Ⅰ. Free-Cutting Carbon Steels: 12L14, 1215, SUM22, SUM23, S12C
Key Features:
- Based on low carbon steel with sulfur and/or lead added for improved machinability.
- Low cost, widely available, and extremely easy to machine.
- Ideal for small screws, pins, shafts, and non-load-bearing fastener components.
Material Comparison:
| Grade | Additives | Machinability | Strength | Weldability | Cost |
| 12L14 | Lead | ★★★★★ | Medium | Poor | Low |
| 1215 | Sulfur + Phosphorus | ★★★★☆ | Medium | Moderate | Low |
| SUM22 | Sulfur (JIS) | ★★★★ | Medium | Moderate | Low |
Pros:
- Excellent chip-breaking, suitable for automatic lathes and CNC.
- High productivity and reduced tool wear.
- Lower production cost for high-volume fastener runs.
Cons:
- Poor corrosion resistance, requires surface coating (e.g., plating, blackening).
- Weldability is limited, especially with leaded steels.
- Mechanical strength is limited, not suitable for high-stress components.
Ⅱ. Free-Cutting Alloy Steels: 11SMnPb30, 11SMn37, Modified S45C, 1144, Type 416
Key Features:
- Medium carbon or alloy steel base with improved strength and toughness.
- Balanced properties for both machining and structural performance.
- Commonly used for high-strength fasteners such as studs, non-standard bolts, and pins.
Representative Grades:
| Grade | Machinability | Heat Treatability | Strength | Application Examples |
| 11SMnPb30 | Excellent | Limited | Medium | Precision shafts, small screws |
| 1144 | Very Good | Yes | High | High-load screws, pins |
| Modified S45C | Good | Good | Medium-High | General-purpose fasteners |
Pros:
- Good combination of strength and machinability.
- Suitable for parts requiring heat treatment or surface hardening.
- Ideal for parts needing tighter tolerances and consistent quality.
Cons:
- Higher cost than basic carbon steels.
- Slight reduction in toughness due to added sulfur or lead.
- Still limited weldability — preheating may be necessary.
Ⅲ. Free-Machining Stainless Steels: Type 303, Type 416, Type 430F
Key Features:
- Improved machinability over standard stainless steels by adding sulfur or selenium.
- Offer moderate corrosion resistance along with good surface finish quality.
- Suitable for decorative screws, precision connectors, and components exposed to mild environments.
Stainless Grade Comparison:
| Grade | Steel Type | Machinability | Corrosion Resistance | Heat Treatable | Magnetic |
| Type 303 | Austenitic | ★★★★ | ★★★★ | No | No |
| Type 416 | Martensitic | ★★★★★ | ★★★ | Yes | Yes |
| Type 430F | Ferritic | ★★★☆ | ★★☆ | No | Yes |
Pros:
- High machining speed and stable cutting performance.
- Retain basic corrosion resistance properties.
- Can achieve smooth surface finishes and mirror polishing (especially Type 303 and 416).
Cons:
- Corrosion resistance is lower than standard 304 or 316 stainless steel.
- Some grades (e.g., Type 303) cannot be hardened by heat treatment.
- Welding can be difficult, especially for sulfurized or leaded stainless steel.
Ⅳ. How to Choose the Right Free-Cutting Steel for Fasteners
Choosing the right free-machining steel for fasteners depends on your application requirements, including mechanical load, corrosion environment, machining method, and cost.
Selection Guide:
| Application Scenario | Recommended Grade | Notes |
| General low-load fasteners | 12L14 / 1215 | Lowest cost, excellent machinability |
| Medium-strength screws or shafts | 11SMnPb30 / 1144 | Stronger with good machining |
| Heat-treated high-strength parts | Modified S45C / 1144 | Heat-treatable and dimensionally stable |
| Decorative and mildly corrosive use | Type 303 | Bright appearance, smooth finish |
| Load-bearing + corrosion resistance | Type 416 | Martensitic, heat treatable |
| Low-cost stainless with magnetic properties | Type 430F | Suitable for magnetic field applications |
Ⅴ. Conclusion: Balance Machinability and Performance
Using free-cutting steel materials in precision fasteners allows manufacturers to significantly increase efficiency, reduce tool wear, and lower machining costs. However, material selection must be aligned with functional requirements, including strength, corrosion resistance, and weldability.
Different precision fastener steel grades offer trade-offs between machinability and mechanical performance. Understanding these differences ensures better product quality and more reliable long-term performance.
