When purchasing castings, have you ever encountered situations where black residue remained on the casting surface, red rust spots appeared, or the dimensions were distorted? This is because the manufacturer failed to perform shot blasting correctly or used the wrong blasting media entirely. This article will tell you everything you need to know about shot blasting in stainless steel investment casting, especially 304 and 316L stainless steel casting, helping you better understand the importance of this process and reduce additional costs.
What is Shot Blasting
Shot blasting is a surface treatment process that primarily uses a high-speed rotating blast wheel to propel metal shot media at high speed and high density onto the surface of stainless steel castings by centrifugal force, thereby removing surface impurities. Understanding how shot blasting works, as well as the different types of shot media and blasting equipment, will help you select the appropriate shot blasting method for your castings, ensuring that their surface quality meets your requirements.
How does it work
Shot blasting relies on mechanical centrifugal force. The entire system features an automated internal circulation process that cleans the surface in 10–15 minutes per batch, thereby significantly improving your surface finish.
The specific process is as follows:
- Ejection: The blasting media is ejected from the blasting head at 50–80 m/s due to the powerful centrifugal force generated by the blades.
- Impact: While striking the stainless steel castings, the powerful energy is converted into mechanical impact force, removing surface impurities.
- Fall: After striking the castings, the shot falls into the hopper at the bottom of the shot blasting chamber along with the surface impurities.
- Recirculation: The screw conveyor and bucket elevator at the bottom transport the shot vertically back to the top of the shot blasting machine. There, it removes impurities from the air wash separator and selects undamaged shot in the storage hopper.
Types of Blasting Media for Stainless Steel Castings
There are many blasting media options available for shot blasting in stainless steel investment casting, and each type produces a different surface roughness and is suitable for different applications. Selecting the appropriate blasting media based on the operating environment can help you avoid cost losses from unsuitable media.
| Blasting Media | Physical Form | Surface Roughness | Surface Effect | Advantages | Application |
| Stainless Steel Shot | Spherical stainless steel particles, typically made of 304, 430, or other high-chromium stainless steel. | Ra 3.2-6.3μm | A standard, uniform industrial matte finish | Contains no free iron, eliminating the risk of rust during shipping transport | Standard industrial castings, such as pump castings, valve body castings, pipe fitting castings, and so on. |
| Stainless Steel Cut Wire Shot | Cylindrical pellets made by mechanically cutting stainless steel wire; when they collide, their edges are smoothed out, causing them to become spherical. | Ra 3.2 – 8.0 μm (Decreases as wire edges round off) | Features a clean, matte silver-gray finish | It can remove shell residue more efficiently and is less likely to break or create dust. | Stainless steel castings with thick walls and stubborn, hard-to-clean dead corners |
| Glass Beads | Extremely small, high-strength spherical sodium-calcium glass particles. | Ra 1.6-3.2μm | A satin finish with a smooth texture and a soft, subtle sheen | Suitable for precision castings with extremely high hygiene requirements | Precision stainless steel castings for medical castings, food-grade components, and more. |
| Aluminum Oxide | Synthetically produced corundum grit with extremely hard and sharp particles, which are angular grit. | Ra 6.3-12.5μm | Appears rough and dull, with a deep gray color | Improves the adhesion of anti-corrosion coatings on castings | Used for cleaning stainless steel castings with extremely complex structures and deep internal cavities. |
Types of Blasting Equipment for Stainless Steel Castings
The type of shot blasting machine directly determines the cleaning efficiency and the risk of damage to your castings. Selecting the right shot blasting machine allows you to process your castings efficiently, reducing the need for recasting due to damage caused by machine errors, which in turn reduces your costs.
- Tumble Belt Shot Blasting Machine
This is the most common type of shot blasting machine. It operates with an internal, continuously circulating rubber track that drives stainless steel castings to roll along it; at the same time, a shot blasting head at the top propels blasting media downward at high speed to abrade the castings.
This machine can efficiently clean large batches of castings with low costs and without leaving any blind spots. However, because the castings are prone to impact while rolling on the track, it is only suitable for small, high-volume castings with a sturdy structure, such as stainless steel pipe fittings, valve bodies, and small brackets.
