Overview: ISO 11124 Cast Steel Shot Standards
ISO 11124 is the primary international standard series governing specifications for metallic blast-cleaning abrasives. Parts 3 and 4 of this series specifically address cast steel abrasives — the most widely used metallic abrasives in industrial blast cleaning operations worldwide.
ISO 11124-3 specifies requirements for high-carbon cast steel shot and grit, while ISO 11124-4 covers low-carbon cast steel shot. Although both products are manufactured from steel, their differing carbon content creates significantly different mechanical properties, service behavior, and optimal application domains.
Selecting the wrong grade can result in inadequate surface profile, excessive abrasive consumption, substrate damage, or suboptimal coating adhesion — all of which have direct cost and quality implications in industrial coating projects.
ISO 11124 specifies requirements for metallic abrasives used in the blast cleaning of steel substrates in preparation for the application of paints and related products. It does not cover peening applications, which are governed by SAE AMS standards. Testing methods referenced in ISO 11124 are detailed in ISO 11125 Parts 1–7.
Chemical Composition Requirements
The fundamental difference between the two grades lies in carbon content, which directly determines hardness, toughness, and brittleness.
| Element | ISO 11124-3 (High-Carbon) | ISO 11124-4 (Low-Carbon) | Engineering Significance |
|---|---|---|---|
| Carbon (C) | 0.85% – 1.20% | ≤ 0.20% | Primary determinant of hardness and brittleness |
| Manganese (Mn) | 0.60% – 1.20% | ≤ 1.20% | Improves strength and hardenability |
| Silicon (Si) | 0.40% – 1.50% | ≤ 1.50% | Deoxidizer; improves castability |
| Sulfur (S) | ≤ 0.05% | ≤ 0.05% | Controlled to prevent hot cracking |
| Phosphorus (P) | ≤ 0.05% | ≤ 0.05% | Controlled to prevent embrittlement |
Hardness Specifications
Hardness is the single most important mechanical property governing abrasive performance. Higher hardness produces more aggressive cutting action and greater surface profile depth, but at the cost of increased brittleness and breakage rate.
| Product Form | ISO 11124-3 Hardness | ISO 11124-4 Hardness | Test Method |
|---|---|---|---|
| Shot (spherical) | 40 – 51 HRC (390–530 HV) | 100 – 250 HV10 | ISO 11125-4 |
| Grit (angular) | 56 – 65 HRC (620–830 HV) | N/A (not produced as grit) | ISO 11125-4 |
| Test load | HV10 (10 kgf Vickers) on polished cross-section, minimum 10 particles | ISO 11125-4 | |
| Result basis | Median of all measurements; specification requires individual values within range | — | |
ISO 11125-4 requires a minimum of 10 particles per sample to be mounted, polished, and measured individually. The median value must meet specification. Hardness converted between scales using ISO 18265 — direct comparisons between HRC and HV are approximate only. Always specify and test in the same scale.
Head-to-Head Comparison
- Carbon: 0.85%–1.20% — high hardness and aggressive cutting
- Shot hardness: 40–51 HRC; Grit hardness: 56–65 HRC
- Achieves Rz 40–110 μm surface profiles (grade dependent)
- Higher breakage rate than low-carbon; replenishment more frequent
- Angular grit produces sharper, higher profile — better coating anchor
- Standard grade for structural steel, shipbuilding, oil & gas
- Available as both shot (spherical) and grit (angular) forms
- More economical per tonne; high recycling rate in centrifugal equipment
- Carbon: ≤ 0.20% — tough, ductile, low brittleness
- Shot hardness: 100–250 HV10 — significantly softer
- Generates compressive residual stress (peening effect)
- Very low breakage rate — extended service life per tonne
- Produces smooth, rounded profile — Rz 20–60 μm typical
- Preferred for aerospace, precision components, thin-section steel
- Shot form only — not produced as grit
- Higher cost per tonne but reduced consumption
Density and Physical Properties
| Property | ISO 11124-3 | ISO 11124-4 | Test Method |
|---|---|---|---|
| Particle density (g/cm³) | ≥ 7.2 | ≥ 7.4 | ISO 11125-5 |
| Defective particles | ≤ 10% by count | ≤ 10% by count | ISO 11125-6 |
| Water-soluble chlorides | ≤ 25 mg/kg | ≤ 25 mg/kg | ISO 11125-7 |
| Moisture content | ≤ 0.5% by mass | ≤ 0.5% by mass | ISO 11125-3 |
| Oversize (top sieve) | ≤ 2% by mass | ≤ 2% by mass | ISO 11125-2 |
| Undersize (bottom sieve) | ≤ 5% by mass | ≤ 5% by mass | ISO 11125-2 |
Surface Profile Performance
The choice of abrasive grade directly determines achievable surface profile (roughness), which must be matched to the coating system's specified profile class per ISO 8503.
