Nuclear power plant cast steel parts have high requirements in terms of safety performance, surface quality, internal quality, and mechanical properties. However, during the smelting, transfer, and pouring processes, impurities such as steel slag, ladle refractory materials, and oxide inclusions inevitably mix into the molten steel. Using foamed ceramic filters to filter the molten steel effectively removes or reduces the amount of steel slag and inclusions mixed into the molten metal, improving the purity of the casting alloy. This results in castings with smooth surfaces, uniform and reliable internal quality, stable performance, low scrap rates, reduced rework costs, and shorter production cycles.

In sand casting, with the gating and riser process already determined, the quality of the casting largely depends on the quality of the molten steel poured into the mold. However, the molten steel inevitably contains slag from the smelting process, as well as refractory materials from the ladle and oxide inclusions formed during tapping and pouring. All of these can create defects in the casting, potentially reducing its mechanical properties, machinability, and appearance, ultimately leading to rework or scrap. Using foam ceramic filters to purify the molten alloy and reduce or eliminate these inclusions is undoubtedly a crucial technique for obtaining high-quality castings.

This application case comes from a long-term client of Xinda. The castings produced are made of austenitic stainless steel, characterized by large gating weight, high pouring temperature, and easy oxidation of the alloy. Based on these comprehensive operating requirements, Xinda foam ceramic filters were selected. This foam ceramic filter possesses a unique three-dimensional interconnected curved-pore mesh skeleton structure, resulting in an open porosity of 80%-90%. It employs three filtration and purification mechanisms: first, mechanical interception, blocking larger inclusions larger than the mesh size from entering the mold cavity; second, rectification of slag, where the filter's rectifying effect keeps the upstream gating system full, ensuring a stable laminar flow of the filtered molten steel, reducing oxidation and scouring reactions, thus facilitating the floating and capture of inclusions and reducing the amount of secondary inclusions downstream of the filter; and third, deep adsorption, where fine inclusions entering the filter are adsorbed onto the skeleton or trapped in dead zones due to full contact with the complex three-dimensional ceramic network. Through these three filtration and purification mechanisms, it can efficiently remove large inclusions and a large portion of tiny suspended inclusions as small as tens of micrometers from molten casting alloys, thereby significantly improving casting quality.

The valve body casting used in this application has extremely high quality inspection requirements, with specific indicators as follows:

Casting Name: Valve Body;

Material: CF8M;

Weight of Molten Steel: 1000kg;

Non-destructive Testing Requirements: 100% volumetric radiographic testing (RT) of the casting, Level 1 for weld ends, Level 2 for the rest;

100% liquid penetration (PT) testing of the casting's outer and accessible inner surfaces must meet the following requirements:

Any linear indentation no greater than 2mm;

The size of a single circular indentation no greater than 4mm;

Dense indentations no greater than 2mm within any 100mm*100mm area.

Based on the flow rate per square centimeter of the foam ceramic filter, two 125*125*30mm foam ceramic filters from Xinda were ultimately selected and placed in the horizontal runner. To ensure process rationality, the completed process design was analyzed using MAGMA simulation software for solidification analysis, mold flow analysis, and flow and heat transfer coupling calculations. Simulation results showed that the molten steel filling was smooth, and there was no splashing of molten steel entering the casting.

Actual production verification shows that using a gating system with a filter can effectively filter and purify the casting alloy molten metal, remove or reduce inclusions in the casting alloy molten metal, improve the purity of the casting alloy molten metal, make the cast parts with clear outlines and uniform structure, reduce the scrap rate, improve the internal and surface quality of the castings, reduce rework costs, and significantly improve the quality of the castings.