- Hanger Type Blast Machines
This machine operates by hanging stainless steel castings to special hooks. As the hooks rotate within the shot blasting chamber, high-speed shot blasting heads on all sides propel blasting media to strike the castings. Since the castings are hung independently, there is no risk of them bumping into others. This makes the machine suitable for large, precision stainless steel castings with complex structure, such as impeller castings, marine castings, and automotive castings.
The Influence of Shot Blasting on Stainless Steel Castings
Shot blasting has a significant impact on stainless steel castings, including improving the quality of surface finish but also enhancing their mechanical properties. Understanding these effects will help you better appreciate the importance of shot blasting and reduce the costs associated with recasting due to cracking caused by excessive surface impurities or excessive internal tensile stress.
Removing Surface Residue
After decoating, stainless steel castings often have mold residue and a layer of quartz sand that are extremely difficult to remove from their surfaces. Shot blasting could thoroughly remove these surface residues through high-speed friction against the casting surface. This is the most fundamental effect of shot blasting on stainless steel castings.
Achieving a Uniform Appearance
After pouring, stainless steel castings may exhibit localized high-temperature oxidation on their surfaces due to variations in wall thickness and cooling rates, resulting in black or dark gray discoloration and traces of the riser. Shot blasting could use extremely high kinetic energy to remove this uneven oxide scale and grinding marks from the casting surface.
At the same time, the countless tiny impacts from the shot not only remove mold shell residues but also force microscopic surface burrs deep into the metal, giving the surface a consistent, fine, and uniform matte finish, thereby significantly improving the surface quality of stainless steel castings.
Improving Fatigue Life
During the cooling process of stainless steel castings, internal casting shrinkage occurs due to uneven cooling rates. Combined with the force exerted by the external mold shell, a large amount of residual tensile stress is generated on the casting surface.
However, when shot impacts the stainless steel casting intensively, it performs a form of micro-forging on the surface, causing the surface grains to elongate. Since the internal metal is not subjected to these impacts, the surface grains develop inward compressive stress. This inward compressive stress, known as residual compressive stress, can eliminate the internal tensile stress and prevent cracks on the casting surface from expanding, thereby significantly improving the fatigue life of components.
Preparing for Subsequent Processes
Shot blasting removes the thick high-temperature oxide scale from the surface of stainless steel castings, exposing the metal and preparing it for subsequent passivation. This allows the castings to react directly with the chemical solution during passivation, forming a dense, chromium-rich passivation film on the surface. It prevents excessive surface residues from influencing the casting’s corrosion resistance.
Common Defects in Shot Blasting
Shot blasting offers many benefits for your castings, but improper shot blasting can also cause numerous problems. Understanding these common issues that arise during the shot blasting process will not only give you a better understanding of the operation but also allow you to avoid these problems in advance, thereby reducing the need for recasting due to shot blasting errors.
Iron Cross-Contamination
When you open the packaging, you may find the surface of the stainless steel casting covered with dense red rust spots. This is because the foundry used the wrong blasting media, that is, carbon steel shot. When the carbon steel shot is propelled at high speed and rubs violently against the stainless steel surface, the free iron ions in the shot become deeply embedded in the casting surface, destroying the passivation film. At the same time, if the subsequent passivation is not performed properly, the casting’s corrosion resistance will be significantly reduced, making it prone to chloride ion pitting during maritime transport.
Dimensional Deformation
In addition to using incorrect abrasive media, using the wrong shot blasting machine can also lead to major issues. For example, if placing complex, thin-walled castings in a tumble belt shot blasting machine, the castings will collide violently with one another during the process, causing severe deformation of the casting structure.
What’s more, if the shot blasting parameters are set incorrectly, excessive force, overly coarse blasting media, or a lack of protective measures can directly cause the castings to exceed dimensional tolerances or render threaded surfaces unusable. This could lead to failed final assembly and inoperability, which could incur many costs for recasting.
Incomplete Ceramic Shell Removal
If perfectly spherical stainless steel shot is used for surface treatment of castings, some stubborn sintered residues on the casting surface will not be completely removed. This will result in a layer of white, rock-hard silica sol-type shell residue, commonly known as quartz sand skin, which remains in the casting’s internal cavities and dead corners.
