Mastering the Improvement of Stainless Steel Casting Surface Quality
I. Fine Surface Treatment Technology of Stainless Steel
(I) Microstructure Control
To effectively reduce the depth of metal liquid infiltration into the surface pores of stainless steel and thus decrease the surface roughness, fine control of the microstructure on the surface of stainless steel is one of the key measures. This requires efforts from multiple aspects. Firstly, control the crystal diameter and filler particle diameter. By optimizing the solidification process of stainless steel, methods such as controlling the cooling rate and adding grain refiners can be used to reduce the crystal diameter. For example, during the casting process, using a titanium-boron alloy as a grain refiner can significantly inhibit the growth of grains, reducing the crystal diameter by 20% - 30%. At the same time, selecting finer and more uniform filler particles and optimizing their distribution state can also effectively reduce the size of surface pores. Research shows that when the filler particle diameter is reduced by 10%, the average diameter of surface pores can be correspondingly reduced by 8% - 10%.
Secondly, control the surface curvature radius of the alloy liquid. During the pouring process, by adjusting the design of the pouring system, such as changing the shape, size and position of the gate, and optimizing the setting of the riser, the flow state of the alloy liquid on the surface of stainless steel can be affected, thereby changing the surface curvature radius of the alloy liquid. For example, using a tapered gate design can enable the alloy liquid to form a smoother flow state when entering the mold cavity, reducing the surface curvature radius and decreasing the accumulation and infiltration of the metal liquid at the pore entrance.
(II) Scientific Application of Additives
Additives play a unique role in improving the surface roughness of stainless steel. Selecting additives that can be enriched on the surface of stainless steel and making them effectively fill larger pores during the pouring process is an effective method. For example, some nanoscale ceramic additives, such as silica nanoparticles or alumina nanoparticles, have high surface activity and filling properties. Before the metal liquid is poured, these nanoparticles are evenly dispersed on the surface of stainless steel. During the pouring process, as the temperature of the metal liquid rises, the nanoparticles will have certain physical and chemical interactions with the metal liquid, gradually filling the pores and forming a relatively flat covering layer on the surface. Experimental results show that after adding an appropriate amount of silica nanoparticles, the pore filling rate of the stainless steel casting surface can be increased by more than 30%, and the surface roughness is significantly reduced.
In addition, some additives with self-repairing functions can also be developed. These additives can automatically release active components when the surface of the stainless steel casting is slightly damaged during use, repairing the damaged area and maintaining the surface smoothness. For example, a composite additive containing metal oxides and organic polymers. When a scratch appears on the surface due to external force, the organic polymer will decompose at the scratch, releasing metal oxide particles. These particles react with the surrounding metal to form new metal compounds, filling the scratch and making the surface flat again.
II. High-Quality Manufacturing Technology of Molding and Investment Mold
(I) Precision Revolution of Molding
As the basic mold for investment mold manufacturing, the surface quality of the molding directly determines the quality of the investment mold, which in turn affects the surface roughness of the stainless steel casting. To achieve high-precision molding manufacturing, a series of advanced technologies and processes are required.
Firstly, in the mold design stage, fully utilize computer-aided design (CAD) and computer-aided engineering (CAE) technologies to optimize the structure design of the molding. By simulating and analyzing the stress distribution, heat conduction and material filling situation during the molding process, potential defects and problems can be predicted in advance and targeted improvements can be made. For example, when designing a molding for a stainless steel casting with a complex shape, CAE software can accurately simulate the flow state of the metal liquid in the mold cavity, helping designers optimize the layout of the runner and exhaust system, avoiding adverse phenomena such as air entrapment and turbulence, thereby ensuring that the molding can produce a high-quality investment mold.
