Have you ever encountered a situation where the stainless steel castings were covered in dense rust spots, resulting in the entire batch being scrapped during procurement? This is because the manufacturer failed to polish them before shipping them. This article will explain everything you need to know about polishing stainless steel castings, helping you avoid financial losses caused by skipping this crucial step.
Main Advantages of Polishing Stainless Steel
Polishing not only enhances the surface luster of stainless steel castings but also offers numerous benefits. These benefits stem from the removal of surface impurities through polishing. Understanding the advantages of polishing and the principles behind them will help you appreciate its importance and reduce the costs associated with rework caused by excessive surface impurities.
Maintaining Surface Finish
Although stainless steel has a distinctive metallic luster, this sheen tends to fade over time. However, once polished, the surface of stainless steel can retain its luster indefinitely. This is because polishing can improve the surface quality of the casting; when combined with grinding, it can achieve a mirror-like finish.
Removing Contaminants
During the machining of stainless steel castings produced by investment casting, trace amounts of iron and other external impurities inevitably remain on the casting surface. If unremoved, these contaminants can damage the passivation layer on the stainless steel surface, leading to pitting corrosion. However, polishing can remove these surface contaminants, preventing them from reducing the casting’s corrosion resistance.
Ensuring Ease of Cleaning
After machining, castings often have fine burrs on their surfaces, which can easily trap bacteria and other microorganisms. These residues are extremely difficult to remove during cleaning. Instead, polishing can thoroughly eliminate these burrs, bringing the surface up to sanitary standards and significantly reducing cleaning difficulty.
Reducing Surface Corrosion
Because the surface of stainless steel castings contains microscopic pits upon completion of the casting process, these pits are highly prone to accumulating chloride ions and water molecules from the air, leading to pitting corrosion. Furthermore, due to the investment casting process, these castings typically have complex internal structures, and these internal dead corners are the most susceptible to bacterial growth and even rusting. Polishing could eliminate these pits and dead corners, creating a smooth surface that ensures smoother operation and prevents stagnation in these areas, thereby reducing the risk of rust formation.
Improving Mechanical Fatigue Life
These pits and burrs not only cause corrosion on the castings but also shorten their service life. Because once these defects appear on the casting surface, they become stress concentration points during operation. However, polishing can remove these defects, significantly improving the castings’ fatigue life in high-pressure operating environments.
Methods of Polishing
There are many different polishing methods. Understanding these methods will help you select the most suitable and efficient one based on the specific structure of your components, thereby effectively reducing the risk of suboptimal results and increased costs that can result from using an inappropriate method.
Mechanical Polishing
Mechanical polishing involves applying a polishing wheel coated with polishing wax to a stainless steel casting at high speed, generating significant friction and removing microscopic protrusions. Different polishing compounds or waxes can achieve varying surface roughness standards; for example, using green wax and white wax in order from coarse to fine, surface roughness can be reduced to as low as Ra < 0.1 μm.
Although this polishing method is relatively simple and can achieve a high surface gloss, it cannot reach deep into complex internal cavities such as flow channels, and it requires a high level of skill.
Electropolishing
Electrolytic polishing is a form of electrochemical polishing in which stainless steel castings are immersed in a specialized acidic electrolyte. When an electric current is applied, the “tip-seeking effect” causes the highest current density to occur at microscopic protrusions on the casting’s surface, resulting in the fastest rate of metal dissolution.
Furthermore, because the process relies on immersion, burrs, dead corners, pits, and wherever the electrolyte can reach can be thoroughly removed. It also concentrates chromium elements, forming a chromium-rich passivation layer on the surface that enhances the stainless steel’s corrosion resistance. However, if the casting has too many surface defects, electrochemical polishing cannot be performed, and the resulting finish will not achieve the gloss of mirror polishing.
Barrel Polishing
Barrel polishing involves placing a batch of stainless steel castings into a rotating drum along with special polishing abrasives, such as ceramic beads, stainless steel needles, and plastic particles, as well as a polishing brightener. Through high-frequency rotation, the castings rub vigorously against these abrasives, which remove surface burrs and pits.
