In the production of stainless steel balls, the presence of internal porosity significantly weakens their mechanical properties, such as tensile strength, yield strength, and fatigue life. Porosity formation primarily stems from gas release and retention during the melting, casting, and solidification stages. Therefore, coordinated control across multiple stages—including raw material handling, melting process, gating system design, shell preparation, and post-processing—is necessary to reduce porosity defects and improve mechanical properties.
The purity and dryness of the raw materials are the key to porosity control. If the raw material surface is corroded, oily, or damp, gases such as hydrogen and oxygen will be introduced during melting, leading to gas release and porosity formation during solidification. Therefore, raw materials must be rigorously screened to remove surface impurities and thoroughly dried before melting to reduce gas sources. Simultaneously, the furnace atmosphere must be controlled during melting to prevent air entrainment, and a suitable deoxidation process should be employed, such as using a silicon-calcium alloy instead of aluminum for final deoxidation, to reduce the risk of deoxidation products acting as a nucleation substrate for bubbles.
Optimizing the melting process is crucial for reducing porosity. During the smelting stage, it is crucial to control the heating rate and temperature uniformity to avoid localized overheating that could lead to drastic changes in gas solubility. Simultaneously, argon-protected pouring or argon agitation in the ladle promotes gas flotation and removal, reducing the gas content in the molten steel. Furthermore, smelting time must be strictly controlled to prevent prolonged high temperatures from increasing gas absorption, while ensuring the full dissolution and uniform distribution of alloying elements to improve the material's density and mechanical properties.
The design of the gating system directly affects gas venting. Excessive pouring speed can generate turbulence and entrain air; conversely, insufficient speed may result in prolonged baking time in the upper part of the mold cavity, increasing gas release. Therefore, an open gating system is necessary, ensuring that the cross-sectional areas of the sprue, runner, and ingate are progressively enlarged to achieve stable molten metal filling. Additionally, slag-collecting and venting risers at the end of the gating system or at mold cavity bends effectively collect gas and impurities, preventing them from accumulating inside the casting. Furthermore, controlling the pouring temperature and speed, avoiding interruptions or excessively slow pouring, allows sufficient time for gas to be expelled from the mold cavity. The permeability and strength of the mold shell are crucial for gas venting. Poor permeability prevents gas and residue from escaping the cavity, leading to porosity under filling pressure. Therefore, the mold shell manufacturing process needs optimization, such as using low-modulus water glass or adding iron oxide powder to reduce gas generation and improve collapsibility. Simultaneously, ensuring the raw sand is dry and has a moisture content below the standard value prevents moisture evaporation and porosity. Furthermore, densely placing vents at the highest point and corners of the mold shell, and ensuring unobstructed venting channels in the sand core, can significantly improve permeability and reduce gas retention.
Post-processing can further eliminate porosity defects. Heat treatment, by controlling the heating rate, holding time, and cooling method, promotes pore closure and healing. For example, high-temperature solution treatment can refine austenite grains and eliminate some martensite or ferrite, thereby improving the material's density and mechanical properties. Simultaneously, machining or shot peening can remove surface porosity and defects, improving surface quality and fatigue strength.
Environmental control during the production process is also crucial. In humid environments, 304 stainless steel readily absorbs hydrogen, increasing its susceptibility to hydrogen embrittlement. Therefore, it is essential to control the humidity in the production workshop to prevent raw materials and molds from becoming damp. Furthermore, a secondary drying of the molds before casting ensures complete dryness, further reducing gas sources.
Reducing internal porosity in stainless steel balls requires a coordinated effort across multiple stages, including raw material handling, smelting processes, gating system design, mold preparation, post-processing, and environmental control. By optimizing process parameters at each stage, porosity defects can be significantly reduced, improving the material's density and mechanical properties, thereby meeting the stringent performance requirements of high-end applications for stainless steel balls.
