, it is important to improve the strength and compactness of molding sand. According to the experience of zhycasting senior engineers, the strength of molding sand is generally increased to more than 10kg / cm2, and the resin content is increased to 1.23%. In addition, attention should be paid to sand impact treatment in operation.

5) Casting effect

Through the above process, the casting basically has no slag hole, shrinkage hole, shrinkage porosity, crack and other defects, and the internal and external surfaces of the casting are smooth and flat, which fully meets the technical requirements of the user for the surface quality of the casting; at the same time, the user reflects the use of the casting well and increases the order.