Secondly, in the processing and manufacturing link, adopt high-precision processing equipment and advanced processing technologies. For example, using a five-axis linkage machining center, high-speed milling technology and electrical discharge machining technology can achieve precise processing of the complex curved surface and tiny details of the molding. The five-axis linkage machining center can complete the processing of multiple surfaces in one clamping, avoiding the positioning error caused by multiple clampings and greatly improving the dimensional accuracy and shape accuracy of the molding. High-speed milling technology, with its high cutting speed and high feed rate, can obtain better surface processing quality, making the surface roughness Ra value of the molding controllable below 0.2μm. Electrical discharge machining technology has unique advantages for processing some high-hardness and complex-shaped mold parts and can precisely machine fine structures and contours, further improving the accuracy and surface quality of the molding.
In addition, the selection and treatment of molding materials are also crucial. According to different stainless steel casting processes and requirements, select mold materials with appropriate properties, such as high-quality mold steel or new ceramic materials. At the same time, conduct strict heat treatment and surface treatment on the mold materials to improve their hardness, wear resistance and corrosion resistance. For example, using a vacuum heat treatment process for mold steel can effectively reduce the internal stress of the material, improve its toughness and hardness uniformity. Nitriding treatment, chrome plating treatment or physical vapor deposition (PVD) coating treatment on the surface of the molding can further improve the surface hardness, wear resistance and demolding performance, reduce the wear and adhesion of the molding during use, and thus ensure the surface quality and dimensional accuracy of the investment mold.
(II) Fine Process of Investment Mold Making
The optimization of the investment mold making process also plays a crucial role in reducing the surface roughness of stainless steel castings.
In terms of investment mold materials, continuously develop and select materials with better performance. For example, new high-molecular polymer investment mold materials have better fluidity, formability and surface smoothness. These materials can more evenly fill the mold cavity after being heated and melted, and the surface of the investment mold formed after cooling and solidifying is smoother and flatter. At the same time, optimize the formula of the investment mold material and add an appropriate amount of additives such as lubricants and plasticizers to improve its processing performance and surface quality. For example, adding a small amount of silicone oil as a lubricant can effectively reduce the friction coefficient of the investment mold material during the injection molding process and reduce scratches and defects on the surface of the investment mold.
In terms of investment mold making methods, select appropriate processes according to different casting shapes and requirements. For example, for stainless steel castings with complex shapes and high precision requirements, using the injection molding method to make investment molds can achieve better results. During the injection molding process, by precisely controlling process parameters such as injection pressure, temperature and speed, it can be ensured that the investment mold material evenly fills every corner of the mold cavity and maintains good dimensional stability and surface quality during the cooling process. At the same time, using advanced rapid cooling technologies, such as liquid nitrogen cooling or high-pressure gas cooling, can accelerate the cooling speed of the investment mold, reduce crystal growth and shrinkage, and thus reduce the surface roughness of the investment mold.
In addition, the post-treatment process of the investment mold cannot be ignored. After the investment mold is formed, appropriate grinding, polishing and cleaning treatments can further improve its surface smoothness. For example, using micro-nano abrasive for polishing treatment can remove tiny flaws and scratches on the surface of the investment mold, reducing the surface roughness Ra value to below 0.4μm. Cleaning treatment can remove oil, impurities and release agent residues on the surface of the investment mold, ensuring the cleanliness of the investment mold surface and creating good conditions for the subsequent pouring process.
III. Alloy Optimization and Cooling Enhancement Technology
(I) Precise Regulation of Alloy Composition and Properties
The composition and properties of the alloy have an important impact on the surface roughness of stainless steel castings. Therefore, precise optimization and regulation of the alloy are required.
Firstly, adjust the types and contents of alloy elements to improve the thermal conductivity and crystallization characteristics of the alloy. For example, adding an appropriate amount of copper, molybdenum and other elements in stainless steel can improve the thermal conductivity of the alloy, accelerate the cooling speed of the metal liquid in the mold, thereby inhibiting the coarse growth of grains and refining the grain structure. Research shows that after adding 2% - 3% of copper element, the grain size of the stainless steel casting can be reduced by 15% - 20%, the depth of the grain boundary groove is significantly shallower, and the surface roughness is effectively reduced. At the same time, reasonably controlling the contents of carbon, nitrogen and other elements can optimize the crystallization process of the alloy, reduce the occurrence of dendritic segregation, pores and slag inclusions, and further improve the surface quality of the casting.
