Yes you can "grind" out a knife from a lawn mower blade without having to heat-treat it again - just relying on the original heat-treat of the mower blade. But it will be softer than a regular knife blade, so the edge will dull down in use a little faster. But you will also be able to re-sharpen it with a common file. Just be sure to cool it often while grinding. Don't let it get too hot to hold in your hands.
The original heat-treat on lawn mower blades is more for toughness than edge holding ability. They have to take A LOT of abuse without breaking! So that original heat-treat is more like for a good axe/hawk/hatchet than for a quality knife. You will just need to touch up the edge a little more often. But, for a "working blade", that ends up being a better option anyway. And if you make a BIG knife, it will hold up to any heavy chopping and log splitting you do - what that modern survival guy on British TV named Ray Mears calls "batton-ing" (using your heavy knife to split kindling.
Of course, if you take new high-carbon tool steel and grind out a knife blade, it will also work fairly well as-is without heat-treating. That's just part of the nature of high-carbon tool steel.
Most knife makers us 1095 steel for making their "carbon steel" knives. Lawnmower blades are generally 1085 carbon steel, or 5160 carbon alloy steel. So they only have a few point less carbon than that popular knife steel. For reference, common "welding shop" steel is around 1018 or 1020 steel. 18 to 20 points of carbon compared to 85 to 90 points of carbon.
So, yes, you can carefully grind a knife blade from a lawnmower blade and not have to heat-treat it. You will just have to touch up or re-sharpen the edge a little more often.
Just my humble thoughts to share. Take them as such.
Mikey - that grumpy ol' German blacksmith out in the Hinterlands
p.s. Many original cheap "trade knives" were made of steel with a low enough carbon content, or heat-treated soft enough, that they were fairly easily sharpened with a common file. The same with the trade axes and hawks. It's why traders also carried and sold files. They were commonly re-sharpened with those files.
The original heat-treat on lawn mower blades is more for toughness than edge holding ability. They have to take A LOT of abuse without breaking! So that original heat-treat is more like for a good axe/hawk/hatchet than for a quality knife. You will just need to touch up the edge a little more often. But, for a "working blade", that ends up being a better option anyway. And if you make a BIG knife, it will hold up to any heavy chopping and log splitting you do - what that modern survival guy on British TV named Ray Mears calls "batton-ing" (using your heavy knife to split kindling.
Of course, if you take new high-carbon tool steel and grind out a knife blade, it will also work fairly well as-is without heat-treating. That's just part of the nature of high-carbon tool steel.
Most knife makers us 1095 steel for making their "carbon steel" knives. Lawnmower blades are generally 1085 carbon steel, or 5160 carbon alloy steel. So they only have a few point less carbon than that popular knife steel. For reference, common "welding shop" steel is around 1018 or 1020 steel. 18 to 20 points of carbon compared to 85 to 90 points of carbon.
So, yes, you can carefully grind a knife blade from a lawnmower blade and not have to heat-treat it. You will just have to touch up or re-sharpen the edge a little more often.
Just my humble thoughts to share. Take them as such.
Mikey - that grumpy ol' German blacksmith out in the Hinterlands
p.s. Many original cheap "trade knives" were made of steel with a low enough carbon content, or heat-treated soft enough, that they were fairly easily sharpened with a common file. The same with the trade axes and hawks. It's why traders also carried and sold files. They were commonly re-sharpened with those files.
