Abstract: This paper analyzes the influence of white patterns and coatings in the lost foam casting process on large and medium-sized castings such as cooling walls. By adopting corresponding control and treatment measures at key points in the mold making, molding, and pouring processes, defects such as sand burning and mold collapse are effectively prevented. The production efficiency of blast furnace cooling walls using the lost foam casting process is improved, and product quality can be guaranteed.
1. Introduction
Lost foam casting technology is a precise forming casting technology that has developed rapidly in recent years. It is more market-competitive than traditional sand casting technology, and the lost foam casting process is demonstrating its strong vitality, being hailed as the “casting technology of the 21st century” and the “green revolution in the casting industry.”
Cooling walls are an important component in the cooling system of a blast furnace, installed in the shaft, belly, bosh, and hearth sections of the blast furnace. They play a crucial role in supporting refractory materials, cooling the furnace shell, and maintaining a reasonable furnace shape. Cooling walls not only endure high temperatures but also withstand the abrasion of furnace burden, erosion by molten slag, and冲刷 by gas flows. They must possess good thermal strength, resistance to thermal shock, and resistance to rapid cooling and heating. The material and performance of cooling walls determine their service life and even the life of the blast furnace body. Production situations at domestic and foreign steel enterprises have proven that one of the keys to the longevity of blast furnaces is the longevity of cooling walls.
Traditionally, cooling walls are produced using resin-sand casting, but this method is complex, requires a high level of worker skill, involves high mold costs, has a long production cycle, and low production efficiency, making it difficult to meet market demand. The dry sand lost foam casting process can simplify operations, shorten production cycles, reduce costs, and improve the surface quality and production efficiency of castings. However, medium and large castings produced using lost foam casting often exhibit defects such as porosity, surface sand burning, mold collapse, and cold shuts and wrinkles. The key factors in controlling these issues are the quality of the white pattern model, coating performance, gating system design, vacuum degree, and molding and pouring methods. Based on production practice and targeting product defects, we have conducted some explorations into the production of cooling walls using the lost foam casting process.
| Aspect | Description |
|---|---|
| Product Structure | Cooling walls are long rectangular plate-like components consisting of a cooling wall body and cooling water pipes cast inside. The main materials are gray iron, ductile iron, and cast steel, with ductile iron cooling walls being the most commonly used in this paper. They weigh between several hundred kilograms and several tons and have wall thicknesses ranging from 80 to 200 mm, classifying them as large and medium-sized thick-walled castings. |
2. Model Making
2.1. Selection of Model Material
For ductile iron castings, copolymer material (STMMA) is generally selected, which is several times the price of expandable polystyrene (EPS). While ensuring casting quality, we selected EPS modules of appropriate density to save costs. The selection of EPS module density is an important aspect of ensuring casting quality. Higher density results in higher model strength and less deformation. However, EPS with high density has a large gas generation volume during pouring, leading to severe backsplash, and the casting surface is prone to wrinkles and obvious bright carbon. When using lower-density modules, wrinkles and bright carbon phenomena are eliminated. However, the strength is poor, leading to easy deformation and inability to guarantee product quality. Through comparison, we found that modules with a density of 16~18 g/L are more suitable.
Table 1: Comparison of EPS Module Densities
| Density (g/L) | Strength | Deformation | Gas Generation | Surface Quality |
|---|---|---|---|---|
| High | High | Low | High | Poor (wrinkles, bright carbon) |
| Medium | Medium | Medium | Medium | Good |
| Low | Low | High | Low | Good |
2.2. White Pattern Cutting and Bonding
Cooling walls have various specifications and are not suitable for mass production through molding machines. We use a numerical control white pattern cutting machine for layered and segmented cutting, then install the prepared cooling water pipes, lifting rings, sand cores, etc., and manually bond and repair them.
2.3. Bolt Hole Processing
Each cooling wall has four square-headed bolt holes for installation, with a cylindrical upper part and a square lower part. During pouring, due to the long thermal action, sand burning and nodulation are prone to occur, making it difficult to clean and even resulting in waste products. We solved this problem by pre-making sand cores, brushing and drying them, and filling them in during model making.
2.4. White Pattern Brushing and Baking
For thick-walled large castings, it is essential to ensure the strength of the coating at high temperatures to prevent the phenomenon of mold collapse caused by a decrease in vacuum in the sand box due to a large gas generation volume during pouring. At the same time, sufficient refractoriness must be ensured to prevent sand burning under the long-term scouring of high-temperature molten iron. Due to the large size of cooling wall castings, brushing is adopted. The baking room temperature is maintained at 4550 ℃. To ensure strength, four coats are applied, with the coating thickness reaching 1.01.5 mm after drying. Each coat must be allowed to dry completely before the next one can be applied.
3. Bending of Cooling Water Pipes
The cooling water pipes cast into blast furnace cooling walls use seamless cold-drawn steel pipes for fluid transportation in accordance with GB/8163. The cooling water pipes are bent into shape using a pipe bending machine from an entire steel pipe, with no wrinkles, flattening, peeling, or scratches allowed at the bends. The cooling water pipes are bent according to relevant drawings, with a bending radius tolerance of ±2 mm and a water pipe center distance tolerance of ±2 mm. The wall thickness reduction caused by bending should be less than 18% of the wall thickness.
| Specification | Requirement |
|---|---|
| Material | Seamless cold-drawn steel pipe |
| Bending | No wrinkles, flattening, peeling, or scratches |
| Bending Radius Tolerance | ±2 mm |
| Center Distance Tolerance | ±2 mm |
| Wall Thickness Reduction | <18% of wall thickness |
The bent cooling water pipes undergo ball passing and hydrostatic testing. After passing the tests, a 0.2~0.3 mm thick anti-carburizing coating is applied. Effective anti-carburizing measures must be taken for the cooling water pipes and protective pipes (including lifting rings and cast-in nuts), and the anti-carburizing coating must not have any peeling or vacancies. Before anti-carburizing treatment, the steel pipe surface is derusted to expose the metal光泽.
4. Molding and Pouring
4.1. Gating System Design
Based on the shape of the casting, side pouring is adopted, and top-pouring, bottom-pouring, and stepped gating systems can be used, selected flexibly according to the casting and sand box sizes. Cooling walls generally have large wall thicknesses. If the molding speed is slow, the molten iron will quickly vaporize a large area of EPS after entering the mold cavity, causing an instant drop in vacuum. Even with high-strength coatings, mold collapse may still occur. Therefore, in the design of the gating system, we use a pouring basin, enlarge the sprue, and configure corresponding ingates to achieve rapid molding.
4.2. Molding
2040 mesh zircon sand is used, with a bottom sand thickness of 200 mm, which is vibrated and leveled. A special lifting tool is used to lift the model into the sand box, connect the gating system, and add sand and vibrate it in 34 layers.
Due to the repeated scouring of a large amount of high-temperature molten iron at the connection between the sprue and the pouring cup, it is easy to damage the coating and cause sand washing. At the same time, during pouring, the negative pressure sand box is under a high negative pressure state, and air enters the sand box along the gap between the bottom surface of the pouring cup and the plastic film, reducing the vacuum at the junction of the pouring cup and the sprue to zero. Under the action of molten metal, it collapses and disintegrates.