Car leaf springs are a great source for knife steel. They are typically made from 5160 steel which is known for its high carbon content and durability.
While they can make excellent blades, used leaf springs aren’t always available and can be a risky choice. That said, if you’re in the market for a knife and are willing to gamble on leaf springs, it can be worth it!
Heat Treating
Heat treating is a process that uses controlled heating and cooling to alter the physical properties of a material. It can be used to increase the strength, hardness or ductility of a metal.
The process can also be used to improve wear resistance. The hardening process can either be done all over (case hardening) or only in one area of the metal, leaving the rest untouched.
Tempering is a process that is often used to change the hardness, ductility and strength of iron-based alloys. It can make the materials easier to machine and increase their tensile, yield and fracture toughness.
The first step of the process is to heat the material up to a certain temperature. Then it is cooled down by using a cooling medium, such as water or oil.
Tempering
When you heat a metal to a certain temperature, it can become strong or flexible. This process is known as quenching and tempering.
Quenching hardens a steel by changing the microstructure of the material. It also makes it more resistant to wear and tear.
But it can also make the material very brittle. This is especially true for carbon steel that is heated and rapidly quenched.
Tempering is a heat treatment that occurs after the quenching process and allows for carbon diffusion to occur within a steel’s microstructure. It also reduces extra hardness and improves ductility.
It is important to temper after a quenching process because it can help the steel retain its hardness and strength while still being ductile enough for use in your project.
A tempered knife is useful for many tasks that require a tough, durable tool. Tempering also is necessary for other metal products, including tools and springs, that will be subjected to heavy stress.
Sanding
When you sand your car leaf spring, it’s important to take the time to sand it down until you’ve removed both paint and primer. This will make the new coat of paint go on much smoother.
You can either hand sand your spring with 1200-2000 grit sandpaper or you can buy a power sander. Either way, you’ll need to sand it until the bare metal shows through.
Before you sand the spring, wipe it down with acetone to remove any contaminates, like grease oil and dirt. Then, you can spray the springs with three, four, or five light coats of high-temperature engine paint.
You can also replace the bushings on your springs. This is a simple process that can be accomplished with an Advance Auto Parts loaner suspension press and an impact gun/driver.
Finishing
A lot of custom knife makers use car and truck leaf springs to make their blades. These discarded automotive parts are made from 5160 carbon steel, which is a good steel to work with for making knives.
Many used leaf springs are rusted and damaged from normal use, but they’re still strong enough to be forged. This means that they’re a great source of knife steel because you can heat treat them and get the right thickness with reasonable equipment.
However, you have to be careful when using a used spring for this project. There are a lot of small fatigue cracks in these springs, which don’t grow when you forge them but can cause major problems during the quench process if you don’t get them removed.