Because this method can polish dozens or even hundreds of small castings at once, it is extremely cost-effective. However, it is limited to deburring, rounding edges, and overall “brightening,” and cannot achieve a high-precision mirror finish.
Chemical Polishing
Similar to electropolishing, this process involves immersing the casting in a solution that dissolves microscopic surface protrusions.
Compared to electropolishing, chemical polishing requires a smaller initial cost but has a shorter service life, necessitating frequent replacement. Overall, it is more costly, and its polishing precision and brightness are typically slightly inferior to those of electropolishing. Furthermore, the process is prone to environmental issues such as acid mist.
Abrasive Flow Machining
This refers to the process of forcing a mixture containing abrasive materials through the internal cavities or channels of a casting under high pressure, using the friction generated by the abrasive materials within the fluid to smooth the casting’s interior.
This method maintains the smoothness of complex internal flow paths and reduces fluid resistance, thereby improving operational efficiency. However, it requires the use of specialized fixtures, which are prohibitively expensive, so it is typically used only in industrial sectors requiring extremely high precision, such as aerospace.
Application of Polishing
Different casting applications require different polishing techniques. Understanding which polishing techniques are suitable for specific conditions will help you select the right method for your components, enabling you to efficiently get castings with high-quality surfaces that are well-suited to their intended operating environments.
Chemical Industry
Due to the exceptional corrosion resistance in the chemical industry, electrolytic polishing is the most suitable method. While removing microscopic burrs and dead corners on the surface of castings, this process also forms a passivation layer. This film will prevent the possibility of corrosion by small pits on the surface of stainless steel castings and significantly enhances the castings’ corrosion resistance.
Medical Industry
The medical industry requires components that can withstand repeated cleaning and sterilization while maintaining the highest standards of hygiene. Electrolytic polishing is ideally suited to meet both of these requirements. Unlike mechanical polishing, which leaves behind nanoscale scratches on the surface, electrolytic polishing can completely remove dead corners from castings, making them easy to clean and sterilize repeatedly. Furthermore, this process ensures a smooth finish, achieving a mirror-like polish under the casting with few surface defects.
Food Industry
Since stainless steel is primarily used to manufacture large valve bodies and pump castings for the food industry, the most cost-effective polishing method must be selected. Because the food industry also requires a surface roughness of Ra < 0.8 μm, mechanical polishing is the most suitable option for this industry. It could maintain a high level of surface finish while enabling rapid, large-area polishing of castings, thereby preventing food residue from adhering to the surface during operation. In contrast, using electrolytic polishing exclusively requires a significant amount of chemical solutions, which would result in high costs.
If the internal structure of the casting is complex, it is recommended to first perform large-area mechanical polishing on the outer layer, followed by electrolytic polishing on the complex internal structures. This is the most cost-effective approach for this industry.
Questions in the process of polishing
The polishing process may reveal sub-surface defects in the casting caused by improper procedures in earlier stages, or it may affect the casting due to improper polishing techniques. Knowing these potential issues and their corresponding solutions will help you prepare in advance and have technical leverage when discussing quality standards or reworks with the foundry.
Subsurface Defects Revealed by Polishing
Some castings appear to have an intact surface before polishing, but once the surface layer of metal is removed under polishing, various subsurface casting defects are exposed. However, most of these can be resolved through appropriate optimization measures. Understanding these defects helps you address them early in the casting process, thereby reducing the cost of remaking castings.
- Gas Porosity
This condition manifests as a sudden appearance of numerous tiny, pinhead-sized black pores on the surface after polishing. To address this, manufacturers should ensure that the mold shells and molds are thoroughly dried, apply the coating evenly, and prevent air from being entrapped during casting.
- Shrinkage Cavities
This condition manifests as irregular, sponge-like micro-pits or a network of micro-pores on the polished surface, which can even cause localized uneven reflectivity. To address this, the manufacturer could optimize the design of the gating system and riser, and adjust the feed channels to achieve sequential solidification from the outer edges toward the center.
