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
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.
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.
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