1. Atomization (Most Common)
Process:
1. Molten steel is poured through a tundish into a water-cooled atomizer, creating small droplets.
2. Droplets solidify into spherical particles as they fall through a controlled atmosphere.
3. Particles are heat-treated (annealed, quenched, and tempered) to achieve desired hardness and ductility.
Advantages: Consistent shape, high production speed, and scalability for different sizes.
2. Cutting and Rounding
Process:
1. Steel wire is cut into short cylindrical segments (shot blanks).
2. Blanks are tumbled in rotating drums with abrasives to round their edges.
Applications: Specialty shots for niche requirements, though less common due to higher costs.
3. Casting
Process: Molten steel is poured into molds to form spherical shots.
Use Case: Older method, now largely replaced by atomization for its superior consistency.
Types of Steel Shots and Their Characteristics
1. Carbon Steel Shots
SAE Classification:
S110–S700: Designates shot size (e.g., S110 = 0.3 mm, S700 = 1.7 mm).
Hardness Grades:
Soft (40–45 HRC): Gentle cleaning, suitable for aluminum or soft steel.
Medium (48–55 HRC): General-purpose cleaning and peening.
Hard (58–62 HRC): Aggressive cleaning on tough surfaces like cast iron.
2. Stainless Steel Shots
Grades:
304 Stainless: Non-magnetic, corrosion-resistant, ideal for non-ferrous metals and medical equipment.
430 Stainless: Magnetic, higher hardness (50–55 HRC) for heavy-duty applications without rust risk.
Advantages: Reusable in harsh environments, no risk of introducing iron contaminants.
3. Alloy Steel Shots
Examples:
Manganese Steel Shots: Enhanced impact resistance for prolonged use in high-volume blasting.
Chrome Alloy Shots: Higher toughness for peening high-strength alloys in aerospace components.
Selecting blasting media requires analyzing physical and chemical properties to balance performance, durability, and material safety. Below is a detailed breakdown of key characteristics and their impact on blasting operations:
| Characteristic | Impact on Performance | Practical Considerations |
|---|
| Hardness (HRC) | - Wear Resistance: Higher hardness (e.g., 50–60 HRC for steel shot) prolongs media lifespan, reducing replacement frequency.
- Abrasion Intensity: Harder media (e.g., steel grit) excel at removing tough contaminants (scale, rust) but may cause excessive surface roughness or substrate damage on delicate materials.
- Machine Wear: Aggressive media increase nozzle and wheel abrasion, requiring more frequent maintenance. | - Use medium-hardness media (e.g., 40–50 HRC) for general cleaning to balance efficiency and equipment longevity.
- Opt for softer media (e.g., ceramic beads, 20–30 HRC) for non-ferrous metals or polished surfaces. |
| Sphericity | - Surface Finish: Spherical particles (e.g., steel shot) produce uniform, low-roughness impacts, ideal for peening or delicate cleaning.
- Cutting Efficiency: Angular particles (e.g., steel grit) create sharp impacts, enhancing material removal but risking scratches or deeper abrasion. | - Prioritize high sphericity (e.g., ≥90% roundness) for applications requiring consistent surface profiles (e.g., aerospace components).
- Use angular media for heavy-duty stripping (e.g., ship hulls, industrial equipment). |
| Density (g/cm³) | - Kinetic Energy: Higher density (e.g., steel: 7.85 g/cm³; stainless steel: 8.0 g/cm³) increases impact force at a given velocity, improving cleaning efficiency for tough deposits.
- Media Recycling: Dense media retain velocity better in recycling systems, reducing energy loss. | - For low-velocity systems (e.g., manual blasting), use dense media to compensate for lower speed.
- Lightweight media (e.g., plastic beads: 1.2–1.4 g/cm³) are suitable for gentle cleaning of fragile parts (e.g., electronics). |
| Chemical Purity | - Contamination Risks: Low sulfur/phosphorus content (e.g., <0.05%) prevents embrittlement in high-strength steels or stainless alloys.
- Corrosion Resistance: Stainless steel media (low iron content) avoids rust contamination on non-ferrous metals. | - Critical for food-grade, medical, or aerospace applications where trace contaminants could cause failure.
- Avoid carbon steel media on stainless steel to prevent galvanic corrosion risks. |
| Particle Size Distribution | - Cleaning Speed: Mixed sizes (e.g., S170 + S230 shot) improve coverage by filling gaps between larger particles.
- Surface Texture: Narrow size ranges (e.g., G25–G40 grit) ensure uniform roughness for coating adhesion. | - Use narrow sieves (e.g., ±10% variation) for precision applications (e.g., coating pre-treatment).
- Broad distributions (e.g., shot peening blends) balance impact force and coverage for complex geometries. |
| Fracture Resistance | - Recycling Viability: Media with high fracture resistance (e.g., heat-treated steel shot) maintains effectiveness through multiple cycles.
- Dust Generation: Fragile media (e.g., glass beads) produce more dust, increasing filter maintenance and health hazards. | - Choose tough media (e.g., forged steel grit) for high-recycling systems to minimize waste and downtime.
- Use disposable media (e.g., sodium bicarbonate) for one-time, low-dust applications (e.g., historic restoration). |
- Cross-Reference with Standards: Use industry specifications (e.g., SAE J444 for shot/grit sizes, ISO 8502 for surface roughness) to validate media compliance.
- Conduct Abrasion Testing: Measure media wear rate using a blast cabinet test (e.g., compare weight loss after 10 hours of use).
- Assess Environmental Impact: For outdoor or regulated sites, prioritize low-dust, recyclable media (e.g., steel shot) to meet air quality standards.
Selection Criteria for Steel Shots
Choosing the right steel shot involves balancing multiple factors to achieve optimal results while minimizing costs:
1. Workpiece Material
Mild Steel/Heavy Metal: Use medium to hard carbon steel shots (48–62 HRC) for efficient scale removal.
Aluminum/Non-Ferrous Metals: Soft carbon steel (40–45 HRC) or stainless steel shots to avoid surface damage.
High-Strength Alloys: Alloy or stainless steel shots to withstand high-impact peening.
2. Desired Surface Finish
Cleaning Intensity:
Light cleaning (e.g., removing light rust): Smaller shot sizes (S170–S230) with lower hardness.
Heavy-duty cleaning (e.g., mill scale removal): Larger shots (S460–S700) with higher hardness.
Roughness Profile: Larger shots create deeper anchor patterns (50–100 microns), while smaller shots produce finer finishes (20–50 microns).
3. Machine Type and Media Recycling
Centrifugal Wheel Machines: Require durable shots that withstand high-velocity impacts; carbon steel shots are ideal.
Air Blast Systems: Smaller shots (S110–S330) are preferable to avoid nozzle clogging.
Recycling Frequency: Harder shots (60+ HRC) last longer in recycling loops, reducing replacement costs in high-volume operations.
4. Environmental and Safety Considerations
Corrosion-Prone Environments: Stainless steel shots prevent rust contamination in final products.
Food/Medical Industries: Non-magnetic, non-corrosive stainless steel shots are mandatory to meet hygiene standards.
Waste Management: Recyclable steel shots align with sustainability goals, minimizing abrasive waste.