| ISO 11124-3 Grade | Nominal Size | Typical Rz Profile (μm) | ISO 8503 Class | Application |
|---|---|---|---|---|
| S110 (Shot) | 0.30–0.60 mm | 25–45 μm | Fine (F) | Thin primer coats, <100 μm DFT |
| S170 (Shot) | 0.42–0.85 mm | 40–65 μm | Medium (M) | General anti-corrosion coatings |
| S230 (Shot) | 0.60–1.00 mm | 55–85 μm | Medium (M) | Industrial coatings 150–300 μm DFT |
| S330 (Shot) | 0.85–1.40 mm | 70–105 μm | Coarse (C) | Heavy-duty offshore, zinc silicate |
| G25 (Grit) | 0.50–1.40 mm | 60–100 μm | Medium–Coarse | Angular profile for coating adhesion |
| G40 (Grit) | 0.71–2.00 mm | 90–140 μm | Coarse–Extra Coarse | Thermal spray, metallizing |
Select an abrasive grade that will produce a surface profile (Rz) of approximately 20–35% of the specified total dry film thickness (DFT). This ensures the profile provides sufficient anchor without peaks protruding through the coating. Profile exceeding 40% of DFT risks corrosion initiation at exposed profile peaks.
Application Selection Guide
The following matrix summarizes which ISO 11124 grade is appropriate for major industrial applications:
| Application | Recommended Grade | Reason | Typical Sa Grade |
|---|---|---|---|
| Shipbuilding — hull blasting | ISO 11124-3 Shot S230 | High throughput, large area, medium profile | Sa 2½ |
| Offshore topsides | ISO 11124-3 Grit G25 | Angular profile for high-build coatings | Sa 3 |
| Structural steelwork | ISO 11124-3 Shot S170/S230 | General industrial anti-corrosion | Sa 2½ |
| Aerospace components | ISO 11124-4 Shot | Low profile, compressive stress, no distortion | Sa 2½ |
| Precision thin-section steel | ISO 11124-4 Shot | Soft shot prevents substrate distortion | Sa 2 |
| Pipeline external coating (FBE) | ISO 11124-3 Grit G18/G25 | Angular profile for FBE adhesion | Sa 2½ |
| Thermal spray / metallizing | ISO 11124-3 Grit G40/G50 | Extra coarse angular profile required | Sa 3 |
| Shot peening (fatigue life) | Per SAE AMS standards | ISO 11124 is for blast cleaning — not peening | N/A |
Breakage Rate and Abrasive Consumption
Breakage rate determines abrasive consumption, operating cost, and maintenance of particle size distribution in recycling systems. ISO 11125-6 defines the test method for assessing defective particles and breakage characteristics.
High-carbon cast steel shot (ISO 11124-3) has a higher inherent breakage rate due to its greater hardness and brittleness. In centrifugal blast wheel systems, manufacturers typically report breakage rates of 0.1%–0.4% per cycle for standard grades. Low-carbon shot (ISO 11124-4) typically exhibits breakage rates of 0.02%–0.08% per cycle — up to 5× lower consumption per tonne of steel processed.
Despite higher per-tonne cost, low-carbon shot's dramatically lower breakage rate often makes it cost-competitive over a full blasting campaign in applications where it is technically suitable.
In both grades, the working abrasive mix in a recycling blast system should be monitored by sieve analysis (ISO 11125-2) at regular intervals — typically every 2 hours of operation. The size distribution of the working mix determines effective surface profile; degraded mixes produce finer profiles. Make-up additions should restore the original size distribution.
How to Specify per ISO 11124
When writing project specifications referencing ISO 11124, always include the following elements:
- Standard designation: ISO 11124-3 or ISO 11124-4
- Product form: Shot or Grit (for ISO 11124-3 only)
- Nominal size grade: e.g., S230, G25 (using SAE J444 or ISO equivalent designation)
- Hardness requirement: Specify HRC or HV range per standard
- Certificate of conformance (CoC): Require from manufacturer with each delivery lot
- Incoming test requirements: Reference ISO 11125-1 for sampling, with specific tests listed
- In-process monitoring frequency: Define sieve analysis interval
"Metallic blast-cleaning abrasive shall conform to ISO 11124-3, high-carbon cast steel shot, grade S230. Abrasive shall be supplied with a certificate of conformance confirming compliance with all ISO 11124-3 requirements. Incoming abrasive shall be sampled and tested per ISO 11125-1 and ISO 11125-2. Working mix shall be monitored by sieve analysis at intervals not exceeding 4 hours of continuous operation."
Summary and Selection Decision
The choice between ISO 11124-3 and ISO 11124-4 is driven primarily by the substrate type, coating system, and performance requirements:
- For structural steel, shipbuilding, offshore, oil & gas, and pipeline applications requiring medium-to-coarse surface profiles for heavy-duty anti-corrosion coatings — specify ISO 11124-3 high-carbon cast steel shot or grit.
- For aerospace components, precision thin-section steel, or applications requiring compressive surface stress with a fine profile — specify ISO 11124-4 low-carbon cast steel shot.
- Where angular profile is critical for coating adhesion (e.g., zinc silicate, FBE, thermal spray), ISO 11124-3 grit grades provide the necessary surface texture that spherical shot cannot achieve.
Always verify that the specified abrasive and resulting profile are compatible with the coating manufacturer's application requirements and the project specification's surface preparation standard.