- Cracks
Cracks are classified into hot tears and cold cracks. Hot tears appear as winding, elongated cracks with a blackened surface after polishing. Cold cracks appear as sharp, distinct, straight, and fine cracks with a metallic luster after polishing. To address this issue, the manufacturer could optimize the structural design and implement simultaneous cooling of the casting.
Defects Caused by the Improper Polishing
Some issues are only revealed during polishing, but most defects are caused by improper polishing techniques.
- Surface Burns
During mechanical polishing, if the operator allows the polishing wheel to remain in one area of the casting for too long or applies excessive force, it can cause severe friction, leading to intense oxidation of the metal surface and resulting in purple or black spots.
To solve this issue, operators must move the wheel evenly and can reduce severe friction by lowering the speed of the polishing wheel or adding lubricant.
- Over-polishing
This is caused by excessive force applied to the edges of castings during mechanical polishing, which can result in sharp edges being rounded off, deformation of thin-walled sections, and dimensional defects that render the castings unusable.
To address this, manufacturers should strictly limit the polishing time and design specialized fixtures to protect sharp edges that do not require polishing, thereby reducing errors associated with fully manual polishing. Additionally, by optimizing the polishing process, including strictly separating the rough grinding and fine polishing stages, and using soft cloth wheels during fine polishing, could reduce the pressure exerted on the castings during polishing.
- Surface Pitting
During electrolytic polishing, if the current density is too low, a dense passivation layer will not form on the casting surface. This significantly reduces the casting’s corrosion resistance, allowing the acid solution to continue corroding, and a dense network of pinpoint-sized black pits will appear on the surface.
To address this issue, the equipment current must be appropriately increased to ensure it remains within the optimal density range during electrolytic polishing. Different materials need different ranges. For example, the optimal current density range for austenitic stainless steel is approximately 15–50 A/dm². This precise control is crucial when polishing 304 and 316L grades to prevent acid over-etching.
The Standards of Polishing
Different polishing standards apply to components with varying surface roughness requirements. Understanding these standards allows you to provide specific instructions to the manufacturer during production, ensuring that the polished parts meet your specifications.
Common Castings
For common castings, the polishing standard follows ASTM A480 No. 4 Finish. Castings meeting this standard exhibit distinct, uniform, fine straight lines and are non-reflective. The surface roughness is typically Ra ≤ 0.8 μm.
High-Grade Mirror-Finish Castings
These castings adhere to the ASTM A480 No. 8 Finish standard. Castings meeting this standard must achieve a mirror-like finish, meaning the surface is completely free of scratches and is even reflective. The surface roughness is typically required to be Ra ≤ 0.05 μm.
Castings for Chemical and Pharmaceutical Applications
For castings used in the chemical and pharmaceutical industries, which have stringent requirements, the standard followed is ASME BPE SF4. Under this polishing standard, castings must undergo mechanical polishing followed by electropolishing to ensure surface luster and absolute internal smoothness. The requirement is Ra ≤ 0.38 μm.
The Equipment of Polishing Inspection
Surface roughness tester
- Stylus Roughness Tester
It is an industry-standard contact measuring instrument that uses an extremely fine diamond stylus to gently glide across a stainless steel surface. A sensor converts the microscopic surface irregularities into electrical signals, directly calculating the Ra value. It’s an extremely fast process.
- Optical 3D Profiler
This high-end non-contact measurement system uses laser confocal or white-light interferometry to perform 3D contour scanning of metal surfaces, generating 3D images to measure Ra values without scratching highly sensitive surfaces.
Gloss and Reflectance Tester
- Gloss Meter
Under specified light sources and angles, a beam of light is directed at the stainless steel surface, and the proportion of the reflected luminous flux is measured. The resulting value is expressed in GU (Gloss Units) to ensure that the visual gloss of the casting surface is consistent.
- DOI Meter
Designed specifically for high-gloss surface casting. It not only measures the amount of reflected light but also accurately determines the degree of distortion in the reflection. The closer the value is to 100%, the flatter the surface.
