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Should I Forge a Knife From a Lawnmower Blade?

Blacksmithing is both a hobby and a useful skill. I initially planned to forge knives, but smaller projects are easier when first learning.

Basic Blacksmithing Tips for Beginners

In this article, you'll learn:

  1. Whether or not you should forge a knife from a lawnmower blade
  2. The kind of steel you should use for forging a knife
  3. Why you shouldn't use "mystery steel" as a beginner
  4. The type of steel used for lawnmower blades
  5. Why guessing at the best heat treatment is a risk factor
  6. Why stress points are risk factors
  7. The best options for repurposing steel
Basic Blacksmithing: Should I forge a knife from a lawnmower blade?

Basic Blacksmithing: Should I forge a knife from a lawnmower blade?

Should I Forge My First Knife Using a Lawnmower Blade?

The short answer is no, you should not forge your first knife from a lawnmower blade. I will go over the reasons below.

What Steels Should I Use Instead?

Instead of using a found lawnmower blade, use these steels:

  • 5160
  • 1070
  • 1080
  • 1084

These steels are inexpensive, simple to work with, and have heat treatment recipes that are easy to follow, using commonly available ingredients. All these steel types contain at least 0.6% carbon, but not above 0.84%. Once the carbon content gets above 1084, heat treatments may become more complicated.

If you're feeling confident, 80crv2 is another option. 80crv2 is like a combination of 5160 and 1080. All the above steels create tough knives that can be made to be very sharp. However, they can rust. When you are learning, using a known, simple steel is great for getting the fundamentals of blacksmithing down.

Why Should I Avoid Using "Mystery Steel"?

When you are first getting the fundamentals of blacksmithing down, it is important to introduce as few variables as possible, in order to develop your skills. If you're using mystery steel, you don't have a way to consistently predict how it will act under the hammer. Little inconsistencies add up, and they can make the process of getting the basics down take longer.

By using the same steel when you are making your first few (hundred) knives, you can make things go easier. No surprises. This will give you a starting point that you can compare once you branch out to different steels.

Make Sure Your Steel Can Harden (Otherwise, You Can't Make a Knife)

Mower blades are made from different metals. Depending on the metal, some don't even have the ability to harden, while others do. If the steel you use cannot harden, it cannot make a suitable knife. It might look like a good knife, but it will never hold an edge to cut consistently, and it may bend easily or even break.

This does not mean you shouldn't save mower blades. Just treat them as mild steel (which they likely are).

How Do I Know If My Mower Blade Steel Can Harden?

You can use a spark test in order to determine if the steel can harden and see if it passes. It may harden, or at least contain some amount of carbon. But let's explore the next problem.

Old mower blade - NOT KNIFE STEEL

Old mower blade - NOT KNIFE STEEL

Can I Heat Treat Mower Blade Steel?

Some mower blade steels can harden. You've demonstrated with the spark test that the blade can be hardened. But how are you going to heat treat it? You're going to have to take a guess at what kind of carbon steel it is.

The Risks of Guessing at a Generic Heat Treatment

If it can harden, it is likely something like 1084, but it isn't really known. Should you use water? Brine? Should you use oil? Parks 50? What temperature should the quenchant be? These are questions to which you will need to know the answer, or at least a good guess.

You might be leaving some performance on the table by using a generic heat treatment. Worse yet, perhaps you used the wrong quench liquid, causing it to fracture your first knife. The "tink" sound you hear during a quench can be heartbreaking.

Why Stress Points Are Risk Factors

Let's say you found a mower blade. It's high carbon and hardenable. Finally, you guessed correctly that it was 1084, so you managed to use the right heat treatment. After all that luck, time, work, and skill, you still are using a piece of metal with an unknown history. That may, and likely will, include hitting rocks, roots, sticks, and other debris.

This continued use and abuse can lead to stress points and small fractures in the steel. Once completed, your handcrafted knife may include a weak spot that can cause it to fail prematurely, and under normal use.

Top: Nicholson Black Diamond File.  Possible Knife Steel

Top: Nicholson Black Diamond File. Possible Knife Steel

Best Options for Repurposing Steel

Purchasing known steel from a company like New Jersey Steel Baron, Amazon, or even eBay will lead to more consistent results. Consistency is what is important when you first start forging. That being said, I can understand the desire to repurpose scrap materials. I have a pile of metal (including some mower blades) myself. That is what led to me researching what kind of steel mower blades generally were.

So, if you want to use scrap steel to make a knife, I've found some options that might work. These will still have some of the drawbacks mentioned above, but you will have a better likelihood of having a good steel.

Scrap Options for Making Your First Knife

  • Leaf spring from truck suspension (possibly 5160)
  • Coil spring from suspension (possibly 5160)
  • Old file, especially USA-made Nicholson. See the image below—this steel requires a careful heat treatment.
  • Old saw blade. Not a newer, carbide-tipped saw blade. This also works well for stock removal.
Nicholson Black Diamond File.  High carbon steel for forging a knife.

Nicholson Black Diamond File. High carbon steel for forging a knife.

So, You Have Decided to Make a Knife from a Lawn Mower Blade...

I have a few suggestions if you have decided to use a mower blade to make a knife, despite all the reasons avoid it.

If you are going to make a knife from a lawn mower blade, choose a knife style that benefits from excessive toughness. Think of knife styles that are used for chopping as opposed to slicing.

Examples of tough knives:

  • Bowie Knife
  • Machete
  • Parang
  • Khukri

Thinner, sharper knife styles should not be used. A mower blade is less likely to do well for a traditional chef's knife.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2020 Devin Gustus

Comments

Danny from India on October 11, 2020:

Very informative article, on forging metals