Orange Peel Effect
Sometimes you may find rough, orange-peel-like cracks on the surface of stainless steel castings. This is caused by excessive shot blasting time, during which the castings are subjected to excessive impact and friction, leading to peeling or even cracking of the surface. This not only damages the luster and texture of the casting’s surface but also affects the quality of subsequent electropolishing.
Industry Standards for Shot Blasted Surfaces
ASME B46.1
This standard is used to measure Ra value using stylus-type roughness testers. For example, if you have an Ra 3.2 surface finish on stainless steel investment castings, you must select glass beads or a specific blend of stainless steel shot.
ASTM A802
We use SCRATA (Steel Castings Research and Trade Association) surface quality comparators to establish ASTM A802 acceptance criteria for shot blasting, ensuring your components are free from microcracks, excessive grinding, or severe orange peel.
How to Specify Shot Blasting Requirements for Stainless Steel Castings
Specify the blasting media.
Approved blasting media shall be limited to stainless steel shot, stainless steel cut wire shot made of 340/430, and glass beads. The use of carbon steel shot is strictly prohibited to prevent rust.
Definition of Surface Roughness
Surface roughness of the stainless steel castings shall be evaluated and defined in accordance with ASME B46.1.
Acceptance Criteria
Surface finish visual acceptance testing must be performed using a SCRATA comparator according to ASTM A802.
Requirement of Protective Measures
Masking protective measures must be used to protect the precision-machined areas of the castings.
Final Thought
We believe you now have a general understanding of shot blasting. This process is crucial for the surface treatment of stainless steel castings and has a significant impact on improving the surface quality. If you are looking for a reliable stainless steel investment casting manufacturer, please contact us. Pingheng Machinery, with its excellent surface treatment and strict quality control, will provide you with perfect castings.
FAQ
Q1. What type of blasting media should be used for shot blasting in stainless steel castings?
Stainless steel shot, stainless steel wire cut shot, glass beads, and aluminum oxide can be used as blasting media for stainless steel castings. However, carbon steel shot must never be used, as the free iron ions on its surface will damage the casting’s passivation film, causing the casting to corrode and rust.
Q2. What protective measures can be used in shot blasting to protect stainless steel castings from dimensional deformation?
You could use masking protection measures, which refer to protecting areas of stainless steel castings that have already undergone precision machining, such as precision threads and bearing seats, during the shot blasting process.
The most common methods of masking protection involve inserting specialized wear-resistant rubber plugs and protective sleeves into internal threads or precision bores, or using silicone caps to cover external threads and bearing seats.
For high-volume castings with fixed shapes, manufacturers also custom-make metal protective fixtures to cover the precision parts of the castings and prevent them from being struck.
Q3. What’s the difference between shot blasting and sandblasting?
The main difference between shot blasting and sandblasting lies in their power sources. Shot blasting uses the centrifugal force of a blast wheel to propel blasting media onto the surface of stainless steel castings, making it suitable for large-area cleaning. Sandblasting, on the other hand, uses compressed air to propel the media through a nozzle, allowing for adjustments in pressure and direction, making it suitable for precision and complex stainless steel castings.
In addition, the abrasives used in the two processes differ. Shot blasting typically uses spherical metal particles, such as stainless steel shot, while sandblasting typically uses angular aluminum oxide or fine glass beads.
Q4. What is the correct sequence for shot blasting, passivation, and polishing?
The standard sequence is shot blasting, pickling and passivation, and polishing. By removing investment shell residues through shot blasting, pickling and passivation can react directly with the bare metal surface to fully reform the protective oxide film, thereby ensuring the corrosion resistance of the stainless steel. Finally, polishing is performed to enhance the passivation effect on the stainless steel casting surface.
Q5. How can we address incomplete sand removal and mold shell residue after shot blasting?
Using entirely spherical stainless steel shots is not recommended. While this medium provides high impact strength, its low cutting capability makes it insufficient for completely removing residual investment shell material. To thoroughly clean hard-to-reach areas such as internal cavities and sharp corners, sharp-edged conditioned stainless steel cut wire shot or aluminum oxide must be blended, improving the quality of surface finish.
Q6. Do you have a dedicated workshop for shot blasting operations?
Yes, we have a dedicated post-treatment workshop and offer one-stop service; we handle everything from mold casting and surface treatment to CNC machining, providing perfect castings.
