Characteristics of Casting Process for Stainless Steel Castings

Characteristics of Casting Process for Stainless Steel Castings

The mechanical properties of stainless steel castings are higher than those of cast iron, but their casting performance is worse than that of cast iron. Due to the high melting point of stainless steel precision casting, the steel liquid is prone to oxidation, poor fluidity, and large shrinkage. Its body shrinkage rate is 10-14%, and linear shrinkage is 1.8-2.5%. To prevent defects such as insufficient pouring, cold shut, shrinkage and porosity, cracks, and sand sticking in steel castings, more complex process measures than cast iron must be taken:

1. Due to the poor fluidity of molten steel, in order to prevent cold shuts and insufficient pouring in steel castings, the wall thickness of steel castings should not be less than 8mm; The structure of the pouring system should strive for simplicity and the cross-sectional size should be larger than that of cast iron; Using dry or hot casting molds; Appropriately increase the pouring temperature, generally between 1520 and 1600 ℃, because the pouring temperature is high, the superheat of the molten steel is high, the liquid state is maintained for a long time, and the fluidity can be improved. However, if the pouring temperature is too high, it can cause defects such as coarse grains, hot cracking, porosity, and sand sticking. Therefore, for small, thin-walled, and complex shaped castings, the pouring temperature is approximately the melting point temperature of steel+150 ℃; The pouring temperature of large, thick walled castings is about 100 ℃ higher than their melting point.

2. Due to the significant shrinkage of stainless steel precision casting compared to cast iron, in order to prevent shrinkage and porosity defects in castings, measures such as risers, cold iron, and subsidies are mostly used in the casting process to achieve sequential solidification.

In addition, to prevent shrinkage, porosity, porosity, and crack defects in stainless steel castings, the wall thickness should be uniform, sharp and right angle structures should be avoided, sawdust should be added to the mold sand, coke should be added to the core, and hollow cores and oil sand cores should be used to improve the yielding and breathability of the sand or core.

The practical application of modern investment casting methods in industrial production was in the 1940s. At that time, the development of aviation jet engines required the manufacture of heat-resistant alloy parts with complex shapes, precise dimensions, and smooth surfaces, such as blades, impellers, and nozzles. Due to the difficulty of mechanical processing of heat-resistant alloy materials and the complex shape of parts, it is impossible or difficult to manufacture them using other methods. Therefore, it is necessary to find a new and precise forming process. Therefore, drawing on the wax loss casting that has been passed down from ancient times, through the improvement of materials and processes, the modern investment casting method has achieved important development on the basis of ancient technology.

The so-called investment casting process, in simple terms, is to use fusible materials (such as wax or plastic) to make a fusible model (referred to as investment mold or model), coat it with several layers of specially made refractory coating, dry and harden it to form a whole shell, then use steam or hot water to melt the model from the shell, then place the shell in a sand box, fill it with dry sand to shape it, and finally put the mold into a baking furnace for high-temperature calcination (such as when using high-strength shells, the demolded shell can be directly calcined). After calcination, the mold or shell is poured with molten metal to obtain the casting.

The dimensional accuracy of investment castings is relatively high, generally up to CT4-6 (CT10-13 for sand casting and CT5-7 for die casting). However, due to the complex process of investment casting, there are many factors that affect the dimensional accuracy of castings, such as shrinkage of the mold material, deformation of the investment mold, linear changes in the mold shell during heating and cooling, shrinkage rate of the alloy, and deformation of the casting during solidification. Therefore, although the dimensional accuracy of ordinary investment castings is relatively high, their consistency still needs to be improved (the dimensional consistency of castings using medium and high temperature wax materials needs to be greatly improved).

When pressing the investment mold, a high surface finish of the mold cavity is used, so the surface finish of the investment mold is also relatively high. In addition, the mold shell is made by coating a refractory coating made of high-temperature resistant special binders and refractory materials on the melt mold, and the surface smoothness of the mold cavity in direct contact with the molten metal is high. So, the surface smoothness of investment castings is higher than that of general castings, generally reaching Ra. 1.6~3.2m.

The biggest advantage of investment casting is that due to its high dimensional accuracy and surface smoothness, it can reduce mechanical processing work. Only a small amount of machining allowance can be left for parts with higher requirements, and even some castings can be used without mechanical processing, leaving only grinding and polishing allowance. It can be seen from this that using investment casting method can greatly save machine equipment and processing time, and greatly save metal raw materials.

Another advantage of investment casting method is that it can cast complex castings of various alloys, especially high-temperature alloy castings. Like the blades of jet engines, their streamlined outer contours and cooling inner chambers are almost impossible to form using mechanical processing techniques. Production using investment casting technology can not only achieve mass production, ensuring the consistency of castings, but also avoid stress concentration caused by residual tool lines after mechanical processing.

The process of stainless steel investment casting is as follows:

Spray wax, shell making, dewaxing, casting, shaking shell, sanding, acid washing, sandblasting, re acid washing, polishing.