What Is Free Machining Steel?

Free machining steel refers to steel alloys that break into small chips during the machining process. This results in better machinability, better surface finishes, and increases the lifetime of the cutting tools. These steel alloys also have sulfur, lead, and/or bismuth, which each act as a dry lubricant when the metal is machined.

The opposite to free machining steels are steel grades that produce long strings of metal as it’s machined. The longer metal shavings can get tangled in the spinning cutting tools causing issues.

List of free machining steel alloys

There are 3 main groups of free machining steel alloys: Leaded, Rephosphorized, and Resulfurized.

The SAE numbers indicate whether the alloy has added lead, sulfur, or sulfur and phosphorus. The “L” indicates that lead was added to the steel. This is different from the “L” in stainless steel alloys, which indicates low carbon.

The second number in the designation indicates if the alloy was re-sulfurized (1) or re-sulfurized and re-phosphorized (2).

Type | AISI Grade

Leaded | 11L41

Leaded | 12L13

Leaded | 12L14

Leaded | 41L40

Resulfurized | 1117

Resulfurized | 1118

Resulfurized | 1119

Resulfurized | 1137

Rephosphorized & Resulfurized | 1211

Rephosphorized & Resulfurized | 1212

Rephosphorized & Resulfurized | 1213

Rephosphorized & Resulfurized | 1215

Advantages of using free machining steel

Free machining steel is the most common choice for precision machined steel parts, or parts with complex features. These alloys are easier to cut and drill, which lowers the machining costs and reduces lead times. Because the steel produces smaller chips, the machines require less human interactions. This makes free machining alloys ideal for high volume machining and Swiss screw machining.

The biggest advantages are:

• Lower machining costs
• Faster, more efficient machining
• Better surface finishes
• Reduced wear on cutting tools

1. Cost-Effective Production:
   • While the initial investment in free machining steel might be higher, it offers a long-term economic advantage. The increased efficiency in the machining process reduces labor hours and machining time, thereby driving down overall project costs. Over the course of multiple projects, these savings can be substantial.
2. Rapid Turnaround:
   • The machinability of free machining steel ensures that parts can be produced at a faster rate. This efficiency is particularly crucial for projects with strict deadlines or when a quick prototype turnaround is needed.
3. Precision and Consistency:
   • Free machining steel’s ability to produce smaller and more consistent chips reduces the chance of machining errors. This results in parts that adhere closely to design specifications, ensuring that batches maintain uniform quality.
4. Scalability for Large Orders:
   • Given the minimized interruptions due to less tool wear and other issues, businesses can expect a smooth production flow. This is advantageous when scaling up production quantities without compromising delivery times.
5. Enhanced Surface Finishes:
   • The quality of the finish often determines the performance and aesthetics of a part. Free machining steel’s properties often result in a superior finish, reducing or sometimes even eliminating the need for additional finishing processes.
6. Reliability:
   • Consistency in production reduces the chances of batch variations or production defects. This reliability ensures that clients can expect a steady supply of quality parts.

Disadvantages of using free machining steel

Like any metal alloy, free machining steels have disadvantages as well. Free machining alloys cost more than regular steel alloys. This offsets the lower machining costs. The ductility and impact resistance is also reduced, as these alloys are more brittle.

1. Material Cost Consideration:
   • While there are cost savings in the machining process, the initial outlay for free machining steel is higher. This might be a consideration for businesses with strict budget constraints.
2. Potential Limitations for Specific Uses:
   • Not all components are best made from free machining steel. For instance, parts requiring exceptional ductility or high impact resistance might be better suited to alternative materials.
3. Environmental Considerations:
   • Eco-conscious businesses need to be aware of the environmental footprint of their material choices. Free machining steels, particularly those containing lead, might raise concerns regarding disposal and recycling.

What are free machining alloys used for?

12L14 steel is the most common steel alloy used for many different precision machined parts. This alloy, and other FMAs are used for parts that require extensive machining, and tight tolerances.

These alloys are used to make:

• Pins
• Shafts
• Spacers
• Bushings
• Nuts and bolts
• Fittings and Couplings

Free machining steel is more commonly used for smaller parts because the smaller chipping results in better automation. When thousands of identical parts are needed quickly, avoiding machining downtime is essential.

Free machining alloys are used in industries that require high volumes of precision steel parts, like the automotive, defense, and aerospace industries. For parts that require welding, FMAs aren’t the best choice. 1008 or 1018 steel has lower machinability, but is much better for strong welds.

FAQs

Can free machining steel be anodized, heat treated, or electroplated?

Free machining steel, like other steels, can be heat treated to modify its mechanical properties. Heat treatment can enhance characteristics like hardness, toughness, or wear resistance. However, it’s essential to note the specific alloy and its constituents because some alloying elements can affect the heat treatment results.

Regarding electroplating, free machining steel can be plated to provide additional corrosion resistance, aesthetic appeal, or other specific properties. Common plating methods include nickel, zinc, or chrome plating.

Anodizing, on the other hand, is a process primarily used for aluminum to form a protective oxide layer. It’s not applicable to steel, including free machining steel. Instead, steel can undergo processes like galvanizing or bluing for enhanced corrosion resistance.

How does the conductivity (thermal and electrical) of free machining steel compare to other metals?

Free machining steel’s electrical and thermal conductivities are generally lower than metals like copper or aluminum. The addition of certain elements to improve machinability can also reduce the steel’s conductivity. However, it’s still more conductive than non-metals or insulating materials. When considering applications where conductivity is a primary concern, materials like copper are typically preferred.

How does free machining steel perform under extreme temperatures?

The performance of free machining steel under extreme temperatures varies based on its specific alloy composition. Generally, free machining steels maintain their strength and toughness at elevated temperatures, but they might lose some hardness. At very high temperatures, like those in some industrial processes, the steel can soften or lose tensile strength. Conversely, at extremely low temperatures, some free machining steels can become more brittle. It’s essential to consult with a metallurgist or material expert when considering free machining steel for applications with temperature extremes.

Can free machining steel be welded?

Yes, free machining steel can be welded, but it comes with challenges. The additional sulfur, lead, or bismuth used to enhance machinability can create issues during welding. These elements can lead to porosity, reduced weld strength, or unfavorable weld bead appearance. Pre-welding preparations, such as using appropriate filler materials or adjusting the welding process, can mitigate some of these challenges. However, for critical applications where welding strength is paramount, alternative steel grades with better weldability might be preferred.

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