Microscopic Defect and Cleanliness Inspection
- Industrial Endoscope
Using a miniature camera with high-intensity lighting and a coiled cable, this system can magnify internal metal surfaces that are hard to reach by tens or even hundreds of times and display them clearly on a high-resolution monitor. It enables non-destructive testing of the interiors of complex castings to detect the presence of cracks.
Testing of Surface Corrosion Resistance
- Passivation Tester
A handheld electrochemical or chemical probe that can measure the open-circuit potential on a stainless steel surface within seconds, or detect the presence of free iron molecules on the metal’s surface, ensuring that electro-polishing fully meets the requirements for castings with extremely high corrosion resistance, such as marine castings.
Final Thoughts
We trust you now have a thorough understanding of polishing. If you are looking for reliable stainless steel casting polishing services in China or a full-service investment casting foundry, please contact us. Pingheng Machinery, with its advanced polishing technology and stringent quality control, can provide you with perfect castings that meet your requirements.
FAQ
Q1. Will polishing increase my procurement costs?
Yes, polishing will slightly increase your procurement costs. However, in the long run, polished castings are much less likely to be scrapped due to rust, which reduces the cost of frequent replacements. Overall, the total cost is actually lower.
Q2. Why is polishing suitable for investment casting?
This is because investment casting produces a smoother surface finish than sand casting. Furthermore, the metals used in investment casting, such as austenitic stainless steel and copper alloys, are highly sensitive to the electrochemical reactions involved in electropolishing, making it very easy to form a dense passivation layer on their surfaces.
Q3. Which polishing methods are suitable for stainless steel?
Mechanical, electrolytic, and barrel polishing are most suitable for stainless steel castings. This is because the properties of stainless steel, combined with the need for high-volume production, make these three methods ideal. Especially electrolytic polishing, which is highly recommended for 316L stainless steel castings, as it significantly boosts pitting corrosion resistance in marine and medical applications.
However, because stainless steel has a complex composition and the surface of castings is prone to coarse-grain structure, chemical polishing often fails to achieve a mirror-like finish and involves high environmental costs. Therefore, when high corrosion resistance and a mirror-like finish are required, electrolytic polishing is typically the preferred choice.
Abrasive Flow Machining, on the other hand, suffers from extremely low cutting efficiency due to the high toughness of stainless steel and its tendency to work harden, and the cost of fixtures is also prohibitively high. Therefore, neither of these two polishing methods is suitable for stainless steel castings.
Q4. Which polishing method is best suited for castings with complex geometries?
For castings with high requirements for surface finish and corrosion resistance, such as marine castings like pump housings, electropolishing is recommended.
If the surface finish of the casting is not a top priority, but the internal structure is complex and requires extremely high smoothness, and if you have a substantial budget, then Abrasive Flow Machining is recommended.
Q5. What is the difference between Mechanical Polishing and Electropolishing for stainless steel?
The most obvious difference between the two methods lies in their fundamental nature: mechanical polishing is a physical process, while electrolytic polishing is a non-contact electrochemical process.
The primary objectives of the two methods also differ: mechanical polishing uses abrasives to remove material from the surface, focusing on appearance, whereas electrolytic polishing focuses on creating a chromium-rich passivation layer on the casting surface, significantly enhancing the casting’s corrosion resistance.
Q6. Why do stainless steel castings rust after polishing?
Stainless steel castings rust after polishing primarily due to two reasons: the destruction of the chromium-rich passivation layer during mechanical polishing or the lack of secondary passivation.
To be more specific, it is possible that issues were identified during polishing or that problems arose in improper handling during polishing. Both scenarios can damage the passivation layer on the stainless steel surface, reducing the corrosion resistance of the stainless steel castings.
Another possibility is that the manufacturer did not perform secondary passivation. Mechanical polishing could damage the original passivation layer, and if passivation is not performed promptly, it’s difficult for the stainless steel to form a new passivation layer on its own. This makes the casting highly susceptible to corrosion by chloride ions in seawater during transportation, leading to rust.
