Machining operations can get difficult when working with harder materials.
To ease the process, manufacturers engineered the structural properties of materials to improve their machinability. Free-machining steel is one such material.
Free machining steel is a grade of carbon steel having sulfur, phosphorus, and manganese as its building blocks to lower cutting resistance. They are also engineered with lead, tellurium, bismuth, or selenium as additives. Free-machining steel's soft, malleable, and ductile nature provides high machinability.
This article provides detailed information about free-machining steel by discussing its composition, properties, types, and applications.
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Free-machining Steel: What is It?
Composition
Free machining steel is a type of carbon steel. It has low and high-carbon variants.
Low-carbon steels have only up to 0.25% carbon content, while high-carbon steels can have up to 1.5% carbon content.
The low carbon content makes low-carbon steel soft and easy to machine. Unfortunately, this softness results in long chip formation as they don't break off easily.
It's important to timely remove the chips from the workpiece as they are made. Otherwise, it can damage the tool and the workpiece.
The steel is engineered with sulfur and phosphorus to break off and remove the chips effectively. Sulfur reacts with manganese and forms manganese sulfate, which promotes chip-breaking.
So as the sulfur content increases, you can achieve faster chip removal, promoting tool life.
Comparatively, high-carbon steel has low machinability, but you can improve it by adding lead.
The maximum composition of lead they can have without affecting their fundamental structure is 0.35%.
Lead has a spherical structure that promotes chip breakage. Hence it increases the machinability of high-carbon steel.
The soft and ductile nature of free-machining steel also makes it suitable for forging steel applications.
The general composition of free-machining steel includes sulfur, phosphorus, and manganese.
| Material | Composition |
|---|---|
| Carbon | up to 0.35% |
| Sulfur | up to 0.6% |
| Phosphorus | up to 0.12% |
| Manganese | up to 4.8% |
The manganese content is decided based on the amount of sulfur in the composition. It can be about five to eight times the amount of sulfur.
Properties of Free-machining Steel
Low-carbon variants of free-machining steels are very soft. Hence they are ductile or malleable, providing good machinability.
High-carbon free-machining steels are tougher than low-carbon steels. But they show good formability.
Free-machining steels will have a uniform distribution of manganese sulfide, providing dimensional quality to its structure.
The surface hardness of free-machining steels is decided by the pre-processing it goes through. Meaning you can adjust its tolerance for a specific machining application.
For example, you can perform steel annealing to enhance its ductility, or tempering to enhance its hardness.
Similarly, special tools are required for machining steel, such as grinding wheels labeled as Inox are suitable for grinding stainless steel and free machining steel.
In general, free-machining steels are cold-processed through grinding, cold rolling, drawing, etc., to convert them into a bright bar.
The extent of such processing dictates their machining performance.
Types of Free-machining Steel
There are four types of free-machining steel
- Leaded
- Resulfurized
- Rephosphorized
- Super
Leaded free-machining steel
Leaded free-machining steel has lead added to its composition. Lead acts as a shearing accelerator and solid lubricant, reducing the chip load during the machining process.
Under the SAE (American) standards, these steels are graded as 12LXX (12L13 and 12L14). They have a machinability rating of 170%.
Resulfurized free-machining steel
Resulfurized steels are graded as 11XX (1117, 1118, and 1119) under the SAE standards. They have added refined sulfur (maximum 0.4%) for better machinability and short chip formation.
The machinability rating of resulfurized free-machining steel is 91%.
Rephosphorized free-machining steel
In rephosphorized steels, refined phosphorus is added to improve their corrosion resistance and machinability.
Under the SAE standards, these steels are graded as 12XX (1211,1212, 1213). They have a machinability rating of 136%.
Super free-machining steel
Super free-machining steels have bismuth, tellurium, and selenium as additives. They reduce the toughness of steel and improve its machinability.
The maximum composition of additives on super free-machining steel is as follows.
- Bismuth - 0.10%
- Tellurium - 0.03%
- Selenium - 0.15%
Applications of Free-machining Steel
The ease of machinability is the major advantage of using free-machining/cutting steel. It provides a faster machine cycle.
Because of that, free-machining steels are used to make small and complex parts that are otherwise hard to machine with other metals.
Some of the general applications include making automobile parts, machine tools, instruments, nuts, bolts, screws, pipes, appliances, key stock, gear, etc.
Final Thoughts - The Pros and Cons
Free machining/cutting steel is a great material to work with. It's easy to machine and only produces small chips that easily break away from the stock.
This allows for long machining run times and reduces the cycle times.
Manufacturers supply free machining steels as cylindrical bars, and are primarily used for turning jobs on compact part-making machines like a Swiss lathe.
Because of the engineering behind the making of free-machining steel, they are priced higher than regular steel.
Unfortunately, since the hardness of free-machining steel is reduced for ease of machining, it lacks other properties like ductility, toughness, etc., and can get brittle over time.
Frequently Asked Questions
Can I weld free-machining steel?
Yes, you can weld free-machining steel, but it's prone to dilution and cracking. To weld free-machining steel with minimal damage, you should work with low-current settings and use special low-hydrogen electrodes.
Is steel easy to machine?
Yes, steel is easy to machine. Low-carbon steels are the easiest to machine because of their soft nature. The hardness of steel is directly proportional to its carbon content, which in turn lowers its machinability.
What is the hardest metal to machine?
Tungsten is the hardest metal to machine. This is because of their hard and brittle nature. Its material density is also very high (19.28 g/cm3).
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