What are files made of?
As abrasive tools, files need to be resistant to wear and have a high tensile strength (resistance to bending).
They also need to be hard enough to cut into other metals, including other types of steel. If the steel that the file teeth were trying to cut into was harder, they would just break.
For these reasons, files are made from high carbon, water-hardened, chrome-alloy steel.
Why is carbon used in file-making?
The steel used for files is very high in carbon content, as it makes up 1.35-1.4% of its composition. 1.5% is normally the maximum for carbon steels.
Carbon molecules reinforce the structure of steel, making it harder than normal.
This also makes the steel resistant to being worn down by abrasion.
Why is chromium used in file-making?
Chromium makes up around 5-7% of the steel used in file making.
When more than one element is used to alloy with steel, the resulting material benefits from all qualities of each added element cumulatively.
Like carbon, chromium also has hardening properties. When both are used, this increases the hardness of the resulting steel alloy to a great degree.
This not only helps the tool to be able to cut through other metals, but also helps it to stay sharp.
Chromium also helps to increase the toughness of the steel, making it less likely to shatter or crack under the application of extreme force.
Adding carbon can often make steel less tough. Adding chromium counteracts this potential weakness.
How does water-hardening benefit steel use in file-making?
Files are tempered after they have had their teeth cut by being repeatedly heated until they are red hot and then plunged into a bath of brine.
This is an inexpensive way of heat treating steel. It also tends to make the metal harder on the outside, leaving the core relatively soft in comparison.
With its tough outer shell and its soft, squidgy insides, the armadillo is the perfect illustration of this particular concept!
This is a great advantage to files, as the softer core supports the outer shell, during the abrasion process, preventing damage to the file and increasing its resistance to being bent.
Spring steel and flexible files
Spring steel is a low alloy, low carbon steel with a high yield point.
This means that it will return to its original shape if bent or twisted.
This type of steel is used in a particular type of file that is designed to be mounted on an adjustable handle that will bend the file’s face into a convex or concave surface. This type of file is called a flexible file.
Alternative abrasive materials
Rather than being cut with teeth, some files have grit made of hard materials such as diamond or tungsten carbide embedded into their surface. Irrespective of which material is used, these tools are referred to as diamond files.
Diamond is the hardest material known to man, which makes it perfect for abrasive tools. Files with diamond grit can be used on hardened steel surfaces and are sharp enough to file brittle materials such as glass or ceramic, which would break if pressure or excessive force was applied through regular file teeth.
While not as hard as diamond, tungsten carbide is still hard enough to work on tool steel and ceramic but is much cheaper. This makes tungsten carbide files more affordable.
This may be a preferable option to DIYers, as the difference in performance between tungsten carbide and diamond should not be noticeable in the home.