Secondly, adopt advanced alloy melting and refining technologies to ensure the purity and uniformity of the alloy liquid. For example, using vacuum melting technology can effectively remove gases and impurities in the alloy liquid and reduce the formation of pores and slag inclusions. During the melting process, through technologies such as electromagnetic stirring or ultrasonic vibration, the uniform distribution of alloy elements is promoted, avoiding local composition segregation, thereby ensuring the uniform and stable performance of the casting and reducing the difference in surface roughness caused by uneven composition.
In addition, modifying the alloy is an important means to improve the grain structure and surface roughness. The addition of a modifier can change the crystallization mode of the alloy, promote non-spontaneous nucleation and refine the grains. For example, adding titanium, boron, zirconium and other modifiers in stainless steel can form a large number of heterogeneous nuclei in the alloy liquid, resulting in a significant grain refinement effect. Experimental results show that after modification treatment, the average grain size of the stainless steel casting can be reduced by more than 30%, and the surface roughness Ra value can be reduced by 20% - 30%.
(II) Effective Improvement and Control of Cooling Speed
To obtain a fine and uniform grain structure and reduce the surface roughness of stainless steel castings, it is necessary to effectively improve and control the cooling speed of the casting.
In terms of mold design, adopt mold materials and structures with good heat dissipation performance. For example, using zirconite, graphite and other materials with high thermal conductivity as fillers or linings of the mold can accelerate the transfer and dissipation of heat and improve the cooling speed of the casting. At the same time, optimize the wall thickness and structure design of the mold, increase the heat dissipation area and heat dissipation channels, and promote the rapid dissipation of heat. For example, using a hollow structure mold or installing cooling water pipes, heat sinks and other cooling devices in the mold can further improve the heat dissipation efficiency of the mold, enabling the casting to cool faster during the solidification process.
In terms of pouring process, reasonably control the pouring temperature and pouring speed to match the cooling capacity of the mold. A lower pouring temperature can reduce the heat brought into the mold by the metal liquid, reduce the solidification time and is beneficial to refining the grains. At the same time, appropriately increasing the pouring speed can make the metal liquid quickly fill the mold cavity in the mold, reduce the temperature gradient change during heat dissipation, avoid local overheating or undercooling phenomena, and thus obtain a uniform grain structure and lower surface roughness.
In addition, using forced cooling technology is also an effective method to increase the cooling speed. For example, during the solidification process of the casting, using air cooling, water cooling or spray cooling and other methods to externally cool the mold can quickly take away the heat in the mold and the casting, further accelerating the cooling speed. At the same time, combined with computer simulation technology, real-time monitoring and regulation of the cooling process are carried out. According to factors such as the shape, size and material of the casting, the cooling time, cooling intensity and cooling position are precisely controlled to achieve precise control of the cooling speed and ensure that the casting obtains the best grain structure and surface quality.
In summary, the surface roughness of stainless steel castings is a complex issue involving multiple factors such as materials, manufacturing processes and alloys. By deeply exploring its causes and comprehensively applying excellent practices and innovative measures in material research and development, manufacturing process control, personalized customization and quality inspection, as well as a series of technology integrations such as fine surface treatment of stainless steel, high-quality manufacturing of molding and investment mold, alloy optimization and cooling enhancement, the surface roughness of stainless steel castings can be effectively improved, and the quality and market competitiveness of stainless steel castings can be enhanced. In the future stainless steel casting field, with the continuous progress and innovation of science and technology, it is believed that more and more effective methods and technologies will emerge, further promoting the improvement of the surface quality of stainless steel castings and the sustainable development of the industry.